Mold Design Series: Material Selection

Plastic Injection molding is a commonly used process in manufacturing many of the plastic product in the market today. Compression Molding process is perfect for high volume production runs, as many molds are multi-cavity that produce multiple parts per cycle. Plastic injection molding is advantageous when requiring a part that needs to be repeatable, high tolerance, can vary in material selection, and require low labor costs. The biggest obstacle to injection molding is the large up front tooling cost.

The plastic part is when a plastic material is heated until it becomes soft enough to be injected into a closed mold. The finished part is ejected out once the material has cooled enough to solidify and is completely formed. It is possible to create a part with intricate features, although the more complex the part the more complex the mold, and ultimately more expensive it is. The next series of posts will discuss important factors to consider when designing a part for injection molding.

How Material Selection Effects Mold Design

Material selection is an important part of designing the mold for production. One aspect to consider is that each material has its own specific shrinkage factors. Every material with the exception of water expands when it is heated and contracts when it is cooled. In injection molding we label the contraction as the materials shrinkage. The shrinkage factor of the material is taken into account when designing the cavity of the mold so that when the part contracts it will meet the expected dimensional requirements of the original design.

Some materials dissipate heat differently than others, which can result in variation in material cooling during the molding process. This has a big impact on how the water cooling lines are placed in the mold to ensure even cooling. Another related factor is the viscosity of the material. Viscosity is the measurement of the thickness of the material in its liquid form. Higher viscosity means thicker material, and greater injection pressure to push it through the mold. Also these materials require larger diameter in the runners and gates to allow the material to flow easily into the cavity. The melt index is a good tool in determining the flowability of a particular plastic.

At Chemplast, we encourage our customers to reach out to us with their inquiries about plastic injection molding so that they can make a better informed decision on what manufacturing process best suits their needs. Located in near Houston, Texas Chemplast serves a variety of customers in the cities of Houston, Dallas, Austin, and San Antonio.