5 Tips to Reduce Variation in Tight Tolerance Injection Molding

In tight tolerance injection molding, process parameters must be controlled tightly and monitored closely to ensure the final part properties and dimensions meet all specifications and a quality product is molded.  As W. Edwards Deming said, “uncontrolled variation is the enemy of quality.”  Therefore, to maintain the quality we need and reduce scrap rates, we must do everything we can to eliminate variation in our process or do everything we can to control the variation we cannot eliminate.  Below are 5 tips to reduce variation in the injection molding process.

  1. Part Confirmation: The ideal production run would consist of no downtime during the run.  Various events can interrupt the process causing a process engineer to investigate and restart.  This interruption adds variation that can lead to opportunities for quality issues and a common interruption is caused when a part or runner sticks in the mold and triggers mold protect.  To 100% eliminate this interruption from happening, utilize a robot and End of Arm Tool (EOAT) that via air pressure or other sensors detects that all parts and runner systems are on the EOAT before the next molding cycle occurs.  Not only does this prevent that interruption it also protects the mold from potential damage.
  2. Material Blending: Blending colorants, regrind and other additives to the virgin material should take place as close to the feed throat as possible.  The longer the distance blended material needs to travel before it gets to the barrel is opportunity for the blend to separate because of the different size and bulk density between the blended components and is especially true when blending regrind back into the process.  This separation causes layering of the materials and can result in pressure swings as the recipe changes based on this layering.  Ideally you will blend directly at the feed throat to provide the most homogenous mix.
  3. Regrind Size: The size of regrind is important.  Introducing large chunks of regrind that are much larger compared to the virgin material will change the melt profile of the resin.  It takes longer for a large piece of regrind to melt than it does smaller virgin pellets.  This changes the final temperature of the plastic as it enters the mold, which in turn effect the pressure loss and final molded part properties.  This variation can be controlled by ensuring regrind is of consistent size and as close to the same size as the virgin resin.  Picking a granulator that is low rpm and has a cutting chamber designed to match the virgin resin size as closely as possible will reduce variation seen when introducing regrind to the process.
  4. Moisture Percentage: Moisture in resin can create what is often described as pressure trending.  On a molecular level, when moisture is absorbed by plastic, the length of the molecular chain is essentially cut in half and the H2O molecule attaches itself to the shorter molecular chains.  Molecular chain length is directly related to viscosity.  The longer the molecular chain is, the higher the viscosity is (resistance to flow).  Materials with high viscosities require higher pressures to mold.  If moisture level in the plastic isn’t consistent, pressures won’t be consistent either.  To ensure moisture level stays consistent during the molding process make sure material throughput is calculated to ensure the resin spends enough time in the dryer to fully dry before making its way to the feed throat.  Purchase hygroscopic materials in bags instead of gaylords.  Bags help keep the material protected from gaining moisture so when it is introduced to the dryer it doesn’t take as long to dry.  If you do have a large amount of material exposed to humid air before it enters the dryer, do a moisture check frequently throughout the run to again ensure the right size dryer is being used.  Moisture level of a material in a bag freshly opened is going to be less than moisture level of resin that has been opened for a few hours, and the more moisture in the resin the longer it will take to dry.  You may have checked moisture at the start of the run to confirm 2 hours was enough time to try it, but later in the run it may take 3 hours due to the new moisture level from resin being exposed.
  5. Temperature Control: Changes in the temperature of the facility can greatly affect the injection molding process.  I remember one winter a technician on the production floor opened an overhead door in the facility and one by one each machine started to alarm (starting with the machine closest to the door.)  When molding tight tolerance processes, the molding parameters are also held to tight tolerances to achieve the part repeatability necessary.  Although small changes in temperature within the facility likely won’t cause your scrap rate to increase, larger swings can.  When molding tight tolerance components controlling the temperature of the production floor will keep your process engineers from chasing process variation due to the temperature changes in the environment.

Author:  Adam Smith, President of Millennium Plastics.

Millennium Plastics is a high-volume injection molding company that specializes in tight tolerance molding.  Utilizing the latest technologies and engineering expertise, Millennium Plastics provides best-in-class quality.

To learn more about how Millennium Plastics reduces variation to achieve process control, visit www.millennium-plastics.com

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