Injection molding shrinkage

Molding and injection Compression ratio Compression molding pressure (lbf/in. ) Compression molding temperature (°C) Injection molding pressure (Ibf/in. ) Injection molding temperature (°C) Molding qualities Mold (linear) shrinkage (in./in.) Specific volume (Ib )... 

Shrinkage can influence product performances such as mechanical properties. Anisotropy directional property can be used when referring to the way a material shrinks during processing, such as in injection molding (Fig. 2-62) and extrusion. Shrinkage is an important consideration when fabricating... 

Fig. 2-62 Example of directional shrinkage in an injection molded product.

As an example the shrinkage and in turn its tolerance of injection molded TPs will be affected as follows. (1) Higher cavity pressures will cause lower shrinkages. (2) Thick sections will shrink more than thin ones. (3) A cooler at the time of the product being ejected from the mold cavity will bring about a lower shrinkage. (4) A melt temperature of the material at the lower end of the recommended... 

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)... 

SD HD PE colorations containing 1 % TiOz are heat stable up to 290°C. The shrinkage of injection-molded articles is noticeably affected in the direction of flow if the processing temperature is low (220°C). This influence, however, is already very slight at 260°C. 

P.Br.23 shows excellent heat stability in polyolefins. 1/3 SD samples containing 1 % TiOz, as well as transparent colorations at 1/3 SD in HDPE are stable to exposure to 300°C for 5 minutes. In injection molding, P.Br.23 considerably affects the shrinkage of the plastic at 220°C, an effect which diminishes with increasing temperature (Sec. 1.8.3.2). 

More than 0.3% pigment is required to formulate 1/3 SD HDPE samples. P.Gr.36 is thus much weaker than P.Gr.7 types, although it withstands more than 300°C and is thus equally heat stable. Considerable influence on the shrinkage of injection-molded polyethylene parts necessitates some caution in using P.Gr.36 for this purpose. Many types are difficult to disperse in plastics, especially in polyolefins. 

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