Processing in plastic injection
can sometimes be a tricky business. It takes a strong and knowledgeable approach towards process set up when adjustments are being made to a process. Materials respond in different ways to process change, and every adjustment made needs to be accomplished with a solid understanding that a part’s dimensional, aesthetic and even function can either be improved or degraded as variables are changed.
A good comparison to process adjustment in regards to machine response is the older style radio with a knob designed for both broad and fine tuning. The larger knob can be adjusted for aggressively adjusting frequencies to get to the station you want. The smaller 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 that 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 will outline various process changes a processor has available to him when a process requires change. It will outline specific parameter changes, their potential outcomes.. as well as specific problems to look for. It will also provide insight on how long it takes for specific changes to take effect .
Barrel temperature would be an adjustment that can be either a broad or fine adjustment to process. It is important to remember that the best way to gauge the end result of a temperature change is 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 defects occurring, 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 affect. 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 set point. 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 as the press runs due to the length of time required for heat changes to take affect with a press in running condition. It is important to note that when a press is in a running condition, it could take several hours for a large change to be confirmed as the actual molding condition.
Much like barrel temperature, mold temperature also serves a dual purpose as both a broad or small change to an ongoing process. Much like the barrel, changes can not be ruled as good or bad without first allowing the mold to heat soak or cool for a minimum of 20 minutes after the set point 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 start up to be sure that both the barrel and mold have reached a heat soaked state.
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Making changes to back pressure are generally a broad adjustment, resulting in larger changes to barrel temperature. It is important to remember that while increasing back pressure raises barrel temperature and lowers viscosity, it also breaks down the material more aggressively, which results in shorter chains. This can adversely affect part strength. Making large changes to back pressure during production can take several hours for the barrel to soak or cool to a consistent result.
Changes to cut-off would be used for fine tuning a process, and are achieved quickly. After making the change, the result can generally be viewed by verifying that 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 increment following verification that the part fills out to 95-98 % of total fill. The part should have a small short, or sinky appearance prior to adding pressure.
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 effect cycle time.
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 veruified 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 overal cycle time.
Mold & Ejection Speeds:
Mold and ejection speeds are best made with the press out of production to prevent mold damage. Changes made should be reviewed critically while cycling the clamp manually. Slower speeds will affect overall cycle.
In closing, both broad or fine tuned changes that are made are critical to process validy, and any change that is made requires that great care be taken to prevent from 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 that are handling or inspecting the parts are aware that changes have been made, and identify potential hazards to them so they know what to look for. When process changes reach the desired effect, monitor the process for a full shift prior to changing set up data. As a scientific molder it is important to review all process monitoring data to verify that any changes that exist are documented to assure process validation has been accomplished.
Our goal as processors is to eventually reach our constant goal of zero rejects, 100% efficiency and repeatable success during start up 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 (see the article, ” The 5 M’s of Molding”) could be causing process deviation. Evaluate these areas prior to making any changes to a process that has been proven and validated. Careful approach to process change will save alot of headaches, and prevents poor quality from reaching the customer.
Garrett MacKenzie is the owner and editor of www.plastic411.com. and also a technical writer for both Plastics Today and Plastics Technology magazines. He currently offers 2 day processor training programs and training-based webinars.
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