Meccom Industrial Molded Rubber Products

Compression Molding is a method of molding in which the moulding material, generally preheated, is first placed in an open, heated mould 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. This controlled process can be used to manufacture industrial molded rubber products where consistent shape, material performance, and durability are important.

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, including compression molded rubber parts designed for demanding industrial applications. 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. For applications requiring continuous profiles rather than molded geometries, extruded rubber products provide an alternative manufacturing approach for producing seals, shapes, and other profile-based components.

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 same ability to form detailed components makes the process practical for producing industrial molded rubber products for equipment and manufacturing applications. These capabilities can also support industrial rubber solutions for heavy equipment where components may be exposed to vibration, mechanical stress, dust, and demanding operating conditions. 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. Proper material distribution is especially important when producing compression molded rubber parts that must maintain the intended dimensions and shape.

Thermoplastic matrices are commonplace in mass production industries. One significant example is automotive applications where the leading technologies are long fibre reinforced thermoplastics (LFT) and glass fiber mat reinforced thermoplastics (GMT). In applications where vibration and movement must be controlled between connected components, shock absorbing rubber bushings can help cushion mechanical interaction and reduce transmitted vibration.

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.

Careful control of these considerations helps maintain repeatability when manufacturing industrial molded rubber products and other compression-molded components.

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. Once molding is completed, excess flash is removed in order to get the best finish. For compression molded rubber parts, controlling material quantity, pressure, temperature, and curing conditions is important for achieving the required final geometry and finish.

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. These process characteristics can be important when evaluating manufacturing requirements for industrial molded rubber products and other precision molded components.

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 thereafter the hydraulic press compresses the pliable plastic against the mold, resulting in a 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. This sequence provides the controlled forming conditions required for many compression molded rubber parts used in industrial applications.

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. Selecting the appropriate mold configuration depends on the geometry, material, finish, and production requirements of the industrial molded rubber products or other components being manufactured. Further details are explained in Ref 3.