Plastic Part Design for Economical Injection Molding – Part II

The process of designing a plastic product to be made by injection molding is the same as designing a product for any other manufacturing process or material.

This article originally appeared on the SPE Product Design and Development Division website, written by Glenn Beall. SPE has recently announced that the ANTEC® 2024 program will feature a Symposium Honoring the Career Achievements of Mr. Glenn Beall. His indelible legacy has significantly contributed to the growth of plastics and SPE.

Each combination of material and manufacturing method has its own unique design guidelines. However, the design process itself is independent of the kind of material or manufacturing method being used.

All new product design projects start the same way. To begin with, someone has to recognize a marketplace need or opportunity. This could be a whole new product or a line of new products. It can also be a redesign to reduce cost or improve quality and eliminate customer complaints. If a new product or a new part is required a product designer or a team is assigned to the project.

An Overall Observation

There are hundreds of interrelated details that combine to produce an injection molded plastic part. All of these details can be divided into four broad categories or elements, which are:

1. The design of that part.
2. The choice of the optimum plastic material.
3. The design and construction of the mold.
4. The actual molding of the part.

There may be additional elements present such as trimming, decorating, assembly, etc., however the four basic elements will already be present even on something as simple as a poker chip.

All four of these elements must be handled correctly in order to produce the optimum part. All four elements are of equal importance, however everything starts with the design of the part.  Without a part design, there is no need for a material, a mold, or a molding process. The only element that is more important than the others is the one which is not handled correctly.

All the different plastic materials, molds and processing techniques have their own advantages and disadvantages. A successful part design is the result of the design engineer’s awareness of these capabilities and limitations, coupled with meticulous attention to design details. If the design engineer does not have this awareness it behooves him or her to consult those who have that knowledge. Those consultations are a valuable learning experience for those who are paying attention and who want to learn.

Product Design

New products are created using many different approaches. No matter what technique is employed, the design engineer should always start by developing a thorough understanding of all of the functional and market place requirements of the product. A designer needs to know how and where the product will be used and by whom.  Indoor, outdoor, or both. What is the maximum use temperature?  What are the required service life, cost constraints, and quantity to be produced?  What is the competition doing in this market area?  What regulatory agencies are involved and what will their counterparts require in South America or Europe?  Etcetera…  The actual process of designing a product must never be started until the designer has learned as much as possible about the application. Once the designer understands what is required, he or she, or the team are then in a position to design a product that will satisfy that marketplace need or opportunity.

Regrettably some new products are designed without the benefit of a thorough analysis of the marketplace requirements or trends. IBM’s continuing to concentrate on mainframe computers and the Ford’s Edsel are classic examples. In other cases, creative designers have designed what they liked or what was easy, or what would impress the design competition judges without carefully following the established customer’s wishes. While designing products to sell for a profit, the customer is the sole judge and jury. Every effort must be expended to satisfy or exceed the customer’s wants, needs and whims.

As the product requirements become defined, they should be recorded. This will create a new product specification sheet or design check list. This list can be used throughout the new product design and development process. It is also a useful communication tool that allows everyone else in the company to sign off on the description of the new product being developed. Marketing people have been known to agree to a new product’s specification and then change their minds about what a new product should be. The list won’t prevent that from happening but it will place the blame where it belongs.

Once the design engineer has a clear understanding of the functional requirements of the product in the hands of the end user, the actual design process can begin.  The first step is for the designer to simply think about what is required and how to achieve it. This is the critical time, when prior knowledge of the marketplace, past experience and education become so terribly important. This leisurely appearing activity is actually very intense. This part of the process should not be rushed.  The decisions made at this time will determine the course of the project from that point on.  This is especially important work. The company will eventually choose one of the designer’s concepts to take to the market. The company’s financial success and the designer’s reputation and maybe his or her job, will be determined by the marketplace success or failure of the product.

Creative designers have the ability to see things that do not exist. As a designer thinks or daydreams about the new product, random ideas, possibilities, and recollections of prior successes and failures will begin to come together to form a mental image or picture. The designer then begins to record these mental pictures so that other’s can see what he sees in his mind’s eye. This takes the form of doodling, then sketching, and maybe renderings.  What the designer is actually doing is experimenting with different shapes in sketch form, looking for a physical structure that will satisfy the functional requirements of the product. This trial and error process continues until the designer has evolved one or more structures that he feels comfortable in being able to develop into a commercially successful product.

Today everyone is in a hurry. Many new products fail because a company proceeds with the first concept that appears to be acceptable without waiting for the best concept which will be evolved later, after the designer has had a longer time to think about the product. The creative process takes time and it is unwise to rush this critical phase of a project.

As the shape of the new product emerges, the designer will simultaneously  be thinking about a lot of other details, such as what would be the best process for producing this shape, in what material. The inherently lower cost tooling and quicker delivery of thermoforming will be mentally compared to injection molding’s lower part cost and ability to produce precision dimensions but at a higher mold cost. These preliminary choices in process and material will immediately influence how the designer sketches the product from that point on.  Injection molded parts are not designed the same as thermoforming parts.

A host of other details will be passing through the designer’s mind, while the new product possibilities are being sketched.  Where will the product be assembled and by whom?  The appearance of the product, mold design, cost, delivery, decorating, packaging, etc.. The designer will also be considering the standards established by such organizations as the Food & Drug Administration, Consumer Product Safety Commission, Underwriters Laboratories, the Federal Trade Commission, etc..  In other words, everything having to do with the product is being considered during the design concept phase of the project.

Before proceeding any further with the project, it is desirable to review the concept with an industrial designer.  Industrial design is different from engineering design.  In recent years, the industrial design function has expanded to encompass many aspects of product design. Two of those functions that are important to the success of many new products are appearance design and human engineering.

Human engineering has to do with proportioning a product so that it fits or can be conveniently used by human beings. Is the height of the chair seat suitable for both adults and children?

The size, shape and color of a consumer product sitting on a store shelf is what first attracts your attention. The appearance of an industrial product may not seem to be as important of a consideration. That is an incorrect assumption. Raymond Loewy, one of the founders of the North American industrial design profession, correctly observed that, “Between two products, equal in price, function, and quality, the better-looking one will outsell the other.”

Industrial designers receive special training in appearance design, human engineering, and many other subjects. The early input of a qualified industrial designer can greatly increase the customer acceptance of a product.  It is beyond the scope of this article to do justice to the industrial design function. However, industrial designers are another valuable resource that design engineers are not using to its full advantage.

It is important to recognize that during this part of the project, the design engineer is considering and making instant trade-off decisions on a large number of details.  It is easy for the designer to overlook one of these details.  At this phase of the project, the designer must concentrate on satisfying all of the product’s functional requirements. At this point, customer acceptance takes priority over efficient manufacturing.

The best of these sketches is then cleaned up and reviewed with the Marketing Department and other concerned parties. Theoretically, the Marketing Department knows what the customer wants. Marketing will review the design checklist or product specifications. They will also study the concept sketches or renderings and make a decision as to whether or not the designer has created a structure that will satisfy the customer’s expectations.  If the proposed designs are not acceptable, the designer will be sent, as they say, “back to the board” to try again. If Marketing approves of the structure shown in the sketches, the new product will move onto the next part of the design process.

The Marketing Department has important functions to perform, but designing new products is not one of them. Most experienced marketing people realize that creating a new product concept is important work and they tend to allow time for this phase of the project. Never having developed a product themselves, they tend to believe the work done between concept approval and market introduction is just routine engineering. This is not true, but this is what Marketing chooses to believe.

Once the Marketing Department sees a sketch that looks like it will do what they want it to do, they are understandably in a hurry to get the product into the marketplace at the earliest possible date.  From that point on, the product will be on the “fast track”. There will no longer be time enough for the designer to sit back and contemplate improving the product beyond its present state.

It is important to recognize that once the Marketing Department approves one of the new product concepts, that design tends to be locked in.  The designer has then lost the freedom to alter the design without securing Marketing’s agreement. This means that if the designer does not incorporate ease of manufacturing into the original product sketches, the single best opportunity to do so will be lost forever.  Studies by the Institute of Competitive Design have indicated that the decisions made up to this point will determine 75% of the product’s cost.

As the product works its way toward production, more and more people will become involved and it will become increasingly difficult to change the original design.  This is especially true when the project is handled by simultaneous or “concurrent engineering”.

Many injection molders have been producing commercially successful plastic products since the Second World War. These experienced molders are very knowledgeable concerning the tricks of the trade for producing plastic parts.  In most instances they know more than their customers about molding high quality, low cost plastic parts.  It is highly desirable to invite an experienced molder to critique the manufacturability of a new plastic product before proceeding with the project.

Considering this brief review of the first phases of the new product development cycle, it is obvious that in order to significantly influence the product design, an injection molder has to be in contact with the product designer before the original design concepts are submitted for Marketing’s approval. The early supplier involvement program is a proven method of keeping an open line of communication between a designer and an injection molder. This procedure will allow a molder to make efficiency improvement suggestions before new product design concepts are approved and locked in by Marketing. Electronic data transfer and on-line communication has now simplified the early supplier involvement to the point that there is no longer any credible excuse for not taking advantage of this free resource.

Read Part I here.