The major advantages of compression molding are that it is fairly simple, cycle times can be relatively fast, repeatability is excellent, high volume production is easily obtained, surface finish of the parts is very good, and parts with tight tolerances can be produced. In addition mixing the resin and fibers before the actual process allows for good control over the chemistry and the mix of the final product.
The major disadvantages are that a large initial investment in molds and presses is necessary, the material must be stored under certain environmental conditions and must be used within a certain time, several flow related problems can arise during the process so mold design is difficult and tedious.
Several minor defects can arise in the part such as residual stresses, delamination, warpage, and flow orientation of fibers; all this gives the process a large number of variables.
Sheet Molding Compound (SMC) is a specific form of compression molding. It is currently the most developed and used processing technique to obtain structural composites, particularly in the automobile industry. Almost 1 million FORD Explorers and Rangers will have SMC beams in them by the end of 1995, and it is likely that this number will increase to 1.75 million by the end of 1996. It is possible to use both thermoplastics and thermosets in SMC but the majority of SMC is done using thermosets.
To form the sheets of SMC material a specific procedure is used. The resin is placed on a sheet of nonporous material, such as nylon, as a thin layer. As the nylon moves along, as in a production line, fibers are added to it, these can be in random form, unidirectional, or other orientations. Next a layer of resin, placed on a cover sheet, is applied onto the fibers so that the resin is in contact with the fibers.
This sheet, enclosed between the nylon sheet and the cover sheet, is then passed through several compaction rollers. These serve two main purposes: they mix the resin and fibers together, and they compact the sheet.
The resin is now in a continuosly changing state (i.e. it is slowly curing), it is left to thicken for approximately 5 days, after which the SMC is ready. At this point the SMC sheet must be used within a certain amount of time, which can be up to several weeks, and must be stored under certain emvironmental conditions, such as low humidity.
Several types of SMC are currently used in industry: SMC-R (reinforced with fibers oriented randomly), SMC-C (reinforced with unidirectional continuous fibers), SMC-C/R (reinforced with both randomly oriented and continuous unidirectional fibers), SMC-D (reinforced with directional but discontinuous fibers).
The actual manufacturing process to form the part begins with a large roll of SMC, the material is then cut to the desired size and shape and the two thin nylon sheets on the outside are removed. The material is then placed inside a mold, and the regular compression molding procedure previously discussed is followed.
Temperatures for this procedure are between 130 and 160 degrees C and pressures can reach 21 MPa. The mold is usually made of steel and it is hardened in key areas where the mold can wear out more easily. This is important because the mold is subject to high pressures and temperatures and also undergoes many cycles continuously. For this reasons mold design is very important and the overall cost of the molds is usually high. Cycle times are between 1 and 4 minutes. Several different resin systems can be used in SMC, vinyl ester and polyester are the most used in the automotive industry, epoxy resins are widely used in the aerospace industry.
There are several variations and modifications that different industries have developed over time, in order to improve the process and to fit it to their own needs. The automotive industry, for example, has a specific need for parts with excellent surface finish. To get this a technique known as in-mold coating was developed. In in-mold coating, after the part is partially cured inside the mold, the mold is opened slightly and a resin, such as a urethane, is injected in the mold. Subsequently the mold is closed again, causing the resin to coat the outside of the part filling any voids on it. This greatly improves the surface finish on the part and can save several stages in the painting process.