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.
|
| 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. |
| 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. |
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.
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.
The mold responds similarly to barrel temperature changes in that it could take a couple of hours for the mold to settle into the actual running state condition when large changes are made. Small changes can generally be confirmed 20 minutes after the setpoint has been reached. It should also be noted that mold temperature changes should be avoided until at least 20 minutes after startup to be sure that both the barrel and mold have reached a heat-soaked state.
Back pressure. Making changes to back pressure is generally a broad adjustment, resulting in larger changes to barrel temperature. While increasing back pressure raises barrel temperature and lowers viscosity, it’s important to remember that it also breaks down the material more aggressively, which results in shorter chains. This can adversely affect part strength. When making large changes to back pressure during production, it can take several hours for the barrel to soak or cool to a consistent result.
Cut-off. Changes to cut-off would be used for fine tuning a process, and are achieved quickly. After making the change, the result generally can be viewed by verifying that the cut-off position was made on the next cycle. Large changes to cut-off should only be made with hold and pack pressure removed to prevent tool damage. Hold and pack are added back into the process in increments following verification that the part fills out to 95 to 98% of total fill. The part should have a small, short or sink-like appearance prior to adding pressure.
Cool time. Cool time can be either a broad or fine-tuned change. Cool time effects can be verified 20 minutes after the change has been made, but larger changes may require more time for the overall result to take effect. Take note that longer cool times adversely affect cycle time.
Screw speed. Screw speed is a change that is used while fine tuning the cycle time of a process. The general rule of thumb is that the screw rotate time should be 1 1/2 to two seconds faster than the actual cool time setpoint. It is also important to remember that changes to back pressure can result in longer or
shorter screw rotate times. Screw rotate time should be veriified every time a change is made to back pressure. A screw that recovers too quickly can result in splay defects. Too long a recovery will affect the overall cycle time.
Mold and ejection speeds. Mold and ejection speeds are best made with the press out of production to prevent mold damage. Changes should be reviewed critically while cycling the clamp manually. Slower speeds will affect overall cycle.
In closing, both broad and fine-tuned changes are critical to process validation, and great care must be taken to prevent straying from the overall verified process. Any time a change is made, parts should be viewed as suspect, and proper precautions should be taken to verify that parts meet quality standards. Change results should never slow the overall cycle time, unless defect conditions exist that can only be corrected by lengthening the cycle time. Always remember that a single change can result in multiple outcomes, such as a temperature change resulting in burns and/ or warping or a pressure change resulting in flash or sticking parts.
Be sure that all personnel handling or inspecting the parts are aware that changes have been made, and identify potential hazards so they know what to look for. When process changes reach the desired effect, monitor the process for a full shift prior to changing setup data. As a scientific molder, it is important to review all process monitoring data to verify that changes are documented to ensure process validation has been accomplished.
Our goal as processors is eventually to reach our constant goal of zero rejects, 100% efficiency and repeatable success during startup and production. Repetitive manufacturing results in business success through process standardization and consistency.
Any changes to established processes should be viewed as suspect, and searching for a root cause is key to maintaining process consistency. Man, mold, machine, material or maintenance—the five Ms of molding—could be causing process deviation. Evaluate these areas prior to making any changes to a proven, validated process. A careful approach to process change will ward off many headaches and prevent poor-quality products from reaching the customer.
Garrett MacKenzie is the Owner/Editor of www.plastic411.com and a technical writer for PlasticsToday. His 31- year plastic injection career includes engineering and management for the automotive, medical and handgun industries. He currently offers in-plant processor training programs and training-based webinars. E-mail training@plastic411.com for more information or a quote.