Molding compound processing

TS reinforced molding compounds processed in IM require water- or oil-cooled barrels, rather than the electrically heated barrels used for TP. The heat of the TS during screw plastication has to be kept lower than its curing heat. The mold heat is higher than that of the resin, causing the TS finally to react and solidify. If there is excess heat in the barrel, the resin crosslinks and the machine stops operating. A cleanup is required, which can cause machine downtime to be extensive. With a liquid heat control, better control of melt heat is maintained. Also the compression ratio of the screw used to process the TS is one, which helps to keep the heat low and, more important, under control. 

Figure M.5 Sheet molding compound process. (Ref Harper, C.A., Thermosels, Reinforced Plastics, and Composites , Modem Plastics Handbook, C.A. Harper, ed., McGraw-Hill, New York, 2000)...

These characteristics have to be considered for processing. The processing method itself, e.g., temperature control, thus also controls properties and quality of the cured molding compound. Processing is essentially characterized by gelling and total resin curing. 

Figure 5. Sheet molding compound processing machine [from Ref. 30].

Lubricants. Process aids or lubricants promote smooth and rapid extmsion and calendering, prevent sticking to extmders or calender roUs, and impart good release properties to molding compounds. In some cases use of lubricants allow slightly lower processing temperatures (see Vinyl polymers). 

To make a decorated plate, the mold is opened shortly after the main charge of molding compound has been pressed iato shape, the decorative foil is laid ia the mold on top of the partially cured plate, ptinted side down, and the mold closed again to complete the curing process. The melamiae-treated foil is thus fused to the molded plate and, as with the decorative lamiaate, the overlay becomes transparent so that the ptinted design shows through yet is protected by the film of cured resia. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

Understand the critical properties of the mixed compound that affect processability. It should be understood that the properties and acceptable operating values of those properties, which control the compound processability, are dependent on the processes under consideration, e.g., hot or cold feed extrusion, compression or injection molding. 

Excess flash—Indicates the compound flow was excessive during the mold closing process (shelf loaded compression molding) [3]. 

TS plastic products that are injection, transfer, or compression molded combine thick and thin sections relatively easily since the hardening process is a chemical reaction (Chapter 6). Annular shapes are best made by compression to gain best dimensional control and freedom from distortion. In the compression process, the molding compound is compressed and reduced to the plastic state in the mold. During this process, portions of the material may lie in hard forms in the mold while other portions are flowing rapidly with great force. 

The usual process for processing TP-short glass fiber RPs is IM consuming about 55wt % of all RPs. Specially designed IMMs process TS-RPs with materials such as BMCs (bulk molding compounds). The other processes primarily use TS plastic matrices. 

Frequently, small amounts of additives have to be admixed as processing aids or to improve the properties of the polymers, before molding compounds or granulated compounds can be made out of them. But also polymers accruing as dispersions have to be converted into a ready-to-use form by additives before they can be applied. 

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