What is flash?

Flash is a defect where excessive material is found at locations where the mold separates, notably the parting surface, movable core, vents, or venting ejector pins.

FIGURE 1. Flash

Causes of flash
Low clamp force  
If the clamp force of the injection machine is too weak to hold the mold plates together during the molding process, flash will occur.

Gap within the mold  
Flash will occur if the parting surface does not contact completely, due to a deformed mold structure, parting surface defect, improper machine and mold set up, or flash or foreign material stuck on the parting surface.

Molding conditions  
Improper molding conditions, such as a high melt temperature (which makes a thinner melt) or high injection pressure, will cause flash.

Improper venting  
An improperly designed venting system, a very poor venting system, or a venting system that is too deep, will cause flash.


Adjust the mold set-up

Set up the mold to seal properly. A mismatch or undesirable gap between the cavity and core sides of the mold will result in flash.

Make sure the mold plates are strong enough to avoid deformation during molding.
Add pillar support or thicken the mold plates if there is any deformation of the mold plate during the molding process.

Check for adequate venting dimensions.
The recommended venting size is 0.025 mm (0.001 inches) for crystalline polymers, and 0.038 mm (0.0015 inches) for amorphous polymers.

Clean the mold surface.
Flash can be caused by the mold surface not sealing well due to foreign material remaining between the parting surfaces.

Mill out the surface to keep the sealing pressure of land area around the cavities high enough.

Adjust the machine settings

Set up the machine and mold to seal properly. Flash can be caused by a poor seal between the cavity and core sides of the mold, and machine platens that are not parallel.

Increase the injection molding machine size. Flash can result from insufficient machine clamp force.

Adjust the clamp force if the machine capacity does have enough clamp force.

Adjust the molding conditions

Decrease the barrel temperature and nozzle temperature.
A high melt temperature reduces the melt viscosity, making a thinner melt, which causes flash. But beware: avoid melt temperatures so low such that the resulting high injection pressure required causes flash.

Reduce the injection and packing pressure to reduce the clamp force requirement.

Reduce the feed setting (stroke length) to reduce metering (over-fill).

Increase the injection time or slow down the injection speed.