How to Prevent and Troubleshoot Short Shots
In this blog post, RJG goes into more detail on the causes, prevention, and troubleshooting of short shots in plastic injection moulding, offering valuable insights for addressing this common quality issue. Short shots are among the most common and visually obvious quality issues most moulders 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 moulded part does not fill out—a portion is missing. A few issues can cause short shots, and the fix is not always straightforward.
Some things that can cause them include:
- Poor venting
- Material variations
- Improper machine settings
- Blocked gates
- Mechanical failures
As you can see, the issue has numerous 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:
1. Proper venting- If vents are crushed or non-existent, air can’t escape, causing short shots.
2. 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 complete the fill and packing out the part.
3. Hold time- This one may seem obvious, but we have seen many parts produced with the hold time off, even with the hold pressure on. This will likely make a part with many sink marks and possibly short shots. This can be caught with an Injection Integral PSI alarm on a process control system.
4. Pack and hold pressure- Having enough pressure to complete the fill and pack the part. As the material’s viscosity increases, indicating a thicker plastic, it gets harder to push and transfer the plastic pressure to the end of the cavity during the pack and hold phase. It is critical to have the bag and keep pressure high enough to do its job when the viscosity increases.
5. Pressure-limited process- If your process does not have enough injection pressure or the injection pressure limit is set too low. 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 abundant 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 (check out this video on how to make 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.
We aim for this part to appear 99% complete, with no second stage being applied. If the part is significantly smaller than that, we need to start the root cause analysis of the problem:
1. Verify the machine settings are correct by comparing them to the process setup document.
2. Make sure that there are not any mechanical problems.
3. Check for foreign material that could block the gates and impede flow into the cavity.
4. While the machine is down, it’s a good time to give the mould surface and vents an excellent wipe-down to ensure they haven’t been clogged by contamination.
5. If everything seems in good working order, the problem could be material variation. If this seems to be the cause of the short shot, then add material to the cavity.
6. Adjust the shot size to achieve 99% complete. You can view the procedure on our YouTube channel.
7. Put back the second stage pressure settings, look at the parts, and compare them to the target part weight if you have it.
8. If the short is still present, or the part is underweight, then increase the second stage pressure to drive more material into the cavity. As we do this, 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.
