By Design: The new plus the old: A winning combination

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

The end of World War II heralded the beginning of the wide use of plastics for peacetime products. However, the engineers who designed these early plastic parts had little or no training in how to use these new materials, and there was very little technical literature available. Most designers had learned their trade by designing products to be made of non-plastic materials; many of this new generation of products were designed using the guidelines developed for metal, rubber, and wood.

In some cases, the designer simply replaced the existing material with plastic and kept the original design. This approach worked well with die-cast parts, but it was disastrous in the case of thick wooden parts and machined or stamped metal parts. Many new products suffered accordingly. It is a tribute to the adaptability of plastic materials that the industry survived and prospered.

The best of these early designers learned from their mistakes and did not repeat the same errors. Gradually, a set of part design rules evolved. Plastic material manufacturers formalized and published these design guidelines, and the quality of plastic parts improved. Even today, these old brochures are some of the best basic design information available to the industry.

Sequential Engineering

For the following 40 years, design engineers continued to design plastic parts using the same sequential procedures as in the past. Engineers created products and laboriously calculated strength and performance criteria. The pocket calculator replaced the slide rule and speeded up this process. The resulting concept designs were then turned over to a draftsman, who worked out the details and finalized the individual part designs. The resulting drawings were then released to production. The drafting machine replaced the T-square and the triangle, but little else changed.

In the mid-1980s, product and piece part designers were being encouraged to adopt a whole new set of product design tools and procedures. Computer-aided engineering (CAE) was slowly adopted and has now become the norm.

Concurrent Engineering

In the 1990s, concurrent engineering replaced the old sequential approach to product design. The well understood distinctions between product design and part design became blurred as new products began to be designed by teams. The wide acceptance of computer-aided design (CAD) has now resulted in a situation where product and part design are often simultaneously done by the same person.

This is as it should be; the new CAE procedures are more efficient than the old way of doing things. Properly utilized CAE produces lower cost, better quality parts that can be put into production on shorter schedules. This sounds too good to be true, and, in some cases, the new techniques fail to deliver as promised. The failures do not indicate flaws in the new systems. The team approach to product design, concurrent engineering, and CAE are capable of doing precisely what they promise to do if they are properly utilized. The published case histories of smoothly flowing, paperless projects that progress from the CAD drawing through tooling to a finished, molded part in an unbelievably short time are true stories. These are the successes, and no one publishes the horror stories. So what is wrong with the new systems?

Corporate management is bombarded with a never-ending stream of new management philosophies and tools. The first mistake is that management rarely adopts a complete, well-thought-out, new system. It accepts whichever part of the system it likes and pretends the unpleasant part of the system isn’t important.

For example, OEMs like “just in time” delivery, but they resist making long-term projections of their future needs. They like the idea of low part costs, but they reject the concept that long production runs are the most efficient. They demand the shortest possible tool delivery while continuing to make part design changes even after the molds are done and in production. CAD databases can move flawlessly into cutter path programs but only if the file is complete, accurate, and compatible with the moldmaker’s software.

Members of corporate America jumped at the opportunity to downsize and replace their well-paid engineers with less costly recent graduates. They ignored the fact that they were losing people with 10, 20, and 30 years of experience designing their company’s products. Management liked the idea of using low-cost, entry level designers who knew how to apply all of the new CAE tools. They did not like the idea of investing time and money in training these bright young people. Management’s answer to this problem was the team approach: Everyone on the team would contribute a little bit, and no one team member was required to take the time to learn everything.

Concept Design and the New Designers

Conceiving a new product is the most important part of the product design process. It establishes parameters the rest of the design and development process must follow. With the exception of brainstorming, concept design does not lend itself to the team approach. So far, no one has come up with an artificial intelligence software program that can take into account all the things a creative designer mentally considers while conceiving a new product concept.

It is worth remembering that a team is just an old-fashioned committee with a new name. The clumsy, ill-tempered, out-of-proportion camel is a special purpose horse, conceived and designed by a committee.

Today’s engineering graduates normally arrive on the job trained to use all of the newest CAE tools, but only a few of them were trained in plastic technology. An even smaller percentage have been through a course on plastic part design. It is a foregone conclusion that, without on-the-job experience, these well-meaning novice designers will make mistakes applying CAE to the design of their company’s new plastic products. Management’s answer to this fact of life is that the analytical aspects of CAE, such as finite element analysis and mold filling analysis, will detect and correct these problems before the product gets to market. There is a flaw in this theory; concurrent engineering dictates that these errors in judgment are not found until after the part is being tooled for production.

CAE tools and concurrent engineering have now been in use for a long enough period of time for many original equipment manufacturers to have developed procedures for avoiding these common problems. A shocking number of others are repeating these mistakes over and over again.

The New Design Tools

The new designers and the old designers they replace both performed basically the same function, and they eventually arrived at the same conclusion. The primary difference is that today’s designers are using a whole new box of tools to do the same work. Regrettably, the effort required to learn how to use these new tools does not leave time enough to include the basics of part design in the average engineering curriculum.

There is no question that the new CAE tools are an advancement over the drafting machine and the calculator. These advantages can be maximized even further by combining CAE techniques with the tried and proven part design guidelines developed in the past.

Part Design

Anyone who has been through the laborious process of finalizing the design of a plastic part is aware this is not a simple undertaking. There are hundreds of little details that must be correctly considered in order to end up with a successful product. The novice engineers who have concentrated on mastering the use of CAE tools will not necessarily know the answers to the questions that will arise. The critical balance between designing for functionality and manufacturability can be an insurmountable problem for a novice designer who has no prior experience with the production of injection molded parts.

In the past, most draftsmen had prior production experience. They knew how their companies’ products were manufactured. The best and most creative of these drafters became designers. Today, most designers are graduate engineers with no hands-on production experience. The use of CAD has resulted in a situation where these engineers are both designers and drafters. There are no software programs or interactive training courses that can quickly teach these designers what they need to know about the capabilities and limitations of the injection molding process. There are, however, some simple tricks of the trade that can allow these inexperienced designers to create part designs that can be efficiently injection molded.

The multi-functional plastic parts being produced today incorporate a multitude of details. These complex parts are difficult to visualize in a drawing or on a CAD screen. It is helpful to recognize that these complex parts can be broken down into three recurring types of structures.

All parts have a nominal wall section. This nominal wall is the basic structure of the part that supports and locates all the other features. Anything added to the nominal wall is a projection. Anything removed from the nominal wall or a projection is a depression or a hole. Different sizes and shapes of these three basic design elements—nominal walls, projections, and depressions—are combined in an endless variety of ways to produce all the injection molded parts now in existence. Each of these three structures can be correctly designed by following a few simple design guidelines. Once a designer has mastered these rules, he or she will be empowered with the knowledge of how to correctly design any injection molded part encountered thereafter. These design guidelines will be the subject of future articles.