In a previous article blog, Injection Molding Process Validation Using Scientific Molding, the standard (baseline) process was established. Coupling the established standard (baseline) process, scientific molding principles and monitoring of the four plastic variables during production runs, helps to Ensure Reliable and Repeatable Product.
Injection Molding Process
For this article I am using references to the human body, because like the human body an Injection Molding Process is complex, and also has a lot of variables and interactions that effect the way it runs. To simplify the complexity of both we can externally monitor and observe key variables.
Standard procedure for a doctors visit starts with a check of your vitals (four plastic variables) temperature, heart rate, blood pressure and weight. Observation of how you look and sound, a quick look at your chart (baseline standard process) and a diagnosis is made. This was accomplished with external data of the internal process from common inexpensive instruments when in properly trained and licensed hands can Ensure a Reliable and Repeatable diagnosis.
At Crescent we pride ourselves in having the Scientific Injection Molding Knowledge and Training Certifications that help us to Ensure Reliable and Repeatable Product.
Plastic Melt Temperature
Use the material manufacturers recommended melt temperature for process development and verify with a calibrated melt probe to check the plastic melt temperature. At Crescent, as a general guideline, the nominal melt temperature is established 10 degrees Fahrenheit above the lowest suggested temperature for the material. The nominal melt temperature is then given a +/- ten (10) degree tolerance. As part of our start up and shut down procedure the established melt temperature is verified and documented.
Plastic Flow Rate
Inject as fast as possible, consistent with quality, until the cavity is 95 to 99% full and then transfer via position to pack and hold. The established (plastic flow rate) machine nominal fill time for this standard process is given a +/- .02 second tolerance and is not changed. Example, a mold with an established fill time of 1.00 second is moved to a comparable molding machine with an acceptable barrel capacity. The injection speed would be adjusted accordingly to match the established nominal fill time of 1.00 second with a low of .98 and a high of 1.02.
Crescent buys only world-class injection molding machines with proven repeatability that gives us the confidence of consistent, (plastic flow rate) machine fill time, through closed loop control despite the normal viscosity variation of plastic. Simply put, closed loop control is high performance cruise control, that will not allow you to get a speeding ticket at the bottom of a steep hill (low viscosity).
The CNC electric molding machine, along with ever greater computing power, has pushed that confidence of consistency to an even higher level versus hydraulic machines. As part of our start up and shut down procedure the established fill time is verified and documented.
Plastic Pressure Gradient
Pack the part to finish filling while adding enough plastic to compensate for shrinkage. Establish the hold pressure that gives the part the desired cosmetic look and size. Coupled with the established hold pressure and the proper amount of hold time. We then push the hold pressure to its high and low limits to establish the range for the standard process. The CNC electric molding machine, along with ever greater computing power, has pushed that confidence of consistency to an even higher level versus hydraulic machines. As part of our start up and shut down procedure the established hold pressure is verified and documented.
Plastic Cooling Rate and Time
Cooling rate is the difference between melt temperature and mold temperature. The mold temperature is established using the material manufactures recommended ranges and the customers working temperature of the part. The delta T of each water circuit during process development is established at 4 degrees Fahrenheit or lower and documented on the process sheet. Fine tuning the cooling time during process development is directly related to the customers expectations of part appearance, size and desired properties. Once established it is documented on the process sheet
For further information on the benefits of scientific molding principles for plastic injection molding, contact the experts at Crescent Industries Inc., the ISO 13485:2003 and ISO 9001:2008 certified, single-source supplier of custom plastic injection molding solutions.
-Written by Rich Wire, Process Engineer
Please share how scientific molding principles will help your injection molding project.
About Frank Spears