Part and Mold Design

Once the part is formed, the excess material must be removed from the part. This can be done in a wide variety of ways, but is most successful when the stress-strain characteristics of the plastic at the trimming temperature are considered. Thin gauge parts in a roll-fed process are usually trimmed in the mold. Heavy gauge materials, which are usually sheet fed, are often trimmed in an external process step. Many packaging applications are thin gauge, roll-fed parts and are therefore trimmed in the mold. Cutting of the polymer involves tensile fracture, so the choice of trimming process depends on the thickness of the material, and on temperature, which affects the modulus of the material. 

For most packaging applications, compression cutting or shear cutting are used. LDPE is most often compression cut because its low modulus leads to necking in a shear die, and therefore a ragged edge is formed. For HDPE, PP, PMMA, PS, nylons, and PET, shear cutting can be used very effectively. However, the same die will not work equally well for each material because of the differences in modulus. 

In designing a new thermoformed part, it is necessary to consider the application, the function of the part, and its lifetime. Each of these areas has many subparts that must be considered also, in order to select the material and refine the design 

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MoldShrinka.ge. Mold shrinkage of unreinforced crystalline plastics is high. This is critical in part and mold design, especially for large... 

Table 25 presents the opportunities and risks of a part/and mold design based on a numeric simulation, compared to analytical/empirical dimensioning. 

Experienced part and mold designers need to understand the flow phenomena of various plastics during the molding process in order to minimize the occurrence of unwanted defects in the final product. But even with the perfect design of part, and the perfect design of mold, the processing parameters of temperatures (of fluid plastic and of mold), fill rate, fill pressures, and in-mold dwell are equally critical in achieving quality parts. 

Parting lines. Parting lines on molded parts require special consideration in part and mold design, especially where two molded parts must come together as, for example, on each half of a molded box with a hinged opening. 

When inserts are to be molded into plastic parts, close coordination is obviously needed between the part and mold designer and manufacturer to ensure the desired results. 

In short, compared to other production processes, injection molding demands more of part and mold designers—experimentation after the mold is built is expensive in terms of time and money. Injection-molding simulation is relatively inexpensive in terms of project cost and offers great benefits to those using it early in the manufacturing process. 

The quality of injection parts is determined by several important factors, which results in complex combination of the materials used, the part, and mold designs along with processing conditions such as temperature and pressure. 

Quality control of plastic molded parts can use optical techniques. In this procedure thin slices of the material are cut from the part and microscopically examined under polarized light transmitted through the sample. Study of the microstructure by this technique enables rapid examination of quality-affecting properties. This kind of approach can provide the molder with information for failure analysis, part and mold design, and processing optimization [12]. 

Modeling Methods Applied to Part and Mold Design... 

ADDITIONAL CAD/CAM FEATURES USED IN PLASTIC PART AND MOLD DESIGN... 

Nelson C. Baldwin, How to Avoid Warpage Problems in Injection Molded Parts. Part 1 — Part and Mold Design , Plastics Design and Processing, 14, No. 1 (January 1974). 

Mold filling is considered with various factors of part and mold designs. Material selection and process setup have to be considered to ensure that the mold can be filled volumetrically. 

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