Good Reasons to Make a Short Shot, and How to Do It Right
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:
- For your own safety.
- To prevent damage to the mold from overpacking if you set shot size incorrectly.
- To confirm that the cutoff or switchover position from first to second stage (fill to pack) is set appropriately. The first stage should fill most of the part but not begin packing.
- So you can do a Scientific Molding viscosity curve.
- To evaluate machine response or amount of screw over-travel.
- To understand the balance of filling in a multi-cavity tool.
- So you can check the parting line for flashing in the first stage.
- To evaluate viscosity of different resin lots, colors, or grades or wet vs. dry resin. In turn, this provides information for applying appropriate second-stage pressure.
- For troubleshooting flash, shorts, sinks, bubbles, and splay.
- To calculate pressure loss over the flow path in filling.
- To see how the press handles second-stage or packing velocity.
- To check non-return valve function.
- To evaluate the machine function for minimum hold pressure.
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:
- Is the second-stage pressure at the set value or high enough to keep the screw moving forward?
- Is the backpressure during screw rotation extruding plastic into the part? Check this by adding a few seconds of screw-rotation delay time. This should ensure the gate solidifies and prevent backpressure from influencing packing and thus your part size.
- Do you have a cushion? If not, the screw is bottoming and your short shot is not representative of the process (assuming you normally run with a cushion).
- Does your machine have a pack velocity setting? If it does, note the size of the current part, then change the pack velocity setting. Go through the entire range allowed and note the results. This will show you the influence of pack velocity on your shot size. With some machines it makes no difference, with others it can over- or underpack your part. This is important to know so you can figure out what value to use to make the same part on different machines.
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.
About the Author
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.
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