Making a proper short shot is an important aspect of practicing “scientific molding.” Upon startup of almost any mold, the first step is to make a short shot. Why? The list of reasons is longer than you might think:
You can learn a lot about your process by making a proper short shot. So how do you make a short shot correctly? The first question is: Can you eject a short shot? How many of us have attempted to make a short shot, only to find that it will not eject? How many hours have you wasted digging a short shot out of the mold? Worse, how much damage did you do to the mold in getting it out?
Once you are sure you can eject a short, the first step is to take off second-stage pressure (pack and/or hold). There are three ways to do this on most machines:
A. Take off second-stage time.
B. Reduce second-stage pressure to a very low value.
C. Do both.
The correct procedure is to reduce the second-stage pressure and leave some time on the second-stage timer. Why? Try it both ways and look for a difference in part size. Ninety percent of the time you will see a significant difference even though you did not touch the shot-size setting. The part made with hold time at zero will be smaller than the part with hold time at some value and pressure reduced as low as the machine will allow.
Now is a good time to check that the hold pressure is truly at this low setpoint. Many machine controllers allow you to input a low value but do not actually go down that low. I don’t recommend setting the second-stage pressure to zero: Use 5-10 psi for hydraulic machines, 50-100 psi for electrics, or use the lowest setting allowed.
Another reason to have some second-stage time is that 99% of all molds run with some hold time and you need to see how the machine responds as it goes from the first to second stage. Imagine that in setting up a job, you have established the first stage at your desired fill time and you have zero second-stage time. Now you are ready to add the second stage to finish filling and pack out the part. You now set some time on the second-stage timer and guess what happens? The part is flashed and you overpacked a slide.
If you have followed my advice, we now have the machine set up correctly—but are we getting we want? Some checks:
The seemingly simple task of making a short shot is not so simple after all. But it’s a critical aspect of developing a “production-capable” process to get identical parts on different machines and it thus needs to be mastered.
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties.