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Aluminum vs Steel Injection Molds: Which One to Choose?

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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 Mold

    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.  

     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.

    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.

    Considerations Before Choosing 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

    It is another critical consideration before choosing between steel and aluminum injection molds. Steel is suitable for high-volume production due to its longevity and capability of withstanding high temperatures continuously. It can create parts in millions with consistent quality and accuracy.

    On the other hand, aluminum molds are better options if you are looking for small-batch production or rapid prototyping.

    Cost Analysis

    Aluminum toolings are less expensive if we see steel price vs aluminum price in the context of injection molding. Steel might cost twice or more compared to aluminum mold of the same size and specification.

    Although steel molds have high upfront investment, they return the value in long-run production. What’s more, steel is a wise option for low per-unit cost during mass production. Conversely, the aluminum option is cost-effective for prototyping to a few thousand runs.

    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.

    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.

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