Introduction
In the manufacturing industry, molds are a critical tool that forms the foundation of mass production. Especially in the casting of light alloys like aluminum and zinc, the choice of mold material significantly influences product cost, quality, and time-to-market. However, relying solely on common perceptions—such as “aluminum molds are cheap and fast” or “steel molds are durable and long-lasting”—is not enough to make the optimal choice. This article provides a detailed comparison of aluminum and steel molds from three perspectives that are most crucial to manufacturing executives and procurement managers: “cost,” “lifespan,” and “accuracy.” We will delve deeply into the advantages and disadvantages of each mold type and provide guidance to help you choose the best mold for your company’s products and production plans.
Cost: A Comprehensive Evaluation of Initial Investment and Running Costs
Cost is one of the biggest concerns in mold selection. At first glance, aluminum molds appear to be significantly cheaper than steel molds, but this is a complex issue involving not just material costs but also manufacturing processes, maintenance, and production efficiency.
Initial Production Costs: The Difference in Material and Processing Fees
Due to its high hardness, steel molds are time-consuming and costly to machine. Machining a hard material requires high-power machine tools and a long cutting process. In contrast, aluminum molds are softer than steel, making them easier to machine. This excellent machinability not only shortens the production period to one-half or one-third that of steel but also significantly reduces processing costs.
The material cost itself is also a major component of the total mold cost. Steel (SKD11, S45C, etc.) generally has a higher cost per unit weight than aluminum alloys (A7075, A6061, etc.). In addition, aluminum has a specific gravity of about one-third that of steel, so for a mold of the same volume, the material weight is lighter, further reducing the total material cost. For these reasons, aluminum molds can reduce initial production costs by approximately 30% to 50% compared to steel molds.
Running Costs: Production Efficiency and Maintenance Costs
A low initial cost is meaningless if significant costs are incurred during production. Running costs are primarily influenced by “cycle time” and “maintenance fees.”
Aluminum molds have a high thermal conductivity, about five times that of steel. This property allows heat from the injected molten metal to be quickly dissipated, significantly shortening the cooling time. This leads to a 15% to 30% reduction in molding time per cycle, directly improving production efficiency.
On the other hand, steel molds have low thermal conductivity, which lengthens the cooling process. However, due to their hardness, they have the advantage of requiring relatively little maintenance. The ability to easily repair wear and tear with welding and material build-up is also a strength of steel. In contrast, the high thermal conductivity of aluminum molds makes welding repairs very difficult, requiring highly skilled technicians.
Lifespan: Material Properties Affecting the Number of Shots
The lifespan of a mold is a critical factor that forms the basis of a production plan. Molds are consumables, and their useful life (number of shots) determines the final product unit price.
Comparison of Durability and Number of Shots
Steel molds, with their high hardness, wear resistance, and heat resistance, boast a long lifespan. In the case of a typical die-casting mold, it can withstand production of hundreds of thousands to over a million shots. This allows for stable, long-term production, even when casting abrasive aluminum alloys at high temperatures.
Conversely, aluminum molds have lower hardness than steel, leading to faster wear and thermal fatigue. For this reason, they were previously considered unsuitable for high-pressure, high-temperature environments like die-casting. However, recent advancements in high-strength aluminum alloys (like A7075) and special surface treatment technologies have dramatically improved their lifespan. While a typical lifespan is a few thousand to tens of thousands of shots, there have been reports of durability exceeding 100,000 shots under ideal conditions.
Selecting the Appropriate Application
Considering mold lifespan, the appropriate uses for both become clear:
- Steel Molds: For products requiring large-scale production of hundreds of thousands or more, such as automotive parts, home appliances, and industrial machine components.
- Aluminum Molds: For prototypes, small-batch products, parts with frequent minor changes, and products where a shortened development period is desired.
For example, for new car prototypes or products for emerging markets with uncertain demand, where the priority is to minimize initial investment and get to market quickly, aluminum molds are the overwhelmingly superior choice.
Accuracy: Quality Driven by Material Properties and Processing Technology
Mold accuracy directly determines the quality of the molded product. The optimal mold material varies depending on the required accuracy, including dimensional tolerance, surface finish, and fine details.
Dimensional Stability and Surface Finish
Steel molds, with their high hardness and rigidity, can withstand high temperatures and pressures during molding and maintain high dimensional stability over a long period. They are also easy to polish and surface-treat (chrome plating, nitriding, etc.), allowing for various finishes from a glossy mirror finish to specific textures (such as graining).
Aluminum molds are softer than steel and have a slight risk of deformation during molding. In die-casting, which uses high pressure, this deformation can affect accuracy. However, in low-pressure casting processes like gravity casting or low-pressure casting, aluminum’s superior machinability allows for high-precision processing of molds with complex shapes and fine details.
Maintainability and Quality Preservation
Molds wear out with use, and their accuracy gradually declines. At this point, the ability to perform quick and accurate maintenance is key to preserving quality.
Steel molds are highly resistant to wear due to their hardness, so their accuracy declines slowly. However, once damaged or when wear progresses, repair requires a high level of skill and time.
On the other hand, while aluminum molds are more prone to wear, their excellent machinability makes it relatively easy to repair and re-process worn parts. This allows for quick correction of minor defects that occur during manufacturing, helping to maintain quality. However, as mentioned earlier, welding-based material build-up is difficult, and in some cases, it may be necessary to re-process the entire mold depending on the extent of the damage.
Conclusion
Aluminum and steel molds have different characteristics and advantages. The key is not to decide which is superior, but to use them strategically according to the application and production plan.
Aluminum molds are the best choice when the goal is to reduce initial costs and shorten the development period. They are particularly suitable for prototype production, small-batch runs of a few thousand to tens of thousands of units, or agile development processes where products are refined based on market feedback.
Meanwhile, steel molds are an indispensable tool when you require long-term mass production and high quality stability. For parts in industries like automotive, where production runs are in the hundreds of thousands, or for precision parts that demand strict dimensional tolerances, the durability and accuracy of steel are essential.
The final decision should be made by considering not just immediate costs, but also the product’s lifecycle, production volume, and required quality level in a comprehensive manner. We hope this comparison will assist you in selecting the optimal mold for your needs. At Daiwa Aluminum Vietnam, we offer professional advice on the most suitable casting processes and molds for our customers’ products. Please feel free to contact us.