A closer look at scientific molding theory: Page 4 of 4

A primer on the science of process development, recording, standardization and repeatability
By: 
September 28, 2016
  • your machine, mold and auxilliary equipment are in a correct state, make changes to your process that replicate your process monitoring variables (fill, peak, etc.).

  • Prior to process changes, monitor your molding variables for significant fluctuations. A cushion that varies sporadically can be a sign of a worn check ring or barrel wear. Barrel temperature fluctuations can point toward bad heater bands or thermocouple positioning/ failure. Also consider recent mechanical changes made while servicing a press, and whether they might impact the validity of your process.
  • Always consider whether your operator could be affecting your process. Inconsistent cycles can wreak havoc on your consistencies. Improper part handling can cause defects that might be mistaken for a processing problem.
  • Part weight is a key recordable variable. Once a process has been validated, part weight (full shot, including runner) should be recorded. The data should not only include part weight after cycle, but also part weight with pack and hold removed. This can help you identify where in the process you are experiencing a change. Part weight should be verified at the beginning of each start up.
  • A clean and well-serviced mold is imperative to any successful molding operation. Tools should be cleaned no less than once per shift, and materials that are prone to gassing may need to be cleaned twice per shift. Slides and guide pins should be lubed, but it is important to remember that overgreasing can be detrimental to your process efficiency. Always clean your mold prior to any process change: Defects could be directly related to dirty vents.
  • Material moisture is a key function that is often overlooked when process defects occur. Moisture analysis should be part of your start-up procedure, and is completed prior to start up. Upkeep of your dryers is essential to your success. Dryer filters should be cleaned every shift, and you should routinely feel the supply and return hoses on your dryer. When a dryer is functioning properly, the supply hose will be hot and the return will be warm.
  • It is imperative that you analyze the effectiveness of your fill time. Some materials require a fast fill, but if you max out your velocities, you lose control of consistency. Monitor your fill and verify that your setpoint is being reached consistently. Whenever possible, your fill speed should be determined by performing a fill time study.
  • Hold time is a crucial element of your cycle. Establishing a time that is too low results in part-weight variations and process inconsistency. Too much time adds time to your cycle that isn't needed. Performing a gate seal study not only verifies that you are achieving gate seal consistency, but is a crucial step toward process optimization.
  • It is important to understand that adding regrind to your process changes material response. The best approach to introducing virgin/regrind into your molding equation is to treat it as a different material. Determine the optimal virgin-to-regrind ratio that reutilizes your regrind effectively without increasing your scrap rate. Once you've established an effective blend and process, record the process separately from a virgin run. Also, record the process monitoring data separately, then consistently repeat the blend ratio. You can further your ability to mold consistently by re-extruding your regrind with virgin base. This will reduce the potential of drop-down inefficencies (pellet size/ weight vs. regrind size/weight) and promote consistency.

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 experience with U.S./ Japanese automotive OEMs and handgun manufacturing. He can be contacted at [email protected].

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