Question

A designer must anticipate and understand the manufacturing processes that will be used to fabricate the created components. Describe differences in the model for a part designed for polyethylene (plastic) injection molding versus one for machined aluminum. Can you anticipate unique model requirements for a part created using hand lay-up resin-transfer molded composite (a process similar to that used for fiberglass)?

Answer 1

In order to understand the pros and cons of choosing aluminum or steel for injection molding, it is important to note that all mold materials have different properties and applications. To achieve best results one should choose the most appropriate material for the mold base and core based on project requirements.

P20 steel is the most commonly used material for molds in the plastics manufacturing industry due to its unique characteristics. Some of its main properties include:

Hardness;
Good polishing ability;
Heat treatment possibility after machining;
Good corrosion resistance;
Good wear and tear resistance;
Good thermal stability.

P20 material is a versatile low alloy steel that is characterized by its hardness and moderate strength level. It is typically sold in pre-hardened condition at a hardness of approximately 40 to 50 HRC (Rockwell Hardness Scale goes up to 100). Having a homogeneous mixture of alloys allows the P20 steel to obtain uniform hardness within the whole material. In P20 steel, the presence of metals such as chromium and nickel enhances its strength and hardness, making it more desirable than other materials.

With that said, the major impact these properties will have on the finished part is the ability for a mold to produce thousands of parts at minimum wear and tear. This is ideal for large production runs of 50,000+ parts.

During your search for injection mold tooling materials, you may have come across Maraging 300, PAS 940, and Berlin Copper. They have similar properties as P20, however, their selection is very specific to niche industries. These materials can be quite costly due to their special makeup and properties.

Now let’s get to aluminum. To most people’s surprise, aluminum can offer leveraged advantages. Some of them include:

Aluminum has a 5x better heat transfer rate. With a better conductivity, the position of cooling lines becomes less critical thus allowing for more mold components to be placed.
Aluminum has demonstrated a reduction in production cycle time by up to 30%.
The heat treatment process for aluminum is much better than that of steel since it does not have to be sent out to be annealed. Annealing is a heat treatment process for metals to remove internal stresses and to toughen the material.
Aluminum is more cost effective than steel.

Although aluminum molds have its advantages, it is only capable of producing between one to two thousand units, whereas steel can guarantee a minimum of 50,000 units. So if you have a small production run, aluminum will certainly be your preferred tooling method at a fraction of the cost of steel.

However, aluminum does have its downfalls. Due to its low- density nature, texture selection is also fairly limited when compared to steel. Additionally, carving more than one undercut can become an issue due to consistent wear and tear on the surface of the mold. This leads to making a modification to the entire mold rather than one particular area, in turn causing more differences in the output quality of the final product. After a few thousand production cycles the cavities of the mold start to wear out, causing nonconformities in your product, consequently leading to more tooling costs. In order to get aluminum up to the level of steel in terms of hardness, it would have to be anodized or plated with nickel.

Whether you are looking into steel or aluminum, it’s important to keep in mind that most molds are precisely made using a CNC (computer numerical control) tooling machine. A CNC is a computer programmed machine that runs automatically, making cuts, cavities, and special finishings to a block of material using drills, saws, and/or other tools. With that said, it’s especially important to pick a material that the CNC machine is able to cut. Additionally, harder materials like P20 require special tools and more effort to be milled than its aluminum counterpart, which in turn increases its overall cost.

Although it’s up to you to determine the route of tooling to go with, looking at overall project requirements such as production volume can serve as a great guide on picking the correct material for your project. Your selection can save you cost you money in both the long and short term.

Hope it will help you.