In his book Failure of Plastics and Rubber [1], David Wright conducted an analysis on 5,000 failed parts and came up with the causes for the failures shown in Figure 1.1.
As discussed in Chapter 1, according to David Wright, material failures account for 45 % of the failures in plastic products. It is, therefore, important to get a good understanding of this “wheel” as a start. One of the themes that I am going to repeat again and again is that plastics are not metals. For most of us, our formal engineering education has been mostly in the area of metals with very little emphasis on the distinction between metals and plastic materials. We, therefore, tend to count on the same properties that we expect from metals when designing plastic parts.
The designer of a product is like the quarterback in American football. The quarterback is responsible for the outcome of each individual play and the ultimate outcome of the game. Even the strongest offense and defense team members cannot make up for the deficiency in the initial play by the quarterback. Similarly, the designer controls the ultimate performance of her/his product. The rest of the members of the team—resin supplier, tool-maker, and molder—cannot make up for the deficiencies in the design.
Continuing the theme of a holistic approach, let us now talk about the third wheel—tooling. Designers should have a good concept of the tooling basics to facilitate synergy with the tooling personnel, in order to prevent deficiencies in the area that can affect a good design and vice versa.
Injection molding makes possible the mass production of intricate, high performance, tight tolerance parts with very little secondary labor and minimal waste. Continuing the theme for a holistic approach, let us proceed to the final wheel—injection molding.
There are few injection molded plastic parts or assemblies that do not have some kind of secondary operation applied to them. At the highest level, these can be divided into three broad categories, namely:
Assembly or joining
Machining or some other kind of material removal process
One of my professors used to say that a well-designed part should not only meet or exceed its functional requirements, it should also be optimized for cost.
The Six Sigma methodology for improving processes and designs has been around since the eighties. As a firm believer in the methodology, I have successfully used it to improve products and performance on multiple projects, saving millions of dollars in the process. I will also be the first to say that in a lot of cases Six Sigma has been used more as a marketing ploy—“We are a Six Sigma company!”—than out of a real desire to improve products or processes.
