Choosing Injection Mold Materials: A Selection Guide

What is an Injection Mold?

An injection mold is defined as the section of an injection molding machine where part formation takes place. In other words, this is where the molten thermoplastic or other raw material is injected.

A typical injection mold tool is built in two halves. One half is called the core, and the other cavity. The two sections—mold and cavity—fit together during the molding process, pressing and shaping the melt to produce the needed part.

Other mold sections include venting, melt cooling, and part ejection systems. The cooling system serves to remove heat from the mold, allowing it to solidify within the required period and prevent temperature-related defects.

Injection mold venting systems help release any gases that might be trapped in the melt, which could lead to voids and other issues. The ejector system, on the other hand, serves to remove the already molded part.

Injection Mold Tooling

Injection mold tooling is an important step of any molded parts project. This is time when engineers work to find the best ways to produce mold tools. It involves using design software and choosing the right materials to use. The injection molding tooling process can, therefore, be divided into these main steps: design and material selection.

Injection Mold Tool Design

During injection mold design, engineers use digital tools—which also include simulation software—to determine the mold geometry and test its suitability. The design is then optimized to ensure production efficiency and prototypes built before the actual production can begin.

What are Injection Molds Made of?

Injection molds are generally metal parts. That’s because they must stand up to the high pressures of the injection process, be wear resistant, and maintain the initial shape and geometry through molding cycles.

A metal injection mold can also transfer heat efficiently, which helps to quickly cool the molten plastic. This is while also ensuring a uniform distribution of heat. Different injection mold tool materials may be used, as explained next.

Injection Mold Tooling Materials

Manufacturers use different materials to achieve specific levels of parts making efficiency. These are usually metal alloys, and include the following: the alloys of steel, aluminum, and copper. These injection mold tooling materials, together with their different characteristics, are discussed below.

Steel Injection Mold

A steel injection mold is one of the most used, owing to the material’s favorable properties. Different steels are also available for mold making and these include soft or semi hardened and hardened steel.

Examples of soft or semi-hardened injection mold steel grades are NAK80 and P20, while hardened steel grades include H13 and S136. Their properties as injection mold tooling materials are listed below:

• NAK80 Steel: hard and durable mold material that produces glossy parts
• P20 Steel: hard wearing, does not require heat treatment, and easy to weld
• H13 Steel: similar to P20 steel but with the added advantage of producing high-quality finish parts
• S136 Steel: this is a type of stainless steel injection molding mold material, extremely hard, durable, resistant to corrosion, and able to produce glossy parts.

Aluminum Injection Mold

An aluminum injection mold is an alloy of aluminum metal. Based on its various characteristics, the material may or may not suit specific injection or part making requirements. Overall aluminum offers these advantages over other injection mold tooling materials:

• The material is easy to machine (2-5 times faster than steel), which means shorter lead times for the tooling process and fewer costs to produce the mold
• With its good thermal conductivity, an aluminum injection mold offers exceptional heat transfer that allows molten plastic to cool quickly to offer optimal production cycles.
• Owing to its excellent machinability, an aluminum injection mold is also a good material for mold prototypes

However, since aluminum is a soft metal, it wears out too quickly. So it’s mostly used when only a few parts are needed. Some plastics, such as PVC or POM, may also erode it, while high temperature polymers may not be usable with the mold.

Beryllium Copper Injection Mold

Copper alloys make good injection mold tooling materials, mostly due to their high thermal conductivity and machinability among other properties such as increased hardness and strength. The alloys include beryllium copper, aluminum bronze, spinodal copper, and Corson bronze.

A beryllium copper injection mold is the most popular, since the materials exhibits superior characteristics for both hardness and thermal conductivity. Here are its benefits.

• Easy to machine when compared to steel (about 2-5 times)
• Easy to weld, which makes the tooling process and maintenance easier
• A high thermal conductivity that makes a suitable material for different parts of the injection mold, including inserts
• Ability to resist the corrosion that mold cooling fluids and plastics may cause
• Excellent hardness level that makes it a durable tooling material

Injection Mold Material Selection

Injection mold material selection involves finding right material to use in a given process. This is to ensure dimensional accuracy, visual appeal, and surface finish of the finished parts. The mold material also determines cycle time and tooling costs, which translates to the overall cost of production.

Injection Mold Tooling Standards

Injection mold tooling standards define tooling requirements and categorize molds into 5 different classes (101 through 105) based on the level of hardness and durability. These standards are written by the Plastic Industry Association, and used by manufacturers to classify molds. That way, the customer can know what to expect.

Injection Mold Tool Life

Different injection mold tooling materials offer different levels of durability. Depending on the project needs, a less expensive but less durable material may be used, or it can be costly yet durable mold.

When it comes to injection mold tool life, steel offers the best choice owing to its superior level of hardness. Aluminum molds are the least durable since the material is soft and wears easily.

But hard mold materials are also the most durable. So manufacturers use them when production volumes are high. Softer molds are best suited for lower volumes and prototyping needs.

Injection Mold Tooling Cost

Injection mold cost can range from a few to several thousand dollars. Since this adds to the cost of production and that of the manufactured units, it becomes important to consider how the cost would affect your project.

Aluminum offers the most cost effective solution when it comes to injection molding mold cost. Steel and copper alloy molds are the more costly options, but more durable. Therefore, your production needs for both volume and quality will determine the amount to spend on the mold.

Aluminum vs. Steel Injection Molds

Aluminum and steel are the most used injection mold tooling materials. But how do they compare in terms of durability, part quality, versatility, and cost?

Durability: a steel mold is more resistant to wear, and lasts a long time. Aluminum is a softer material and less durable. However, given their cost differences, each material has its best application as an injection mold

Part Quality: steel can be polished to a level that will produce parts with gloss or superior quality finishes

Versatility: aluminum injection molds cannot be used with all plastics. Some, like PVC plastic, may corrode and reduce the mold’s effectiveness. Aluminum molds are also not the best to use with plastics that mold in high heat levels.

Cost: Steel injection molding costs more, given the material’s superior characteristics, with the mold costing up to $5, 000. Aluminum-based molds are less expensive (up to $1000) and best suited for low production runs and prototyping.

Overall, the question of whether to choose a steel or aluminum mold will depend on your specific project requirements: steel for higher part volumes and quality and aluminum for lower production runs and prototypes.

Conclusion

Different injection mold tooling materials offer different benefits and downsides when used to make parts. So it’s important to make proper selection, usually based on the required cycle time, production volumes, and other factors such as part geometry and surface finish. Equally important is the cost of the mold material, as that can have a major impact on the initial cost to produce parts.