By Robert (Bob) Kowalczyk
Published in NADycasting – March 19, 2020
A simple way to avoid the trap of using the wrong prototyping process is to ask yourself honestly why you favor one process over another. Review the real reason(s) behind the choice, and don’t be fooled by fancy rationalizations. If the process is not ideal for that part design, and you are not fully convinced that your prototype supplier is giving you the best advice, then take the time to investigate further. The characteristics of the part and the reason for producing the prototype (such as mechanical testing) should guide your final choice of prototyping process.
To prosper in today’s highly competitive marketplace manufacturers protect their market position by continually improving their products and being quick to market. The ever-decreasing product cycle time puts enormous pressure on designers to ensure a successful launch.
To create a prototype of a part before launching into full production is a process familiar to anyone involved in the designing of parts that will eventually be produced as production die castings. Before considering what process should be used for a particular prototype, it is important to examine the rationale behind prototyping.
As a designer, you are best served by obtaining the “Design Insurance” that is available by using prototypes. They eliminate false starts and costly design changes that can hinder the production start-up and can significantly shorten the time to market. You never want to be the designer of a component that “looked good on paper” but did not function as required and potentially jeopardized an entire product or project release. Imagine the personal nightmare because your part has now become the pacing item, your design is being questioned, you are being second guessed, and numerous design changes continue to delay production.
It is extremely important to understand all criteria pertinent to a specific prototype. For instance, will it be subjected to mechanical testing? Is it for appearance or “fit” only? Will it be tested for RF leakage? Is corrosion resistance an issue? Will heat dissipation be a critical factor? All of these questions are relevant to the selection of a suitable and therefore successful prototype process.
Horror stories of a project gone badly are far too numerous. Here are some examples:
The design of a component had both structural and heat transfer considerations. Although the prototype functioned well in heat transfer testing, the production die-casting did not. Subsequent research determined that the alloy chosen for the prototype had significantly different heat transfer characteristics from the eventual production alloy.
Another example is the procurer of prototypes who didn’t take-into-account the different mechanical properties of the production alloy when prototyped in a different process. This caused an over-design of the die casting because the prototype was not as strong as the designated die casting material.
How about the designer who sent his design to an SLA service bureau, sent the subsequent pattern to a casting facility that promptly produced good-looking prototype parts—but they lacked machining stock for the performance of secondary operations?
All of these have happened, and unfortunately continue to happen today. These cases exemplify why it is so important for you to understand and communicate to the prototype supplier the special requirements of each prototype. You need to make sure the supplier understands all available options in-order-to render prototypes that most accurately simulate the eventual die cast part. That said, let us now explore what options may be available in prototyping aluminum or zinc die casting designs.
Hog-outs:
“Hogging out”, (a prototype machined from solid aluminum stock) can be a viable option. Advantages include cost effectiveness in low quantities, precision, and time efficiency. Disadvantages can be higher cost in higher volumes, possible distortion of mechanical testing results from differences between wrought and cast materials, and difficulty in simulating the effects of draft, fillets, and radii.
Using this method, part configuration will be the major cost driver after the initial programming expenses. Minimizing material removal helps to limit costs.
Extra caution must be exercised when considering this method for prototyping structural components due to mechanical property differences.
Examples of part components prototyped via this method include electronic enclosures, and low temperature heat sinks.
Sand Casting:
Sand casting continues to be utilized for die cast prototypes. Advantages of this method include cost, timeliness, and ease of design changes. Disadvantages include dimensional integrity (as compared to production die castings). Most configurations can be cast effectively, and new advances in precision fine grain sand casting processes can accommodate almost all die cast wall thicknesses and draft angles.
The sand-casting process has proven to be significantly effective in structural designs. Together with appropriate alloy and heat treatment choices, the prototype can provide close approximations of die cast mechanical properties. It has also been considerably effective for heavy-walled castings, and much thicker sections than are customary in conventional die casting. The heavy section castings are often conversions from grey or ductile iron into aluminum for production in the advancing “squeeze casting” processes.
Investment Casting:
Although primarily a production process, die cast designs are occasionally prototyped through the investment casting process. Advantages can include cost effectiveness on longer runs of smaller parts. The primary disadvantage is longer lead-time. The exception to this is when an SLA wax pattern can be used for casting of a single part through investment casting.
Part configurations produced via this method include hinges, valves, and switch enclosures.
3D Printing:
Currently 3D printing is an evolving process with new applications and materials being discovered almost daily. It produces a three-dimensional part from a CAD file or 3D model by adding layer upon layer of material, which is why the process is also referred to as “additive manufacturing.” 3D printing can be used to prototype eventual die castings. In many cases the 3D process is used to produce a master pattern in combination with available casting processes. Advantages include shorter lead time and lower cost (particularly in low quantities). Disadvantages include the requirement for a “watertight” or complete design prior to beginning work, some size restrictions, material restrictions, and the need to produce a master in 3D printing technology that best suits a part’s geometry. 3D printing of sand molds has provided added opportunities for cast prototypes. This can be cost affective in low quantities since no tooling is involved. It can be cost prohibitive in larger quantities, and still is developing in respect to surface finish and wall thickness considerations.
Die Insert or “soft die” processes:
These processes are occasionally used for production under certain conditions. A distinct advantage of these processes is that prototypes are truly representative of the actual production. Disadvantages include longer lead-time and higher tooling costs as compared to other prototyping methods. These processes can be effective for larger quantity prototype runs (1000 + parts).
Plaster Mold Process:
The plaster mold process is a viable method of prototyping eventual die cast parts. Advantages include inexpensive tooling, ease of design changes, smooth surface finish, and the ability to cast thin walls. Disadvantages can include increased part cost. Recent advances in sand casting technology appear to be relegating the plaster mold process to special appearance applications.
Whatever prototyping process you choose, be sure to fully understand its limitations. Your final selection should be based on which process offers the best simulation of the production cast part. Be sure to discuss in detail all the critical measurements and tests your prototype part must undergo before it is approved for production.
Secondary Processes:
Technological advancements in secondary machining capabilities have increased the demand for finished prototype castings to reflect more closely the final production part. It is not uncommon for suppliers of cast prototypes to offer turnkey services that give their customer a part that not only looks and feels like the die cast production part, but also performs like it. Turnkey service can save time and money, as well as the aggravation of trying to coordinate different secondary operations. Examples include additional machining, inspections, certifications, painting/coating, and partial or full assembly. In the prototyping process it is important to determine which eventual die cast features should be cast in the prototype process and which features should be machined. Often the most timely and cost-effective route may include a combination of casting and secondary machining.
Bob Kowalczyk
President
Aluma Cast, Inc., 3200 E. Pershing – Appleton, WI 54911
Phone: 920 739 6282 – Email: bob@alumacast.com – www.alumacast.com
(published in NADycasting 3/19/2020)