Definition: Gate 1. A barrier for limiting the passage of fluid. Location: 1 where something is at
Since the gate is in fact the area where our plastic enters the cavity, its location needs to be looked at for many reasons. Some of those are:
Flow distance
Uniform filling of the cavity
Minimum weld line formation
Part structure
Venting pattern in the cavity
Should avoid jetting of material
Ideally located in thickest area of part
In practical purposes or what we can call what is allowed by the OEM, tool design, part design specification, and or just costs the ideal gate location is many times not allowed. That being understood let us examine some the reasons for a few of the list above.
Flow Distance:
This was discussed in a topic last issue and should be self evident. Ideally one gate should be enough to fill most parts when in fact the F:T ratio is too great many issues come to play in the functionality of the part. This needs to be examined as to not only filling of the part but also the stresses that may be involved in the finished part.
Uniform filling of the Cavity:
Again when we look at the filling of the part it is ideal that we reach all extremities of the part at the same time thus achieving a uniform pressure in our part and being able to pack the part uniformly. This is not to say the pressure is uniform across the part (gate to end of fill) but the drop in pressure is uniform and the plastic has reached the furthest points in equal volume.
Minimum weld line formation
Can the gate in conjunction with points brought up earlier be place to avoid the formation of weld lines? Can it be placed to where the weld line can be orientated in another direction or off kilter? In many cases speed and mold temperatures are constantly played with to place the weld lines in non critical areas. (Or at least they try, limiting the process window)
Structure
In the functionality of the part the gate should not be placed in the area of high stress within the part. Also given the gate location can it be located so as to allow the perceived stresses in the part to function properly?
Venting
Since the material entering the part at the gate displaces the air within our cavity, how is its effect on our flow? Reviewing points above as to uniform flow and weld line formation venting of air out of the cavity is just as important. We do not wish to trap air within the part due to our fill pattern and what can be done to avoid this.
An example of a simple part should explain some the issues above. The product was a simple water pitcher. It was gated onto the bottom of the part, which should have been fine. The issue though was a movement within the core of the mold, as the part was not filling uniformly. What was not considered was the handle in the part, which when the flow reach it would allow material to flow into it, this than started an uneven fill pattern up the opposite side wall, which when looking at square inches of surface than pushed the core to one side continually exaggerating the condition. Though parts were produced full, upon opening of the mold and ejecting the part they would scuff as the core tried to come back to it original position.
So the issue was looking at the volumetric flow of the material. The issue was solved using flow leader type solutions and new core support capabilities.
For other information on gates look at vol. 1 #1, vol. 2 no 12, vol. 3 no. 4, no.6
