Strategies for Diecasting Prototyping
Until recently, diecasting prototypes were widely considered impractical in most instances because of the greater costs involved with diecasting and the additional lead time needed to develop the dies necessary to make the prototypes. However, innovations in the diecasting industry have made the use of the diecasting prototype process more affordable and efficient.
One notable advancement was the introduction of CNC machining. This high-speed machinery makes it possible to produce the tools needed for die casting much faster. Depending on the manufacturer, a company using this technology can produce a four-slide die in just two weeks, an achievement that used to take at least eight to 10 weeks using more traditional production methods. If a client has a condensed timeline, CNC machining can be used to produce a prototype in less than two weeks, if circumstances permit.
The use of 3D design and simulation software has also had a positive effect on the diecasting industry by making prototype diecasting tooling more affordable. With the use of 3D CAD technology, the time necessary for tooling design can be reduced from several days to a few hours. Additional software makes it possible to virtually prototype a concept and prevent models that are destined to fail as a function of their own design from proceeding to hands-on production.
Basic diecasting prototyping methods include single-cavity prototype die, gravity casting, rapid prototyping, plaster mold prototyping, and machining. New 3D printing technology has added other options, which will be covered in a future article.
Single-Cavity Prototype Die
If you’re going to conduct extensive testing, the single-cavity prototype die process is likely your best choice from among all of the current diecasting prototyping strategies, except for actual diecasting. Arguably, the biggest advantage of this process is that it allows you to thoroughly evaluate your end product’s critical characteristics, including the finish on its exposed surface.
With approximately 75% of production dies requiring either minor or major changes, another benefit of the single-cavity prototype die process is the ability to make certain changes to the design of the prototype’s die after the initial round of parts is produced, thus avoiding costlier changes down the line.
The original die’s insert often can be used in the final production phase, as well. With the single-cavity prototype die process, it normally takes less time to create your final dies and secondary trim tools compared to other diecasting prototyping strategies.
Due to the cost involved and the time it sometimes takes to create dies, the single-cavity prototype die process may not be suitable if you don’t have sufficient lead time or if the design of the product has some uncertainties.
The gravity casting process—which includes most investment casting and plaster mold casting strategies—is the most popular choice to create die cast prototypes because it is less expensive than the single-cavity prototype die strategy for small amounts of product. In addition, the gravity casting process does not require as much lead time as single-cavity prototyping does.
While gravity casting may be more affordable, it has some drawbacks that need to be considered when deciding which diecasting prototyping strategy to use. Since they generally have less porosity, a gravity casting typically has greater fatigue strength than a die casting does, for instance. Diecasting can produce an end product with precise dimensions, but additional machining will be required to achieve the same exacting dimensions when gravity casting is used. Gravity casting also cannot achieve the very thin wall widths that can only be produced by diecasting, although this casting technique can yield walls with greater widths.
Rapid prototyping for die-casting is often associated with various processes, including stereolithography, laser sintering and fused deposition modeling. Contingent upon a prototype’s geometry, prototyping for die castings using one of these methods can usually yield an initial part in as little as five to eight weeks. These prototyping strategies for diecast parts use a stereolithography model to create H-13 steel dies using pressure diecasting instead of gravity-fed diecasting.
Since the alloys, as well as the physical and thermal properties, used in rapid prototyping are the same as in the production run, rapid prototyping enables you to perform a thorough and accurate analysis of your product before investing in the construction of intricate, expensive dies. For the same reason, rapid prototyping is typically your best choice if you want to produce up to several thousand units while your production dies are being fabricated.
Diecasting rapid prototyping is often referred to as the “steel process.” This method is not usually appropriate for work involving thin and/or tall standing detail on parts.
Plaster Mold Prototyping
Also known as rubber plastic mold casting (RPM), plaster mold prototyping is a gravity-based casting strategy that works with aluminum, magnesium, zinc and ZA alloys. Working with a stereolithography model, an initial prototype can be produced in just a few weeks. This technique enables you to make any necessary changes to a part’s geometry quickly and simply. At about 10% of what it costs to construct a production die, plaster mold prototyping can be more economical than other strategies for diecasting prototyping.
Although it can produce parts in many sizes, RPM is generally most appropriate if your part is in the range of 2 to 24 cu.in. Plaster mold prototyping is capable of producing a few or up to several hundred working diecast prototypes, making it an appropriate method to produce a prototype if the quantity of product needed is not large enough to justify the cost of hard tooling.
Plaster mold prototyping can replicate any castable geometry, as well. While that is an obvious benefit, it can also lead to problems because it allows designers to erroneously use geometry that can drastically increase diecasting costs or make a shape that is ultimately impossible to diecast.
Machining From Similar Die Castings
Machining from similar die castings involves creating prototypes from existing die castings that have a size and shape similar to the prototype you want to have made. If you want multiple prototypes of small parts, it’s possible for them to be machined out of a single, large die casting’s thick areas using this strategy. This method is appropriate for creating small gears and screw-machined items, as well as other instances calling for a large number of prototypes and access to the automatic machining processes and materials needed to make them.
While this approach to diecasting prototyping may seem convenient, it does have drawbacks that can be significant. First, the dimensions and shape of the prototype you want to make are restricted by the size and form of the die castings that are on hand. This diecasting method also removes the dense, thick skin a production die casting normally has.
In the paper, “The Significance of the Die Cast Skin Pertaining to the Fatigue Properties of ADC12 Aluminum Alloy Die Castings,” Briggs & Stratton reported that when their skin was machined off, the yield strength and fatigue strength of die castings made from aluminum were reduced by more than 10% and 39%. Additionally, a study performed by the U.S. National Energy Technology Laboratory revealed that if all or a portion of the skin was removed from a zinc die casting, the casting’s yield strength dropped by almost 10%.
Machining From Wrought or Sheet
The list of strategies for diecasting prototyping includes machining from wrought or sheet when you want a prototype made from sheet or extruded aluminum and magnesium. When compared to a die casting, wrought and sheet materials have higher ductility but lower compressive yield strength. These materials may also have an undesirable directional quality caused by the direction of the sheet or the extruded alloys used to make them.
Picking the Best Diecast Prototyping Process
In order to determine which diecast prototyping process is the right one for you, you must understand that the diecasting technique used to make production castings is inherently different from the methods commonly used to construct prototypes. For this reason, and due to the differences between the alloys used in diecasting and other casting methods, a prototype is probably going to have different characteristics than a production casting would have.
For instance, a component made from diecasting is going to have a skin that is normally around 0.5-mm thick. This skin gives diecast parts a considerable amount of their tensile strength and fatigue life. While the skin is a vital component of a die casting, a finished machined prototype must have had either a portion or all of its skin removed just to be created.
Given that the diecasting process involves certain steps such as rapid cooling, quick solidification and high pressure on liquid metals, among others, diecasting gives a component mechanical characteristics that a prototype made with another method might not have. Although the difference is normally negligible, the core of a diecast item may contain porosity, which might make it less dense than castings created using alternative techniques. Depending on the diecasting prototyping strategy you choose, it may be able to closely replicate many of the mechanical properties associated with die castings.
As a general rule, the alloys typically used in diecasting are not appropriate to use with the gravity casting prototype process or machining from wrought or sheet because their chemical makeup is different from the chemical composition of the alloys used in the prototyping techniques. The zinc alloy group used for diecasting includes Zamak 3, 5 and 7, which contain 4% aluminum. Since this group is touchy when it comes to the rate of solidification and the strength and hardness of a gravity casting are measurably less than they are in die castings, this group of Zamak alloys should not be used in the gravity-casting prototyping process.
Instead of Zamak 3, 5 and 7, ZA alloys should be used with the gravity-casting prototyping technique to better replicate the mechanical properties produced by diecasting. It is okay to use Zamak 3, 5 and 7 to produce the ornamental elements of your prototype, however, as long as the mechanical properties of these pieces is irrelevant to the functionality of your prototype. CS
Click here to see this story as it appears in the July/August 2020 issue of Casting Source.
Leonard Cordaro is president of Premier Die Casting/director of sales development at Appalachian Cast Products (Abingdon, Virginia).