Molded Part Design

Computer-aided engineering applications have been developed to optimize the molded part design with respect to manufacturing and end use performance. Many, if not most, commercial molding applications utilize mold filling analyses to analyze the melt flow and pressure distribution in the injection mold before the mold is manufactured. Typically, these analyses... 

With wall thicknesses in the range of 0.040 to 0.060 in, it is clearly impossible to adhere to all the ftindamental rules of proper injection-molded part design (rib thicknesses will be greater than 70% of the nominal wall). This means that surface finish problems will result and texture is required to mask these flaws. In any case, wall thickness variations should not exceed 25%, inside radii should not be permitted to go below 0.020 in, and design details that restrict flow should be avoided. Furthermore, the thinner parts are less rigid at the time of ejection from the mold. Thus, much larger ejector pins must be used than would be used for normal injection molding and there would be more of them. 

Alvord, Lincoln J., Engineered Blow Molding Part Design, GE Plastics, Pittsfield, Mass., 1989. 

Alvord, L. J. Engineering Blow Molded Part Design. Paper presented at GE Plastics SPI Structural Plastics Conference, 1989. 

Polypropylene Mold Part Design, supplier technical report. Huntsman Chemical. 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

These possible sources of problems in a molded part should be marked on the product drawing and explained to the mold designer for corrective action or creating an awareness of possible product defects. This is a necessary step in the chain of events in which the aim is to produce a tool that will provide useful products. Even if the mold s design, workmanship, and operation are carried out to the highest degree of quality, they cannot overcome a built-in weakness due to the product design. 

Part Design Blow Molding Casting Compression Extrusion Filament Winding Injection Matched Die Molding Rotational Thermo- forming Transfer Compression Wet lay-up (Contact Molding)... 

Brookfield, CT 06804-0403 Series of SPE sponsored seminars and workshops are held in various locations throughout the year. Subjects range from Die Design Principles to Plastic Part Design for Economical Injection Molding. A full range of seminars are also conducted in conjunction with SPE s annual technical conference (ANTEC). http //www.4spe.org... 

Design Considerations for Injection Molded Parts (Parting lines, draft angles, wall thickness, fillets and radii, bosses, ribs, opening formations, shrinkage, gating, vents, potential knit lines)... 

Malloy, R. A., Plastic Part Design for Injection Molding, Hanser, 1994. 

To meet the products functional needs the wall thickness of the part can be varied. However, to obtain optimum processing cycles and reduce the chance of sink- and flow-mark formation, wall thickness should be kept as uniform as possible. Where variation in wall thickness is unavoidable, gradual transitions in the wall thickness should be provided to ease the polymer melt flow transitions. Part design, mold design, and gate location should work together to allow the material from heavier section into thinner walls for best appearance and process latitude. 

The simplest molds consist of two parts one fixed and one moveable. We use such molds to fabricate items, such as disposable stadium cups, compact disks, and coat hangers, which have simple designs with no undercuts. As the complexity of the product increases, so must the complexity of the mold. The molds used to produce large items, such as dishwasher tubs and automobile bumpers, may consist of multiple moveable components and weigh several tons. Even apparently simple items, such as screw tops for bottles, require a complex ejection system that unscrews the product from part of the mold. In order to facilitate cooling, virtually all molds are designed with liquid cooling channels. 

Contents Introduction to Materials. Manufacturing Considerations for Injection Molded Parts. The Design Process and Material Selection. Structural Design Considerations. Prototyping and Experimental Stress Analysis. Assembly of Injection Molded Plastic Parts. Conversion Constants. 

A problem had arisen with molded parts from a medical products company. Some injection-molded sheets were exhibiting small translucent areas (gels) within the opaque polymeric material, which had been designated as non-melt areas. One theory for the source of the non-melt was that a contaminant was present, while another postulate was that these areas represented a cold slug coming from the sprue. One sheet was chosen for analysis since the area of non-melt within it was largest. Both the non-melt area as well as a "normal" area were tested. 

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