By Garrett MacKenzie, Plastic411 Editor
There are many variables in the beginning stages of process development that determine the longevity of process standardization in plastic injection. Establishing a repeatable process that provides 100% efficiency with little to no scrap is certainly an important step towards standardized process.
However, there is another stage in developing a process that can be sometimes overlooked in haste or poor approach. A true process certainly requires normal process consistencies, such as fill time, peak pressure, etc. to assure that the initial process developed is still holding true to its original standard. But keep in mind that recording historical data can be just as critical to standardization as process development itself.
Historical data is a group of key measurements that not only confirm that standardization has been achieved. These data points also can identify changes that are affecting the initial process, and help to analyze what steps need to be taken to revert back to the original process.
This article outlines the value of historical data as it relates to physical conditions outside of the normal process control measurements normally monitored. It will provide four primary examples of what physical data should be recorded, and how to use that information to identify changes occurring within the original molding conditions. It will also provide some insight into how to analyze that data to pinpoint what has changed, and steps that might be taken to correct the inconsistencies.
BARREL & MELT TEMPERATURE:
Barrel temperature is a normal recordable, that is commonly included on the set-up sheet for most companies. Zone set points are repeated, to assure that temperature set points remain consistent.
However, barrel temperature itself is a key indicator of process change. For instance, if a process that has been proven over months of time as repeatable starts to run short/ unfilled parts, verifying set points might not provide adequate information as to what has changed. Heater bands can wear out, leading to poor heat performance in certain areas of the barrel.
Historical barrel temperature should be measured between bands using a band-type thermal indicator. Laser measurements will work, but are much less reliable. It is also important to record melt temperatures for each process you run. It is imperative that when recording melt temperature, the press should be shut down during a running condition to assure that the barrel has been properly heat soaked. An example of this data being helpful would be when a heater band is working, but under-performing. The band will register on the controller as being at temp, but a temperature variance on the barrel itself might help to identify a changing condition.
Mold temperature is a frequently under-valued and forgotten form of historical data that offers real value to change analysis. Measurements should be taken with the mold in a running condition to allow proper heat soak. Band and probe- type thermal measuring devices are again preferred as the best way to record accurate, controlled measurements. Temperatures should be measured in various areas of the cavitation, runner, bushing and hot drop areas. A good way to record this data is to take a picture of the mold faces themselves, then record the temperature data defining the correct positioning of the temperature probe/ band. An example of this data helping to identify a problem is a poorly performing water circuit with reduced flow. The flow reduction would cause an increase in mold temperature in a specific area. Increasing historical data for the mold surfaces can help to identify these types of changes.
PROCESS CONTROL MEASUREMENTS
Process control limits are essential to verifying validation. Here are the key control variables that should be recorded and verified every time a process is repeated:
Peak Pressure: Peak pressure is a primary variable that should be repeated. Peak is measured at the switch over point as injection transitions from injection cut off into hold pressure. Pressure variance limits should be no more than 100 psi above or below the recorded pressure established in the original validated process.
Screw Recovery: Screw recovery is a time measurement that records how long it takes the screw to rotation to complete. Recovery limit window should be 2 seconds above or below the original recorded process validation time.
Cushion: Cushion is a position measurement that has a direct relationship to shot size. The general cushion measrement is .25-.4 (6 to 8mm) on presses 400 Tons and lower. Larger presses may require a larger cushion (.5 to .75) set point due to larger volume capacities.
Support equipment, such as thermolators and hot runners are frequently overlooked as a key source of variation that require control to achieve process standardization. Two pieces of equipment can be identical brand/make etc., yet perform completely different than on another. Whenever possible, marry this type of equipment to a specific press to reduce potential variances.
Thermolator performance can be measured by recording GPM/ LPM both on supply and return of the unit itself. It is also important to verify that actual temperature meets set points. For instance, if a thermolator pump starts to fail, lower GPM would be a clear indicator that a change has occurred.
Hot runner performance is another key measurable that can be used to analyze variance. Actuals should be the same as setpoints. Another key recordable is temperature variance. Preferably, there should be no swing above and below actual setpoints. In cases where the swing is inevitable, the normal swing should be recorded to help identify future variances. The hot runner itself should be measured similarly to barrel temperature recording. With the hot runner exposed, temperatures between bands should be measured and recorded for future reference. It is important to note that when bands on the hot runner are replaced, great care should be taken to install the new band in the same location that the old band was in.
These are only a few examples of how historical data can help to increase a molder’s ability to quickly identify and respond to changing molding conditions. Increasing the amount of data recorded improves analysis capabilities and improves response time to process change. There is a clear difference between a photograph in black and white, to that of a colorized photo. Similarly, change response is fully dependent upon the amount of data available to identify process variance. Record all data available when a process has been validated to assure that when future variances arise, the changes can be quickly responded to with the proper approach.
Garrett MacKenzie is the owner and editor of www.plastic411.com. MacKenzie has held engineering/ management positions for 20 years. His plastics career spans over a 31-year time frame, including Glock, Stanley Electric (Honda sister), Rubbermaid and Johnson Controls. He currently works in plastic injection trainer capacities. His next training event is a Scientific Molding Work Shop, being held in Dalton, GA October 27-28, 2018 at CH3 Solutions. For more information about the event, click the following link:
He can also be reached at: [email protected]tic411.com .