MOLDING
How to prevent and troubleshoot short shots
Image courtesy of RJG
Deliberate processes to identify and remediate the problem can yield high-quality products and keep patients safe.
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S h a n e Va n d e k e r k h o f | RJG |
hort shots are among the most common and visually obvious quality issues most molders face, but they are especially hazardous in the medical industry. A short shot could mean life or death. Imagine if a luer lock connector was short. You run the risk of introducing air into a vein, which could be fatal. Even if the medical staff caught it before harming someone, how can health providers be confident that all the parts they have in inventory are not the same way? Do they stop the procedure to check each part? What if it was an emergency that could be fatal if the procedure is paused for product evaluation? The risk is too great, so it’s vital to ensure a good, robust process is in place to prevent short shots. So how do we do that? And how do you troubleshoot them when they do occur? Let’s start with what causes them in the first place. What causes short shots? A short shot occurs when a plastic injectionmolded 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 straightforward. Issues may include: • • • • •
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Medical Design & Outsourcing
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Poor venting. Material variations. Improper machine settings. Blocked gates. Mechanical failures.
It’s often hard to pin down which issue you’re battling, and there are just as many fixes, depending on the root cause of the problem. But the best-case scenario is avoiding them altogether. Prevention is key There are many ways to prevent short shots that range from equipment to processes to material. Here are five of the most common fixes: • •
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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 can’t complete the fill and pack out the part. Hold time — Producing a part with the hold time off, even with the hold pressure on, may produce a part with sink marks and possibly short shots. An Injection Integral PSI alarm set on a process control system can catch this problem. Pack-and-hold pressure — 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. You need enough pressure to complete the fill and pack out the part.
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