Short shots are among the most common and visually obvious quality issues most molders face. So, what causes them? How can we prevent them? And how do we troubleshoot them when they do occur?
What causes short shots?
A short shot is when a plastic injection molded part does not completely fill out—a portion of it is missing. A few issues can cause short shots, and the fix is not always straight forward.
These issues include:
–Improper machine settings
As you can see, there are many causes, so it’s often hard to pin down which one you’re battling. Unfortunately (or fortunately), there are just as many fixes to these issues depending on the root cause of the problem. But the best-case scenario, of course, is avoiding them altogether.
How do you prevent short shots?
There are many ways to prevent short shots that range from equipment to processes to material. Here are five of the most common fixes:
- Proper venting—If vents are crushed or non-existent, air can’t escape causing short shots.
- Proper fill volume—Always ensure that 1st stage (or fill) is similar to the setup sheet. If there is not enough fill volume, then the pack pressure won’t be able to do its job of completing the fill and packing out the part.
- Hold time—This one may seem obvious, but often parts are produced with the hold time off, even with the hold pressure on. This is likely to produce a part with many sink marks and possibly short shots. This can be caught with an Injection Integral PSI alarm set on a process control system.
- Pack and hold pressure—Having enough pressure to complete the fill and pack out the part is important. As the material’s viscosity increases, indicating a thicker plastic, it gets harder to push and harder to transfer the plastic pressure to the end of the cavity during the pack and hold phase. Having the pack and hold pressure high enough to do its job when the viscosity increases is critical.
- Pressure limited process—If your process does not have enough injection pressure or the injection pressure limit is set too low, then the machine may become pressure limited and won’t be able to inject the plastic at a constant velocity when the viscosity increases too much. As it is good practice to have your pressure lowered from maximum to act as a safety net, you must still have an abundant amount of injection pressure during a good production cycle.
How do you troubleshoot short shots?
One of the first steps in troubleshooting a short shot is to turn off second stage pressure and time, and make what is referred to as a fill-only part. This will give you an idea of the flow pattern of the material inside the cavity and help with the troubleshooting process.
The goal is for a part to appear to be 99% full with no second stage being applied. If the part is significantly smaller than that, you will need to do a root cause analysis of the problem:
–Verify the machine settings are correct by comparing them to the process setup document.
–Make sure that there are no mechanical problems.
–Check for any foreign material that could be blocking the gates and impeding flow into the cavity.
While the machine is down, it’s a good time to give the mold surface and vents a good wipe down to make sure they haven’t been clogged by contamination.
If everything seems to be in good working order, the problem could simply be material variation. If this seems to be the cause of the short shot, then add material to the cavity.
You can put back the second stage pressure settings and take a look at the parts, compare them to the target part weight if you have it.
If the short is still present, or the part is underweight, then increase the second stage pressure to drive more material into the cavity. The short will hopefully disappear and the part weight will come up.
The ultimate goal is to follow a systematic troubleshooting procedure that allows everyone to work through the process in the same manner for the same reason to fix the problem.
(Source: RJG Inc., www.rjginc.com)
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