Design limitation and constraint

As reviewed throughout this book, designing acceptable products requires knowledge of the different plastics and their processing limitations such as individual advantages and disadvantages. Although there is no limit theoretically to the shapes that can be created, practical considerations must be met such as available and size of processing equipment and cost. These relate not only to the product design, but also the mold or die design, since they must be considered as one entity in the total creation of a usable, economically feasible product.

One of the earliest steps in product design is to establish the configuration that will form the basis on which strength calculations will be made and a suitable material selected to meet the anticipated requirements. During the sketching and drawing phase of working with shapes and cross-scctions there are certain design features with plastics that have to be kept in mind to obtain the best cost-performances and avoid degradation of the properties. Such features may be called property detractors or constraints. Most of them are responsible for the unwanted internal stresses that can reduce the available stress level for load-bearing products. Other features may be classified as precautionary measures that may influence the favorable performance of a product if they are properly incorporated.

As an example a weld line(s) can exist in a product that could have met design requirements if the weld line(s) did not develop. The designer did not contemplate the potential for weld line (s). However the person designing the mold took a logical approach to simplify its construction and reduce cycle time to mold the product based on the requirements specified for the product. Result was in reducing the cost of the mold and fabrication time. To meet this objective the design of the mold causcd one or more weld lines to develop in the product. With conventional injection molding, molded products can be designed that create unwanted weld line(s). The so-called line forms when two melt flow fronts meet during the filling of an injection mold cavity. This action can also occur during extrusion through a die, etc. Depending on how the weld line forms it could have very litde strength and under the most ideal molding conditions it may obtain up to possibly 85% strength retention. To eliminate any problem the product requirements have to account for loss in properties if weld line(s) occur or specify that no weld lines are to occur.

Weld lines are also called knit lines. During processing, such as by injection molding and extrusion, weld lines can occur. They can form during molding when hot melts meet in a cavity because of flow patterns caused by the cavity configuration or when there are two or more gates. With extrusion dies, such as those with "spiders" that hold a center metal core, as in certain pipe dies, the hot melt that is separated momentarily produces a weld line in the direction of the extrudate and machine direction. The results of these weld lines could be a poor bond at the weld lines, dimensional changes, aesthetic damages, a reduction of mechanical properties, and other such conditions.

The top set has a single gate for each specimen, the center set has double gates that are opposite each other for each specimen, and the bottom set has fan gates on the side of each specimen. The highest mechanical properties come with the top set of specimens, because of its melt orientation being in the most beneficial direction. The bottom set of specimens, with its flow direction being limited insofar as the test method is concerned, results in lower test data performance. With the double-gated specimens (the center set) weld lines develop in the critical testing area that usually results in this set's having the potential lowest performance of any of the specimens in this diagram.

Fabricating techniques can be used to reduce this problem in a product. However, the approach used in designing the product, particularly its mold (relocate gates), is most important to eliminate unwanted orientation or weld lines. This approach is no different from that of designing with other materials like steel, aluminum, or glass.

With moldings that include openings (holes), problems can develop. In the process of filling a cavity the flowing melt is obstructed by the core, splits its stream, and surrounds the core. The split stream then reunites and continues flowing until the cavity is filled. The rejoining of the split streams forms a weld line. It lacks the strength properties that exist in an area without a weld line because the flowing material tends to wipe air, moisture, and/or lubricant into the area where the joining of the stream takes place and introduces foreign substances into the welding surface. Furthermore, since the plastic material has lost some of its heat, the temperature for self-welding is not conducive to the most favorable results.

A surface that is to be subjected to load bearing should be targeted not to contain weld lines. If this is not possible, the allowable working stress should be reduced by at least 15%. Under the ideal molding conditions up to about 85% of available strength in the solidified plastic can be developed. At the other extreme where poor process controls exist the weld line could approach zero strength. In fact the two melt fronts could just meet and not blend so that there is relatively a microscopic space. Other problems occur such as influencing aesthedcs.

Prior to designing a product, the designer should understand such basic factors as those reviewed in this book. Recognize that success with plastics, or any other material for that matter, is directly related to observing design details.

The important factors to consider in designing can be categorized as follows: shape, part thickness, tolerances, ribs, bosses and studs, radii and fillets, drafts or tapers, holes, threads, colors, surface finishes and gloss levels, decoradng operadons, parting lines, shrinkages, assembly techniques, production volumes, mold or die designs, tooling and other equipment amortizadon periods, as well as the plasdc and process selecdons. The order that these factors follow can vary, depending on the product to be designed and the designer's familiarity with particular materials and processes.

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