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Injection Molding Reference Guide
5 Seven Steps to Scientific Troubleshooting
5.1 STEP 1: Develop a Scientific Molding Process
A scientific molding process consists of the following attributes:
5.1.1 1st Stage Injection
During this stage, the mold is filled using screw velocity control. There should always be enough injection pressure available to ensure the machine can maintain the desired velocity setpoint.
5.1.2 1st to 2nd Stage Transfer
Transfer should take place using screw position. The mold should be approximately 95% full at the time of transfer. The resulting part should be a visual short shot.
5.1.3 2nd Stage Packing Pressure
Pressure must be high enough to finish filling the mold cavity and pack out all sinks and voids. 2nd Stage Packing Pressure is typically 50-75% of 1st Stage Pressure.
5.1.4 2nd Stage Time
Determine the appropriate 2nd Stage Time for your process by per-forming a gate seal study. The Gate Seal Study will help determine an adequate 2nd Stage Packing Time at which the part weight does not increase with an increase in 2nd Stage Time.
5.1.5 Screw Delay or Decompression before Recovery
After 2nd Stage Pack there is a large amount of pressure present in front of the screw. Either screw delay or screw decompression should be used before recovery to prevent damage.
5.1.6 Screw Recovery
During screw recovery, screw recovery should consume 80% of the overall cooling time. If there is a long cooling time, then a significant screw delay can be used to reduce the time the material remains in the barrel.
5.1.7 Screw Decompression after Recovery
When the screw travels forward for injection the pressure holding the check ring in the forward position can interfere with the check ring movement. Screw decompression is necessary to prevent in-terference. The proper amount of ‘screw suck back’ should be equal to the amount of ‘check ring travel’.
5.2 STEP 2: Properly Document the Process Outputs
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
• 1st Stage Fill Time
• 1st Stage Fill Weight
• 2nd Stage Packing Time
• 2nd Stage Plastic Pressure
• Gate Seal Time
• Cooling Time
• Plastic Back Pressure
• Screw Recovery Time
• Overall Cycle Time
• Final Part Weight
• Clamp Tonnage
You can also document any important information such as photo-graphs, observations, and quality information.
5.3 STEP 3: Examine Defective Part and Rule-Out Obvious Causes
Once a non-conformance occurs, the first thing to do is to thor-oughly inspect the part to ensure other defects are not present. If any aspect of the process, mold, machine, or material is obviously suspect, then this should be investigated first.
For example, let’s suppose a part may have been identified to have flash. With only this information, the traditional troubleshooter may investigate aspects such as the clamp tonnage, transfer position, packing pressure, or barrel temperatures. Now, let’s assume further inspection took place, and the scientific troubleshooter determines the part also has splay. With this additional information, it may be possible to conclude that both the flash and splay could be the result of moisture in the material.
5.4 STEP 4: Compare Current Process with Documented Process
Once obvious causes are ruled out, the next logical step is to compare the current process with the documented standard. Since a well-documented process contains a variety of parameters, it is best to start by reviewing the parameters which most likely relate to your defect.
As you compare the documented standard to the current process, you will determine the aspects of the process which have changed from the standard. A scientific troubleshooter can use this infor-mation to help make educated decisions about which parameters need to be changed to bring the process back to the documented standard.
For example, if the part is showing degradation, you should first compare parameters such as melt temperature, back pressure, and 1st stage injection time.
Likewise, if flash, short shots, or sinks are present, it is best to turn off second stage packing to verify all the process outputs related to injection such as 1st stage fill time, 1st stage fill weight, plastic pres-sure at transfer from 1st stage to 2nd stage.
5.5 STEP 5: Return the Process to the Documented Standard
With accurate knowledge about which processing parameters have changed, the scientific troubleshooter cannot begin to make changes to return the process outputs to the documented standard. Always be careful of related process parameters.
For example, if the 1st stage fill time is too high and the 1st stage fill weight is too low, then an increase in the injection speed may bring both parameters back to the documented standard.
The goal here is not to just fix the defect, but to return the machine independent process outputs back to the documented standard. When steps 1 and 2 are conducted properly, the scientific trouble-shooter has confidence that the parts will be acceptable when the process is returned to the documented standard if the mold, machine, and material are behaving properly. This troubleshooting method will also help the scientific troubleshooter to quickly identify and isolate a problem with the equipment or material.
5.6 STEP 6: Verify the Part and Process
After the parts are brought into conformance, it is best to ensure that both the part and process are brought to the standard. This means the scientific troubleshooter should take a few minutes to check each of the parameters which are easy to verify.
It may be impractical to check every process output, but any information which is easily obtained such as 1st stage fill time, 1st stage fill weight, cycle time, cooling time, etc. will help increase the confidence that the process will remain stable and reliable over time.
5.7 STEP 7: Document All Changes Made
This is one step any troubleshooter, scientific or traditional, will derive benefits from. A trail of documentation should begin when the process was established and approved, and continue to build as the process is adjusted to create a portfolio of information to draw upon when trouble arises. A scientific troubleshooter should be able to see the full history of the process, machine, mold, during this time.
For example, if an employee on first shift corrected the process by increasing transfer position, this would be invaluable information for the second shift technician when they encounter a defective part, such as a sink.
If a systematic approach to processing and documentation is used when the process is established a scientific troubleshooter will be able to correct the problem in a relatively short time with a signifi-cantly high degree of confidence. Ultimately, good troubleshooting is just an extension of good processing. The more effectively you process and document, the more efficiently you will troubleshoot when non-conformances occur.
PlasticsEdge™ microsite sponsored by...
Injection Molding Guide
- Part 1 - Understanding Plastics
- Part 2 - Plastics Materials Overview
- Part 3 - Properties, Additives, & Preparation
- Part 4 - Establishing a Scientific Molding Process
- Part 5 - Seven Steps to Scientific Troubleshooting
- Part 6 - Defects
- Part 7 - Basic Mold & Part Design Guidelines
- Part 8 - Units and Conversions
- Part 9 - Frequently Used Calculations
- Part 10 - The Importance of Training
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