Molded part shrinkage

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. 

Table II giveB a comparison of actual XYDAR molded part shrinkages vs shrinkages predicted upon cooling from melt and mold temperatures for 45% glass fiber (G-445) and 50% mineral/glass fiber (MG-350) compositions. These predicted values were estimated using equations (5) S (6) in the theory Bection.

Fischer, J. M., Handbook of Molded Part Shrinkage and Warpage, William Andrew Publishing, Norwich, 2003. 

Fischer JM (2003) Handbook of molded part shrinkage and warpage. Plastics Design Library, William Andrew, Norwich, NY... 

Fischer, Handbook of Molded Part Shrinkage and Warpage,... 

Process details may be summarized powder sizes are fine (usually < 20 Ilm) low (generally < 69 MPa (10,000 psi)) injection pressure low (ca I49°C) mol ding temperature shrinkage (molded part to finished size) typically 20% final part densities are usually 95—98% + of maximum pore-free density and ductility is exceptionally high, elongation values are > 30%. 

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)... 

Bushko, W. C., et al., Estimates for Material Shrinkage in Molded Parts Caused by Time Varying Cavity Pressure, SPE-ANTEC1997. 

PVDF resins can be molded by compression, transfer, and injection molding in conventional molding equipment. The mold shrinkage can be as high as 3% due to the semicrystalline nature of PVDF. Molded parts often require annealing at temperatures between 135 and 150°C (275 and 302°F) to increase dimensional stability and release internal stresses.24... 

This analysis reveals that measurement of shrinkage or linear coefficients of thermal expansion (CTE s) in just flow and width directions, as is done for conventional polymers, is not sufficient for liquid crystal polymers (LCP s) and can lead to erroneous shrinkage predictions. This is a consequence of inherent LCP anisotropy, resulting in a relatively large linear CTE and shrinkage in the thickness direction of associated molded LCP parts. Linear and volumetric CTE data for neat and filled LCP molded parts of different geometries are presented. 

Two extreme cases can be predicted for the shrinkage of an LCP molded part based upon 3-dimensional CTE data. In the first case the part is assumed to shrink from the melt temperature (Ts) to room temperature (Tf). Here it is postulated that no real part packing occurs during molding. For the second case, the part is assumed to cool from the mold temperature (Tm) to room temperature. For this case, maximum packing would occur. For cooldown from melt temperature, the equations are ... 

COMPARISON OF ACTUAL AND PREDICTED SHRINKAGES FOR XYDAR MOLDED PARTS... 

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