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Tech Tip April 1996

Gate:


Though this tip has to do with design, it is important that we all understand the components of the gate, what makes it work, and what can be important about it.

The gate itself is composed of three (3) areas/parts, land, height and width. You must understand that each is related to the others and can have an overall effect on the processing window. I will state now that the gate should not be used as a balancing tool until all other options have been tried, and even then don't do it until you have generated the necessary data.

Land: The land is the distance the gate covers from the end of the runner to the part. This length should be as short as possible and in most cases is only 0.040" to 0.060" in length. The longer the land length the higher the pressure drop in the gate.

Height: This is the thickness of the gate and typically may be used to determine gate freeze time. In looking for a starting point it is usually recommended to start at 40% of the wall thickness for the height of the gate. This then ensures that the gate will freeze off first prior to the part, thus letting the screw recover while the part continues to cool.

Width: The width is how wide the gate is at the part. A starting point would be at 40% of the wall thickness. If you consider that you need more volume of material in the part, than making the width wider, if acceptable will accomplish this without increasing gate freeze time.

Please note that there are various formulas available to determine the actual size of the gate. MOULD DESIGN by PYE has some very good ones in it. But the knowledge of your practical experience, past experience and that of the tool maker, molder and others is invaluable.


Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers