Polypropylene mold shrinkage

There are several reports on the influence of fillers on shrinkage. " Figure 8.56 shows mold shrinkage vs. concentration of filler. Mold shrinkage can be reduced to half of the value for unfilled resin by the incorporation of mica. Additional reduction of shrinkage is possible if the interaction between filler and the matrix can be increased. This can be achieved by reacting polypropylene with maleic anhydride. ... 

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.]...

It apparently depends on the amount of the fillers (hence, amount of the plastic) and ability of the fillers to interfere with crystallization of the plastic. The less the crystallites in the filled plastic, the less the shrinkage. The less the plastic in the filler composite, the less the shrinkage. At the same filler loading, fillers with nucleation effects lead to lesser mold shrinkage. For example, when polypropylene, showing mold shrinkage of 1.91%, was fllled with some mineral and cellulosic fillers, its mold shrinkage was as follows [2] ... 

Comparing the processing behavior of PP to PE, it is found that polypropylene is more non-Newtonian than PE and that the specific heat of PP is lower than polyethylene. The melt viscosity of PE is less temperature sensitive than PP. Mold shrinkage is generally less than for PE but is dependent on the actual processing conditions. 

See Table 14.41 for formulations that give the best HDT, melt index, and mold shrinkage values. Maximum predicted heat deflection temperature, 143°C, was obtained with 40 wt% mica, 53 wt% polypropylene, and 7 wt% Unite MP-1000. 

Table 14.41 Formulations for Best Predicted HDT, Melt Index, and Minimum Mold Shrinkage Properties HIMod-360 Mica in Polypropylene...

In injection molding, the pol5uner is cooled from the melt temperature to a temperature below the solidification, which is accompanied by a reduction of pressure in a mold before the molded item is ejected from the mold. Since the ejected molded part is still hot, its cooling to room temperature occurs under atmospheric pressure. As a result of temperature and pressure changes, the cooled polymer shrinks according to the PVT (pressure-specific volume-temperature) relationship or the thermal expansion coefficient of the polymer. The linear mold shrinkage is 0.1-0.8% for amorphous polymers and 1.0-2.0% for crystalline polymers. For polypropylene, it is about 1.7%. Shrinkage affects the dimensional accuracy and dimensional stability of molded items. 

Estimate the volume shrinkage of an object made from a 50-50 blend of polyethylene and polypropylene. Molding conditions are 1.1 X 10 N/m and 530 K. The mold temperature is 30 C. 

Apparatus for the measurement of this property according to DIN 53464 [24] is available from ATS FAAR (Table 4.1). Bertacchi and coworkers [26] reported computer-simulated mold shrinkage studies on talc-filled polypropylene, glass-reinforced polyamide, and a polycarbonate-acrylonitrile butadiene styrene blend. 

Typical thermoplastic binders which are found in literature for injection molding of ceramic bodies are, styrene-butadiene, polyethylene, polypropylene, polybutene, ethylene vinyl acetate, polymethylmethacrylate and polyoxymethylene. When selecting one of these binders for thermoplastic extrusion of ceramic bodies, it should be noted that the shrinkage of par-tially-crystalline polymers is higher than for amorphous polymers, and hence warping during cooling is more critical in the former case. This is, however, not the only criterion for selection price and processability at adequate temperatures are also important factors to consider. 

Hyperfom HPN 210 M is a new nucleating agent for injection molded high-density polyethylene apphcations. It improves mechanical performance to the levels approaching those of polypropylene impact copolymers. It increases stiffness by up to 50%, heat distortion temperature by 10-40°C, and productivity. It has ability to create lamellar crystal orientation of PE in the polymer flow direction reducing shrinkage. ... 

---