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Choosing the Right Material for Injection Molding Molds: Aluminum vs. Soft Steel (P20)

When it comes to creating prototype molds for injection molding, choosing the right material is crucial. Two common options for prototype mold construction are aluminum and soft steel (P20). Today, we’ll compare these two materials to help you make an informed decision. For the purposes of today’s discussion, we’ll be talking about “late stage” prototype molds. These are molds that are likely to be used for medium to high volume production runs, not just to knock out a few early-stage prototypes.

Durability/Production Life – P20 steel, although a soft steel, is stronger and more durable than aluminum. It will certainly endure a higher number of injection cycles as result. Further, steel’s strength means better mold integrity over time. Since aluminum is softer, molds made from it are more likely to deform eventually, requiring either repair or replacement, depending on how severe the deformity is.

Heat Conduction – aluminum dissipates heat quickly and efficiently, meaning reduced cooling time during the injection molding process. Steel holds heat longer, leading to extended cooling times, which could lengthen production time. Also consider the required melting point of the material being molded. For those that require higher values, steel is the way to go.

Machinability – since aluminum is a softer material, it’s generally easier to machine than steel. This is particularly useful if changes still need to be made to the mold before putting it to work on longer production runs. However, when it comes to high precision, steel would be the better choice.

Part Geometry/Size – since steel can be more easily machined to high precision, and is durable, it works much better for parts with complex geometries, especially over longer runs.

Cost – we saved the best for last. In fact, if we started with the cost aspects, you might not have read this all the way through! As you could probably guess, a domestically sourced aluminum mold is going to be less expensive than a steel mold. No doubt about that. But I’m going to share a valuable secret with you: if you source a mold from a quality oversees mold maker in China, there is little difference in the cost between an aluminum and steel mold. This is simply since labor costs are much lower. While there is still a difference in the cost of the material itself, and it takes longer to machine P20 steel, the differences become negligible when considering the final cost of the mold.

I know what you’re thinking – what about shipping and tariffs? Don’t they wipe out the reduced labor cost advantage? The answer is no. In fact, molds sourced overseas still typically cost about half, saving tens-of-thousands of dollars, even after the extras are added to the price. They are also able to produce them quickly, negating the longer shipping distance. And quality? That’s always going to be a concern. No matter where you source from, you’ll still need to do your homework to make sure you are working with a reputable supplier that has a documented quality program in place.

Who Wins, Aluminum or Steel?

So, what to choose? In most cases, an outsourced mold made from P20 steel will be your best decision. When price is essentially off the table, why wouldn’t you want a mold that’s more durable, will maintain its integrity, and can handle considerably more injection molding cycles? Sure, there are exceptions where locally sourced aluminum molds are a better choice, such as earlier stage prototyping. For some jobs where there are ITAR or other domestic sourcing mandates at play, there’s no choice. Otherwise, if you’re getting ready for mid-high volume/repeat production runs, we recommend an outsourced P20 steel mold. It will last longer, and have a lower lifetime cost.

Not sure which material is best for your mold?

Contact us, we’re happy to help you make the right selection!

Self-Contained Molds vs. Modular Insert Molds

With plastic injection molding, there are two types of mold configurations that may be used: standard self-contained molds and modular insert molds. Today, we’ll compare the key aspects of both types.

Self-Contained and Modular Insert Molds – Defined

Let’s start with a definition for each. Self-contained molds consist of all components integrated into a singular unit or a mold frame. These include the core, cavity, and other necessary components within a single base. Conversely, a modular insert mold is comprised of separate interchangeable components. The primary parts are the frame and the mold itself, which gets inserted into that frame.

Part Complexity

Self-contained molds are custom-made and, therefore can facilitate more complex part designs. When compared to modular insert molds, self-contained ones allow for intricate cavities, cores, and cooling channels.

There are some limits to what modular insert molds can handle. They can’t facilitate complex
operations involving for example gear assemblies. However, some insert molds can support
cam actions (side action).

Tooling Lead Times

Since self-contained molds are typically designed and fabricated from scratch, they take longer to produce. Conversely, insert mold frames are readily available, and while the insert itself does take some time to make, it’s much less than what’s required to build an entire self-contained mold from scratch.

Longevity

In the world of injection molding, longevity translates to cycles. In other words, how many impressions can be made in that mold before it needs to be replaced? Note that, regardless of configuration, longevity also depends on various factors including material and maintenance. Self-contained molds tend to outlive insert molds, as they are usually comprised of higher-quality materials.

Cost

Conversations around cost are rarely simple these days, and this topic is no exception. Let’s start here: self-contained molds have higher tooling costs due to their custom nature. Modular insert molds are lower in cost since they have interchangeable inserts, thus reducing the need for mold frames. Remember, with insert molds, you are essentially paying for the “guts” of the mold, and not the entire assembly including the frame, helping to keep a lid on tooling costs.

If design changes are made, insert molds win again. However, keep in mind that self-contained molds generally last longer, and the lifetime cost may be lower than an insert mold. Similarly, if only specific hi-wear components of the tooling need to be replaced over time, insert molds offer more flexibility since the entire tool doesn’t need to be rebuilt.

Before we wrap up, we need to talk about part volume. Let’s put tooling costs aside for a
moment. If the injected component is a high-run part, in the hundreds of thousands, then the
price of the components themselves will be lower with a multi-cavity self-contained mold. This
allows for multiple pieces to be made at one time, which significantly reduces cycle time and its
associated costs.

Which is Best?

So, what will it be, self-contained mold or modular insert? It depends. For instance, we have a customer who supports the aviation market, but their parts are simple and are ordered in short runs. Perfect for modular insert molds. Conversely, we have an automotive customer with an uncomplicated part, but we use a self-contained mold. If you’ve read this far, you know why – it’s a high-volume run (500,000+) produced with a multi-cavity mold.

While some jobs may use either, in the end, the decision will rely on the requirements of the job, including volume, complexity, frequency of design changes, and related considerations.