Compression Molding Basics
Compression moldingis a manufacturing process whereby a heated polymer resin is placed between two dies and cured under pressure while maintaining the proper temperature (at least 300°F [150°C]). It is the most common method by which thermosetting plastics are molded.
In compression molding, matching metal dies are mounted into a hydraulic molding press and heated. The material charge is then placed into the mold, the two halves are closed, and up to 2,000 psi of pressure is applied (depending on the material type and part thickness).
It is a high-volume, high-pressure method for molding complex, high-strength fiber-reinforced products. The advantage of compression molding is its ability to mold large and intricate components, such as those for the aerospace and automotive industries.
And it’s also more economical than injection molding.
Types of Compression Molds
There are basically three types of compression molds.
- The open flash mold has a simple design and construction. A slight excess of molding powder is loaded into the mold cavity. On closing the top and bottom platens, the excess material is forced out and flash is formed, which must then be removed. One disadvantage to this type of mold is that the flash prevents full closure of the mold. As a result, it generates minimal back pressure (needed to control part density and dimensions).
- The fully positive mold is used for high-bulk materials and parts that require maximum density. With these molds, no allowance is made for placing excess powder in the cavity. If excess powder is loaded, the mold will not close. This would result in an insufficient charge and, ultimately, reduced thickness of the molded article. Fully positive molds typically have one cavity, and they must use an accurately weighed charge of material. One advantage is that they produce minimal flash.
- The semi-positive mold combines certain features of the open flash and fully positive molds, and makes allowance for excess powder and flash. Semi-positive molds are more expensive to manufacture and maintain than the other types, but they are much better from an applications point of view. They are typically used to produce close-tolerance parts of maximum density, while controlling flash generation.
How Are Compression Molds Made?
Compression molds are produced from special steel; they’re created by using either standard/CNC machining or Electrical Discharge Machining (EDM) processes.
While standard machining requires the manual use of lathes, milling machines and drill presses, CNC machining automates all of those processes. With CNC, computers are used to control the movement and operation of the mills, lathes, and other cutting machines.
In modern CNC systems, the compression mold design and manufacturing processes are both highly automated. The mold’s mechanical dimensions are defined using computer-aided design (CAD) software, and then translated into manufacturing instructions by computer-aided manufacturing (CAM) software. “Post processor” software then transforms these instructions into the specific commands necessary for each machine used in creating the mold. The resulting commands are then loaded into the CNC machine.
Electrical Discharge Machining
Electrical Discharge Machining (EDM) has recently become widely used in compression mold making. Also known as “spark machining,” EDM is a simple process in which a desired shape is obtained using electrical discharges or sparks.
In EDM, shaped electrode (usually made of copper or graphite) is very slowly lowered onto the mold surface, which is immersed in kerosene. This process is done over a period of many hours. A voltage applied between the tool and the mold causes spark erosion of the mold surface in the inverse shape of the electrode.
There are several advantages to EDM processes. For instance, EDM allows the formation of shapes that are otherwise difficult to machine. The process also allows pre-hardened molds to be shaped so that no heat treatment is required.
Advantages of Compression Molding
Many manufacturers choose compression molding because it’s one of the least expensive ways to mass-produce products. Furthermore, the method is highly efficient, leaving little material or energy to waste.
Other advantages include:
- Very large parts (over 250 lbs. [100 kg]) can be produced with minimal degradation, yielding the strongest parts of all molding processes.
- Material can be placed in the cavity to achieve optimum fiber orientation in critical strength locations. In other words, the process is not limited by gate locations.
- Dissimilar material (such as glass mat or uni-directional glass) can be placed in the mold, to improve part strength.
- Trust the Experts
At CS Tool Engineering, we’re experts in compression molding. We specialize in compression molds for the automotive industry, particularly, vehicle headliners and door panels. Our experienced engineers use only the latest technology to ensure our compression molds are of the very highest quality.
Trust the experts.