Flashing and plunging are terms that could conjure up many images - every industrial process has its terminology and compression molding is no exception. It was invented way back in 1907 by Leo Baekeland, who gave his name to ‘Bakelite’ which used one of the first synthetic resins based on a phenol-formaldehyde. As with so many modern industrial processes, its refinement and usage was driven initially by the automobile industry – particularly tire production.
How does it work?
Simplicity is the key - the mold and process are much simpler than those of injection molding. Tooling is straightforward – there are three main variations, but the basis is that a female mold (or platen) and a male mold plunger (usually hydraulic) come together with a pre-heated, partially-cured and carefully measured quantity of thermoplastic material between them.
The molding compound may be in granular or putty-like form and consistency – and even preformed (for example in a sheet). Pressures used may be several hundred tons, and water cooling may be employed before the mold is opened. Once the cure-time is reached then the mold is opened and the component is ejected.
There may be some excess material (which is known as flashing) to be removed, but there are fewer knit-lines (mold seams) than with other processes. Product quality consistency is sometimes cited as a problem with the process but this usually relates to low cost consumer products where investment in tooling has to be lower than for, say, production of parts for aerospace applications.
Modern tooling, materials and process refinement has enabled reinforcing of the thermoplastic. Widely used reinforcing materials are carbon fiber (light weight, high strength, low flex tolerance), aramid fibers such as Kevlar, a trademark of DuPont Corporation (high flexibility, low abrasion tolerance) and glass fibers (which offer high strength and moderate flexibility but are relatively heavy).
The specific technologies are know as long-fiber reinforced thermoplastics (LFT) and glass-fiber mat-reinforced thermoplastics (GMT).
The modern automobile includes many compression-molded components from Kevlar-reinforced tires to glass-fiber reinforced fenders and dashboard panels.
Consumer products such as component cases, protective foam inserts, radio and TV casings and even gumboots are typical applications of the process. Sports applications include oar blades, bicycle components, skis and even small boats.
There are many medical applications, including hi-tech joints for prosthetics and internal joint replacement structures (using UHMWpe – ultra high molecular weight polyethylene).
Industrial applications are widespread, from farm machinery to conveyor systems and production-line machinery components. In the Aerospace and Defense sector, compression molding delivers products such as helicopter rotor components, jet engine cowlings and wing components.
In fact, there are very few areas of modern manufacturing where compression molding does not play an important part.
Advantages of Compression Molding
The process is now a very widely used manufacturing technique because it is:
- cost efficient
- suitable for high volume production
- capable of producing large components
- efficient in material usage – wastage is minimal
- physically quick and efficient with a low number of process steps
- capable of producing detail features such as holes – which with
- other molding techniques would need additional machining to deliver.
With its ability efficiently to deliver high volumes of components economically, there would seem to be an assured future for the process. As always, the auto industry is a powerful innovator. With the drive towards more fuel-efficient autos, hybrid power technology and lower production costs, then we are likely to see some significant developments and new ways of using the technique. Add to that the almost daily development of exotic materials – both the base resins or plastics and the reinforcement materials – and the increased refinement of the press machinery and processes, then the future is surely one of continued growth.