Tolerance and Dimensional Control

Design drawings Detail or fabrication drawings Materials requirements including composition, quality standards, and minimum structural properties Fabrication requirements and standards, including dimensional tolerances, allowable defects, and minimum structural properties Requirements for prototype and quality control tests and procedures Shipping and handling... 

To meet fabricated dimensional tolerances different approaches are used. They include use of specific fillers and reinforcements and process control (Chapter 3). Popular filler used is short glass fibers (Chapter 15). Over 50wt% of all types of glass fibers used with different plastics and by different processes are used in injection molding compounds. Table 2.2 shows the shrinkage of different unreinforced plastics ad glass fiber reinforced plastics based on ASTM testing procedures. 

We assume that conditions can be controlled to minimize additional relaxation effects such as magnetic dipole-dipole interactions. As the number of relaxation mechanisms decreases, the information necessary for a line shape analysis of the spectra also decreases. Thus, a poorer signal-to-noise ratio can be tolerated, and the signal can be smoothed by curve fitting techniques. Since little is known at the molecular level about two-dimensional transport coefficients, such as the surface viscosity, large uncertainties can be tolerated. In this sense, we believe that much can be learned from monolayer experiments using spin label surfactants. 

Semidovetail joint. The sernidovetail joint is probably the most common joint used for position and contour control in plastic parts. The semidovetail joint is used in place of a full dovetail joint because the latter is not required since location in the inside direction is controlled by the joint on the other side of the part. More importantly, the semidovetail joint requires two-thirds the wall thickness of a fiill dovetail joint. A semidovetail joint around the entire perimeter of the part provides location, tends to mask minor warpage and debris from joining devices (surplus adhesive, solvent, welding flash, etc.), and is reasonably tolerant of dimensional variations. In addition, if designed steel-safe, it can be readily adjusted if the molded parts turn out to have too... 

Flatness control. The very nature of plastics processing makes absolute flatness a virtual impossibility. Therefore, flatness specifications must take this factor into account. However, most plastics are flexible to some degree, thus they will conform to the shape of the mating part, making absolute flatness unnecessary. Flatness recommendations for each material are provided in the tolerance charts discussed in Sec. 8.2.8 under Dimensional Control. Nonetheless, plastic parts are process sensitive and there must be limits or the parts can go out of control and proper fitments can be jeopardized. The following flatness specification is recommended Part must be flat within. XXX in. (or within. XXX in/in). 

It should be noted that with really precise tooling accuracy is much more prevalent, especially with matched male and female molds and careful control of temperature, time, and pressure. The dimensional tolerances with the more conventional single-mold system are generally 0.6 percent ( 0.35 percent for close tolerances) with female molds, 0.5 percent ( 0.3 percent close) with male molds under 3 ft., 0.8 percent ( 0.4 percent close) with male molds over 3 ft., and 30 percent ( 10 percent close) for wall thicknesses. 

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