The finer points of injection molding process adjustments

Processing in plastic injection can be a tricky business. It takes a strong and knowledgable approach toward process setup when adjustments are being made to a process. Materials respond in different ways to process change, and every adjustment needs to be made with a solid understanding that a part's dimensions, aesthetics and even function can either be improved or degraded as variables are changed.

vintage radio
Think of process adjustment like tuning a vintage radio, with one dial that broadly adjusts the frequency and a second dial reserved for fine tuning.

A good comparison to process adjustment in regards to machine response are old-style radios with knobs designed for both broad and fine tuning. One knob is used to aggressively adjust frequencies to get to the station you want. The other knob allows for fine tuning of a particular station. 

Process adjustment is very similar. There are adjustments that can be made for fast and/or broad change(s) while establishing process, and other changes work better when making small adjustments to an established process. It is crucial to note that the time to be making major changes to a specific process is during the engineering phase of process development and validation. 

This article outlines various changes available to processors when a process requires adjustment. Specific parameter changes and their potential outcomes, as well as specific problems to look for, are covered. The article also provides insights on how long it takes for specific changes to take effect.

Barrel temperature. Adjusting barrel temperature can be either a broad or fine change to process. It is important to remember that the best way to gauge the end result of a temperature change is by measuring melt temperatures. Melt temperature variation can result in a deterioration of the overall result even with a modest adjustment. 

Great care should be taken to verify part function, aesthetics and dimensions. Some instances that may require barrel temperature change might be when viscosity is a suspect in the occurrence of defects or if process optimization is being attempted.

One important consideration to note is that these types of changes require time to allow the change to take effect. The best approach to verifying the result of temperature modifications is monitoring the change itself. If a temperature is raised or lowered, allow the temperature to first rise or drop or rise to the setpoint. You must then allow 20 minutes for the barrel to heat soak or cool to see if the change was successful. 

Large temperature changes are best approached with the press idle. This prevents running scrap because of the length of time required for heat changes to take effect with a press in running condition. It is important to note that when a press is in running condition, it could take several hours for a large change to be confirmed as the actual molding condition.

Garrett MacKenzie, the author of this article, will present a free webinar on the key principles of scientific molding methodology on Feb. 13, 2018. He will address such essential topics as process development, recording, monitoring and control; standardization; design optimization experiments; and process optimization. Register now for this webinar. Busy that day? No problem: You can listen to the webinar on demand, but you must register first.

Mold temperature. Much like the barrel temperature process, mold temperature also serves a dual purpose as both a broad or small change and changes cannot be ruled as good or bad without first allowing the mold to heat soak or cool for a minimum of 20 minutes after the setpoint has been reached.

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