Specific molding techniques are commonly used for thermosetting polymers and elastomers. Keep reading for expert information about the processes.
Interested in learning about injection molding? Discover more about it here.
Compression molding is a time-tested, widely used molding process for thermosetting (TS) plastics. Compression molding manufacturers use the technique for applications such as phonograph records, rubber tires and polymer matrix composite parts.
The process, illustrated below for TS plastic, has the following basic steps:
· Load a precise amount of molding compound, or charge, into the bottom half of a heated mold
· Bring the halves of the mold together to compress the charge, forcing it to flow and conform to the cavity’s shape
· Heat the charge using the hot mold to polymerize and cure the material into a solidified part
· Open the halves of the mold and remove the part from the cavity
Compression molding for thermoplastics steps: (1) load the charge, (2) and (3) compress and cure the charge between the mold halves, and (4) open the mold halves and remove the part from the cavity
A molding compound’s initial charge can take a variety of forms, such as:
· Powders
· Pellets
· Liquid
· Preform
Compression molding companies must precisely control the amount of polymer used to obtain repeatable, consistent results in the molded products. It’s common practice to preheat the charge before placing it in a mold because it softens the polymer and shortens production cycle times.
Preheating methods include:
· Infrared heaters
· Convection heating in an oven
· Using a heated rotating screw in a barrel
The latter technique is borrowed from injection molding and serves to meter the amount of the charge.
Compression molding presses are vertically oriented and contain two platens to which the mold halves are fastened. There are two types of actuation the presses use:
· Bottom platen upstroke –or–
· Top platen downstroke (more common in machine configuration)
A hydraulic cylinder usually powers the presses. Compression molding manufacturers can design a cylinder to provide clamping capacities of up to several hundred tons.
Compression molding molds are typically simpler than their injection molding counterparts. However, there are some differences:
· Compressions molds do not have a sprue and runner system
· The compression molding process is often limited to simpler-part geometries because of the starting thermosetting materials’ lower flow capabilities
Manufacturers must make provisions for heating a mold. They usually accomplish this using:
· Electric resistance heating
· Steam
· Hot oil circulation
Compression molds are classified as:
· Hand molds: Used for trial runs
· Semiautomatic molds: The press follows a programmed cycle and an operator loads and unloads the press manually
· Automatic molds: The molds operate under a fully automatic press cycle, including automatic loading and unloading
Materials used for compression molding include:
· Phenolics
· Melamine
· Urea-formaldehyde
· Epoxies
· Urethanes
· Elastomers
Items commonly made using TS plastic moldings:
· Electric plugs, sockets and housings
· Pot handles
· Dinnerware plates
Advantages of compression molding for the respective applications include:
· Molds are simpler and less expensive
· Molds require less maintenance
· Less scrap
· Low residual stresses in the molded parts (making the processes ideal for flat, thin parts, such as phonograph records)
The greatest disadvantage for the respective applications is a longer cycle time, which leads to lower production rates when compared to injection molding.
The custom transfer molding process uses a thermosetting charge, or preform, that manufacturers load into a chamber immediately ahead of the mold cavity to preheat it. The equipment then applies pressure to force the softened polymer to flow into the heated mold, where it cures.
Transfer Molding Processes
There are two transfer molding processes:
1. Pot transfer molding: The charge is injected from a “pot” through a vertical sprue channel into the cavity
2. Plunger transfer molding: A plunger injects the charge from a heated well through lateral channels in the mold cavity
Both processes produce scrap called “cull” during each cycle. The leftover material forms in the lateral channels and the well’s base. The sprue in the pot transfer is also scrap material. Because polymers are thermosetting, manufacturers cannot recover the scrap.
· Polymers types: Because transfer molding is utilized on the same polymer types (thermosets and elastomers), it’s closely related to compression molding.
· Charge preheating: There are similarities to injection molding as the charge is preheated in a separate chamber before being injected into the mold.
· Shapes: Transfer molding is capable of molding part shapes that are more intricate than compression molding, but not as intricate as injection molding.
· Molding with inserts: Transfer molding lends itself to molding with inserts in which manufacturers place a metal or ceramic insert into the cavity prior to injection. The heated plastic bonds to the insert during the molding process.
(a) Pot transfer molding, and (b) plunger transfer molding. Cycle in both processes is (1) charge is loaded into pot; (2) softened polymer is pressed into mold cavity and cured; and (3) part is ejected.