Polymer mold shrinkage

The most serious imperfection that must be accounted for is mold shrinkage, which is the difference in the part dimensions and the dimensions of the mold. The amount of shrinkage is dependent upon several factors, including the temperature of the polymer, the type of polymer, the flow field of the polymer in the mold, injection pressure and hold time, and the presence of additives. Characteristic shrinkages for several common... 

In conventional amorphous polymers, molded part shrinkage can usually be predicted using a single measured coefficient of thermal expansion since the CTE values for the flow, transverse, and thickness directions are very similar. Knowledge of CTE or actual shrinkage behavior in the flow and width directions for a... 

This paper attempts to treat LCP molded part shrinkage first from a theoretical viewpoint related to directional and volumetric CTE s, and then from a more empirical perspective, where both actual shrinkage and CTE data are examined. Differences between LCP and conventional amorphous polymer molded part shrinkages are highlighted. A method is suggested for predicting potential shrinkage in a molded LCP article. 

PVDF resins are readily molded in conventional compression, transfer and injection molding equipment. Typical molding temperatures in the cylinder and nozzle are 180-240°C for injection types and 50-90°C for molds. As a crystalline polymer, PVDF typically shows relatively high mold shrinkage of about 2-3%, reflecting the high degree of crystallization and the difference between the solid and the molten densities. 

Figure 8.56. Mold shrinkage of mica/polypropylene composites vs. concentration of filler. [Data from Chiang W Y, Yang W D, Pukanszky B, Polym. Engng. Sci.., 34, No,6, 1994,485-92.]...

Figure 8.57. Mold shrinkage vs. holding time. [Data from Mamat A, Trochu F, Sanschagrin B, Polym. Engng. .,35,No.l9, 1995, 1511-20.]...

Titanium hydroxide, 129 Wollastonite, 129 Wood flour, 129 Mold and spores, 412,413 Mold growth, 27, 412-416 Mold propagation, 26,412 16 Mold shrinkage, 105, 131 Nucleation, effect of, 131 Plastic crystalllinity, effect of, 131 Moldable thermoset acrylic polymer, 80 Molecular weight distribution (MWD), 63, 636, 649-651... 

Polymer Blends Forming Table 10.31. Mold shrinkage of PA-6 or PA-66/elastomer blends 719... 

The primary motivations for blending the thermoplastic polyesters with other polymers are (a) to improve the solvent resistance and process-ability of amorphous polymers such as PC, styren-ics, PPE, etc., (b) to reduce the mold shrinkage of polyesters associated with their crystallization,... 

PPO has low molding shrinkage. The polymer is used for the injection molding of such items as pump components, domestic appliance and business machines, and electrical parts such as coimectors and terminal blocks. 

SAN copolymers are linear, amorphous materials with improved heat resistance over pure polystyrene. The polymer is transparent but may have a yellow color as the acrylonitrile content increases. The addition of a polar monomer, acrylonitrile, to tbe backbone gives these polymers better resistance to oils, greases, and hydrocarbons when compared to polystyrene. Glass-reinforced grades of SAN are available for applications requiring higher modulus combined with lower mold shrinkage and lower coefficient of thermal expansion. ... 

Polymer Talc (wt%) Mold shrinkage (in,/in,) Change due to talc (%)... 

PMMA and its derivatives have been used most often in the bone cements used for fixation in orthopedic surgeries. PMMA is an amorphous thermoplastic polymer. It gives excellent optical clarity, weather resistance, surface hardness, good chemical resistance, rigidity, dimensional stability, low mold shrinkage, good impact strength, nontoxicity, and tastelessness. PMMA powders can be injected and... 

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