Compression / Transfer Molding of Thermosets

Thermoset plastics undergo a chemical reaction when they are molded, and cannot be directly remelted or recycled, as opposed to thermoplastics. Some of the major advantages of thermosets are their good high temperature properties, very little creep, and excellent dimensional stability.

In compression molding, the parts are made in a mold similar to an injection mold. The mold is opened and closed vertically. The mold is generally heated to around 300F to 400F, because high temperature starts the chemical reaction. When the mold opens, an operator places a predetermined amount of plastic in the bottom half of the mold. The mold closes, forcing the plastic throughout the cavity. An excess amount of plastic must be added so the part can completely fill out, so a lot of excess plastic is produced around the parting line. This excess material is called flash. After the part is removed, it must be deflashed.

Transfer molding is very similar to compression molding, and uses the same type of machien. First, the mold closes. Then, a certain amount of plastic is added to the transfer chamber above the cavity. A ram on the molding machine forces the plastic in the chamber through a runner system, and into the cavity. Because the viscosity of thermoset plastic is extremely low (much lower than thermoplastic), this part will flash too.

Materials

Thermoset materials irreversibly chemically react during the molding process. They cannot be remelted or recycled very easily. However, because they will not melt or soften, they are excellent plastics for high temperature environments. They also have excellent electrical insulation properties, and are commonly used in electrical applications.

As with thermoplastic, there is a host of thermoset materials, each with its own set of properties and applications.

Summary

These processes are usually used to produce electrical components where high heat stability is needed. Thermoset will not burn; they char. They are very well suited for high temperature environments. Thermoset resin is also considerably cheaper than a high temperature thermoplastic, although the manufacturing process for thermoset is usually more expensive because it is labor intensive. Compression and transfer molding is commonly chosen for parts that have metal inserts. Since the clamp is vertical, gravity will hold the inserts in place. Loading material and removing parts requires an operator anyway, so they can also load the inserts.

Some common compression and transfer molded parts are electrical circuit breakers, wall outlets, electrical motor components, and pot and pan handles.

Advantages of Compression Molding

· It is a simple process, and very easy to understand

Disadvantages of Compression Molding

· If the mold has multiple cavities, multiple material charges must be distributed to each

· Generally, the material charge is weighed on a scale. Too much or too little material will cause short shots or excessive flash

· It is very labor intensive

· The process generates a lot of flash and is messy

Advantage of Transfer Molding

· Since the mold has a runner system, multiple cavities are easier

· There are lower molding pressures in this process than compression molding

· Since the parts have gates, engineering a flow path is possible

Disadvantages of Transfer Molding

· The mold is more complex than a compression mold

· Flow induced and orientation problems are more pronounced (similar to injection molding) because the material is essentially injected into the cavity at high speeds

· Not as much, but still a significant, amount of flash is produced, and good housekeeping is difficult

Characteristics of compression and transfer molded thermoset parts are:

· Parts have excellent high temperature properties (low creep, high dimensional stability)

· They will not burn or melt

· Materials are excellent electrical insulators

· Parts must be deflashed

· Manufacturing is labor intensive

· Process is well suited for parts with inserts because of the vertical clamp and the operator