This article was updated on Sept. 4, 2018. It was originally published on Dec. 3, 2017.
In a previous article, I presented 10 scenarios illustrating how companies that seemingly espouse scientific molding principles fail in their implementation. I could have mentioned many more, but to prevent putting readers to sleep, I got off my soapbox when I reached 10 reasons. In revisiting the article here, I have included some additional insights to the first 10 and added another five failures, bringing the total to 15.
Too many “medicine men” and not enough processors claim they practice the scientific molding ideology that John Bozzelli created, while others at the top of the industry have capitalized upon the use of the term. I personally remember sitting in a classroom the summer of 1993 as John taught his program. It changed my entire view on molding, and developed my approach toward true standardization. It gave me a sense of what validation truly means when, as molders, we establish process and develop controls. Beyond that, it defined the next 25 years of my career. I learned the importance of process development and recording. John’s insights have always steered me away from false readings, as well as defined my analysis of process.
It is important to understand what scientific molding is. It is not bells and whistles, with fancy terminology, beautiful pictures and fancy equipment that most plastics controllers are outfitted with nowadays. Outlining all the steps behind establishing true process would be its own article. But at the end of the day, scientific molding is a series of steps that first establish a solid repeatable process, which is then validated. True process is never the first step of the engineering phase. Process is developed following the stages of part design, tool design/building, material choice and press validation. Without these fundamental functions having been proven, process is limited to the failures attributed to these key functions.
|Garrett MacKenzie, the author of this article, will present a free webinar on troubleshooting the injection molding process using a scientific molding approach on Sept. 11, 2018. He will discuss variables that can identify changes within an existing molding process and provide insight into specific approaches that can be used to bring a validated process back into normal process parameters. Register now for this webinar. Busy that day? No problem: You can listen to the webinar on demand, but you must register first.|
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. This process must be repeatable and include a fully standardized setup. True processes are generally easy to start over and over with a minimal amount of startup scrap. In more complex cases, strict startup procedures are clearly documented and enforced to ensure that the procedures are sound and deviation is minimal. This means that the new job is easily changed over without variation and start up is achieved without a major scrap and/or process adjustment phase.
The next step is process control. Process control 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 changes that have occurred. If a process requires multiple changes during startup, or if a process requires frequent changes during production, something is wrong!
There is a reason that the scientific molding approach has been defined as a repetitive, or standardized, process. It is important to remember that it is possible to have more than one working process. Our goal as processors always remains the same: Easy startup, 0 to 1.5% scrap and 100% efficiency based on quoted cycle. This defines true process. If the process we have deemed valid does not provide high efficiencies, low to zero scrap and adequate startup results, we must reevaluate and look for ways to achieve maximum yields. We as molders are not limited only to process changes. We must review mold design, component function and mold modification as potential sources of achieving the goals for which we strive. Consider all available criteria prior to a complete change of process approach. Never assume that the process itself is invalid, until you have ruled out that other criteria are limiting the process requirements and controls.
All historical data must be recorded for future analysis. It is important to note that when processes go “bad,” it isn’t the process that has failed. Data give us direction and insight into changes that have occurred. In most cases, recordable data provide a troubleshooting blueprint used to correct whatever change has occurred. The first thing to remember when documenting a process is that strong data references are key! A great comparison is the difference between a black and white picture and the same picture that has been colorized. The more information that is recorded, the better we can distinguish changes in molding conditions. A poor approach toward process monitoring will always result in vague interpretations of available data, because the data sets are lacking. Limited data lead to poor interpretations of data sets. Evaluation requires more time because the lack of available information inhibits our ability to quickly assess what has happened, and how best to correct the condition.
With this, let’s address the meat of the topic, which is why molders fail.
- Button-pushing cowboys. Many times over the years, I have watched this scenario unfold: Rather than trying to identify what has changed, a molder starts pushing buttons in an attempt to make corrections. The proper approach requires us to first ask, “What has changed”? Process control is set aside, and process limits are totally ignored. Think of the potential outcomes of this approach: Bad parts reach the customer, hours of scrap result from poor evaluation and consistency is non-existent in the molding approach.
- 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 and handled. For instance, we know for a fact that one nylon is not the same as another. They may perform in a similar fashion, but one material’s response may be totally different despite the base similarities. A process is established based on each specific blend. Every time a material change occurs (regardless if the material manufacturer insists they are the same), it is a brand-new material and a brand-new processing approach must be deployed.
- Changing auxiliary equipment. A thermolator, dryer, hot-runner controller and so forth are not the same when we are making every effort to ensure process consistency. It is highly recommended that molders marry equipment, molds and other systems to the same press. What this means is each component is physically bound to the same production press every time, so that there is no deviation in press and auxiliaries. Remember, every variation that is introduced into a process is a new process. It is also important to note that there are changes that occur to auxiliary equipment itself, which requires historical data to be collected: Thermolator valves stick causing overheating or poor heat dispersion, gallons-per-minute readings change, valve gates stop functioning. Monitor every condition available to you for verification of true process.
- Mold modification. Mold modification is generally implemented during the engineering stage, not on the production floor. If a mold is modified in any way, the process should be viewed as new until the modification has been revalidated. Never assume that a change to a mold will not affect the process in any way. Instead, consider the mold to be suspect, and review all historical data you have available to ensure that no real change has occurred. Your quality department needs to be fully aware of the modifications and should perform the proper layout inspections and testing, prior to the press being released for normal production.
- Watering and heating. Molds should be evaluated during the process engineering stage for proper cooling and heating dispersion, allowing base historical data to be established. Setup diagrams should be documented, and frequently updated. Turbulent flow to and from process (supply/return) should be measured in units of gpm/lpm. Steel temperature measurements are also critical components for measuring process changes over time. It is important to note that mold steel temperatures can vary in different areas of the mold due to heat exposure and flow differentials. Measure and record temperatures in multiple mold locations for historical reference.
- Process monitoring. Once a process has been validated as true and consistent, process monitoring is a primary key for measuring historical change. Fill time, screw rotate time, cushion, peak pressure and all other primary sources of process monitoring data quickly identify changes within the process. Monitoring is a reliable identifier that triggers reaction when the troubleshooting system is enacted. Verify all key monitoring measurables, and if you identify a significant variance, use that change to identify other changes that might have taken place, including the five M—man, mold, machine, material, maintenance—system of troubleshooting.
- Barrel temperature. Barrel temperature is not limited to set points—temperature actuals should also be monitored. Controller measurements that monitor actual temperature, as well as manual measurements of true barrel temperatures between heater bands, should be recorded and used to identify changes in temperature over time. In addition, watch for signs of worn heater bands, such as poorly heating zones at start up. Historical data of steel temperature data between bands throughout all zones will identify heater bands that are performing poorly, and quickly identify bands that need immediate replacement.
- 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. Step back and take the time to evaluate precisely where the defect occurs. Don’t be afraid to run a press in semi-automatic and to inspect the part as it is removed from the mold, even prior to removal in a post-ejection state.
- 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 the “fit-to-function principle,” making sure that components meet the needs of your customer to ensure functions meet part requirements. Remember, unnecessary process changes can be just as detrimental as taking the “good Samaritan approach,” trying to adjust for false defects.
- Setup. Standardized setup is fundamental to strong start up and production runs. Poorly executed and/or inconsistent mold and process setups 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. Make sure that all personnel involved in changeovers are consistently performing setup duties and not using multiple approaches to accomplish the same task. Establish clear changeover guidelines, and enforce their implementation.
- Robotics. Never rule out a robot as a potential cause of part defects. Programming and even end-of-arm tooling can cause defects such as drag, pull, scratches and so forth. Inspect parts prior to robot extraction to verify that the scrap event isn’t robot-based. Damaged cups, pneumatic cylinder failures and even poorly cleaned/serviced end-of-arm tooling can lead to unexpected defects.
- Automation. Similar to robotics, never assume that automation can’t be the root cause of a defect event. Inspect parts prior to and post assembly to verify that scrap issues aren’t being caused by poorly performing or maladjusted automation equipment. Look for damaged components, poorly performed automated actions and review that the equipment has been properly cleaned and serviced.
- Material handling. Inspect all aspects of the material-handling operation. Make sure filters are being cleaned, loaders are functioning properly and products are not being moved improperly causing damage. Inspect product just packed by operators and prior to removal from press to ensure adherence to proper handling procedures.
- Mold Cleaning and function. Many processors fail to understand that one of the first things that require analysis prior to process change is making sure the mold has been cleaned and mold components are functioning properly. No process change should ever occur without a full cleaning of the mold and an inspection of mold components for damage and/or improper function. In some cases, inspect the parts just prior to and during ejection to identify problems. Inspect vents that are at or near areas of shorting and/or burns.
- Maintenance. Review press and auxiliaries for changing conditions and poor performance. Machine valves wear, dryers malfunction, carriages get out of alignment, screws wear and check rings crack or break. Learn to identify equipment failure using monitoring data, and get input from your maintenance team to analyze potential faults and determine the best approach to fix them. Remember, planned maintenance events are always much more cost effective than an unplanned repair. Develop a strong preventive maintenance approach to reduce scrap, down time and troubleshooting events.
These are some of the biggest failures that occur while trying to implement a systematic approach to scientific molding. The foundation of scientific molding theory is standardization and monitoring of your operation. These remove chaos from each molding system by simplifying procedures and establishing a concrete molding approach. Lean molding requires consistent replication and thorough documentation of successful runs to ensure each production event is successful. Standardization, monitoring and maintaining process consistency are the keys to a strong molding foundation and solid profits.
Garrett MacKenzie is the owner/editor of plastic411.com, as well as a consultant/trainer to the plastic injection industry. He has provided his process engineering expertise to many top companies, including Glock, Honda, Johnson Controls and Rubbermaid, and currently works for a company that provides automotive products to Yenfeng, Faurecia and other top automotive suppliers. He can be contacted at [email protected].