Prototyping in Cast Metal

Avoiding the pitfalls

Bob Kowalczyk, casting engineer

A prototype-proven design can significantly shorten “time-to-market” by eliminating false starts and costly design changes in the construction of tooling and the die-casting production phase of a project. For the designer, the prototype serves as a design insurance policy for a successful outcome. This article does not attempt to cover all aspects of the prototype decision-making process, but simply draws forth some of the more egregious pitfalls that trip up designers and prototypers of cast metal prototypes.

Differences between casting processes

Parts produced via the metal die-casting process lead the designer into very selective prototyping decisions. If the prototype is to be used simply for touchy-feely trade show displays or for engineering meetings that address issues of size, shape and fit, then one of several prototyping processes may suffice. However, if the purpose of the prototype is to test the part’s strength or heat dissipation characteristics, then the designer will want to choose a prototyping process that most closely simulates the production die-cast part.

Other reasons to select a cast prototype over other prototyping processes is to resolve tooling construction issues and to determine what secondary machining and other finishing operations may be required. When a prototype is intended for performance testing, then the choice of alloy and prototyping process becomes critical. Mechanical and physical properties vary significantly between the cast prototyping process and the die-casting production process, further affecting the selection of prototyping alloy and process. Additionally, some prototype casting processes can be artificially enhanced to accelerate the metal solidification, to more closely simulate the performance of the part being developed. 

Simulating production characteristics 

Cast prototypes are primarily used to simulate the characteristics of the production casting, in order to test and measure critical performance parameters of the part before investing in production tooling. If there is concern with the production part having proper heat dissipation, then the designer has several options for achieving that desired result. Knowing which alloy will be used in the die-casting production process may point to one or more alloys suited to the prototyping process of that particular part. For example, if a part is to be die-cast in 380 alloy, the prototyper may advise 319 alloy to simulate the heat dissipation characteristics of the production part. 

There are, however, complexities below the surface. The alloy that is right for the prototyping process where heat dissipation measurement is critical may not be right for another part where heat dissipation is just one of several concerns. When one combines the need for proper heat dissipation with the need for specific mechanical properties, such as strength, then the alloy selection picture changes completely. If that part that is to be die-cast in 380 aluminum is also prototyped in 380 aluminum, the part may fail in testing, because the alloy produces different mechanical properties in the two different processes. 

Can or should a feature be cast?

One of the most common questions designers ask, “Can this feature be cast?” The parallel question that often is not asked is, “Should this feature be cast?”

Many features can easily be cast in the production process. In the prototyping process most features can be cast but it is not always cost- and time-efficient to do so. Small holes, for example, can be cast in the prototype process, but not without a considerable amount of additional work and expense. It is helpful if the prototyper recognizes this issue and guides the designer by saying, “We could cast this feature, but here is another option that will work as well in less time and lower cost.” 

The same is true for casting mating surfaces between two die-cast components that are to be assembled together. It is often easier and less costly to machine the two features in the prototyping process rather than attempt to cast them. The designer may overlook this fact and simply assume that since it is something that can be accomplished readily in production die-casting, it should be easy to cast in the prototyping process.

Picture two parts, one a box with a groove and the other a base with a mating element. In the die-casting process the groove can be cast without special effort. In the prototyping process it can be cast, but not without a considerable amount of extra work. Beyond that, if one should require a 25 or 100 piece prototype run, casting this particular type of feature may simply not be worth even attempting. It is generally more time- and cost-efficient simply to machine these features. 

The reverse can also be true — a feature that can be cast in the prototyping process may need to be machined in the production process. An example of this situation would be a part with undercut or odd-shaped features. Referred to as a “maverick,” the part feature will show up in the prototype, but will lead the unaware designer into a cul-de-sac when the part goes into production. 

Decisions and choices made in the prototyping process must be extended into production. Likewise, die-cast production decisions, anticipated part usage and performance information must be fully taken into consideration in order to create a prototype that truly simulates the final product in the pre-production part testing phase. This efficacy cannot be achieved if the designer and prototyper do not work closely together. They must fully share their concerns, their expectations and their knowledge. Success can be achieved every time when everyone is focused on the desired outcome.