Compression molding

Compression molding - simplified diagram of the process

Compression molded rubber boots before the flashes are removed.

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.

Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly oriented fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Also, it is one of the lowest cost molding methods compared with other methods such as transfer molding and injection molding; moreover it wastes relatively little material, giving it an advantage when working with expensive compounds.

However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Fewer knit lines are produced and a smaller amount of fiber-length degradation is noticeable when compared to injection molding. Compression-molding is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion techniques. Materials that are typically manufactured through compression molding include: Polyester fiberglass resin systems (SMC/BMC), Torlon, Vespel, Poly(p-phenylene sulfide) (PPS), and many grades of PEEK.

Compression molding was first developed to manufacture composite parts for metal replacement applications, compression molding is typically used to make larger flat or moderately curved parts. This method of molding is greatly used in manufacturing automotive parts such as hoods, fenders, scoops, spoilers, as well as smaller more intricate parts. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed.

Materials may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage.

Thermoplastic matrices are commonplace in mass production industries. One significant example are automotive applications where the leading technologies are long fibre reinforced thermoplastics (LFT) and glass fiber mat reinforced thermoplastics (GMT).

In compression molding there are six important considerations that an engineer should bear in mind:

• Determining the proper amount of material.
• Determining the minimum amount of energy required to heat the material.
• Determining the minimum time required to heat the material.
• Determining the appropriate heating technique.
• Predicting the required force, to ensure that shot attains the proper shape.
• Designing the mold for rapid cooling after the material has been compressed into the mold.

Process definition

Compression molding is a forming process in which a plastic material is placed directly into a heated metal mold, then is softened by the heat, and forced to conform to the shape of the mold as the mold closes.

Process characteristics

The use of thermoset plastic compounds characterizes this molding process from many of the other molding processes. These thermosets can be in either preform or granule shapes. Unlike some of the other processes we find that the materials are usually preheated and measured before molding. This helps to reduce excess flash. Inserts, usually metallic, can also be molded with the plastic. As a side note, remember not to allow any undercuts on the shape, it will make ejection especially difficult. Thermoplastic matrices with an inherent indefinite shelf-life and shorter cycle moulding times are widely used and examples are shown in Ref 3.

Process schematic

The compression molding starts, with an allotted amount of plastic or gelatin placed over or inserted into a mold. Afterward the material is heated to a pliable state in and by the mold. Shortly there after the hydraulic press compresses the pliable plastic against the mold, resulting in a perfectly molded piece, retaining the shape of the inside surface of the mold. After the hydraulic press releases, an ejector pin in the bottom of the mold quickly ejects the finished piece out of the mold and then the process is finished. Depending on the type of plunger used in the press, there will or won't be excess material on the mold.

Typical tools and geometry produced

Three types of molds used are the flash plunger-type, straight plunger-type, and the "landed" plunger-type molds. The flash type mold must have an accurate charge of plastic and produces a horizontal flash (excess material protruding from the mold). The straight plunger-type mold allows for some inaccuracy in the charge of plastic and produces a vertical flash. The landed plunger type mold must have an accurate charge of plastic, and no flash is produced. Further details are explained in Ref 3.

See also

• Matrix molding

References

Bibliography

• Todd, Robert H., Dell K. Allen, and Leo Alting. Manufacturing Processes Reference Guide. New York: Industrial P, Incorporated, 1993 on page 219-220.