Compression molding of thermosets

Solution In compression molding of thermoset, once the resin is cured the molded part can be ejected while still hot and allowed to cool outside the mold. Qn the other hand, compression molding of thermoplastics involves heating followed by cooling to solidify the molded part. Therefore, in compression molding of thermoplastics the relatively longer heat-and-chill cycle times involved are imeconomical for large volume production. 

In-mold decoration can be done with either injection molding of thermoplastics or compression molding of thermosets. Thermosetting plastics are decorated with a two-stage process. For melamine products, for example, the mold is loaded with the molding powder in the usual manner and closed. It is opened after a partial cure, and the decorative "foil or overlay is placed in position. 

For molding urea and melamine materials, pressures of approximately one and one-half times that needed for phenolic material are necessary. In compression molding of thermosets the mold remains hot throughout the entire cycle. After the molded part is ejected, a new charge of molding powder is introduced. Unlike thermosets, thermoplastics must be cooled to harden. So before a molded part is ejected, the entire mold must be cooled, and as a result, the process of compression molding is slow with thermoplastics. 

Reactive Processing n A molding or extrusion operation in which chemical reactions are carried out. Extruders, mainly specialized twin-screw machines, have successfully carried out partial and complete polymerizations on a large scale. Transfer and compression molding of thermosets have always been reactive processes, but see Reaction Injection Molding. 

The materials presented in this chapter will be used in the discussions on the processing of thermosets in Chapters 11-13 of Volume 2, which cover reaction injection molding, pultrusion of thermoset/fiber composites and compression molding of thermoset/fiber composites, respectively. 

Compression molding is the most common forming method for thermosets. Thus compression molding was the preferred processing technique in the early part of the 20th century. Its relative importance diminished as thermoplastics were introduced, and other forming methods better suitable for the new materials were developed. Compression molding of thermoplastics has always been limited to special cases. 

Examples 11.9 Explain why compression molding of thermoplastics is limited to small quantity production while screw injection molding is thermoset is also currently used on a limited scale. 

Tolerances of parts made by injection molding of thermosetting materials are comparable to those produced by the compression and transfer methods described, earlier. Tolerances achieved are as low as +0.001 in./in., although ordinarily tolerance of 0.003-0.005 in./in. are economically practical. 

Compression molding Molding of thermoset plastics, reinforcement fiber and matrix resin placed in mold cavity, mold closed, heat and pressure applied, until material has cured or achieved final form. 

Figure 6.11 Several typical parts molded by compression or transfer or injection molding of thermosetting plastics.

The cure kinetics of thermoset resin is dependent on heat transfer from the mold to the SMC. In general, heat conduction along the thickness direction is much more important than conduction along the planar directions in the case of the compression molding of thin parts (Lee and Tucker, 1987). Moreover, the viscous dissipation is also negligible because the strain rate and the viscosity are sufficiently low in compression molding. Thus, the heat transfer equation in thickness (z) direction can be obtained as follows. 

Today compression molding of non-free-fiowing, mat-like thermosets (SMC) has an established presence in the automotive industry for example, in the production of mostly large-scale auto body parts (under body paneling, etc.). 

Plastics - Compression molding of test specimens of thermoplastic materials Plastics - Thermosetting molding materials — Determination of shrinkage... 

Thermosetting composites can be processed by means of compression molding of uncured resin. Usually fiber reinforcement is used to provide additional strength and stiffness. The application of pressure pushes the resin into the fiber reinforcement and heat crosslinks the resin to form a solid material. The critical factors are the flow of the uncured resin into and around the reinforcement and the uniform and complete cure of the resin throughout the part. This technique is used primarily in the aerospace and automobile industries. 

Compression molding is used predominantly for large flat or curved auto parts such as hoods and fenders and with thermosetting resins. Because compression molding of a thermoplastic resin involves heating and then cooling the mold, most such materials are more efficiently molded in the injection machines described in Section 14.6. 

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