Molded part shrinkage from

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.

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 ... 

Table II also indicates that the actual part shrinkages fall between the limits predicted by the linear CTE s for the melt temperature and mold temperature cooling to room temperature cases. In the flow direction, shrinkage appears to be best modeled as part cooling from the mold temperature. For the transverse and thickness directions, cooling from the melt temperature may more closely approximate the shrinkage. This is an important result because a part of more complex geometry may be more isotropic in nature and could, as a consequence of conservation of volumetric CTE, have considerably larger shrinkage than that predicted from flow and transverse direction shrinkages alone.

It is obvious from this discussion that procedures for reducing part shrinkage work in direct opposition to those required for preventing its warpage. Therefore, the injection molding of a trouble-free part requires a careful balancing of these operating conditions. 

In the molded parts, the maximum temperature attained during injection is in a region near the center and farthest from the gate. The maximum shrinkage occurs at the surface located close to the gate. ... 

Refers to low profile RJ surfaces. These are special TS polyester compounds for RPs that are combinations of thermoset and thermoplastic materials. Although the term low profile and low-shrink are sometimes used interchangeably, there is a difference. Low-shrink contain up to 30wt% TP, while low-profile contain from 30-50wt% TP. Low-shrink offers minimum surface waviness on molded parts, as low as 25 pm (1 mil) per in mold shrinkage whereas low-profile offers no surface waviness of 0-12.7 pm (0-0.5 in) per in mold shrinkage. 

In Fig. 4.18B, there is a radius R, on the core this radius should be large enough to permit easy flow of the plastic. However, the result of such a design is a thickening at the comer which can result in more shrinkage at this point, creatiag sink marks as indicated by heavy lines. Also, unless the product really requires a comer shape such as that shown in Fig. 4.18B, the material there is wasted. In addition, shmp inside comers in any piece made from steel (or any other material, including plastics) are bad for the life of the product (dr mold part) due tin the notch effect created by such an inside comer. 

The dimensional deviations that occur in plastic parts are usually not the result of incorrect mold design or shrinkage accommodation, but rather result from bending and deformation of the injection-molded parts. 

Stress relieving of molded parts by external means, hot air or oil, humidity chambers, or submersion in a fluid. The shrinkage occurring after a part has been removed from the mold is influenced by the material properties of the resin and its molding conditions. 

Flow molding n. (1) A variation of injection molding used for thick-walled parts. A large gate is used and pressure is maintained on the injected melt so as to fore a little more melt into the mold as each part solidifies from the outside inward, thus minimizing shrinkage and improving... 

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