.

Tech Tip May 1996

Gate part Two:

Last month we talked about the land, width and height of the gate. We mentioned what could happen in the tool and why they are important. Now let us get into some definition of the specifics and some examples of what goes on.

The Land. If the gate is properly sized and the land area is too long, we can imagine that the possible pressure drop through this may be too great. This would be like having an extremely long water hose with a T off it and expecting the same pressure that you had at the house where you connected the hose. A case in point was a three pound polycarbonate job that we were not able to fill. The original question that I was presented with was "How large should I open the gate to fill the part?" After discussion and finding the land length was 0. 125", we then only shorten up the land to approximately 0.055", with the results being a full part.

In looking at the height and width of the gate, we need to determine the gate freeze time and then make our assumptions based on that fact. If the gate is freezing off too quickly then we may be having the problems due to no pack this would be shorts, sinks, etc. If it is not freezing off, we may end up with size problems, repeatability and quality problems.

Checking the diameter or size of the gate should be done with calipers and not pin gauges. Consider the fact that a round pin inserted into an oval hole will not go past the tangent point of the two. Thus we could have the same results for holes measured with a pin, and be way off volume-wise.

Worming/Jetting. If in a short shot we notice that the material is entering the cavity uninterrupted in a straight line and then double backs on itself, then we have worming/jetting and need to impinge the material into a side wall or change gate location. If the resultant flow marks are not objectionable, then changing of the gate location need not be done.



Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers