Different Applications for Compression Molding
Compression molding is utilized to form a wide range of different types of parts from a variety of raw materials. Chem-Trend has developed useful experience in helping molders and processors of both rubber and thermoplastics utilizing compression molding in their processes. Our experience has been helpful in reducing scrap, increasing efficiency and improving molded component quality when compression molding is utilized.
Improving Compression Molding of Rubber
Compression molding is the simplest form of rubber molding and involves squashing a chunk of uncured rubber between two mold halves. Heat from the mold, pressure from the press and time ensure that the rubber cross-links or “cures” result in the molded rubber retaining the shape of the mold cavity†. Watch a video of this process.
Although the compression molding process is the simplest form of rubber molding; there are still considerable challenges for the release agent in the process. Compounds to be molded are usually at ambient temperature and can be quite stiff. As well, molding “blanks” (common name for the chunk of rubber that forms the part) are only cut roughly to the shape of the mold cavity. Therefore, when the mold closes, the mold release must encourage the rubber to flow around the mold cavity fairly quickly to ensure that there are no voids in the molded surface. This is particularly important when molding high durometer rubber compounds.
A mold release with high lubricity† can assist in the process of rubber flow throughout the mold. However, the mold release should not be disturbed or scoured away from the mold surface during the rubber flowing process. If it is, defects such as knit-lines can appear in the rubber section. Very often a balance needs to be struck between the level of lubricity that the mold release provides and its tenacity to remain attached and in place on the mold surface. Products that form a very hard, low-transferring mold release film are highly desirable if the rubber is to be bonded to a metal part during molding. However, good rubber flow may be impeded.
First and foremost, the mold release must not migrate to the bond-line. Very often, mold release lubricity (for encouraging good rubber flow) needs to be sacrificed to ensure that good technical quality parts are made. On the other hand, very soft rubber stocks must not be encouraged to flow around the mold cavity too quickly. Insufficient pressure build-up within the rubber section can lead to voids or air-trapping. A less lubricious mold release, one that is more tenaciously attached to the mold surface can assist here.
Molding blanks usually have some anti-tack on their surface. The mold release agent must not allow the anti-tack to contaminate the mold surface, causing build-up. Compression molds often “leak” meaning that thin skins of rubber compound or “flash” can be formed around the periphery of mold cavities. The mold release needs to be highly efficient to prevent these very stubborn rubber deposits from sticking to the mold overflow† areas between moldings. Many different elastomers are compounded, depending upon the application and the mold release needs to be able to handle these different compound chemistries. In cases where customers process multiple elastomers more than one type of mold release may be required.
Over its years working next to compression molders on their shop floors Chem-Trend has been able to develop a wide range of Mono-Coat® branded semi-permanent and Chem-Trend® PM type conventional products. These products address the challenges that compression molded rubber compounds present to the molder. To learn more about our experience in this area, and how that experience can help you, read our main Rubber Molding section or contact us directly to speak with one of our rubber industry experts.
Thermoplastic Compression Molding
Compression molding of thermoplastics 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. All of these provide different molding challenges.
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. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Materials that are typically utilized in compression molding include: Polyester fiberglass resin systems (SMC/BMC), Torlon, Vespel, Poly (p-phenylene sulfide) (PPS), and many grades of PEEK. 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 e.g. automotive applications where the leading technologies are Long Fiber reinforced Thermoplastics (LFT) and Glass fiber Mat reinforced Thermoplastics (GMT).
Release agents used in compression molding often need to be silicone† free or must be NSF® registered, depending on the application. Often, the polymer used in the process is a high temperature polymer, therefore the release agent utilized must be able to both survive in, and pull out, the high temperature. Chem-Trend’s Lusin® brand of products for the thermoplastic industry have been specifically developed for challenging applications such as those found in compression molding. To learn more about how our products can help you improve your efficiency, reduce your waste and improve finished part quality read our Thermoplastic Processing section or contact us to be connected with one of our thermoplastic industry experts.