Elastic recovery molding

ERM elastic recovery molding EVE ethylene vinyl ether... 

PVC has a unique ability to be compounded with a wide variety of additives, making it possible to produce materials that range from flexible elastomers to rigid compounds, that are virtually unbreakable with a notched Izod impact greater than 0.5 J/mm at —40 C, that are weatherable with good property retention for over 30 years, as well as compounds that have stiff melts and litde elastic recovery for outstanding dimensional control in profile extrusion, and also low viscosity melts for thin-walled injection molding. 

The properties of block copolymers differ from those of a blend of the correponding homopolymers or a random copolymer (Chapter 7) with the same overall composition. An important practical example is the ABA-type styrene/butadiene/styrene triblock copolymer. These behave as thermoplastic elastomers. Ordinary elastomers are cross-linked by covalent bonds, e.g., vulcanization (see Chapter 2) to impart elastic recovery property, as without this there will be permanent deformation. Such cross-linked rubbers are therraosets and so cannot be softened and reshaped by molding. However, solid thermoplastic styrene/butadiene/styrene triblock elastomers can be resoftened and remolded. This can be explained as follows. At room temperature, the triblock elastomers consist of glassy, rigid, polystyrene domains... 

Step 2 may be to examine the flow conditions and any shifts in molding. As mentioned earlier and shown in Figure 16C.3, shear rate has a major impact on the elastic recovery of the material. This elastic recovery can be pronounced after demolding where the part acts like the stretched chewing gum and retracts in the direction of flow. In addition, from Figure 16C.5, the effects of increasing shear rate on heat build-up in the stock are pronounced. Essentially, decreasing injection... 

Semiflexible molded polyurethane foams are used in other automotive appHcations, such as instmment panels, dashboards, arm rests, head rests, door liners, and vibrational control devices. An important property of semiflexible foam is low resiHency and low elasticity, which results in a slow rate of recovery after deflection. The isocyanate used in the manufacture of semiflexible foams is PMDI, sometimes used in combination with TDI or TDI prepolymers. Both polyester as well as polyether polyols are used in the production of these water-blown foams. Sometimes integral skin molded foams are produced. 

The preform expands due to relaxation and recovery. The pressure placed on the molded resin exerts three types of changes in the particles of resin. Resin particles undergo plastic deformation and are intermeshed together leading to the development of cohesive or green strength. Particles also deform elastically and experience cold flow underpressure. The air trapped in the space between the resin particles is compressed. Removal of pressure allows the recovery of elastic deformation, which creates a quick snap back of the preform. Overtime, stress relaxation partly reverses the cold flow, and the preform expands. 

Essentially a hinge is composed of three layers caused by a high velocity of the pol)mner melt two highly oriented layers and one isotropic core layer. The mechanical behavior of the oriented layers is similar to that of hard elastic fibers which show a high stiffness and a high strain recovery. The higher the molecular orientation and the layer thickness, the better the hinge properties are. The layer thickness can be increased by low temperatures (melt and/or mold). 

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