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What is causing short shots?

Short Shot is a customer concern or symptom, and root cause could be one of many or combination of other process parameters. I have been out of injection molding for few years, but recall a DOE (design of experiment) completed in late 1990's to resolve the same issue. It could be mold temperature, injection pressure, barrel temperature, inconsistent cycle time (manual part removal).

Assuming that your process validated with melt cushion range, this range can be monitored if the actual is above the range machine should alarm so that particular shot can be rejected upfront, cavity pressure can be set for each cavity the pressure difference will help to detect short shot defect, root cause of the defect already shared by most of them.

If your customer is asking you to correct the problem then you must drive backward through the entire process (material, machine, process) to determine what is causing the short shots. If your only goal is to catch the defects and not fix the problem (don't ask me why anyone with common sense would think this way but I've worked for a few of them) then you can install a vision system to inspect the parts as they are ejected and kick any short shots out of the finished goods part stream.

If the short shots come and go during production your injection molding process is not under control and you need to find the root cause/s to ensure a reliable processing window. However another point to consider is all injection molding shops will produce short shots at startup for example, generally this is considered good practice. It might be startup short shots have found their way into your final shipped product. If this has happened then the root cause might be in your procedures for dealing with transition from startup to declaring production.

Based on cavity pressure you will detect every short shot produced and will be able to sort out the respective parts as well. Additionally your people after a very short period of time will understand what they are doing and how to establish a stable process within a very short time. Have in mind your machine is not reliable as well as your operators are not. Cavity pressure monitoring will show you what an opened window or a changed injection speed, or a defective heater band for your quality produced means. So give them a try and you will learn a lot.

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