You will rarely find me writing an article in the first person. I have always felt that the best way to engage my molding peers in the depth of the topics I cover is to put them in the driver’s seat. However, this article is best presented with analogies and 30 years of experience. As such, I will cover 10 scenarios that cause scientific molding–based companies to fail in their utilization of the principles they attempt to practice. Trust me: There are many more I could outline, but to prevent readers from sleeping through my soapbox episode, I will limit my scope to 10.
Too many “medicine men” and not enough processors practice the scientific molding ideology that John Bozzelli created. I personally remember sitting in a classroom in the summer of 1993 as John taught his program. It changed my entire view on molding. Beyond that, it molded the next 30 years of my career. I learned the importance of process development and recording, and it has always steered me away from false readings and refined my analysis of what a true process really is.
It is important to understand the tenets of scientific molding. Outlining all the steps would be an article in itself. At the end of the day, scientific molding encompasses a series of steps that first establish a solid repeatable process, which is then validated. The general rule I use in validation is that if a process is true, a press meets or exceeds production requirements for a period of 24 hours with minimal (1.5%) to no scrap.
|Stay on top of plastics processing trends and business opportunities at PLAST-EX, part of Advanced Design & Manufacturing Expo in Toronto on May 16 to 18. Go to the event website for more information and to register to attend.|
Another key rule of scientific molding is that set up (both changeover and process input) is standardized. This means that the new job is easily changed over without variation, and start up is achieved without a major scrap or process adjustment phase. A true process starts up quickly with, at most, minimal adjustments.
The next step is process control. Process change limits are established to ensure that process consistency is maintained. Process changes that stray outside of those limits are viewed as “red flags,” requiring a deeper assessment of what changes have occurred.
Lastly, any and all historical data are recorded for future analysis. It is important to note that when processes go “bad,” it isn’t the process that fails. Data give direction as to what changes have occurred, and in most cases provide a troubleshooting blueprint, which is used to correct whatever change has developed.
With this, let’s address the meat of the topic—why molders fail:
1. Button-pushing cowboys. Many times over the years, I have seen this situation. Rather than try to identify what has changed, a molder instead just starts pushing buttons in an attempt to correct. They should be asking, “what changed?” Process control is set aside, and process limits are totally ignored. In all situations where a process has been validated, the process control limits should be monitored closely to look for what has changed. It is these limits that will point toward changes that are causing the process variance.
2. Material. It is important to understand the effect that material can have on process consistency. Molders need the ability to trust that the materials they are receiving are consistently produced. In addition, one nylon is not the same as another. A process is established based on a specific manufacturer blend. Every time a material change occurs, regardless if the material manufacturer insists they are exactly the same, it is a brand new material, and a brand new processing approach. A change from one material to a new brand/blend of the “same” base needs to be treated as a fresh start up. Remove the press from production, establish a new process and, once validated, record all new data as it relates to the material change.
3. Changing auxiliary equipment. One thermolator, dryer, hot runner controller and so forth is not the same when it pertains to process consistency. Marry equipment, molds and other equipment to the same press. Every variation that is introduced into a process is a new process. In cases where auxiliary equipment is frequently moved press to press, label your auxiliaries with number identifiers. Record these on your set-up sheet, and, if at all possible, reuse the same equipment every time the job runs.
4. Mold modification. This is generally implemented during the engineering stage, not on the production floor. If a mold is modified in any way, the process is new and should be viewed as such until the modification has been re-validated.
5. Watering and heating. Molds should be evaluated during the process engineering stage so that base historical data can be established. Set up, flow to and from process and steel temperature measurements are critical components of measuring process changes over time. Data should be recorded for each individual circuit to provide best case pressure/flow validation.
6. Process monitoring. Process validation should be based on a 24-hour run at 100% efficiency; 0 to 1.5% scrap (preferably zero). Once process has been validated, process monitoring is key to measuring for change. Fill time, screw rotate time, cushion, peak pressure and so forth quickly identify changes within the process and are reliable identifiers when troubleshooting system changes.
7. Barrel temperature. This is not limited to set points; temperature actuals also should be monitored. Controller measurements, as well as steel temperatures between zones, should be recorded and used to identify changes.
8. Labor. Never rule out the machine operator as the cause of process failures. Defects can sometimes appear to be process related, but eventually part handling/operator procedure becomes the true cause of process change. Machine operation should always be viewed as part of the overall process. If a process and its monitoring appear to be fine, take the time to review operator steps and procedures for change and effects.
9. Quality system. Make sure that quality failures are not misdiagnosed. Check part dimensions and aesthetics to print and customer requirements. Make comparisons between the last shot previous run and first shot from the new run. Utilize fit-to-function principle, when applicable. Remember, unnecessary process changes can be just as detrimental as taking the “good samaritan approach” and trying to adjust for false defects.
10. Set up. Standardized set up is fundamental to strong start up and production runs. Poorly executed or inconsistent mold and process set ups quickly lead to large scrap rates and unplanned down time. Develop the changeover during the process engineering stage to ensure that changeovers are precise and easily repeatable.
These are some of the strongest fails that occur while trying to practice a systematic approach toward scientific molding. The foundation of this molding theory is to standardize your operation and remove chaos from each molding system by simplifying procedures and establishing a concrete molding system. Lean molding requires consistent replication and thorough documentation of successful runs to ensure each production event is successful.
Garrett MacKenzie is the owner/editor of plastic411.com, as well as a consultant/ trainer to the plastic injection industry. He has spent more than 31 years in plastics processing, engineering and development, including scientific molding experience with U.S./ Japanese automotive OEMs and handgun manufacturing. He also offers in-house processor training customized to customer needs. He can be contacted at email@example.com.