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TOPIC: Delamination at the Gate
 

Submitted: August, 2001

We are molding a label, which is basically a flat plate with hooks. We are using curved tunnel gating to the underside of the part. We have problems in delamination at the gate and get high vestiges. We recess the gate area in an attempt to keep the vestige below the adjacent surface, however we cannot recess enough to avoid the problem.

Does anyone have suggestions regarding gate design for this application that will cause a clean break. This material seems to delaminate and leave the vestige on the part as opposed to on the gate.

We are currently molding Cycoloy C6600 or C2800. Processing Temperature is: 500F; Gate Diameter is: .030" - .040"; Wall stock is: .100"; and overall size is: 2.363" X .788". Note, however, we mold parts of a variety of shapes and sizes, with the same problem.

Comments would be appreciated.

Unfortunately, there may not be an easy answer to your problem. It seems that you may be picking up shear heat through the gate which is causing the material to delaminate at the gate. 

Assuming that there is appropriate, turbulent cooling in the mold, especially in the gating area, and that the temperature of the melt and mold is correct for the grade of material you are molding,  one solution might be to make sure the gate geometry lends itself to a clean break. A typical cashew or banana gate may not accomplish this. 

Included is a gate drawing that illustrates the type of geometry which gives the best chance of success. It is designed to create a good break plane and pull the material to the runner side of the gate. This type of gate requires inserting the gate area, which restricts the water. There are standard cashew gate inserts available commercially from several sources.

If the gate geometry is correct and the problem remains,  another idea might be to open the gate to .050" diameter, being careful to maintain the same geometric structure of the gate. This may seem like a counterproductive approach, but delamination is often a shear heat issue. By opening the gate, the shear heat is reduced and the tendency to delaminate is reduced also.