Basic Detractors and Constraints for Design


The successful design and fabrication of good plastic products require a combination of sound judgment and experience. Designing good products requires a knowledge of plastics that includes their advantages and disadvantages as well as some familiarity with processing methods. Until the designer becomes familiar with processing, a fabricator must be taken into the designer’s confidence early in development and consulted frequently during those early days. The fabricator and mold or die designer should advise the product designer on materials behavior and how to simplify processing. The designer should not become restricted by understanding only one process or method and in particular should avoid focusing on just a certain narrow aspect of a design or process. 

Although there is no limit theoretically to the shapes that can be created, practical considerations must be met. These relate not only to part design but also to mold or die design, since these must be considered one entity in the total creation of a usuableeconomically feasible part. In the sections that follow, various phases considered important in the creation of such parts are examined for their contribution to and effect on design and function.

Prior to designing a part, the designer should understand  basic factors . Success with plastics, or any other material, for that matter, is directly related to observing design details. For example, something as simple as a stiffening rib is different for an injection molded or structural foam part, even though both parts may be molded from the same plastic . And a stiffening rib that is to be molded in a low-mold-shrink, amorphous TP will differ from a high-mold-shrinkage, crystalline TP rib, even though both plastics are just injection-molded . Ribs molded in RP/composite plastics have their own distinct requirements:

The important factors to consider in designing can be categorized as follows: part thickness, tolerance, ribs, bosses and studs, radii and fillets, draft or taper, holes, threads, color, surface finish and gloss level, decorating operations, the parting line, gate locations, molded part shrinkage, assembly techniques, mold or die design, production volume, the tooling and other equipment amortization period, as well as the plastic and machine selected. The order of importance that these factors follow can vary, depending on the product to be designed and the designer’s familiarity with particular materials and processes .

Preparing a complete list of design constraints is a crucial first step in plastic part design; failure to take this step can lead to costly errors. For example, a designer might have an expensive injection mold prepared, designed for a specific material’s shrink value, only to discover belatedly that the initial material chosen did not meet some overlooked design constraint. Flammability, glass-fiber fillers to provide a higher modulus, and other requirements are best considered before a tool is made. Otherwise, the designer may have the difficult if not impossible task of finding a plastic that does meet all the design constraints, including the important appropriate shrink value for the existing mold. Such desperation in the last stages of a design project can and should be avoided. As emphasized from one end of this book to the other, it is vital to set up a complete checklist of product requirements,to preclude the possibility that a critical requirement may be overlooked initially. Recognize that the “im-possible,,as well as the “approaches” to be avoided can in most cases still produce excellent products; however, it is easier to follow the direction with the least number of problems.