When most of us look at a house, we may not know whether it is Georgian, colonial, or Cape Cod in style. We will, however, be able to identify the material it is made of -- brick, stone, wood, or stucco. When it comes to plastic, on the other hand, unless we are designers or engineers (and sometimes even if we are) we may have no idea whether we are looking at ABS, PP, PS, HDPE, or any of a hundred other resins. That's fine for end users, but for product developers, the choice of resin can be critical.
Often a designer decides early in the development process what resin to use; for others, resin choice is left until the final stages of development, or it evolves as the design develops. But no matter how the choice is made, the material affects virtually every aspect of the design. It opens and closes doors and influences choices of size, shape, and features. It is, in turn, dictated by the intended uses of the product and the environment in which it will function. In short, material matters, and it matters in many different ways.
A knit line is any line, visible or not, where two resin flows meet. Depending on the design of the mold and the material being injected, a knit line may present no problem at all, may be a cosmetic issue, or may cause a serious structural problem.
A common question in choosing a resin is "What's the cheapest resin?" The question should be "What is the cheapest resin that does everything I want it to do?" Before you can answer that question, you need to know what that "everything" is. Designers may not know that when they start development, and it can take several iterations and a certain amount of calculated trial and error to define it. That is what design and prototyping are for.
Engineers continue to push the boundaries and limitations of plastics in today's market. Plastics play crucial roles in car parts, power tools, and handheld devices. Plastics get pounded, pushed, pulled, twisted, and bent. They may be expected to hold their shape under stress or to deform and then bounce back. As designers push the limits of materials and continue to demand more from their material choices, materials will have to deliver more.
Their strength can be compressive, tensile, or flexural. It can depend on the ambient temperature, the speed with which the stress is applied, or its duration. In other words, strength is a very complex interaction of material, design, conditions, and stress. It can be one of the hardest characteristics to measure without prototyping.