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This chapter describes geometry modeling techniques for the part and melt-delivery systems as well as the mold cooling systems. These techniques are mainly intended for injection molding and gas-assisted injection molding applications. You'll also find helpful information on special-purpose modeling techniques for blow molding and thermoforming, transfer molding for microelectronics encapsulation, and structural-reaction injection molding.
The objective of geometry modeling is to describe the geometry and dimensions of the part, sprue, runners, and cooling channels as a system of finite elements with appropriate attributes and connectivities. The procedures consist of creating the part geometry, runner system, and cooling channels; specifying the associated attributes (such as dimensions, and mesh size); discretizing the geometry into finite elements; and defining the entrances for the polymer and gas, as well as the coolant entrances and exits. This information is stored in the finite-element mesh file, in .fem format, and is part of the required input data for the C-MOLD analyses.
Since most injection-molded parts are thin compared with their global dimensions, they can be approximated by their mid-planes or surfaces. Thickness and shape factor attributes are used to incorporate the actual geometry features as they are modeled by the triangular shell elements.
Similarly, a one-dimensional (1-D) element can be used to model a sprue/runner/gate, cooling channel, or connector by properly assigning its element attributes. For example, 1-D elements can be used to model hot and cold runners, as well as "part runners" (rod-like features in the part such as pins or grills). One-dimensional elements can also be used for the various types of cooling channels, with the appropriate channel type assigned. Connectors are used as multipliers to simplify the modeling of symmetric geometries. Connectors can also be employed to model valve gates used in some special processes, where the valve gate is opened and closed under a timer control.
General Procedures for Injection Molding
This section describes the general procedures for geometry modeling using C-MOLD Modeler. The major task in geometry modeling is to describe the part (e.g., Figure 3-1) by a finite-element mesh (e.g., Figure 3-2) that carries the proper dimension and attribute information.
Figure 3-1. Geometry of an injection-molded part, including sprue, runner, and gate.
Figure 3-2. Finite-element mesh of a single cavity, plus cooling channels. Counterpart is modeled by specifying connector multiplicity factor.
The steps involved in creating a geometry and mesh model are described below.
- Define the part by its mid-plane or surface.
- Create the boundaries of the regions and surfaces by defining points, lines, circles, arcs, splines, and surfaces, and using utility commands such as copy, rotate, reflect, intersect, project, and corner (Figure 3-3).
Figure 3-3. Boundary edges of part geometry.
- Connect end points to create one-dimensional geometry for runners, cooling channels, and connectors.
- You'll need to define the diameter and shape factor attributes. The finite elements generated on each one-dimensional geometry entity will carry these attributes as their element properties (e.g., Figure 3-4).
Figure 3-4. Model of runners, connectors, part runners, and cooling channels.
- Define the topology of the part surfaces.
- Each region or surface region is defined by a closed loop of consecutive edges. A semi-automatic region generator assists in defining the topology. When you pick two edges on a region or surface, the region generator automatically closes the loop to create the region. You can define inner features such as holes within each region or surface region. C-MOLD assigns the element attributes of a region to all the finite elements in the region.
- Generate the finite-element mesh.
- Specify a global mesh size and, if variable mesh density is desirable, assign local mesh sizes to geometric entities, such as points, lines, arcs, and surfaces. Invoke the automatic mesher to discretize the geometry into elements based on the mesh size(s) specified.
- Define the plastics-specific attributes of the finite-element mesh.
- Define attributes, such as entrances for polymer and gas, and coolant manifold entrances and exits (e.g., Figure 3-5). Define the orientation of the parting plane for the clamp force calculation. The default normal direction to the parting plane will be parallel to the z-axis.
Figure 3-5. Plastics-specific attributes for the analyses.
- Save a finite-element mesh file.
- If a modeler other than C-MOLD Modeler is used, an interface program is available to transfer the geometry or mesh to C-MOLD .fem file format. Further details of the FEA interface are discussed in Chapter 4, "Using Translators."
This section describes several of the attributes and how they are utilized in modeling. For an introduction to attributes, see "Attributes Editor" in Chapter 2; for a description of individual attributes, see Chapter 5, "Command Reference," and for a complete list of attributes and their default values, refer to Appendix A.
Diameter and shape factor
Diameter and shape factor are 1-D element attributes used to incorporate the actual geometry features. The diameter is specified as the actual diameter for a circular cross section of the element, and the shape factor is equal to 1. The diameter of a baffle or a bubbler is the diameter of the hole drilled in the mold.
For 1-D plastic elements having a non-circular cross section, the diameter is defined by an equivalent circle with the same cross-sectional area. The shape factor is introduced to account for the extra wall effects (such as cooling and flow restriction) from the surface compared to the equivalent circular shape. Shape factor is defined as the ratio of the actual perimeter of the cross section to the perimeter of the equivalent circle. Figure 3-6 illustrates the definition of shape factor on a rectangular tube.
Figure 3-6. Definition of equivalent diameter and shape factor of a rectangular tube.
When 1-D elements are used to model a section of sprue, runner, or gate, the shape factor should always be equal to 1 (circular) or greater than 1 (non-circular). Allow exceptions, however, when 1-D elements are used in conjunction with the triangular elements for modeling gas channels in the parts. The shape factor can be less than 1, depending on the geometry, as detailed on page 3-55. For the same equivalent diameter, a larger shape factor means the cross section is more irregular and has more cooling effects from the wall, as well as greater flow resistance from the cross section. C-MOLD analyses automatically adjust these two effects for 1-D elements, based on their respective shape factors. Table 3-1 lists the equivalent diameter and shape factor for several commonly used 1-D geometries.
Table 3-1. Equivalent diameters and shape factors for common runner and gate geometries.
Thickness and shape factor
Thickness and shape factor are attributes for triangular elements, similar to diameter and shape factor used for 1-D elements. When a surface has a regular cross-section (flat on both sides), the thickness is specified and the shape factor is equal to 1. However, surface patterns are frequently used in plastic part designs for appearance and functional reasons. There are two ways to model parts with surface patterns: use elements that are smaller than the feature in the surface patterns, or use elements with an equivalent thickness and a shape factor to correct for the additional cooling and flow resistance from the surface patterns. The first method could result in a very large number of elements for models with small features in the pattern. The second method is a more efficient way to solve practical problems.
The shape factor for a triangular element is defined as the ratio of the actual contact surface to that of the equivalent flat surface. A slab-like surface is shown in Figure 3-7. The equivalent thickness and shape factor are calculated from one basic unit of the pattern, which is repeated.
The basic unit of the pattern has a projection area, A; a contact surface, S, which includes the top, bottom, and lateral areas; and the volume, V, between the top and bottom surfaces. For the same equivalent thickness, larger shape factor means a given volume has more contact surface and has more cooling effects from the wall as well as flow resistance from the geometric irregularity. C-MOLD analyses automatically adjust these two effects for triangular elements based on their respective shape factors.
Figure 3-7. Equivalent thickness and shape factor of a slab-like surface.
There are two ways to model geometries with grills, depending on the relative dimensions of the grill and the mesh size. If the length of a grill segment is larger than the mesh size, as in Figure 3-8(a), the grills should be modeled by 1-D elements with the proper equivalent diameter and shape factor, as described above. But, if the grill size is small in relation to the mesh size, the total grid area should be modeled as triangular elements with an equivalent thickness and shape factor, as shown in Figure 3-8(b).
Figure 3-8. Modeling geometries with grills. (a) Grill size is larger than mesh size. Use 1-D elements with equivalent diameter and shape factor to model the grill. (b) Grill size is much smaller than mesh size. Use triangular elements with equivalent thickness and shape factor to model the grill.
The initial fill condition is used to determine whether or not a part element is filled with polymer at the beginning of the injection cycle. Its purpose is to differentiate between hot and cold runners. A part element is
Empty (= 0), unless otherwise specified with this element attribute. Two more conditions can be specified for a part element.
Filled At Inlet Melt Temp. (= 1) indicates that the element is initially filled with polymer at inlet melt temperature.
Filled At Hot Runner Mnfld. Temp. (= 2) indicates that the element is initially filled with polymer at its wall temperature. Note that the initial fill condition applies to both 1-D plastic elements and triangular plastic elements. C-MOLD Modeler automatically assigns
Empty to all triangular elements, cold runners, and part runners, and assigns
Filled At Inlet Melt Temp. to all hot runners.
Hot Runner Manifold ID
When creating a hot runner, C-MOLD Modeler requests a hot runner manifold ID, which points to the hot runner manifold control specified as one of the process conditions. C-MOLD analyses use the temperature specified in the associated hot runner manifold control as the wall temperature of a hot runner.
It is a common practice to control temperatures by heaters at individual sections of a hot runner manifold to achieve a controlled flow distribution. All hot runner elements in one section should then point to one hot runner manifold ID and assign the proper wall temperature and insulation thickness. For modeling hot runners that are separated from the mold base by an air gap instead of glass fiber insulation, consult the C-MOLD Cooling User's Guide.
A special element type called a connector is available for convenience when modeling symmetric parts or multi-cavity molds. The connector, which connects two nodes in the finite-element mesh, can be used in three applications: to include symmetrical branches without modeling all of them (the connector is considered to be a multiplier); to better represent runner/runner or runner/cavity connections to avoid significant volume overlapping that may cause inaccuracy in overall volume calculation and numerical problems for mold cooling analysis; and to model valve (shut-off) gates that open and close at different times.
Using connectors as multipliers to model a multi-cavity mold or symmetric parts reduces redundancy in modeling, thereby reducing the CPU time required for the analysis. Whenever there is a branch leading the flow of polymer to several identical paths or geometries, a connector can be placed at that branch point, and only one path needs to be modeled. The total number of identical paths or geometries is the multiplicity factor of that particular connector.
Figures 3-9 and 3-10 indicate the applications of connectors for multi-cavity systems and symmetric parts. The numbers next to the connectors are the multiplicity factors; the arrows indicate the directions in which the number of cavities should be multiplied. That is, a connector has an orientation when its multiplicity factor is greater than 1.
The direction of a connector is from the first selected point to the second selected point. The length of the connectors has no influence on the numerical results because there is no flow resistance in connectors. All physical conditions are equal at the two end nodes of a connector. Therefore, the connectors can be made short so that the plastic elements will look as if they are still connected in space.
In most cases, the orientation of a connector should be along the polymer flow, from upstream to downstream. There are exceptions, as indicated in Figure 3-10(c), where a connector with a multiplicity factor of 2 at the merging point is oriented against the flow direction. This is because there are two identical branches from upstream, meeting at the merging point.
Note: Because connectors are used only for the purpose of modeling, they must be used with caution. Connectors should not be used to connect two nodes belonging to the same element, or any two nodes on which the physical conditions might not be identical. You should also keep in mind that, in some cases, the cavities/parts are symmetric, but the cooling system is not.
All surfaces in C-MOLD Modeler must be topologically equivalent to a rectangle (i.e., they must be bounded by four edges). Degenerate cases will fail to mesh. For example, revolving a 90 degree arc around a line that shares an end point with the arc to create a hemisphere creates a degenerate case (one edge of the equivalent topological rectangle collapses to a single point). A bounded surface consisting of three curves that form a triangular patch would be another illegal surface. Note that creating geometrical surfaces is a different issue from creating surface regions, which do not have to map to rectangles.
Figure 3-9. Use of connectors to model multi-cavity systems.
Figure 3-10. Use of connectors to model symmetric parts.
General Modeling Tips
Tips for Surface Creation
When creating a revolved surface, the position on the axis of revolution where you pick with the mouse is important. The line chosen to be the axis is considered a directed line, whose starting point is the end point closest to where you picked. The right-hand rule of rotation is then applied, using this directed axis (Figure 3-11).
The starting point is the end point of the entity closest to where you picked. The curves generated to complete the lofted surface will go from starting point to starting point, so if you pick, for example, two lines at "opposite ends," the resulting surface will be "twisted" and any mesh generated from it would be illegal (e.g., Figure 3-12).
Figure 3-11. Creating a revolved surface.
C-MOLD Modeler will not attempt to extend one entity to meet another.
Figure 3-12. Creating a lofted surface.
This occurs because C-MOLD Modeler smoothly interpolates control points for the surface boundary, whereas the entity used might not have such smoothing. (This is most likely to happen when using splines as boundaries). While displays could show some discrepancy as a result, mesh nodes along the boundary are guaranteed to lie along the original entity.
Tips for Surface/Surface Intersection
A simple instance of this case is two cylinders of equal diameter whose axes of revolution are coplanar (i.e., an X-joint pipe piece). Another simple case is two cylinders that touch each other: the intersection would be the common tangent line. If you really need such a tangent point or curve for your model, form the surfaces such that the tangents fall on the boundary curves of both surfaces (i.e., split the cylinder into four "swept arc" surfaces), as shown in Figure 3-13.
The entity must not extend beyond the edge of the surface, nor can it cross the surface's seam, if any. If you need to do such a projection, first create a swept surface; use the entity to be projected such that the intersection of the swept surface with the original surface will be the desired projection of the original entity. (Note that such a swept surface intersection will then be subject to the limitations noted above). Then intersect the surfaces to create the desired curve.
Figure 3-13. Surface/surface intersection.
Tips for Projecting Entities to Surfaces
C-MOLD Modeler assumes that the intersection set is a finite set of points, which is not the case when the entity lies on the surface.
Tips for Surface Region Generation
Use the command Edit Geom.; Break Edges; Break Edges on Surfaces to pick two curves on the same surface. The order in which you pick the curves is not important (both entities get broken), but the position where you pick on the curves is. You need to pick close to the intersection point where you want the curves broken, because C-MOLD Modeler breaks at only one intersection point, and uses the pick location to decide where to break. Then, before creating a surface region, you might want to delete superfluous entities on the surface. Do not delete entities that form the boundary edges of the surface.
The auto. search for region generation will occasionally be unable to determine what constitutes a region. We haven't isolated any hard-and-fast rules to warn you when this might occur; if it does, turn off Auto. Search and pick all the edges manually, in sequential order of the desired region.
Part and Delivery System in Injection Molding
This section discusses the general geometric modeling techniques for the part and delivery system. These modeling techniques provide basic guidelines for constructing finite-element meshes of typical injection molded parts. The objectives are to yield more accurate predictions, as well as to allow more efficient computation. These techniques are applicable to any geometry modeler and mesher.
Model the part on the mid-surfaces; model runners along the axes.
There is no need to create a solid model (a model with part thickness). C-MOLD analyses use two-dimensional, triangular elements, and 1-D elements with the attributes of thickness and diameter, respectively. To maintain correct spatial relationships, triangular elements should be modeled on the mid-surfaces and one-dimensional elements should be modeled along the axes of a part and its delivery system.
When C-MOLD analyses import geometry from a CAD modeler other than C-MOLD Modeler, only a surface model should be used. This significantly reduces the time required to construct topology information in C-MOLD Modeler before meshing. If the mid-surface is not available in the CAD database, you can use the surfaces on one side of a part, as long as the part's thickness is less than its global dimensions.
Define major surfaces first, then add the details.
Elaborate reproduction of geometric details is not necessary. Don't labor over the small details of the geometry, such as round corners or small holes (e.g., Figure 3-14); for modeling, these details require many small elements, even with a variable mesh density, but do not influence the overall melt-front and pressure predictions. They can, therefore, be simplified by using a square corner to approximate a round corner, and so on. Care should be taken to match the volume of the modeled part and the actual part as closely as possible.
Figure 3-14. Details smaller than the mesh size on the part can be omitted in modeling
The flow analysis is based on a fixed finite-element mesh, so computed variables, such as the moving melt front, must be interpolated within the elements. Thus, the inherent inaccuracy of melt-front advancement, weld-line location, and gas or core polymer penetration predictions is limited to one element length. Any features in the geometry that are smaller than the mesh size, such as a small hole, need not be modeled unless there are other considerations. Broadly speaking, the CPU time for the simulation is approximately proportional to n2, where n is the number of nodes.
Although finer mesh size typically results in better resolution, a model with a large number of elements and nodes is not recommended, especially for the initial iterations. In general, a model with 1,000 to 5,000 elements is appropriate for a relatively complex part. Variable mesh density is desirable if small features of a geometry are modeled, because it provides the flexibility to refine the mesh over critical areas and regions, while maintaining relatively coarse mesh for other areas.
Using 1-D Elements for the Delivery System
A delivery system can be described by using runners and/or connectors. To create a runner, supply the runner attributes and two points to define the runner. Runner attributes include runner type, starting diameter, tapered half-angle, and shape factor.
In a delivery system, the runner type can be either cold or hot. (A hot runner must also be given the hot runner manifold identification number.) A tapered runner is defined by the diameter at the starting point and the tapered half-angle. The tapered half-angle can be either zero, if the runner is straight, or non-zero, in which case C-MOLD will calculate the diameter at the ending point to define the runner. The geometrical definition of a tapered runner is given in Figure 3-15.
Figure 3-15. Geometrical definition of a tapered runner.
A sprue is a typical tapered runner with a positive tapered half-angle. Some gates can also be described by using a tapered runner with a negative tapered half-angle. A tapered runner can also be approximated by using smaller straight runner segments with different diameters, in which case no information about tapered angle is assumed. The tapered half-angle is defined in degrees by default, but it is saved in radians in the finite-element mesh file, to be consistent with the SI standard.
Note: The order in which you define the end points is important since this determines the direction of the runner. For the flow direction to be correct, you must define the starting point first, followed by the ending point.
To modify runner attributes, see "Attributes Editor" in Chapter 2. For convenience in modifying a tapered runner, you can specify the ending diameter, instead of using a tapered half-angle.
If the volume of a delivery system is significant in the overall mold design, it is important to conserve the runner volume in the model. We strongly suggest that connectors be used for this purpose. In Figure 3-15(b), connectors are used between a sprue and a primary runner, and between a primary runner and a secondary runner, to avoid overlapping. We also suggest you use connectors to model naturally balanced multi-cavity systems or symmetric parts, as discussed in "Connector Multiplicity" on page 3-15.
If a naturally unbalanced delivery system is to be modeled, and runner balancing is required, it is important that the delivery system have a tree structure, and that a slug well be used properly, as mentioned in the C-MOLD Filling and Post-Filling User's Guide.
The aspect ratio of triangular elements is important to the accuracy of the solution, especially in sensitive areas such as gates, or gas channels in a gas-injection simulation. The rule of thumb is that the average aspect ratio of triangular elements should be less than 5. Triangular elements with an aspect ratio greater than 10, as reported by the analysis, should be modified.
Figure 3-16. Modeling of delivery system, where (a) runner volume is not conserved, and (b) runner volume is conserved.
Pin gates are modeled by a small section of a cold, solid runner.
Figure 3-17. Pin gate, shown with hidden element lines.
Submarine gates are modeled by tapered runners.
Figure 3-18. Submarine gate, shown with hidden element lines.
Tab gates are modeled as regions with a small gate thickness. Connectors are used to ensure the tab gate entrance nodes have the same conditions as at the end of the runner.
Figure 3-19. Tab gate, shown with hidden element lines.
Fan gates are modeled as regions. If the thickness of the fan gate varies, two or three regions should be used, each with a different assigned thickness.
Figure 3-20. Fan gate, shown with hidden element lines.
Valve (shut-off) gates are modeled as solid runners with an equivalent diameter and shape factor. A connector must be attached to the end of a valve gate to control the delayed opening in sequential process control, as discussed earlier.
Figure 3-21. Valve gate, shown with hidden element lines.
Edge gates (film gates) are an extension of the part and are designed to fill uniformly across the width of the part. They should be modeled by cold runners with connectors to the film-gate region. The runner length should be equal to the desired mesh size in the film gate, so a connector can be placed at every node in the runners to connect to the film gate.
Figure 3-22. Edge gate, shown with hidden element lines.
Ring gates are modeled by cold runners and linked to the cavity with connectors. Similar to edge gates, the runner length should be equal to the desired mesh size in the ring gate. Connectors are placed between nodes in the runners and the cavity.
Figure 3-23. Ring gate, shown with hidden element lines.
Tips for Modeling the Part and Delivery System
The mesh size for one-dimensional runners should be in the same order of magnitude as for the triangular elements. The aspect ratio (length/diameter) for one-dimensional channel elements, however, should be between 2 and 3 to justify the line-source assumption for the cooling analysis (see "Select the mesh size of cooling channels properly." on page 3-43).
The aspect ratio of a triangular element is defined in Figure 3-24. Triangular elements with smaller aspect ratios are more suitable for the analyses. The minimum aspect ratio is equal to 1 and occurs with an equilateral triangle.
The flow calculation in a triangular element considers only the drag force and heat conduction from the top and bottom surfaces. The side wall effects are not included in the flow calculation in these elements. Decreasing the element size gives a better definition in terms of melt-front advancement, but at the expense of more CPU time. In contrast to the element aspect ratio, as described above, the element size-to-thickness ratio (Figure 3-25) does not change the accuracy of the solution, or the fact that the side wall effects are not considered. Such effects are important in certain geometries, and are discussed in later sections.
Figure 3-24. Aspect ratio of a triangular element.
The thickness of a triangular element or the diameter of a one-dimensional runner element has a significant influence on flow resistance and heat transfer. In isothermal filling (without considering the cooling of polymers), the sensitivity of the flow resistance to the thickness or diameter is given by:
Figure 3-25. Element size-to-thickness ratio does not change the accuracy of the solution
where t is the thickness of the triangular element, and d is the diameter of the one-dimensional runner element. Reducing the thickness of triangular elements by 10 percent results in a 37 percent increase in flow resistance.
Pressure, however, is not directly proportional to the flow resistance, due to the shear thinning effect of polymers at high shear rates. Assuming the polymers are injected in the shear thinning region with a Power-law index equivalent to 0.3 and under isothermal conditions, the sensitivity of the pressure drop to the thickness or diameter is given by
Reducing the thickness of triangular elements by 10 percent results in about an 18 percent increase in the pressure drop. If cooling effects are included, however, the increase of the pressure drop is even greater. Note that the pressure drop is more sensitive to the diameter of runners than to the thickness of triangular elements.
The sensitivity of thickness or diameter to the cooling time can be estimated as:
Reducing the thickness of triangular elements by 10 percent results in about a 19 percent saving in cooling time. The cooling time of a runner is proportional to a value between the diameter and the square of the diameter. Since there is no simple expression for this correlation, the average exponent, 1.5, is used as a good approximation in Equation 3.6.
The shape factor is used to correct the cooling effect and flow restriction from the additional contact surface in a cross section, which can also affect the pressure drop and cooling time. Shape factor is discussed under "Attributes," starting on page 3-8.
Thickness (diameter) and shape factor have major affects on the pressure and cooling time predictions; thus, correct dimensional information must be specified in the model to obtain accurate predictions. Surfaces with different thicknesses or with surface patterns should be modeled as different regions with the appropriate information assigned to each region, as shown in Figure 3-26. For large parts with variable thickness, divide the surface into several regions and specify the average thickness and shape factor over each region.
Pins that extend from a surface can be modeled as part runners. Bosses with a diameter smaller than or equal to the mesh size can also be modeled as part runners. The equivalent diameter and shape factor can be found in Table 3-1. Bosses or cylindrical surfaces with a diameter larger than the mesh size can be modeled as surfaces, with the actual wall thickness assigned to triangular elements.
Figure 3-26. Modeling a surface with different thicknesses.
In the case of a thin cavity, the shear region from the side wall affects only a small portion of the flow, about two times the part thickness as shown in Figure 3-27(a), and can be disregarded. Triangular elements should be used to model this kind of geometry, and the shear and cooling from the side walls is thus neglected. However, if the thickness-to-width ratio of a flow channel is in the order of 1 to 4, the shear and cooling from the side walls become important and can affect the calculation significantly. In this case, part runners should be used, as shown in Figure 3-27(b), so the shear and cooling from all the contact surfaces are considered.
A typical example can be seen on computer housing grills, where the width of a grill is the same as the thickness. Part runners should be used to model the flow in this region. It is still possible to model such one-to-four ratio flow channels with triangular elements, as shown in Figure 3-27(b). The shear and cooling effects from the side walls can be corrected with an equivalent thickness (t) and shape factor (). The total number of elements in the model increases, however, if triangular elements are used instead of part runners.
Figure 3-27. (a) Triangular elements for thin cavity surface. (b) Part runners or triangular elements for flow channels with thickness-to-width ratio of 1 to 4.
Mold-Cooling System in Injection Molding
Figure 3-28(a) illustrates a typical cooling system in a molding plate. The cooling system consists of two circuits connected to the supply and collection manifolds using hoses. The coolant first enters these circuits at the connection to the supply manifold, flows through the cooling channels and baffles (where the flow rate, or pressure drop, is controlled), and leaves the circuits to a collection manifold. The collection manifold takes the coolant to the temperature controlling unit, where the coolant temperature is regulated. This temperature controlling unit is then connected to the supply manifold by a pump.
The cooling system required for C-MOLD Cooling analysis specifically includes the components between the supply and collection manifolds. A manifold is identified by an ID number and has unique coolant entrance and exit nodes, as shown in Figure 3-28(b). All channel elements in a manifold must be connected. The coolant temperature at the entrance is controlled, and either a total flow rate or total pressure drop is specified at the entrance.
The channel elements in a coolant manifold can have very complex branches. The actual flow rate or pressure drop in each branch is calculated by a channel network analysis: C-MOLD Coolant Flow. The results are then passed to C-MOLD Cooling through an interface file to calculate the heat transfer coefficient in each channel element.
"Channel Element Types" on page 3-39 details the cooling channel elements that are used for modeling. Then, cooling system modeling techniques are described. The objectives of these modeling techniques are to obtain more accurate predictions, and to achieve more efficient computation.
Figure 3-28. Schematic of (a) a typical cooling system and (b) model of a coolant manifold for C-MOLD Cooling analysis.
Channel Element Types
Cooling channels are modeled in C-MOLD Modeler as 1-D elements with an assigned channel type. There are four types of cooling channel elements: regular cooling channels, baffles, bubblers, and hoses. Hose elements contribute only to the pressure drop calculation. The other three participate both in the pressure drop and heat transfer calculations. Note that a connector element cannot be used in modeling cooling channels.
Regular cooling channel
A regular cooling channel is defined by two end points and a diameter that is the size of the drilled hole in the mold. Both circular (shape factor = 1) and non-circular cooling channels are allowed. Circular cooling channels are the most commonly used.
The coolant enters a baffle through the entrance node, flows up through the semi-circular region, then bends around the baffle plate and returns to the exit, which coincides with the entrance node in the model, as shown in Figure 3-29. A baffle is defined by at least two nodes: one for the coolant entrance, and the other(s) for the top free end(s) where the coolant changes direction. The size of the drilled hole is used as the outer diameter of the baffle, and the shape factor equals 1.
Figure 3-29. Schematic of a baffle and its model
In C-MOLD Cooling, the cross-sectional area of a baffle is divided into two halves with zero baffle plate thickness. A semi-circular cross section is used for the heat transfer coefficient and coolant flow-rate calculations. C-MOLD Coolant Flow's pipe network analysis traces the coolant from a baffle entrance node (which is connected to a regular channel), up through the semi-circular region, around the bend at the top of the baffle plate, and back to the exit. Extra pressure losses due to bends at the entrance, at the top, and at the exit are accounted for. Note that a non-circular baffle can also be modeled using a shape factor that is not equal to 1
The coolant enters a bubbler through an inner tube and returns through the annular region between the inner tube and the outer channel surface. A bubbler is defined by at least two nodes: one for the coolant entrance, and the other(s) for the top free end(s) where the coolant changes direction. Similar to a baffle, the size of the drilled hole should be specified as the outer diameter.
In C-MOLD Cooling, the inner diameter of the bubbler is assumed to be optimum at 0.707 times the outer diameter. This optimum diameter gives a minimum total pressure drop for the coolant flows through the outer annulus and the inner tube. The actual cross section is used for the heat transfer coefficient and coolant flow rate calculations, as shown in Figure 3-30.
The mesh size of the cooling channels should be properly selected to obtain accurate results for mold cooling analysis. The rule of thumb is that the mesh size for the cooling channels give an aspect ratio (mesh size/channel diameter) of 2 to 3. For cooling channels that are very close to the mold cavity surface or other cooling channels, use an aspect ratio of about 1. (A distance of less than the channel diameter is considered close.)
Figure 3-30. Schematic of a bubbler and its model.
C-MOLD Coolant Flow's pipe network analysis traces the coolant from a bubbler entrance node (which is connected to a regular channel), around the bend at the top of the bubbler, and back through the outer concentric annular region to the exit. Extra pressure losses due to bends at the entrance, at the top, and at the exit are accounted for. Note that a non-circular bubbler can also be modeled by a shape factor that is not equal to 1.
Baffles and bubblers are often used in the core region, where it is difficult to incorporate enough drilled U-shaped cooling lines. Heat can be extracted more efficiently by baffles or bubblers than by regular cooling channels. Baffles are preferred, except when the length-to-diameter ratio is large, in which case bubblers are used.
Hoses are external connecting channels, which account for pressure drop in the coolant flow, but assume no heat loss. A hose element is defined by two end nodes and a diameter.
Tips for Modeling the Cooling System
If a baffle (or bubbler) is very long compared to its diameter (Figure 3-31), it can be modeled and meshed as more than one element to give an aspect ratio of about 2 to 3. Two or more baffle elements can be used to model this baffle, with the same diameter assigned to each baffle element.
Use a hose element to connect two cooling channels of different diameters, as shown in Figure 3-32. The hose element contributes only the pressure drop of the coolant flow and does not participate in the heat transfer calculation. Note that a connector element cannot be used in modeling cooling channels.
Figure 3-31. Modeling a long baffle with two connecting baffle elements.
Just as small features can be ignored when modeling the part, channels that have small surface areas can be ignored as well. The channel connectivity must be maintained, though. Figure 3-33(a) shows a small end feature in a channel element that can be ignored in the model. If the channel to be neglected leads to constriction of the coolant flow, then it can be modeled by using a hose element, which still accounts for the pressure losses due to a small dimension, as shown in Figure 3-33(b).
Figure 3-32. Connecting cooling channels with diameter changes.
Each element can be part of one (and only one) coolant manifold. The cooling system model shown in Figure 3-34 is illegal because one element is shared by two coolant manifolds.
Figure 3-33. (a) Cooling channel to be ignored, and (b) cooling channel to be modeled by hose element.
It is occasionally necessary to use hose elements to consolidate multiple coolant entrances and exits when modeling the cooling system, as shown in Figure 3-35. In the model, each coolant manifold can have only one entrance node and one exit node, with all the associated channel elements defined in between. Also, entrance and exit nodes can be connected to only one channel element. In other words, an entrance node connected to two hose elements, as shown in Figure 3-35 (second iteration) is illegal.
Figure 3-34. Model of coolant manifolds is illegal because of shared element.
Isolated elements that cannot be reached from any manifold entrance node (Figure 3-36) will cause C-MOLD Coolant Flow analysis to stop. The program reports a list of such isolated elements before stopping execution. If this occurs, check the element connectivity.
Figure 3-35. Consolidating multiple entrances using hose elements.
For part surfaces that are in close proximity, choose a smaller mesh size to ensure the accuracy of surface integration used in C-MOLD Cooling, but not too small compared with the channel diameter. It is occasionally necessary to use variable mesh densities for different regions and cooling channels.
Figure 3-36. Isolated channel causes C-MOLD Coolant Flow to stop execution.
The example in Figure 3-37 shows six regular channel elements with nodes that are not shared by more than one channel element. Nodes corresponding to the free end of these elements are called dead ends. In reality, such elements are plugged, so coolant cannot pass through them. Since stagnated flow in the cooling channels does not contribute to removing heat from the mold, dead-end nodes and elements can be ignored in the model. C-MOLD Coolant Flow searches for and reports all such dead-end nodes in a warning message. The analysis continues without calculating the heat transfer in these elements.
Overlapping regions or surfaces in the geometry model will introduce numerical problems for C-MOLD Cooling. When building the part model, check for overlapping regions or surfaces and delete them. The analysis checks for overlapping and intersecting elements before any heat transfer calculation begins. If these exist, an error message is given and the program stops execution. Figure 3-38 illustrates geometries that are illegal because of overlapping and intersecting elements.
Figure 3-37. Dead-end nodes and elements should be ignored in the model.
When the width of the gas channel is greater than four times its thickness, shear and cooling effects from the side walls can be neglected. This kind of gas channel should be modeled as a region with the proper thickness and shape factor, and meshed with triangular elements as shown in Figure 3-39. It is important that the mesh size over and adjacent to the gas channel be kept small to ensure numerical accuracy and better resolution of the gas-penetration display. The rule of thumb is that the mesh size should allow at least two layers of triangular elements across the width of the gas channel.
Figure 3-38. Illegal geometry due to overlapping and intersection.
Special Procedures for Gas-Assisted Injection Molding
Gas-assisted injection-molded parts are similar to traditional injection-molded parts, except they typically have a network of heavy-sectioned gas channels to guide gas penetration. Gas channels are modeled in the same way as any other plastic element. Whether to use triangular elements or part runner elements depends on the aspect ratio of the gas channel cross section. The rules discussed in this section are applicable for the modeling of gas channels only. Because of the dynamic nature of the gas-assisted injection-molding process, accurate modeling of gas channels is critical to the numerical prediction of gas penetration.
Tips for Modeling Gas Channels
Figure 3-39. Gas channel modeling when the channel width is greater than four times the thickness.
One way to achieve small elements adjacent to the gas channel is to use variable mesh density. Specify a finer local mesh size in the gas channel regions, as well as neighboring regions. Another way is to create lines parallel to the gas channels and to create additional regions in the neighboring thin-section areas. As shown in Figure 3-40, these additional lines constrain the elements adjacent to the gas channel from the mesher. This mesh configuration also reduces the artificial zigzag gas penetration resulting from the mesh configuration when the predicted gas penetration is displayed.
When the flow width of the gas channel is less than four times the thickness, shear and cooling effects from the side walls cannot be neglected. The gas channel should be modeled as illustrated in Figure 3-41. First, separate the thin plate from the gas channel, which is to be modeled as a part runner with an appropriate equivalent diameter and shape factor. Then restore connectivity by using the connectors.
Figure 3-40. Confine the mesh size adjacent to the gas channel.
Refer to Table 3-1 on page 3-10 for the derivations of the equivalent diameter and shape factor for the part runner that is used to model the gas channel. Note in Table 3-2 on page 3-55 that the cross-sectional area (A) and the perimeter (C) are derived from the detached gas channel. In particular, the perimeter does not include the edge that is in contact with the thin part.
For simplicity, the part runner can be merged with the thin section, as shown in Figure 3-41(d), thereby eliminating the extensive use of connectors.
Table 3-2 shows the commonly used geometries for gas channels, with the corresponding models with connectors, and the simplified models without connectors. The cross-sectional area and perimeter are also illustrated; they are used to calculate the equivalent diameters and shape factors.
Figure 3-41. Gas channel modeling when the channel width is less than four times the thickness.
Table 3-2. Commonly used geometries for gas channels
Note that the perimeters (heavy curves) exclude the edges in connection with the thin sections because such boundaries do not contribute to the cooling of gas channels. However, the simplified model introduces extra volume that is embedded within the gas channels.
Also notice that the resultant shape factor based on the suggested perimeter and cross-sectional area is sometimes less than 1. This means that those adjacent edges between the gas channels and thin sections have provided some "insulation" that physically reduces the overall cooling of the gas channel, as compared to the cooling in a circular cross section with the same equivalent diameter.
Blow Molding & Thermoforming
C-MOLD Blow Molding and Thermoforming analyses require modeling that is very different from that used for other C-MOLD analyses.
The geometric model must include the polymer domain, i.e., the parison, preform, or sheet, and the mold surfaces. The polymer and mold domains are usually disjoint, sharing no nodes or elements in common. In other C-MOLD analyses, the mold cavity is modeled and no distinction is made between polymer and mold domains; the resin is injected from one or more "entrance nodes" and fills the cavity elements during the simulation. In contrast, during C-MOLD Blow Molding and Thermoforming simulations, the polymer domain expands, i.e., the polymer elements deform and stretch, until all the polymer nodes touch the mold surfaces or are otherwise restricted from moving further. The deformation (or flow) of the polymer is not tracked by filling an empty mold cavity, as in other C-MOLD analyses, but by the distortion of the polymer mesh itself.
Polymer elements suitable for blow molding and thermoforming simulation will be created when topologies that have the attribute Region Type: Parison or Region Type: Sheet are meshed. These attributes indicate that these topologies belong to the parison, preform, or sheet domain.
Mold surface elements will be created when topologies that have the attribute Region Type: Mold Face or Region Type: Mold Parting Plane are meshed. The former attribute is used to identify mold surfaces that will comprise the part after molding and the latter is used to identify mold surfaces that do not correspond to the part.
The thicknesses of the topologies that correspond to mold surfaces are not used by C-MOLD Blow Molding and Thermoforming. However, thicknesses would have been specified in order to create the topology using C-MOLD Modeler.
Shape factors are not used by C-MOLD Blow Molding and Thermoforming. However, shape factors would have been specified in order to create topologies for both the polymer domains and mold surfaces.
The polymer domain must be adequately "clamped." This means that free edges must be fixed, in order to prevent them from moving freely during mold closing or pressure application. If the polymer domain is not clamped adequately along its free edges, instability in the computations performed during the simulation may result. To clamp a node or edge, assign to it the attributes Nodal X Displacement, Nodal Y Displacement, and Nodal Z Displacement. Assign a value of 0 to each of these x, y, and z displacement boundary attributes. As a result, these entities will not be allowed to move during the simulation.
A crude mesh can be used on the mold surfaces if the mesh output from C-MOLD Blow Molding and Thermoforming is not directly used for structural analysis. In this case, the mold elements need to be fine enough only to the extent needed for a reasonable representation of the mold geometry, because the mold surface mesh would not be part of the computational domain, and would be used only for enforcing contact conditions with the polymer.
On the other hand, if the mesh output from C-MOLD Blow Molding and Thermoforming is used for structural analysis, the mold elements must be of sufficient refinement for the performance of post-molding structural analysis. C-MOLD Blow Molding and Thermoforming writes a mesh file for structural analysis using the mold surface elements (that are not identified as parting plane elements as mentioned above) to define the geometry; thicknesses are mapped from the deformed polymer mesh to the mold mesh for this purpose. This is necessary because the user does not have explicit control over the quality of the polymer mesh at the end of the simulation, when it conforms to the mold surfaces. Without control, polymer elements could undergo substantial distortion, rendering them unusable for structural analysis purposes. By using the mold surface mesh, with thicknesses mapped from the polymer, the control necessary for structural analysis can be exerted over the input mesh.
Type of Elements
In C-MOLD Blow Molding and Thermoforming, one-dimensional elements (runners) and two-dimensional triangular elements cannot be used in the same mesh. Two-dimensional elements can be used for a 3-D analysis and 1-D elements can be used for analyzing two-dimensional (plane strain or axisymmetric) cases. If the input mesh has both 1-D and 2-D elements, C-MOLD Blow Molding and Thermoforming will exit, and provide an error message.
Parting Plane Modeling
In blow molding analysis, when mold halves close in on the parison/preform from both sides, the mold surfaces that comprise the parting plane must be modeled and meshed. In C-MOLD Blow Molding and Thermoforming, contact is computed between polymer nodes and mold elements; hence, if the parting plane surfaces are not modeled, there is a possibility of mold surfaces that are "perpendicular" to the parison or the preform surface passing through polymer elements without contact being detected.
Reactive Molding: Microchip Encapsulation
C-MOLD Reactive Molding filling and curing simulation for reactive materials can be applied to transfer molding in microchip encapsulation. But since the chip device and leadframe are inserted inside the multi-cavity mold, special modeling techniques are required. Figure 3-42 is a schematic of a typical multi-cavity transfer mold. Figure 3-43 shows the cross-sectional view of the transfer pot and the flow of the molding compound in a cavity.
Connectors can be used to model symmetric geometry to reduce redundancy and CPU time. For example, the system shown in Figure 3-42 can be modeled using connectors with appropriate direction and multiplicity factor, as shown in Figure 3-44. The identical branches and cavities have been modeled using connectors (indicated by arrows) with corresponding multiplicity factors (indicated by numbers).
Figure 3-42. Schematic cavity and runner layout of a transfer molding tool for microelectronic encapsulation.
The terminology used in the following discussion is borrowed from thermoplastics injection molding. Hot runners are initially filled with resin, and cold runners are initially empty. The coolant temperature is used as the wall temperature for cavities and cold runners. In transfer molding, the wall temperature of cavities and cold runners is actually higher than the temperature of the molding compound to activate curing.
Figure 3-43. Cross-sectional views of the transfer pot and the flow of the molding compound in a cavity.
The transfer pot can be modeled as a hot runner and is initially filled with molding compound, as shown in Figure 3-44. The initial fill condition in the transfer pot can be either
Figure 3-44. Modeling the runner system shown in Figure 3-43.
Filled at Inlet Melt Temp. (=1) or
Filled At Hot Runner Mnfld. Temp. (=2). The hot runner manifold temperature for the transfer pot is given in the hot runner manifold control specified in the process conditions file (filename.prc). If the Initial fill condition is set to 1, the pre-heated temperature is given as the inlet melt temperature specified in the process conditions file (filename.prc). The rest of the runners can be modeled as cold runners with the wall temperatures given in the coolant manifold control, which will also be used as the cavity wall temperatures.
For modeling individual cavities, refer to Figure 3-45. Each cavity is further divided into two sub-cavities by the leadframe. The spacing of the leadframe is assumed to be small throughout the cavity, except at the opening adjacent to the gate that actually allows material to flow through. In this way, the permeation of molding compound through the leadframe for the rest of the cavity is ignored. Each sub-cavity has an attribute of thickness that includes the presence of the paddle as shown in Figure 3-45. In other words, the molding compound divides into two streams flowing above and below the leadframe across the cavity. The two sub-cavities are linked by a connector at the leadframe opening. The distance between these two sub-cavities (or the length of the connector) has no affect on the simulation results, because there is no flow resistance in the connector.
The flow resistance across the leadframe opening can be approximated to predict the "lead-lag" phenomenon of melt fronts in these two sub-cavities. The pressure drop, P, of a Newtonian fluid through a rectangular opening, with dimensions of width, w, and height, h, is estimated by:
Figure 3-45. Cavity with leadframe and paddle is modeled by two sub-cavities with a connector.
where µ is the viscosity, and Q is the flow rate.
There are two ways to incorporate the flow resistance of the leadframe opening. The first way is to replace the connector in the model shown in Figure 3-45 by a part runner. The part runner should have dimensions that provide an equivalent pressure drop as in Equation 3.7. The diameter of the part runner should not be too large, to avoid contributing excess volume that does not really exist. On the other hand, the diameter should not be too small, or considerable shear heating will be generated by the flow.
Following are the steps for finding the proper dimensions of the part runner. First, the pressure drop across a circular tube (such as a part runner) is described by:
where L is the length, and d is the diameter of the tube.
Second, the recommended diameter of the part runner is the smaller side, h, of a rectangle. Having done that, the required length of a part runner, with diameter h and pressure drop as in Equation 3.7, can be derived as:
The resulting model is shown in Figure 3-46, where the part runner has a diameter, h, and length, L, given in Equation 3.9.
Another way of incorporating the flow resistance of the leadframe opening is to first perform the flow analysis with the model in Figure 3-45. From the output displays, estimate the flow length from the gate that has the same pressure drop as given in Equation 3.7. Modify the geometry so the end of the connector is away from the gate for the estimated flow length, as shown in Figure 3-47. Run the analysis with the modified geometry. The flow will not branch into the second sub-cavity until the melt front reaches the end of the connector.
Figure 3-46. Modeling the flow resistance of the leadframe opening with a part runner.
Figure 3-47. Modeling the flow resistance of the leadframe opening with an off-set connector.
Reactive Molding: RTM and SRIM
Resin-transfer molding (RTM) and structural-reaction injection molding (SRIM) are two important processes for manufacturing composite parts using thermoset resins and fiber mats. In RTM and SRIM, resin is forced to flow through a cavity in which reinforcing fiber mat (also called preform) is present. The flow type, characterized by the properties of the fiber mat, can be isotropic or anisotropic, depending on the network of the mat. Although there are some fundamental differences (such as resin used, machine configuration, and cycle time) in these two processes, the geometry modeling techniques for both processes are the same.
In terms of part geometry, the RTM and SRIM parts are similar to regular injection molded parts so that the general geometry modeling techniques described previously can be applied. The major exception in geometry modeling for RTM and SRIM parts is that the user must specify the Fiber Mtl ID (TCODE 30120) to the regions (or surface regions) over which fiber mats (isotropic or anisotropic) are pre-placed. For anisotropic fiber mats, the reference vector that defines the principal direction 1 of the fiber mat must also be specified.
The current release of C-MOLD Reactive Molding analysis does not take into account fiber mat in 1-D elements. Only unreinforced 1-D elements, if any, are permitted in the geometry model.
Fiber Mtl ID
The Fiber Mtl ID, which is represented by an integer, is a parameter that associates the fiber mat in the geometry model with the Fiber mat porosity and permeability data set (TCODE 3920) specified in the material property file (filename.mtl), as shown in Figure 3-48. The Fiber mat porosity and permeability data set consists of
For unreinforced (i.e., empty) regions, there is no need to specify the Fiber Mtl ID. The use of Fiber Mtl ID makes C-MOLD Reactive Molding analysis uniquely capable of simulating the resin flow through more than one kind of fiber mats, as shown in Figure 3-48. Detailed descriptions on the Fiber mat porosity and permeability data set can be found in the C-MOLD Reactive Molding User's Guide.
- An ID, which should match the Fiber Mtl ID of the corresponding fiber mat in the
- Porosity of fiber mat
- Permeability of the fiber mat
- A reference thickness at the measurement of the fiber mat properties (such as
porosity and permeability).
Figure 3-48. Schematic representation of a simple cavity that contains an isotropic fiber mat, an empty region, and an anisotropic fiber mat. Note that the associated Fiber Mat IDs of the fiber mats point to the corresponding Fiber mat porosity and permeability data set in the material property file (filename.mtl).
When using C-MOLD Modeler to model the geometry, areas with different kinds of fiber mats or with no fiber mat should be created as separate regions (or surface regions), as shown in Figure 3-48. To assign the Fiber Mtl ID, follow these steps:
Keep in mind that the same Fiber Mtl ID can be assigned to different regions (or surface regions) over which the same fiber mat is used, and that the ID number does not have to be continuous, as long as it matches the ID in the Fiber mat porosity and permeability data set in the material property input data.
- Click Attributes; Select Attribute; Reactive Molding; Fiber Mtl ID.
- Click Attributes; Set Value to Assign to provide a value for the Fiber Mtl ID for
the regions to be assigned. The default ID value is 1.
- After entering the ID number, select the Attributes; Assign Attribute command
to assign the Fiber Mtl ID to the corresponding region.
Fiber Mat Orientation Node for the Anisotropic Fiber Mat
The structure of a directionally stitched/woven fiber mat is not uniform. In terms of the pore area distribution, it shows a maximum in one direction and a minimum in the other direction that is orthogonal to the former. When the resin flows through such a fiber mat, the flow in the direction of maximum pore area will have longest flow length because of the least flow resistance. If the pore area distribution is smooth, the in-plane melt front exhibits an elliptic pattern, as shown in Figure 3-49. The shape of the ellipse depends on the maximum and the minimum pore areas, which can be characterized by "the permeabilities of the principal directions." One of the primary principal directions corresponds to the major axis of the ellipse. This direction has the largest pore area (and, thus, the largest permeability) and is denoted as the principal direction 1. The other principal direction, the principal direction 2, corresponds to the minor axis of the ellipse, as shown in Figure 3-49.
Figure 3-49. A schematic representation of a cavity showing elliptic melt-front pattern for resin flowing through an anisotropic fiber mat. Principal directions 1 and 2 of the fiber mats are determined by the major and minor axes of the ellipse.
Note that C-MOLD Reactive Molding can handle, at most, one anisotropic fiber mat and multiple isotropic fiber mats. For an isotropic mat, only Fiber Mtl ID is needed and fiber mat orientation nodes are not required. For an anisotropic mat, the fiber mat orientation nodes that determine the principal direction 1 should be defined in the geometry model. The principal direction 1 of the anisotropic fiber mat can be obtained from the permeability measurement.
Defining the fiber mat orientation nodes can be done either before mesh generation or after mesh generation.
Defining the fiber mat orientation before mesh generation
When the mesh is generated, nodes will be created on those two points, which carry the nodal attribute Fiber Mat Orientation (TCODE 20050). Figure 3-50 illustrates how the fiber mat orientation is defined by selecting the two points that determine the principal direction 1.
- Select Attributes; Select Attribute; Reactive Molding; Point Type: Fiber Orientation Vector.
- Select Attributes; Assign Attribute to assign the attribute to two points on a planar region that defines the principal direction 1 of the fiber mat.
Figure 3-50. A schematic representation of a cavity geometry model showing that the principal direction 1 can be defined by selecting two points (P1, P2).
Defining fiber mat orientation after mesh generation
When the mesh is saved, these two nodes will carry the Fiber Mat Orientation nodal attribute (TCODE 20050). Figure 3-51 illustrates how the fiber mat orientation vector is defined by selecting the two nodes that determine the principal direction 1.
- Select Attributes; Select Attribute; Reactive Molding; Point Type: Fiber Mat
- Select Attributes; Assign Attribute to assign that attribute to two nodes on a planar region that defines the principal direction 1 of the fiber mat. Note that care
should be taken to turn off point visibility if the node to be selected coincides with
a point. Otherwise, a point, instead of a node, will be selected to carry the Fiber
Orientation Vector attribute.
Figure 3-51. A schematic representation of a cavity finite element mesh model showing that the principal direction 1 can be defined by selecting two nodes N1, N2).
Note that the anisotropic fiber mat orientation is required for all the triangular finite elements created over the reinforced regions. Once two nodes are specified to define the fiber orientation vector, C-MOLD Reactive Molding will automatically determine the principal direction 1 of the anisotropic fiber mat for the entire fiber mat domain. Since the analysis automatically propagates the fiber mat orientation throughout the reinforced region, correct execution of the fiber mat orientation propagation is only guaranteed when the anisotropic fiber mat is placed in a planar region or a slightly curved surface region. Care should be taken in cases in which the anisotropic fiber mat is placed on a highly curved surface region or several surface regions, due to the lack of a unique solution to propagate orientation on these kinds of geometry. Upon obtaining the analysis results, use C-MOLD Visualizer to verify the fiber mat orientation in the output data.
Using the same fiber mat in a cavity with varying thicknesses
The same fiber mat preform can occasionally be placed on areas with different thicknesses, resulting in different porosities and permeabilities. This kind of situation can be handled by creating different regions that carry the corresponding thickness in a normal way, and assigning the same Fiber Mtl ID to those regions that use the same fiber mat, as shown in Figure 3-52. As a result, these regions will share the same Fiber mat porosity and permeability data set. If the cavity thickness at molding is different from the reference thickness specified in the Fiber mat porosity and permeability data set, the porosity and permeability used in the simulation will be internally modified by the analysis. Keep in mind that this automatic modification of the fiber mat porosity and permeability is based on the assumption that the thickness variation in the cavity is not too large from the reference thickness and that the porosity is within 0.4 and 0.7; otherwise, errors in the predictions could be introduced due to the improper values of the fiber mat attributes.
Figure 3-52. A schematic representation of a cavity with a single fiber mat but different thicknesses. Different regions corresponding to various thicknesses have been created, but they carry the same Fiber Mat ID.
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