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Why make shorts
Definitions: Short Shot: 1) to be not full or completely packed out 2) not full in that some cavities in multicavity tool may not be full while others are. Test: to evaluate, to trial put to process
In injection molding the use of on the machine evaluation is a good way to see what is going on. One of the ones that have been used since I have been in plastics is referred to as shorting the mold, creating on purpose a short shot. While the majority of tools may allow this there are some tools which this should not be done too. These tools are ones which the processor knows well stick to the wrong side, create issues with digging parts out of deep cavities, do not eject if short and the what not.
What the short shot test can show is how the part fills. By this we are saying is that if we stop material flow prior to filling out the mold, we see what the pattern is. If a multi cavity tool it shows the fill pattern for multi cavities individually and as part of all of the cavities and their fill pattern. It is just as critical in a single cavity too.
The correct way to run a short shot is to cut the shot size / volume of material being injected into the tool. The first step is to remove any packing and holding pressure and sometimes time to see how much is used to fill the part, at the transfer position / point.
The second step involves cutting the volume of material. If there is only one injection speed than either cutting shot size or moving transfer point back well work in minimizing the shot volume. If in fact there is a profiled injection speed used than the only way is to move the transfer point back to create this shot volume decrease. The reason for this is that in filling, the speed plays a big part in how material flows into a cavity. One may get poor results if using a multiple speed profile and decreasing the shot size by the total shot size since this if the first speed of filling.
Short shots can be produced from 95% full to 20% full or whatever range one wants, and what one is trying to see.
Important*** always change or create short shots via volume injected. While short shots may be created by changing pressure, and slowing down speed, these changes in effect are also changing the way that the material flows into the mold cavities and is not representative of machine is working in production.
The results of running a short shot are that one can see visually how the part is filling, flowing into the cavity, cavities and mold.
Example 1 hesitation;
Hesitation in flow front can be multiple, in that in one case an area of the mold does not fill and the flow continues to fill the part and then the material has to fill this area of the mold which was partially filled and then stopped filling and later is filled. In a single cavity tool this may be areas which are thinner, trap gas, or both. In multicavity tools this may be one cavity which fills uniformly with all others than stops filling while the others continue to fill and then fills the rest of the way or at least tries to fill the rest of the way but can't.
Example 2 backfill, windowing:
Back fill or windowing is when a part fills in the perimeter of the part and then comes back to fill the thinner section. This can be associated with race tracking in which the thicker section fills first and then the thinner.
Example 3 Balance:
Balance is an important part of filling a tool and especially in multicavity tools. By use of short shot and part weight one may find out how balanced a tool is. It also shows which cavity may be filling first and last.
Example 4 jetting:
Jetting while a fill issue may not be noticed except for lines in a part, while with a short shot one can actually see and visualize this issue.
In running a short shot correctly, (by volume decrease) it is producing a picture as to how the parts fill under the injection profile one has. If one is using a profiled injection sequence than by using transfer position at the various speed change positions one may see at what point this speed change is really taking place. (Too many times when the author has observed profiled sequence part or a large portion is not occurring when they thought it was for example at gate location) what this series does is show exactly where the change occurs, and a conclusion can be made if it is really necessary.
Photo 1
Photo 1 shows an example of a short shot, in which multiple issues are shown in how the part fills.
to be continued.
SL SILVEY
06012014.01
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