You can justify low-volume injection molding, and rapid injection prototyping shows you how.
Traditional injection molding is all about quantity. Making a multi-cavity, high-performance hardened steel tool is a slow, expensive process, but once the mold has been manufactured, presses can crank out hundreds of thousands of parts both quickly and inexpensively. Today’s mass markets have plenty of demand for just that kind of production, but what happens when the required quantity begins to shrink?
As quantity drops, the cost of producing each part remains fixed. The high cost of making the mold, however, must be paid off over fewer and fewer parts, which drives up the piece-part cost. In theory, when cost per part becomes large enough, the process should become impractical from a cost standpoint. However, in many applications, there is no viable alternative to injection molding. Now comes the challenge: you must find a way to reduce the cost of injection molding.
Rapid injection molding was used to make this sensor frame for the Tensymeter–a non-invasive device designed to measure arterial blood pressure in real time, on a beat-to-beat basis.
Photo courtesy of Tensys Medical.
Justifying low-volume injection molding
Several situations require injection-molded parts, but their production may not be in 100,000+ volumes. These include prototyping, low-volume production, and bridge tooling.
Prototyping. Today’s markets make harsh demands on products. To meet these demands, the development process must address “The Four F’s”: form, fit, function, and feasibility. Form is how a part looks and feels. It includes factors such as weight and surface finish. Fit is the precise shape and dimensions of a part and its interaction with other components. Function–the strength, elasticity, chemical resistance, and other performance characteristics of a part–is determined by testing and depends on both the properties of the resin from which a part is made and the process used to make it. Finally, feasibility is a measure of how successfully and cost-effectively the intended production method can produce a part. All four of these factors can, and should be addressed in the development of prototypes.
In the early stages of development, form and fit can sometimes be evaluated using rapid prototyping methods such as stereolithography (SLA), selective laser sintering (SLS), or fused deposition modeling (FDM). Unfortunately, as the development process moves forward and surface finish and tolerance become more important, the layered processes used in rapid prototyping are less effective in matching the characteristics of production parts. The bigger drawbacks to these additive processes, however, are in functional testing, which requires the use of engineering grade resins and production processes. As for feasibility, additive rapid prototyping methods are of no help in determining problems that could arise in production by injection molding.
Low-volume production. Production does not always mean hundreds of thousands of parts. In some cases, production volumes will be low–just hundreds or thousands of parts–but the parts still need to be made by injection molding to achieve the specified performance characteristics. Medical devices are a good example of such a situation. They often require the material properties and aesthetics of injection molded parts, but the high cost of traditional mold production may be prohibitive without the offsetting savings of high-volume production.
Bridge tooling. Proven designs that will be mass-produced are ideal candidates for traditional multi-cavity steel tooling; however, because of today’s fast-paced markets, completing the required development and testing required to get products to market quickly is critical. The production volumes may justify the cost of traditional steel tooling, but the time it takes to make the molds becomes a problem because it does not allow for pilot production or market testing.
In this context, bridge tooling refers to tooling that “bridges” the gap between soft tooling and full-production tooling. The object is to provide from tens to thousands of injection-molded prototypes in the required production material as quickly and as cost-effectively as possible.
Today’s markets demand this type of tooling. It lets companies get initial production and marketing moving while they wait for their steel tools to be made for high-volume production – a process that typically takes up to 12 weeks.
Fast, low-cost injection molding
Rapid injection molding addresses all three challenges. The process entails milling molds from
aluminum rather than steel. It is also automated to eliminate delays and reduce costs. Rapid injection molding is sometimes referred to as “soft tooling,” which is misleading. Nothing is “soft” about aluminum molds.
When your production tooling will not be ready for another 3 or 4 months, bridge tooling can give
you a way to make 100, 1000, or 10,000 parts for pilot production or market testing.
Aluminum molds, in most cases, can be milled to specifications comparable to steel molds. They have been shown to produce up to tens of thousands of parts without discernible degradation.
Aluminum molds suit several production requirements, including short runs. Made from an aluminum mold, this antenna Tilt Mechanism Clip joins two rods that drive a phase shifter to provide electrical tilt on a base-station antenna. Photo courtesy of Andrew Corporation.
Because it is a true injection molding process, rapid injection molding can support any of hundreds of resins. This is critical for bridge tooling or low-volume production, as well as prototyping. By producing parts with all of the characteristics of actual production parts, it lets designers evaluate form, fit, function, and feasibility just as they would with traditional injection-molded parts, but it costs much less than traditional steel tooling.
Because rapid injection molding relies on complex compute software as part of its process, an approved design can be converted directly to a toolpath for automated CNC milling of components, such as this snap-on retaining block used to prevent the buckling of a stylet. Photo courtesy of Endogastric Solutions.
Save time and money
Rapid injection molding has other benefits too. It replaces costly, time-intensive human labor. Rapid injection molding automates the complex process of producing a working mold from a customer design. It requires sophisticated software and a lot of processing power. At the Protomold division of Proto Labs in Maple Plain, Minnesota, an approved design can be converted directly to a toolpath for the automated CNC mills that produce the mold components. Its system was originally designed for simple two-part molds, but has evolved to support up to four side-actions for designs that include undercuts.
Protomold’s system can evaluate whether a customer’s 3D CAD models are moldable. It can identify possible design flaws and recommend modifications, and illustrate them in 3D color-coded diagrams that rotate, independent of human input. What’s more, the system can predict potential fill problems and recommend necessary changes in the design. This is a critical part of the prototyping process, and it begins before the first part is molded.
Rapid injection molding design guidelines
Rapid injection molding can produce many of the same parts that can be made by more traditional methods; however, like any technology, it does have its limitations, which include maximum footprints, depths, and volumes.
These parameters depend on the specific piece of equipment being used to make the part. The following figures are specific to Protomold’s equipment; however, they are a good example of suitable design guidelines for rapid injection molding.
• Maximum part outline: 13.5 x 30.5 in. (smaller for deeper parts)
• Maximum projected mold area: 175 in.2
• Maximum part volume: 59 in.3
• Maximum depth: 3 in. from the parting line (6 in. total if the parting line can pass thru the middle of the part, inside and outside)
• Draft: 1° per in. of depth from parting line; 1/2° minimum for most faces
• Up to four side action cams per mold for production of external undercuts
• Note: side action cams reduce allowable part outline.
This process supports numerous resins. The most common ones can be found online at Protomold’s site. The process can also support a variety of custom-blended and custom-colored resins.
For more information about rapid injection molding in general, as well as Protomold’s process, visit www.protomold.com/DesignGuidelines.aspx.
Fast, low-cost machining for 10 or fewer parts
When the number of required parts drops to 10 or less, an acceptable alternative to additive rapid prototyping is available. First Cut Prototype offers fully automated CNC machining directly from customers’ 3D CAD models. The system converts a CAD model directly to toolpaths for automated milling equipment, which cuts the part from a solid block of resin. As with rapid injection molding, this process produces prototypes from any of dozens of resins, which makes it possible to match the resin that will be used in production. Because the finished part is not layered, its performance is very similar to an injection-molded part. This is ideal for functional testing.