Definition: Gate: 1) a smaller opening though which the polymer melt enters the cavity.2) an opening permitting passage or flow. Function: for what it was designed for
The term gate is that of a passage or opening which can be closed or shut off. It can also be used to monitor the flow, and to control the flow. In many cases the gate is also the point of shear on the material. In location as it regards plastic injection molding it is the point after the runner system and prior to the part which actually connect the runner to the part. It is intended to freeze off prior to the part solidifying and in most cases is a smaller cross section area than that which it is attached to.
Different gate designs and sizes of gates are required for different materials and sometimes a part shall require more than one gate. Material manufactures all have within their specification their requirements of gate sizes versus part volume and also maximum shear rates allowed for their materials. It is critical to understand what the material see's when traveling through the gate, though in many cases the limits suggested by the material supplier are exceeded and then it is up to the experience of the processor / mould builder.
As is commonly noted the gate consist of a diameter or a height and width and in both cases also a land length. In tunnel or submarine type gates the gate shape at the part may be oval, due to the fact that the gate is on an angle as enters the part and is below parting line.
The above illustrates a tab gate and the key features to any gate design, in that they all have a width, height, and land length. In the use of a round or oval gate the height and width are in diametrical form. It is this height (typically) or the thinnest cross section distance which is of a concern in gate freeze studies as this is the dimension which controls the gate freeze time.
On a side note it is the "land" which plays a big part in pressure drop to the cavity. Though approximate gate diameter can have a large influence the author has found that land length can in many cases play a larger influence especially when all gate diameters / heights have been checked and are the same but there is a difference in part.
To measure a gate diameter, the use of pin gauges may be a typical way of measurement. This is very inaccurate in tunnel gate or submarine gate that enters the part cavity at an angle because this angle of entrance creates an oval. The only way to measure is to actually physically measure the runner gate subsystem at the point of breakage and or measure the witness on the part.
In high cavitation molds it is important that all gates be identical so as to achieve the same flow and gate freeze time through them. If in fact one or two of the cavities are not the same than these are the cavities that shall control the process set points.
Though there are many types and configuration of gates they all have in their design, a land, a height, width or diameter.
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