Metal die-castings are a mystery to many manufacturers that use them. That is because the process uses a technology that is so full of complex variables, operating through complicated machinery, that few outsiders understand it.
Indeed, many die-casters, though they know how to make die-castings, do not fully understand the behavior of molten metal in their own machines and dies.
Until recently, high-pressure die-casting was more of an art than it was a science. It was a business marked by incredible successes, but also by heavy scrap costs, long set-up times, and very costly tooling. Quality was highly variable, fluctuating from excellent if the process was under control, to poor if it was not--often on the same machine with the same operator.
But that was yesterday. Today, die-casting is becoming an exact science. The process stands as an increasingly high-tech, sophisticated manufacturing method capable of superb, repeatable quality, and high production rates. New die steels, faster die-making, and innovative design software are beginning to reduce tool and die costs while improving die performance. New technology in die-casting machines is reducing porosity problems and making stronger, more dense castings. Computer controls and robotics are reducing variability and thereby increasing quality and reducing scrap costs. Last, but not least, new furnace designs are giving cleaner molten metal with fuel savings that approach 50 percent.
Die-casting offers the OEM high quality components with all the advantages that metal can deliver. An OEM need not rely on plastic injection moldings to obtain complex, high-precision components: modern metal die-casting can produce similar shapes and dimensional stability, all at comparable rates of production.
While many die-castings are made from aluminum alloys, zinc is a useful material, easily painted or plated. Magnesium is emerging as an excellent material if light weight and strength are important considerations.
In the United States' die-casting industry, the customer traditionally owns the die and contracts with a die-caster to use the die to make and deliver the product. In effect, the die-caster acts as a steward--storing, cleaning, maintaining the die on the customer's behalf, and then running it to make product according to the customer's needs.
In fact, the die-caster may arrange to have the die built for the customer, working from a prototype, drawings, from an existing part, or an old die. Although the customer owns the die, he or she may never actually possess it, and may not even know what it looks like.
The die may stay with a single die-caster for its entire life, or move from one die-caster to another without ever seeing the inside of the OEM's facility. This creates several problems. First, if a die is moved frequently between die-casters to obtain price or service advantages, it often does not receive good maintenance or sound metallurgical treatment because no sense of "ownership" occurs, and maintenance records become inaccurate or even lost.
Second, a die steadily deteriorates with use through the processes of metal fatigue and thermodynamic erosion. Therefore, the die-caster and the customer must constantly communicate to make sure that both parties track the die's life cycle, and rebuild or replace it when its useful life ends.
A die life of 150,000 part-making cycles is historically observed in aluminum die-casting; dies used for zinc parts can last much longer. Modern techniques for die maintenance, which involve periodic heat treating for stress relief, surface hardening, and metallic or ceramic surface coatings, can extend die life to 300,000 or more part-making cycles. New die steels and experimental surface treatments can possibly extend die life into the millions of parts.
Given the high cost of creating a die, die life extension has an immediate impact on the per-unit cost of a die-cast product.
Close customer/supplier relationships are particularly important in die-casting. Given customer ownership of a die, it is vital that a customer involve the die-caster early in the product design to ensure that the product can be readily die-cast at a minimum production cost.
Concurrent engineering and robust design are important trends in die-casting. Without input from the supplier, a customer may design a part that will be very difficult to translate into a steel die, and very difficult to produce on conventional casting machines.
This can result in high product cost and large quantities of imperfect castings. Involving the supplier in the design process will optimize both cost and manufacturing effectiveness, and will usually create a die that will have maximum die life.
If the customer wishes, the die-caster can retain ownership of the die and contract with the customer to supply parts at a piece price built around a guaranteed die life. This frees the customer from the responsibility of die ownership while delivering a set price for the entire die life-cycle.
Research on how liquid metal actually flows into a steel die under high pressure led to new understanding of how turbulence and solidification shrinkage affect product strength and overall quality.
Computer systems can now measure and analyze each variable in every cycle of a die-casting machine. This permits tight process control, high repeatability, and optimization of machine performance. Also, images of the die-casting process inside the die can be created by software. This guides engineers in designing the die and in selecting the process variables that will produce a perfect part.
These advances open the door to obtaining stronger, more complex, and higher quality parts with little increase in cost. As a result, die-casting has become more attractive for creating components that might otherwise use steel or plastics.
The technological improvements in die-casting have contributed to improved die life, resulting immediately in longer production runs from an initial die investment, driving down price per piece and total life cycle costs.
Improved quality, defined as conformance to product specifications, has also been reducing inspection and quality control costs for the customer. These costs were traditionally high because die-casting seldom exceeded 98 percent good parts in deliveries to customers.
Today's process control gives a die-caster much lower scrap rates, less product variation, and more success in meeting customer quality expectations. This is a win-win situation for both sides of the customer-supplier relationship.
The low cost, high strength, and low weight of zinc, aluminum, and magnesium die-castings, coupled with process improvements that have greatly increased quality, make die-casting more attractive than ever for a very broad range of products.
The automobile industry is using more die-castings to reduce vehicle weight. Computers are using magnesium and aluminum for the same reason. Plastics are a substitute material, but they are often more expensive, less robust, and generally cannot be recycled easily--unlike aluminum and magnesium.
Future applications will show die-casting to be alive and well, re-establishing itself as a major production process.
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