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A scientific molding process is an efficient and repeatable molding process which is stable and reliable with a large processing window. Although there are many facets to a scientific molding process, the general attributes are as follows:
• The process uses one injection speed to fill, whenever possible
• The mold fills 95% to 98% full during first stage
• All cavities are short shot during first stage
• 1 st stage fill is velocity-controlled and not pressure limited
• Process uses 20% to 80% of the machine’s available shot size
• Final cushion is approximately 10% of the overall shot size
Since polymer viscosity decreases as injection rate is increased, higher injection speeds allow the polymer to flow into the mold more easily. At these higher rates, shear thinning causes more consistent viscosity. This promotes a more consistent and reliable process.
Using a single injection speed simplifies the process and improves consistency. If defects occur with one speed, then multiple speeds can be used. This is referred to as ‘injection profiling’. The best use of injection profiling is to solve specific defects.
When filling, it is critical to ensure you are not using a ‘pressure limited’ process. If the maximum injection pressure setting is inadequate, the screw will slow down during injection to create what is known as a ‘pressure-limited process’. When injection becomes pressure-limited, the machine can no longer maintain the desired injection rate, resulting in an inconsistent fill rate and injection time.
You should transfer from 1 st stage fill to 2nd stage pack before the mold is completely filled to ensure optimal efficiency and consistency. Such a process will be able to better compensate for variations in material viscosity. With a poorly established transfer, an increase in viscosity is likely to cause a short shot, while a decrease in viscosity results in flash. A process with fluctuating transfer is unlikely to be consistent over time, resulting in flash, short shots, and sinks.
To determine the appropriate 2 nd stage time for your process, you should perform a Gate Seal Study. This study determines adequate 2 nd stage packing time by graphing part weight vs. 2 nd stage time.
When graphed, the part weight will increase until the gate freezes. The optimal 2 nd stage time is the time at which the part weight does not increase with an increase in 2 nd stage time.
To prevent screw damage during recovery, either screw delay or screw decompression can be used. The screw delay option adds a delay after 2 nd stage packing to relieve the pressure on the screw prior to recovery. Although each process is different, the screw delay time should be enough to allow the injection pressure graph to drop to zero before recovery starts.
In some high-speed applications, screw decompression or suckback can be used to relieve the pressure at the front of the screw. This option backs the screw up before starting screw recovery.
To determine the optimal feed zone temperature for your process, you should perform a feed zone temperature study. The purpose of the feed zone temperature study is to determine an adequate feed zone temperature by graphing screw recovery time versus feed zone temperature. When graphed, the screw recovery time will drop and then rise as the temperature is increased. The optimal feed zone temperature is the temperature at which the screw recovery time is the lowest.
Once the feed zone temperature is determined, adjust the rotational speed of the screw so that recovery consumes 80% of the overall cooling time. For example, if your process has a 10 second cooling time, your shot should be recovered approximately 2 seconds before the cooling time is finished.
For most screws, the optimal amount of ‘screw suck back’ should be equal to the amount of ‘check ring travel’. Since the size and configuration of check ring assemblies vary, you should ask the manufacturer or pull the screw and measure the check ring travel.
To establishing cooling, begin with a longer cooling time than should be necessary. You can first determine the lowest mold temperature which provides an acceptable part. Once this mold temperature is established, you can lower the cooling time to determine the lowest cooling time which provides an acceptable part.
The documented process outputs are those which result of a process where acceptable parts are produced. Many of these parameters are the same as the process inputs but each of these parameters would be consistent from one machine to another.
Examples of machine independent process parameters include:
• Melt Temperature
• Coolant Temperature Entering and Leaving the Mold
• Coolant Flow
• 1 st Stage Fill Time
• 1 st Stage Fill Weight
• 2 nd Stage Packing Time
• 2 nd Stage Plastic Pressure
• Gate Seal Time
• Cooling Time
• Plastic Back Pressure
• Screw Recovery Time
• Overall Cycle Time
• Final Part Weight
• Clamp Tonnage
We also document any important information such as photographs, observations, and quality information.