Injection molding is the production method used to produce plastics that you use daily. If you’ve ever found yourself trailing an idle finger over an unusual line in an office chair’s armrest while bored during a meeting, sanding down a model part to make it flush, or putting putty into a plastic seam to create a flat surface you’ve encountered an injection molded part. You’ve actually encountered an undesired side effect of the process that engineers have put a great deal of effort into minimizing. One of the ways they’ve sought to do this is through the choice of mold materials, the most common of which are aluminum and steel.

When it comes to aluminum vs. steel injection molds, which is better depends on the material being molded—resins are injection molded along with plastics—the amount of detail needed in the final product, and the volume of production that is needed.  Which is better is not as simple as one metal being superior to the other but is instead highly situational and dependent on the desired end product.

Comparing the Costs of Aluminum vs. Steel Injection Molds

That aluminum is more expensive and thus more valuable on both the basis of weight and volume is widely known. Aluminum drink cans are grabbed off the streets to be recycled in exchange for case, while much larger steel drums are chiefly recycled into fire drums and novelty barbecues. However, when it comes to aluminum vs. steel injection molds things are much less clear-cut.

Although steel for an injection mold is initially much cheaper than aluminum, it is also much harder and far more difficult to machine into an injection mold. Lead times for producing steel injection molds can be several months, and this additional time and labor cost means that steel injection molds cost substantially more than aluminum ones. Aluminum’s relative softness makes it more easily machined, and aluminum injection molds can be ready for use in a few weeks. The time and labor savings mean that aluminum injection molds cost far less than their steel equivalents. This isn’t to say that steel doesn’t have its own advantages.

The Advantages of Steel Injection Molds

One of the more frustrating aspects of any molding or casting process is getting the material to completely fill the mold. Injection molding is no different, and if you’ve ever had to break out putty or clay to make a plastic model look right or bring a component into spec you’ve had experience with this particular molding flaw. The strength of steel means that steel molds can bear 20 to 30% more injection pressure than their aluminum counterparts. This additional pressure on the injected material means that it is more likely to fill every part of the mold. This extra pressure also shortens injection times as the application of extra force can reduce wait times until the mold is filled even if overall work cycle times may be higher than with aluminum molds for reasons we will discuss later. Additional advantages of steel injection molds include:

• Durability: As a side effect of its hardness steel is far more durable than aluminum. If a production run of injection molded parts is expected to last many years then steel is the preferred choice as its strength and durability means it will last.
• High-volume production: Due to their durability, steel injection molds are also suitable for high-volume production runs that may not last for years, but whose final production number runs into the hundreds of thousands or even millions.
• Fine details or unusual shapes: As a side effect of its ability to take higher pressures steel injection molds are better at producing components with fine details that require extra pressure for the product to fill, producing uneven wall thickness, or unusual shapes that restrict the easy flow of the injected materials.
• Molding of resins: Recent years have seen increasing use of resins and epoxies that are more viscous than traditional plastics and require additional pressure to fill a mold. These can also be reinforced with glass or carbon fibers that require even more pressure to fill all parts of the mold and avoid voids.

These advantages can be something of a double-edged sword though. The same durability that allows for long production runs, high pressures, and high volumes is also responsible for steel injection molds’ lengthy development times. If they are damaged there is no easy repair or replacement of the molds. When comparing aluminum vs. steel injection molds, steel takes far longer to dissipate the heat from the injection process than aluminum and this can magnify issues with the shrinking or cracking of molded parts. These and other advantages are seeing aluminum injection molds finding increasing use as injection molds.

Why Usage of Aluminum Injection Molds Is Increasing

One of the most exciting innovations in manufacturing has been the rise of custom limited production runs made possible by advances in computing, and the ability to share detailed plans and dimensional specifications instantly online. The traditional disadvantages of aluminum injection molds in more traditional long-term and high-volume production become major advantages in small and short-term production runs.

Tooling costs for aluminum are vastly lower than those for steel. Aluminum can be cut and machined to a high degree of precision with the tooling that is available at an everyday hardware store. This opens up the ability to create relatively detailed aluminum injection molds for a much wider range of manufacturers. Essentially, any shop with a CNC machine and a decent machinist can prototype, and eventually produce aluminum injection molds for short-term production runs. Given enough practice turnaround time for aluminum injection can be just a few days. This easy machinability also means that repair and modification are also far easier with aluminum injection molds vs. steel ones.

This speed of production is also matched with the speed of production. Aluminum is one of the best thermal conductors available, and that translates to very quick cooling speeds for injected products as the copper pulls the heat from them quickly. This can also translate to reduced shrinking and cracking of the cast part as the thermal conductivity of the aluminum vs. steel injection molds helps to keep temperatures even throughout the mold while hot and cold spots are more likely to occur in steel. Overall, full production cycle times are much shorter with aluminum injection molds than steel ones due to the ability to cool molded components quickly. With these advantages, and constant improvements in metallurgy improving the strength and other qualities of aluminum injection molds it is likely that aluminum is going to be the more commonly preferred material when it comes to aluminum vs. steel injection molds.