Let's face it we are all human (perhaps some more than others) and one constant that we share, aside from the obvious, is that we make mistakes. Those who work in product development circles are no exception...in fact it is the act of failure that sometimes leads to our greatest accomplishments. Unfortunately, "success challenged development" often results in a seemingly unending cycle of design-test-redesign-test-repeat. This not only eats into the budget, but the project timeline as well. Obviously, one sure-fire way to get to market fast and on budget is to identify design opportunities early in the design process-often easier said than done. Many times errors can be caught during the prototyping phase, when bench models are relatively inexpensive and changes can be quick. But, errors that are discovered further downstream are usually more expensive to remedy. Not to mention that time lost deep in the development cycle is not only difficult to recover, but also very costly. And then there are the errors that are found not in development or testing, but (dare we say it...) after the product hits the hands of the customer.

Plastic components-from internal mechanisms and supports to cosmetic enclosures and interfaces-obviously require testing as well. Life testing, repeated actuation, drop testing, safety, ease of assembly, serviceability, manufacturability, aesthetics, ergonomics are all important factors that apply to plastic parts. In the past, obtaining a short run of actual plastic parts to function test required a lot of money and a long lead time. If a developer wanted to test a few parts he was relegated to SLA (stereolithography) or RTV (cast urethane) parts. These types of processes produced parts that worked well for form and fit, but basing functional test results on them had "recall" written all over the place.

Fortunately, recent technological advances in the realm of Rapid Tooling have presented product designers with new options. A few companies have successfully honed their processes to such a point that they can produce actual plastic components that sometimes compete head-to-head with SLA and RTV in terms of both price and delivery. The players in this game range from vendors who use only CNC machining (which often requires special part design concessions) to those who can produce a near production quality part that can be used for bridging into production as hard tooling is built. Often a simple plastic part produced from a CNC cut tool is all that is needed, but if the requirement is a test part that will closely mirror a production part, it makes sense to use the process that closely mirrors production tooling.

Remember, the goal is to identify design opportunities and challenges early in the process. Before deciding on a process or provider, consider the following items that can have dramatic impact on the plastic parts you receive:

1. Does your material specification fit your application?

Not everyone is a materials expert especially when it comes to plastic resins. The plastics industry changes daily with new developments and advances. However, like most materials, there is usually one or two that will fit your needs better than others. Additionally, consider that custom blended materials will often carry a long lead time and higher price than an off the shelf stock color material. Contact a resin supplier if you have questions about your material spec. Better yet, if your rapid tooling provider has a good knowledge base and experience in development, consult with them on materials. Be prepared to answer questions that refer to environment, application, agency approvals, UV stabilization, cosmetic issues, hardness, toughness, color, fillers, etc.

2. Where should plastic flow into the part (i.e. gating techniques)?

Few designers give this much thought and will leave gate type and location up to the tooling vendor. Depending on their process, many tooling vendors will specify the type and location of gate they will use. Or, they may not give you a choice and simply put it where it is easiest for them or their process. Say you are designing an exterior cover for some highly cosmetic field test units. The last thing you want is for your tooling supplier to put a cold sprue right in the center of the cover. Think about areas such as pivot points, sealing surfaces, bearing and lens seats, touch points and handle areas. Placing a gate in any of these areas could render the molded part unusable. Rapid tooling vendors usually see only parts and pieces of products and often have no idea where the parts actually go or what they may be used for. Discuss gating with your tooling supplier especially if the part from the rapid tool is supposed to mirror the one from the production tooling. How plastic flows into a tool to create a part can impact the cosmetic and structural characteristics. Testing a part that was gated one way and then using a completely different gate configuration in production could spell trouble.

3. Identify critical-to-function areas and expect realistic tolerances.

Many times, when having a part quoted for rapid tooling, designers will send only 3D data to quote from and not include any dimensional information. While most rapid toolers will only guarantee a standard tolerance range in either the molded part or the tool itself, often they can hold tighter tolerances in areas which need special attention, such as bearing surfaces or connector areas. However, this information must be relayed in the project kick-off and it may take a few tooling tweaks to dial the parts in. Also, try to be realistic when specifying tolerance bands on plastic parts. Unlike machined parts, molded components have a few more variables to contend with in their production (i.e. the tool, the processing conditions, the material characteristics, etc.) and trying to hold extremely tight tolerance bands on as-molded plastic parts can be very challenging and time consuming.

4. Include draft on the part designs or specify it at kick-off.

The goal of it is to make plastic parts fast and normally require at least a ½ degree of draft in all areas to be successful. Omitting the draft on part files can create havoc throughout the entire rapid tooling and molding process. While some vendors offer draft addition as a service, many do not and will not accept the project until they have a drafted part file If you are unsure of what draft requirements to put where, consult with your tooling vendor. If your design requires zero draft in certain areas, be sure to convey this to your vendor at the beginning so he can plan accordingly. Keep in mind that well drafted part designs will normally require less tool making time and will mold faster at the press.

5. Think about part texture and finish.

Some vendors offer basic texturing in-house or (depending on their process) can send the tools out for custom textures. Additionally, many can offer high polish for ultra-smooth part finishes. But, texturing and polishing cannot be an afterthought and must be given consideration in both the part and tool design. Think of a highly textured part as having millions of small undercuts all over it. If there is not enough draft, the texture may cause the part to resist ejection or it is going to wipe the texture off, or both. Most texture specifications have minimum draft requirements that must be met. Smooth surfaces, while not as critical from a draft standpoint, may require more labor to polish and can add cost and time. Also, highly polished surfaces tend to reveal a lot of cosmetic issues on a plastic part such as witness lines, sink marks, flow lines, etc.

6. Offer realistic part quantities

Rapid tooling processes are built for speed and the really good ones have a number of different options depending on the type of tool that is needed for a customer's application. Part quantity (i.e. estimated tool life) can have a dramatic effect on the type of tooling built or process used. For example, if you tell your vendor that you will only need 100 parts from a tool and no more, they are going to build a tool that is capable of producing at least 100 pieces in the quickest, most cost effective manner possible. Later, if you determine that you need 10,000 more parts, the tool may be capable of producing it, but it may not maximize material usage, have a slow cycle time and produce a high part cost. To maximize the use of the tool, offer both an immediate need and an annual or total volume estimate.

7. Tie down the timeline as early as possible.

This may sound easy, but many projects have missed due dates because upfront expectations were not clearly set. Most rapid tooling vendors will quote a project based on the information provided in the RFQ and most quotes are a perfect example of "garbage in-garbage out" scenarios. If your RFQ includes a part file, a material spec and a quantity required, then a fairly accurate standard quote can be provided. If after kick-off, the part file changes (in process ECN's), the material spec is changed (with possibly different shrink rates), texture is now required (with draft changes), part quantities change or high tolerances are now required, the tooling process comes to a screeching halt while these changes are addressed and the lead time-and sometimes cost--begins to stretch. Many suppliers will ask appropriate questions in the beginning to make sure these delays are avoided, but some may not.

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Copyright (C) 2007 Alain Georgette / Copyright (C) 2006 Frantisek Hliva. All rights reserved.