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Finding a Fit with Rapid Injection Molding

Medical Device & Diagnostic Industry Magazine MDDI Article Index   Originally Published MDDI August 2005 Cover Story: Molding

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published MDDI August 2005

Cover Story: Molding

Rapid injection molding may be a good fit for design engineers looking for quality first-run parts. But they should first know the basics.

Bradley Cleveland

Suture clamps, like these pictured still in the mold, can be made in small batches for prototyping. At right, a high-speed CNC milling machine is linked to the 3-D CAD file used to generate tool paths for the mold design.

Rapid injection molding is changing the way designers think about prototyping and low-volume production of parts in industries such as medical devices, aerospace, appliances, and electronics. For medical device manufacturers in particular, laborious testing cycles and rigorous product evaluations are needed to submit a product for clinical trials and to bring it to market. Rapid injection molding can help minimize time to market by reducing time spent on the testing and certification processes.

Technological advancements are changing the medical industry. The product design process, for example, uses increasingly powerful 3-D CAD programs that enable more-complex product designs. But these design advances in turn drive demand for more-complex prototypes. Fast-moving competitive markets require frequent design changes, short lead times, and tight budgets. In sum, today's high-tech world means prototyping must be faster, better, and less expensive than ever before.

While traditional molding methods are still widely used, a growing number of design engineers are turning to rapid injection molding. The technique offers designers the ability to create prototypes with high-quality materials. Designers can create inexpensive first-run parts suitable for testing. Rapid injection molding also gives manufacturers the flexibility to create parts that can be used for marketing studies or low-volume production needs.

This article discusses rapid injection molding and related mold-making processes. Understanding these options, their capabilities, and their limits can help designers make better-informed decisions about which processes to employ.

Rapid Injection Molding for Design, Testing, and Production

Table 1. Mold-making technique comparison chart (click to enlarge).

Rapid injection molding can economically deliver from 25 to 10,000 molded prototypes in 3–15 days. Using a CNC-machined aluminum mold that produces the same geometry as subsequent steel production tooling, rapid injection molding can replicate the intended design shape. The resulting prototypes enable engineers to submit parts to the extreme and rigorous testing procedures needed to validate functional properties. The dimensional accuracy of the parts is comparable to that of injection-molded parts made using steel tooling.

Both mechanical properties (e.g., strength, temperature resistance, etc.) and future production costs should be considered when obtaining prototypes.

Because the rapid injection molding process produces functional parts, it can be used in several steps of the new product development cycle. At the earliest stages of product development, an engineer may only need 25 pieces for initial evaluation. During market testing, as many as 250 parts may need to be put into the hands of end-users. And at the bridge tooling stage, a few thousand parts may be created to meet production requirements until steel tooling is ready for full-scale production. In cases where only a few thousand parts per year are required, rapid injection molding can easily perform the necessary machining for all stages.

How Rapid Injection Molding Works

Rapid injection molding is a highly automated method of producing injection-molded parts from a 3-D CAD part model. The core technology of the process involves a software application that automatically converts digital part models into tool paths for CNC milling machines. The CNC milling machines produce metal mold components that are assembled and mounted on an injection molding press. Heated thermoplastic resin is then injected into the mold, where it solidifies to produce the desired part.
Although there are process differences among suppliers, most employ the same basic steps. Understanding the process does not just involve the end product. The path to the end is just as important.

CNC-milled mold components are assembled and mounted on a press. Heated thermoplastic is injected into the mold where it solidifies.

First, the design engineer submits a 3-D CAD part design. Depending on the supplier, the design file can be submitted in any commonly shared file format, including IGES, STEP, native SolidWorks, Parasolids, and ACIS. (IGES is initial graphics exchange specification; STEP stands for standard for the exchange of product model data; and ACIS is Andy, Charles, and Ian's system, owned by Spatial Technologies.)

After a design file is submitted, software typically analyzes the 3-D CAD model, exploring potential changes that might improve the molding process. A quote is sent to the engineer, usually within one business day. If the quote is Web-based, it should provide pricing as well as suggestions for improving design parameters that better fit with the rapid injection molding process. Items that are frequently subject to change include the number of cavities, A- and B-side finish levels, and resin used. The desired delivery schedule may also shift if changes are substantial or complex. Keep in mind that each change may alter the price quote. While software is rapidly evolving to ensure precision, it's always a good idea to double-check the analysis to make certain the part will be manufactured as desired.

Besides offering general geometry improvements, the software may also provide a compatibility review. A compatibility review suggests changes that will improve moldability or reduce tooling costs using the rapid injection molding process. The review should also identify undercuts, wall thicknesses that could cause fill or sink problems, and areas where draft is required. Indications of radii or wall-thickness limitations resulting from the mold-milling process should also be provided.

Although a vast array of geometries can be produced using rapid injection molding, the process does have some limitations. Both part size and part complexities are restricted, largely due to the highly automated nature of the mold-making process. For example, because CNC milling results in rounded external part corners, some part features may not be possible using rapid injection molding. The process is also limited in its ability to produce undercut part-geometry features, which require mold pieces to pull out sideways, perpendicular to the direction of pull. Most problems, however, can be worked around. Design engineers should consult an online design guide or contact the supplier's engineering specialists for more-detailed assistance.

Upon receipt of the final 3-D CAD design and order, the software uses the published shrink coefficient of the selected resin and automatically determines the geometry of the mold components that are required to produce the part shape. During this process, the software calculates factors such as core and cavity geometries, shutoff surface generation, gate-design layout, and ejector-pin placement. The software then outputs tool paths for three-axis CNC machine cells to manufacture required mold components for subsequent assembly into a functional mold. Mold technicians produce the final parts using injection molding presses that typically range in size from 30 to 300 tn. This range of presses can support parts with a projected area of up to 75 sq in.

Small parts, like these suture clamps, are especially well-suited to the rapid injection molding process.

If changes to part geometry are needed after an initial run of samples, design engineers must submit a revised 3-D CAD model. The supplier determines whether the change requires a new mold or whether the initial mold can be modified via additional processing. Typically, molds can be modified relatively easily even if the mold material needs to be removed to support required changes. If additional mold material is required, a new mold may need to be machined. What makes rapid molding convenient is that even when a new mold needs to be created, it can be done quickly and cost-effectively. Creating additional iterations is generally a feasible method for rechecking a design before full production.

Working within Rapid Injection Molding's Capabilities

The rapid injection molding process is best for smaller parts. Although it varies by supplier, a typical limit for a part envelope is approximately 7.5 ¥ 14 ¥ 3 in. deep and is subject to an overall limitation of 75 sq in. of projected part area. Also, because the mold is produced by high-speed three-axis CNC milling technology, rapid injection molding is best for parts that require simple, straight-pull molds or simple mold side actions. A straight-pull mold enables the two mold halves to pull straight away from each other without mold metal passing through the plastic part. Side actions require mold pieces to pull out sideways (perpendicular) to the direction of pull. Rapid injection molding can support up to four mold side actions for undercuts if the undercuts are on the outside of the part geometry, at the parting line, and within certain size requirements. There are several considerations to keep in mind for designing within the limits of the rapid injection molding process:

• Avoid placing detailed features adjacent to deep walls, where it can be difficult to maintain accurate milling control.
• Pay attention to corner design. Some part corners may end up with a radius rather than a sharp edge; reshaping a corner may not require a mold change, but it's a design element to consider.
• Steer clear of deep, thin ribs. They tend to increase the mold-milling time and make hand polishing difficult and time-consuming.

Whether the process involves conventional molding or rapid injection molding, parts should have consistent wall thicknesses to minimize the potential for warped or distorted parts. Also, engineers should design a part using the appropriate draft and reinforcing fillets. Doing so will ensure proper ejection, add rigidity to part ribs, and strengthen the mold.

Additionally, if more than 10,000 parts are needed, high-volume steel tooling in a conventional manner may be a better choice, economically.

Evaluating Available Rapid Mold-Making Technologies

Submitting 3-D CAD part designs via the Web can make it easier to provide feedback and explore mold changes.

As with any selection process, it's important to evaluate mold-making options before making a decision. While milling mold parts speeds the time to market for some medical device manufacturers, it's not a one-size-fits-all solution for every prototype and low-volume-production need. Table I compares other options.

Depending on the definition of rapid, design engineers can also work with vendors who use the electrical discharge machining (EDM) method to manufacture their injection molds. EDM technology has fewer geometrical constraints but tends to be a slower process than three-axis CNC milling, so there may be a trade-off to consider. EDM works by eroding material in the path of electrical discharges. It can create both simple and complex geometries.

There are also some relatively new mold-making technologies for quick-turnaround requirements. These are based on additive rapid-tooling techniques. Such techniques include selective laser sintering (SLS) and ultrasonic welding. In SLS, a laser is used to sinter metal powder into the desired shape of the mold. For complex mold geometries, SLS can be significantly faster than using EDM. SLS is useful for a 1- or 2-day turnaround when testing a part's form or fit. This is especially true if the part is highly complex in shape, and the material characteristics of the prototype do not need to match those required for production. But it may not be a good fit for projects requiring duplication of fine detail or testing in extreme conditions. The final SLS surface finish is powdery and porous unless a sealant is used, and the material is difficult to control in extreme temperatures.

Ultrasonic welding uses high-frequency oscillations to weld together thin strips of metal to form the desired mold geometry. The process has been shown to produce high-quality molds from both aluminum and steel, in competitive time frames.

The key advantage to any additive technique is the ability to create nonmachinable features. Nonmachinable features are the geometric shapes within the mold or on the surface that cannot be created by cutting away metal. Conformal cooling channels in the mold geometry, for example, can effectively reduce injection molding cycle times during production.

Conclusion

Rapid injection molding is a convenient way to quickly obtain real parts for prototyping, initial runs, and low-volume production. The medical device industry uses rapid injection molding to get parts that can be accurately tested at a fraction of the cost of full-scale production materials. However, engineers must understand the possibilities and constraints of rapid injection molding to use the technology successfully.

Following simple rules can help engineers choose the best process technique for individual needs. It's important to understand the technical details and compare geometric requirements, such as size and dimensional accuracy, for mold making. Engineers need to know the business constraints and choose the process that best fits their budget and schedule limitations. And they need to select a qualified provider. It is imperative to research the supplier's ability to meet basic requirements and solve the unpredictable but inevitable technical problems that arise during any prototyping project.

Bradley Cleveland is the president and CEO of Protomold Company, Inc. (Maple Plain, MN).

Copyright ©2005 Medical Device & Diagnostic Industry

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