The first step in repeatable molding is understanding the key fundamentals in process monitoring
Scientific molding demands that process and historical data are repeatable, within the established limits of process monitoring variables. There are also key practices within the procedures in your molding system that can cause process inefficiencies and unplanned maintenance.
Allowing monitoring variables to stray outside of established control variances can potentially turn into large scrap events that threaten the integrity and profit margins of your organization.
This article will outline the key variables of process monitoring, as well as potential failures that can lead to process inconsistencies and scrap. There are simple precautions that need to take place every shift to assure that process and production are on target, World class facilities must police themselves to assure our customers are receiving prime product while preventing down time events associated with poor in-plant floor performance.
It is important to note that process validation procedures must be completed prior to establishing process monitoring windows. A validated process must run at 100% cycle efficiency, producing little (1% max) to no scrap for no less than 8 hours. On jobs that cannot run at a minimum 1% scrap limit, evaluations need to take place on how a <1% scrap rate can be achieved. The other consideration that must be analyzed is whether a saved process can be repeated from one run to the next. In most cases, a repeatable process is dependent upon consistent mold changes.
Process Monitoring Requirements:
Many companies fail to understand that a similar press or mold does not guarantee that process will be repeated. Each press should have its own process monitoring sheet for recording data, and sister molds need to be approached as completely different molds. The same applies to auxiliary equipment. Marry hot runners and thermolators to the press they serve.
Record the current conditions for Peak Pressure, Fill Time, Part Weight and Cycle Time at least once per shift. This is to be compared to the target values. Here is a list of normal target values for these monitoring recordables:
- Fill Time- .20 seconds above/ below
- Peak Pressure: 100psi above/ below
- Part weight: 5% total weight above/ below
- Cycle: 1 second above/ below
Each target value also has upper and lower control limits. The values recorded for current condition MUST be within the window established by these limits to be in control. IF the current value falls outside the established monitoring window, the technician needs to verify the process matches the process set up sheet
If process matches the set up sheet, look for any unplanned production changes (Man, Mold, Machine, Material, Method) that could be responsible for the variation. Changing a process should ALWAYS be a last resort measure! Review set up, historical data, clean the mold, verify press set up, etc. before a process change is made.
If all criteria have been met, adjustments are to be made to process to bring the corrupt value into the established process window. Every change made can lead to a different defect. Adjusting for a short can lead to flash, raising mold temperature for a visual defect can cause feather flash, etc. Learn to think forward, and watch for potential defects that could be associated with a particular process change.
If process changes fail to bring a value into the established process window, the deviant value must be noted.
Values running outside of tolerance must be reported to the Process Engineer and/or Injection Molding Manager. The engineer will make adjustments to get back within the upper/ lower control window. The engineer may need to adjust the upper/ lower control limits to bring the values back into control.
Other Basic Molding Fundamentals:
Here are some of the other key procedures that need to be a part of normal process monitoring protocol:
The key to consistent process consistency is care and inspection of molds for each job. Every mold is to be cleaned and inspected once per shift, taking great care to verify mold components are greased and performing properly.
Every nozzle is to be inspected for blowback once per shift. Failing to do so will lead to catastrophic down time events replacing heater bands. Review the barrel temperature page, confirming actuals meet set point requirements. Hot runner set points should match actuals as well with temperatures varying no more than +/- 3F.
Water is to be verified ON at the beginning of each shift, especially if a mold was changed and started at the end of the outgoing shift. Molds without water can break, or lead to major scrap events.
Following these simple procedures can help to promote solid production events, while avoiding major down time and scrap. Scientific molding protocol requires a true commitment to replication and monitoring. Following these simple procedures will not only avoid scrap and down time failures. They assure the success of your company, and help promote solid profit margins.