Polyethylene mold shrinkage

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. 

As symmetrical molecules approach within a critical distance, crystals begin to form in the areas where they are the most densely packed. A crystallized area is stiffer and stronger, a noncrystallized (amorphous) area tougher and more flexible. With increased crystallinity, other effects occur. As an example, with polyethylene there is increased resistance to creep, heat, and stress cracking as well as increased mold shrinkage. 

Mold shrinkage is dependent on many factors, such as resin density, stock temperature, mold temperature, part thickness, and blowing air pressure. Typical polyethylene blow molding shrinkage is as follows ... 

The addition of inorganic compounds to polyethylene is carried out for a variety of reasons. Fillers are sometimes only low-cost particulate materials used to extend the polyethylene and, therefore, lower the cost of the fabricated item. However, fillers usually affect some other aspect of the finished product such as the stiffness (modulus) or the processability of the polyethylene to increase rates of fabrication. Fillers may also reduce mold shrinkage and the thermal expansion coefficient Filler such as mica may increase heat resistance. Some examples of these compounds are discussed below. 

The degree of crystallinity of a polethylene sample reflects the quantitative distribution of chain segments between ordered and disordered regions. Many of the physical characteristics of polyethylene samples can be related to it in one way or another such attributes include elastic modulus, yield stress, barrier properties, and mold shrinkage. 

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. 

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. 

Indentations in the surface are not undercuts if they are perpendicular to the parting line because they are in the direction of the draw. Therefore, it is desirable to set the parting line such that indentations are in that plane. In Fig. 8.36, the comer Z and the indentation at 17 are imdercuts because they are in the direction parallel to the parting line. The imdercut at U could be stripped from the mold however, the one formed by the comer Z would require a removable section in the tool. Parts which are rotationally molded are often quite large and the materials used (polyethylenes) have high shrinkage rates. It is often possible to strip the undercut if the part will shrink more than the... 

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. ... 

Combination of the solid bicyclo[2.2.1]heptane dicarboxylate sodium salt and calcium benzoate reduced shrinkage of molded polyethylene parts and increased the crystallization temperature (cycle time saving). ... 

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