Plastic Injection Molds

An injection mold is a critical component in the plastic injection molding manufacturing process. In addition to the plastic and the molding machine, the mold allows manufacturers to create parts that match precise CAD specifications. Explore the basics of plastic injection molds, common applications and how they work to provide low cost products with quick lead times.

Plastic Injection Mold Types and Applications

There are two basic types of molds that can be used in the injection molding process. Understanding the differences between these two is the first step to selecting the optimal mold for your manufacturing process:

• Free standing molds
• Insert molds

A free standing design incorporates the mold base, inserts and other components to create a fully functional piece. This mold design requires a longer lead time and production cost, but it’s a necessary option for multi-cavity molding projects.

Insert molds are quick, affordable and convenient for single-cavity applications. Expect a much shorter lead time and production cost than a free standing mold, but also be prepared for a more limited range of design features.

How Molds Work

The first step in the process is to create a detailed rendering of your product. Once you have an exact, three-dimensional model of your new component, molds must be created to replicate the dimensions of the product. This process is more complicated than it seems, since the mold isn’t just an exact copy of the intended product. The raw materials in injection molding expand and shrink at different rates depending on the temperature and type of material.

Once the mold is created, it’s time to clamp the mold into the machine. An injection mold machine closes around the two haves and keeps them forced together throughout the process. The injection process starts with raw plastic, in pellet or another form, that is fed into the machine with a reciprocating screw. The plastic must be heated to a specific temperature as it travels through the feed and heat zones.

Molten plastic travels via the reciprocating screw into the custom injection mold. Once the precise amount of plastic is injected at high pressure levels into the mold, it begins to cool. The cooling process varies depending on the material and the size of the mold, and it can only be ejected once the material is fully cooled.

This marks the end of the plastic injection molding process. Plastic components may need to be assembled or they may be immediately sent to the packaging stage.

Aluminum vs. Steel

Plastic injection molds must be durable enough to withstand the high temperatures of molten plastic, yet manufacturers also need molds that are affordable. The two most common mold materials are steel and aluminum. 

Aluminum Molds

Choose an aluminum mold for fast and affordable manufacturing. Aluminum tends to dissipate heat faster than steel, which reduces the cooling time. Unfortunately, aluminum isn’t as durable as steel, so the mold will have a shorter lifespan and cannot be used in high heat applications. Choose an aluminum mold if you’re working on a prototype or short run of plastic components that don’t require high temperatures during the injection process.

Steel Molds

Invest in steel molds for a long-lasting alternative. Pre-hardened steel can be used in high heat applications with less risk of deforming the mold. All this strength, however, comes with additional manufacturing costs. Consider using steel if you can afford a longer cooling time and you’re planning on long-term use of your plastic injection molds.

Find out the pros and cons of aluminum vs. steel and discuss your molds with our team at Special Tool and Engineering to find out more about our custom mold options.