Shrinkage venting

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)... 

High shrinkage Level of material lower than design in molds and vents Incorrect temperatures of mold and polyurethane High exotherm Wrong ratios System contaminated... 

Molds with open-topped risers and vents, like those used for casting of metals, may be used for certain thick parts. Makeup for polymerization shrinkage comes from the volume of material in the vents and risers which are eventually machined off. 

Fill pattern Weld line position. Air trap position. Position of vents. Overpacking excessive costly part weight. Overpacking warpage due to differential shrinkage. Underflow structural weaknesses. 

Solvent cements should be chosen with approximately the same solubility parameter as the plastic to be bonded. Table 8.8 lists typical solvents used to bond major plastics. Solvents used for bonding can be a single pure solvent, a combination of solvents, or a sol-vent(s) mixed with resin. It is common to use a mixture of a fast-drying solvent with a less volatile solvent to prevent crazing. The solvent cement can be bodied up to 25 percent by weight with the parent plastic to increase viscosity. These bodied solvent cements can fill gaps and provide less shrinkage and internal stress than if only pure solvent is used. 

LCPs offer high strength and rigidity, reduced mold shrinkage, dimensional stability, excellent sol-vent/chemical resistance, wear resistance, high heat resistance, and inherent flame retardancy. Polyester... 

Since shrinkage and cracking are produced by capillary forces, Kistler [94] reasoned that those problems could be avoided by removing the liquid from the pores above the critical temperature (T ) and critical pressure (P ) of the liquid. As indicated in the phase diagram in Fig. 32, there is no longer any distinction between the liquid and vapor phases the densities become equal, there is no liquid-vapor interface and no capillary pressure. In the process of supercritical (or hypercritical) drying, a sol or wet gel is placed into an autoclave and heated along a path such as the one indicated in Fig. 32. The pressure and temperature are increased in such a way that the phase boundary is not crossed once the critical point is passed, the solvent is vented at a... 

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