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Aluminum vs Steel Injection Molds: Which One to Choose?

January 29, 2024 · About 3 minutes
aluminum vs steel injection mold
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Injection mold is the heart of the molding process. It shapes the injected material into the desired parts by withstanding high pressure, temperature, and thermal cycling. Steel and Aluminum are two common metals used to make mold. Therefore, comparing Aluminum vs steel injection molds is essential to decide which mold fits your design and requirements.

You can only make wise decisions on aluminum mold vs steel mold if you know their capabilities, cost, and compatibility with manufacturing scenarios. Let’s break down this comparison by discussing different aspects.

aluminum vs steel injection mold
aluminum vs steel injection mold

Why Need Aluminum or Steel Mold for Injection Molding?

First, why do we need aluminum or steel molds? Well, molds are the cavities of the intended shape that define the contours of a molded part. Thus, we need these molds to manufacture our designed plastic components into exact geometry.

Although they have different capabilities, both have fundamental properties to facilitate the production of injection molded parts. Aluminum and steel alloys provide the strength, thermal conductivity, precision, and surface finish detail required to accept and shape molten plastic on solidification. Furthermore, aluminum and steel molds are corrosion-resistant, which makes them durable.

Overall, using these molds ensures the seamless injection molding of thermoplastics. That’s why we need them, even though they are preferable in distinct molding conditions.

Comparing Aluminum vs Steel Injection Molds

Aluminum and steel injection molds have differences in various aspects, such as thermal capability, durability, dimensional accuracy, surface finish of molded parts, production cycle time, etc.

aluminum injection mold

This comparison is often done as the soft tool vs the hard tool. Next, let’s elaborate on the main differences between them.

Thermal Properties

Thermal properties of mold materials directly impact the process efficiency as it determines how uniform heat will transfer within the mold and the time it takes to cool down. In this context, aluminum has almost five times more thermal conductivity (237 W/ m/K) than steel, which offers faster heat dissipation. Hence, aluminum molds heat and cool in much less time than steel molds.

The fast heat conduction ensures injected material fills the cavity uniformly because it allows liquid to flow across more distance. In comparison, steel cools slower, resulting in longer production cycle time. However, the longer cycle time gives more control over temperature and cooling rate.

Durability and Wear Resistance

The composition of carbon and other alloying elements provides steel superior strength, hardness, fatigue resistance, and thermal resistance. As a result, molds made from steel are highly wear and abrasion-resistant, can withstand injection molding pressure over time, and are more sustainable. They can repetitively go for molding cycles, up to millions.

In contrast, the quick heat flow in aluminum molds leads to rapid expansion and contraction of the material. Furthermore, it causes dimensional instability and misalignment in the molds over time. Additionally, an aluminum softer surface is more prone to scratching and denting. For all these reasons, molds made from aluminum are less durable and relatively less wear-resistant in comparison to steel vs aluminum injection molds. They are suitable to create a few hundred to ten thousand identical injection molding parts.

Surface Finish

The smooth surface of mold cavities enhances the flowability and provides a quality finish to the molded parts. If we compare aluminum and steel injection molds, aluminum molds produce plastic parts with better finish. Although, it degrades over time and alters the surface roughness.

If you want SPI A-1 finish (Ra 0.012 – 0.025 µm), any mold can achieve that. Otherwise, the parts can be processed with further finish options like polishing and powder coating to achieve the desired aesthetic appearance.

Machinability and Modification Ease

Steels are comparatively more challenging to machine than aluminum because of their hardness and stiffness. On the other hand, aluminum is softer and quickly machinable, reducing mold production time and cost.

Next, the aluminum one is easy to modify between aluminum vs steel injection molds. This is again because of the hardness difference. Soft aluminum can be easily machined and modified, whereas steel mold modification requires more time and advanced machining setup.

Impact on Molding Cycle Time

The total cycle time of creating an injection molded part includes tool setup time, injection time, time to fill & form, and cooling time. Among these, mold cooling takes up to 50 to 80% of cycle time.

As aluminum has a higher heat dissipation rate, molds cool down in significantly less time than steel. So, they reduce the overall molding cycle time. Conversely, the low conductivity of and more complicated tooling increases the cycle time while molding with steel molds.  

Heating and Cooling Time

Aluminum molds have a significantly higher heat recovery rate, allowing heating and cooling cycles to be up to 7 times faster than steel molds. This rapid thermal response cuts cycle times, increasing part throughput within the same production period. This advantage is further accentuated because aluminum molds guarantee uniform temperature — making the process defectless by preventing defects caused due to non-uniform heating.

Steel molds are more rigid but take longer to heat and cool, which can increase cycle times and production costs. For fast, high-efficiency production, aluminum molds are often the better choice.

Defect Reduction

Aluminum injection molds offer consistent heating and cooling, ensuring uniform temperature distribution that reduces defects like sink marks, voids, and burn marks. This leads to higher-quality parts and lower rejection rates, minimizing waste. While steel molds are more durable, they can struggle with temperature consistency, which may result in defects, especially in complex designs. For critical areas where precision and quality control are essential, aluminum molds provide a more economical solution, maintaining defect-free production.

 Comparison CriteriaAluminum MoldSteel Mold
Thermal PropertiesFive times higher thermal conductivity than steel mold and faster cooling rate.Relatively low thermal conductivity and slower cooling.
Durability and Wear Resistance Highly wear-resistant and can produce millions of parts.Prone to wear and abrasion, and can be used for up to 10,000 parts.
Surface Finish Better smoothness Less smooth surface  
Machinability and Modification EaseExcellent machinability and easy to modify and repair. Steel is less machinable and modification is also complicated.
Impact on Molding Cycle TimeA higher cooling rate significantly reduces the cycle time. Slower heal release increases the molding cycle.

Advantages and Disadvantages of Aluminum and Steel Molds

Understanding the advantages and disadvantages of aluminum vs steel injection molds helps to identify which mold appeals and which restricts your molding requirements.

Pros of Aluminum Molds

  • It is fast to create and costs less, often 2-3 times less tool investment than steel injection molds.
  • Low upfront cost makes them cost-effective in small-volume injection molding.
  • Its low weight also facilitates easy installation & handling and easy modification to address new issues.
  • A higher dissipation rate of heat lowers the time to production cycle significantly. Also, uniform heating and cooling eliminates the risk of shrinkage, voids, or marks.

Cons of Aluminum Molds

  • These molds are unsuitable for some advanced high melting-point resins like PEEK and PSU.
  • The lifetime is too short, only up to ten thousand cycles.
  • The softness of aluminum makes mold more prone to abrasion and wear. For example, even periodic maintenance can form scratches.

Pros of Steel Molds

  • Steel is superior in injection molds in terms of durability. For instance, a steel mold can produce multiple millions of complex parts under periodic maintenance.
  • They can process any type of resin, including abrasive and high melting-point.
  • Steel-made molds maintain dimensional stability for critical applications such as medical and aerospace.
  • The longevity of mold makes it ideal for mass production, reducing per-part cost significantly.
  • Steel’s structural integrity holds up better than aluminum’s, which is beneficial for more intricate designs and features.

Cons of Steel Molds

  • The high upfront cost restricts its use for startups and small businesses.
  • The low cooling rate of steel mold increases the injection molding cycle time, influencing the production cost.
  • The modification of mold tooling is complex, expensive, and time-consuming.
  • Steel molds tend to have cracks and deformations as the cooling system is not perfect here, leading to defects in the produced parts, so many of them will be rejected.

Things to Think About When Deciding Between Aluminum or Steel Mold

steel injection mold

Do you know what using the wrong mold can cause? It leads to quality degradation, a decline in process efficiency, restriction in production volume, high cost, and even project failure. Nevertheless, it can be avoided by considering the following factors in the selection of aluminum vs steel injection molds.

Tool Life & Maintenance

The strength, hardness, wear & fatigue resistance, and thermal stability of steel & its alloys are higher than aluminum. As a result, steel has a longer injection mold life expectancy. So, you need to identify whether your requirement is mold longevity or not.

Next, both mold types demand maintenance over time for repetitive production of identical items. Some typical maintenance activities include cleaning, lubrication, frequency checks, and wear inspections.

Product Volume & Production Scale

When deciding between steel or aluminum injection molds, the key factor is production volume. Steel molds are built for high-volume runs—they’re durable, can handle high temperatures over long cycles, and produce millions of parts with consistent quality and repeatability.

On the flip side, aluminum molds are perfect for small batches and prototyping. They’re affordable, fast to produce, and ideal for low-volume or rapid iteration projects. Ultimately, your choice should depend on your project’s specific needs and how you plan to scale production.

Cost Analysis

When comparing injection mold costs, aluminum tooling is a more affordable option than steel, making it the clear choice for many projects. Aluminum molds are generally cheaper to manufacture, often at a fraction of the cost of steel molds of similar size and specs.

While steel molds come with a higher upfront cost, they offer long-term value for high-volume production runs. They lower the cost per unit over time, making them a reliable choice for mass production. On the other hand, aluminum molds are perfect for short runs and prototyping, offering an economical solution for producing a few thousand parts.

Ultimately, the decision should balance both initial costs and the expected production lifespan to maximize overall value.

Material Properties

Steel and aluminum have distinct physical, mechanical, and chemical properties. For this reason, they behave accordingly during the injection molding process. Aluminum is soft and ductile, but it offers excellent heat conduction. On the contrary, steel is a robust and highly wear-resistive material with relatively low conductivity.

Compatibility of Advanced Resins

The type of material you will mold also influences the decision of steel vs aluminum injection molds. For example, aluminum molding cannot process some abrasive and high melting point thermoplastic polymers like Polyetheretherketone (PEEK), glass-filled nylon, and Polysulfone (PSU). However, steel mold tooling can create parts from almost any advanced resin. Thus, evaluate the molding material properties and consider its compatibility with both types of injection mold.

Acceptance of Complex Design

Can the chosen mold accept the extremely intricate design? It is another consideration before choosing the mold. The soft nature of aluminum does not allow for complex inserts and cavities. The reason is it cannot maintain the hole’s accuracy over time. In contrast, the steel’s stiffness and strong nature enable the higher complexity. So, it can hold the accuracy of complex cavities for long production runs.

How to Manufacture Aluminum or Steel Injection Mold

Mold creation for injection molding typically has three manufacturing approaches: CNC machining, EDM, and 3D Printing. In many cases, manufacturers combine CNC and EDM machining to leverage the capabilities of both methods. Moreover, raw steel and aluminum are also crafted into plastic injection molds using these techniques.  

3D Printing

Manufacturing steel or aluminum molds with 3D Printing involves adding layer by layer of molten metal to obtain desired cavities. Fundamentally, it prints or physically replicates the computer 3D model (CAD) using a 3D Printer. It can achieve the utmost complex mold shapes in a short time. Saying that mold printing is best in situations where you are continuously changing the designs or for prototyping.

injection mold and plastic parts

Likewise, it is the best alternative when machining cannot achieve the intricate mold inserts or patterns. Also, you can test the mold ideas with 3D printing before EDM or CNC machining. It helps to validate your design and reduce the expensive tooling cost.

CNC Machining

This method can convert raw aluminum (2024, 6061, 7050) or steel (H13, S7, SS 229) blocks into a functional mold by removing the material with computer-controlled tools. In this scenario, the machinability of aluminum makes its CNC machining quick, while the steel hardness makes the same procedure three to five times longer.

cnc machining injection mold

CNC-machined injection molds offer high accuracy and a smooth finish if we see head-to-head aluminum mold vs steel mold. However, they can be post-processed with polishing to enhance the as-machined surface quality. You can use this mold-making method for detailed features such as small cavities, precise cores, undercuts & threads, complex slides, and inserts.

EDM Machining

EDM uses electrical sparks to erode the steel and aluminum injection molding workpiece and converts them into a designed mold shape with detailed features. This manufacturing process involves no contact between the tool and the workpiece, which means no mechanical stress in machining, and mold material keeps its integrity unchanged.

metal tool for injection molding

EDM is renowned for creating mold toolings from hardened steels with precision and superior surface finishes. Furthermore, it is the best choice for deep cuts, internally sharp corners, specific texture, delicate and thin walls, precise lifter & inserts, and basically, all that is beyond the reach of CNC cutting tools.

Suitable Maintenance Practices for Aluminum and Steel Molds

Maintaining those are done in unique ways too to prolong their use. Aluminum and steel molds have different ways of maintenance and endurance. Every material has its own characteristics associated with it and brings its share of challenges and maintenance.

  • Aluminium Molds: Avoid scratches and oxidation by using non abrasive cleaners on Aluminium Moulds. Do not use strong acid or cleaners
  • Steel Molds: Clean with wire brushes or scrapers to remove debris; apply rust inhibitors to prevent corrosion

Inspection:

  • Aluminum Molds: Aluminium is softer, so more prone to abrasion and wear. Warping can be an issue, so you will want to check your molds more often.
  • Steel Molds: Evaluation of Wear and Cracking of the Steel Molds where affecting areas such as parting lines and ejector pins are present.

Lubrication:

  • Aluminum Molds: use light oils or special non-reactive lubricants designed to reduce friction
  • Steel Molds: Recommended with heavy duty lubricants capable of crying loads providing smooth functioning.

Repair and Refurbishment:

  • Aluminum molds: Its high thermal conductivity makes it much harder to weld. If work needs to be done here, then usually skilled agents are needed.
  • Steel Molds: Easier to weld since they can be refurnished through grinding, which leads to easier maintenance.

Storage:

  • Aluminum Molds: Their storage area must be in dry land with no oxidization, don’t stack, and avoid deformation.
  • Steel Molds: Customarily kept in a controlled environment preferably with some layers of oil in between preventing rust.

Temperature Control:

  • Aluminum Molds: Must watch out for thermal expansion issues; need optimized cooling
  • Steel Molds: Endure longer in heat but need to be heated uniformly to avoid thermal stress

Mold users to recognize and perform these maintenance practices will assist in achieving the life and functionality of aluminum and steel molds.

Conclusion: Making the Right Choice

The choice between these two molds depends on several factors, from your production volume, raw resin properties, part size, and complexity to the required precision of molded parts. For instance, aluminum molds are best for small volumes, while steel molds are for mass production.

However, if you are still confused about aluminum vs steel injection molds, we can help you to choose the best fit according to your requirements and specifications. RapidDirect specializes in manufacturing robust molds and high-quality injection molded parts from thermoplastic resins. Although injection molding projects are challenging and time-consuming, our professional engineers and powerful manufacturing networks can handle different manufacturing needs and designs.

If you are looking for an injection molding partner, we can collaborate to achieve your manufacturing needs. Our injection molding services will exceed your expectations. Go to our online platform, get rapid quotes, and produce your prototypes quickly.

FAQs

Why are steel injection molds more expensive than aluminum ones?

The steel material price, longer machining time due to hardness, and some overhead costs make steel injection molds more expensive than aluminum ones.

What are the best aluminum alloys for making injection molds?

The best aluminum alloys for making molds include Aluminum 7050, 7075, 2024, 5083, and 6061.

Can a metal 3D printer create a mold for casting plastic products?

Yes! You can make intricate and customized molds with a Metal 3D printer to create plastic products in small volumes.

Can I use an aluminum injection mold to make chainmail?

Yes! It is feasible to make chainmain with injection molds. However, it might be more suitable with other methods rather than assembling the molded interconnected metal rings.

What material should I use for injection molding?

Suitable materials are selected depending on the application and product properties needed for the final part. Common materials are thermoplastic, such as ABS, polypropylene, and Nylon, which are relatively easy to process and versatile as well. Polycarbonate or PEEK is better suited for high-strength, high-stiffness, and high-usage components.

Why is steel preferred as mold material?

Steel is the most common material for injection molds, offering a strong balance of machinability, durability, and cost. Hardened steel grades are available for more demanding applications, providing extra wear resistance for high-pressure molding. Steel molds are ideal for complex or delicate features, as they’re tough, resistant to damage, and ensure consistent, high-quality results—especially in high-volume runs. While they may cost more upfront, their durability leads to long-term savings, making them the top choice for manufacturers who need reliable, long-lasting molds.

Does aluminum last longer than steel?

Steel is typically more durable and wear-resistant than aluminum, offering a longer lifespan under high-stress conditions. While aluminum is lighter and resists environmental damage, it’s softer and won’t last as long in tough environments. If weight is a key concern, aluminum might be the better option, but for high performance and durability, steel is usually the best choice. Ultimately, it all comes down to your application’s needs and the conditions it’ll face.

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    Written By
    Lucas Bennett is a mechanical engineer with expertise in manufacturing, 3D printers, automobiles, EVs, renewable energy, and HVAC. He has worked on projects from trainee engineer to senior engineer and deputy project manager and specializes in writing manufacturing content. In his spare time, he’s an avid reader and loves tinkering with DIY tech projects.

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