A Guide to Improving Your Plastic Injection Molding

Plastic Resin pellets in holding hands.Plastic injection molding is one of the most common manufacturing processes because of its ability to produce identical parts at a rapid rate. Almost every industry has some demand for injection molded parts; a wide variety of consumer products are manufactured by injection molding, which varies greatly in their size, complexity, and application.

The injection molding process requires the use of an injection molding machine, plastic resin pellets, and a mold. The plastic resin pellets are melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

Why Choose Plastic Injection Molding?

Injection molded parts offer incredible accuracy and repeatability at a cost-efficient price point. Plastic injection molding is also a very efficient way of producing parts; cycle times can range from a few seconds to a couple of minutes depending on the size of the part and the number of cavities in the mold.

There are numerous materials available that offer different, unique characteristics that fit a wide range of applications. The parts can be completely customized with molded-in inserts, custom colors and branded logos. Once injection molded parts are removed from the mold, they are a finished product with the exception of a few post-process steps like sonic welding, UV laser marking/pad printing, or further part assembly.

What is plastic injection molding used for?

Injection molding is used by almost every industry, the majority of plastic products in the world today are injection molded parts. Consumer products like cell phone cases, implantable medical devices, automotive parts, and so many more are examples of how injection molding is integrated into everyday life.

What material are injection molds made of?

There are many factors in determining what kind of mold should be built (prototype, production, single-cavity, or multi-cavity) and most are typically made from aluminum or tool steel.

For building aluminum injection molds, the most commonly used grade of aluminum is 7075; these aluminum molds are typically a great fit for prototype or short-run production. Steel tooling is commonly produced from tool steel in the following grades: P20, H13, A2, D2, and 4140. These steel molds are a great fit for high-volume injection molding or when molded parts are being produced from abrasive material like glass-filled nylon.

Why are injection molds so expensive?

Building an injection mold is a lengthy and potentially expensive process that starts with an understanding of the part that needs to be molded. Potential injection molding problems can be minimized by performing upfront precautions like mold-flow simulations, fill and warp analysis; these measures can highlight potential issues and save time & money down the road.

The block of material, whether aluminum or steel from which the mold will be made, is also a determining factor in the cost of injection tooling. Part geometry and number of cavities will directly correlate to the amount of CNC machine time that is required to make the mold. Parts that have complex geometries (undercuts) that require any additional movement in the tool other than open or close will add to the complexity and cost of the injection mold tool.

How to Optimize the Plastic Injection Molding Process

Optimize the Part Design for Injection Molding

Having a part properly designed for prototype injection molding is critical to achieving manufacturing success. Things to consider include:

  • Material
  • Wall thickness
  • Draft
  • Undercuts
  • Gates & gate locations
  • Part ejection
  • Texture

Utilizing software such as Mold-Flow Simulation & Warp Analysis is extremely beneficial and can highlight potential issues prior to building an injection tool.

Understand Common Injection Molded Part Defects

Understanding the injection molding process as well as the common defects that can happen to injection molded parts:

  • Warp
  • Flash
  • Sink
  • Knit lines
  • Splay
  • Burn marks
  • Short shot
  • Air voids
  • Material degradation

Although some of these defects can happen from improper injection molding processing, properly designed parts and injection mold tooling can minimize these from happening.

Correct Injection Mold Processing

The injection molding cycle occurs by melting plastic resin pellets and injecting the molten material under pressure into a closed metal mold tool. This process is repeated time and time again to produce large quantities of parts at a cost-efficient rate.

  1. Clamping: The two sides of the mold are closed and clamped shut.
  2. Injection: The plastic resin pellets are fed into the machine and pushed towards the mold. While this is happening, the material is melted by heat and pressure. The molten plastic is then injected into the mold — this is called the “shot.”
  3. Cooling: The molten plastic that was injected into the mold is cooled and returns back to a solid state.
  4. Ejection: Once the part has cooled, it is ejected from the mold.

This is a complex process and it takes numerous pieces of equipment and highly trained individuals to oversee the entire process.

Perfect the Process With a Melt Temperature Measurement System Kit

If you do choose to do your own injection molding, you’ll need to be able to accurately measure melt temperatures. Equipped with accurate measurements, you’ll reduce wasted material and minimize variation from lot to lot, saving you more time and money. Schmit offers a Melt Temperature Measurement Kit that helps you eliminate human error in a safe and effective way.

Learn About the Kit

If you’re in need of a prototype or have any additional questions about plastic injection molding or the process, our team at Schmit Prototypes is here to help. Contact our shop and get started on your injection molded part.

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