5 Things to Consider Before Plastic Injection Molding: Aluminum vs. Steel

You've conducted prototyping for a plastic product that you designed. The design is well-liked by your users, and you are pleased with the test results. Now that you want to make a batch of it, you're not sure what to do. First off, prepare your product for plastic injection molding. We covered this in our article 3D printing VS. injection molding: technologies for success. Part 2. Next, create a mold using your design. But here lies the rub: what kind of material should I use? This time, we shall contrast steel molds and aluminum ones.

The choice of a mold material affects part quality, cycle time, cost and time to market. Therefore, we often advise our customers to weigh 2 options: a quickly produced aluminum mold and a “classic” steel mold which takes a long time to build up. We shall examine what is "quick" and "classic" in the next article. For now, let’s move on to discussing the effects of choosing between steel and aluminum molds.

 In a nutshell: things to consider

1. The initial investment in an aluminum mold might be up to 5 times lower, when compared to a steel mold.
2. Aluminum molds are intended for batches of fewer than 50,000 pieces, whereas steel ones can create up to 1,000,000 parts. 
3. Aluminum molds can reduce the cycle time by 30–40%.
4. Engineering and reinforced plastics cannot be molded using aluminum molds.
5. If defects are discovered in the final product, aluminum molds are easily reworked but steel molds are challenging and expensive.

1. Production of molds costs and lead times

The initial investment in an aluminum mold can be up to 5 times less than the cost of a steel mold. Depending on how many production cycles the mold is intended for, the overall cost and return on investment for aluminum and steel molds might vary significantly.  If we need to make a million injection molded parts, one expensive steel mold is more cost-effective than 5-10 less expensive aluminum ones with 25,000–50,000 parts. 

The cost of making changes to the mold can vary depending on the type of mold you choose. For instance, changing aluminum jigs and fixtures is roughly ten times less expensive than changing steel tooling.

An aluminum mold can be made in 15 to 25 working days on average. In contrast, it typically takes 35 to 60 business days to produce a steel mold. As we will explore next, the complexity of production is to blame for this time gap. 

2. Durability: The number of plastic manufacturing cycles for aluminum and steel

Before placing an order for a mold, choose the necessary manufacturing volume. Output should take a significant consideration in selecting the material. 

 An aluminum mold will not be able to make more than 50,000 components, whereas a steel mold can generate between 100,000 and 1,000,000 parts before failing. A steel mold can create more parts before it has to be maintained because of wear or the entire set of molding parts needs to be replaced. Steel tooling will therefore be a preferable choice if you need hundreds of thousands or millions of parts annually.

 The claim that aluminum is a soft alloy and therefore unsuitable for use in plastic molds is one of the commonest. This is true when compared to tempered steel, yet there are some circumstances in which aluminum tooling will result in parts of higher quality than steel molds. Due to its "soft" nature, aluminum tends to level off microscopic molecular cavities and protrusion during the course of use.

Mold wear happens when plastic enters the mold and slides across the surface. This creates intense friction between the surface and the material. A rough comparison of the wear resistance of aluminum and steel can be made by the hardness of these materials. The wear rate and wear resistance decrease and increase respectively with the hardness of the mold material. Molds made of steel have a hardness of 30–60 HRC, but molds made of aluminum have a hardness of 60–70 HRB. To put it into a single measurement system, steel molds are around three times harder than aluminum molds.

Since steel has a coefficient of friction of 0.5 and aluminum has a coefficient of friction of 1.05-1.35, the moving elements of aluminum molds wear down much more quickly over time when rubbing against one another.

 Fatigue failure

Fatigue failure often occurs when a mold is subjected to prolonged cyclic loads. Fatigue results from cyclic or repetitive loading, and causes the steel to fail under loads much lower than its normal yield strength. One of the most severe stresses experienced by injection molds is when the cycle consists of a full loading of the steel during clamping and injection, followed by a full load release during mold opening and ejection of the product. High-strength steels like chromium-nickel steel, which has tensile strengths between 1200 and 2400 MPa, are typically used to make steel molds. On the other hand, aluminum or aluminum alloys with tensile strengths between 100 and 500 MPa are typically used to create aluminum molds.

3. Cycle time

The majority of the entire cycle time of an injection molding process is made up of cooling. Aluminum molds may heat up and cool down up to 5 times faster than steel molds due to a significantly higher heat transfer rate. The capacity of a material to transfer heat is known as thermal conductivity. While steel has a thermal conductivity of roughly 50 W/(m*K), aluminum has a thermal conductivity of about 237 W/(mK). In comparison to steel, aluminum conducts heat 4-5 times better. The quicker reaching of the operating temperature of the mold and the quicker cooling of the plastic melt inside the mold shorten the cycle time.

The thermal control of the aluminum molds can reduce the cycle time by up to 40%.  As a result, choosing aluminum-based molds can drastically cut down on cycle times, a crucial economic consideration.The parts' quality can be raised through better heat dissipation. Parts that are cured in a mold at a lower temperature have less warpage and sustain their dimensions.

4. Will tooling affect the type of material and part surface quality?

Using aluminum as the mold material won’t significantly limit your selections of plastic, but there are some exceptions.

 Engineering plastics

Engineering plastics are polymers with a melting point of roughly 400–450° C and high toughness, such as PEEK or polyamide. For producing parts from these materials, it is preferable to utilize steel molds.

At high temperatures, the aluminum mold will be subjected to rapid wear because high temperatures reduce the strength and rigidity of the material.

However, aluminum molds can be strengthened in a number of ways to improve their tensile strength and tolerance to high temperatures. For instance, specific coatings, like ceramics, can be applied to aluminum molds to boost their resilience to high temperatures.

Reinforced plastics

Reinforced plastics are a type of plastics which are used to reinforce polymers, namely fiberglass, carbon, and other additives. Aluminum is more prone to deterioration and the possibility of scratches or other damage from some additives since it is "softer" than steel. The reinforcing fibers scratch the surface of the mold like millions of tiny knives do by repeatedly running over it. This will affect the quality of molding and the texture of the final part.

 Parts’ quality

As was already established, compared to steel, aluminum has high thermal conductivity. This makes it possible for aluminum molds to achieve homogeneous heating and cooling five times more quickly, which lowers the number of flawed and rejected parts. Uneven heating and cooling are some of the major contributors to defects such as sinks, voids and burn marks. When used properly, aluminum molds can provide a cost advantage by reducing part failure rates.

Steel molds have advantages when molding thin walls and intricate parts, while aluminum molds perform better when molding long and massive sections. For injection molding items requiring very thin walls of 1-2 mm in size, steel tooling performs better than aluminum.

When employing steel, thin components and sections of the finished product can handle pressure during molding better.In one of our projects we had to make a steel mold (Figure 3) precisely because of the wall thickness of the product. The wall thickness had to be 1 mm.

5. Repair and refinement of the mold

Due to the material's extreme hardness, repairing deformed or damaged steel molds is both time-consuming and costly. Steel molds that are worn out are more likely to be replaced than repaired. Depending on the level of wear and the necessity for repair, repairing worn steel molds can be done using a variety of techniques. The principal ones are:

• alloying, which is applying a layer of another metal to the surface to restore the original size and shape;
• drilling and milling, which are removing the worn metal and restoring the size and shape.

Refining follows the same principles. The steel mold will need to be redone if there is a flaw in the product or mold design. Due to the hardness of steel, expensive 3-, 5-axis CNC milling and turning machines as well as electrical discharge equipment are used to machine the steel mold, which only raises the cost of rework.

Aluminum molds can be adjusted and repaired more easily since they are more "soft." If geometry issues are found, the aluminum mold can be modified using a milling or turning machine.

Conclusion

While saving labor and money, aluminum molds have a shorter lifespan than steel molds. Utilizing an aluminum mold could be a wise decision if you only need to create parts on demand for a few days each year or if you want to keep the financial and technical risks in check until the part design has been thoroughly tested.