The 5 M’s of Molding—Part 3: Material

You’ve accounted for variability in two other “Ms”—mold and man—but materials in their lot-to-lot differences and handling challenges pose a distinct threat to repeatability.

Material:  One of the fundamental considerations in processing is the material being used. Processing relies heavily upon the consistency and quality of the plastic that has been chosen for the part’s aesthetic and dimensional make up. Here are some of the primary variables that need to be reviewed during the stages of process development and in some cases continuous improvement.

Properties: Every material has its own unique set of molding properties that must be considered as a molding operation is developed. Here is a list of some of these properties as they relate to part aesthetics and dimensions:

Temperature- Mold and barrel temperatures are a huge part of the molding equation. Some examples of potential heat-related deficiencies would be long cycles, warping, burn at end of fill, gassing, etc.

 Shrinkage- Every material has its own rate of shrinkage and this is one key measurable that should be considered as material choice is being decided.

Melt Flow: Material viscosity is a key component during process development and improvement review. For instance, a heavily ribbed part might be better suited with a material with a low viscosity to assure that the flowfront moves quickly across the ribs preventing an overpack condition.

Aesthetics & Dimension: There are many variations on engineered materials that either benefit or detract from the overall performance and functionality of part production. Utilize the knowledge and resources of your material manufacturer whenever possible to find the appropriate material solutions.

Additives: There are many situations where material additives can create problems or provide solutions. For instance, the use of color mixing may seem cost effective, yet generate scrap due to poor diffusion leading to faulty parts via color swirls. Or from a different perspective, adding a lubricant to polypropylene might eliminate parts sticking in a mold where draft cannot be added. Work with your material supplier to develop and/or improve a part’s molding capabilities.

Drying: Drying is always a major factor in process consistency. Verify that your dryer throughput is sufficient enough to allow adequate time to properly dry the material being used. It is also important to verify that the material is truly dry through moisture analysis.

Characteristics: When choosing a material, it is important to understand what the typical characteristics are regarding a material’s performance and drawbacks. For instance, nylon has a tendency to shrink; polypropylene tends to have sink issues over large or deep ribs. Look for materials that match the functionality of the part being molded while avoiding potential problems associated with material choice.