Compression molding produces molded parts by subjecting them to pressure in a mold. The properties of compression molded parts are different from those of injection-molded parts. Normally, pressed parts have a higher level of crystallinity, and are therefore more rigid and solid but also harder and more brittle.
The form press cycle includes
- Thickening the polymer in a mold
- Heating the system over the melt temperature
- A holding phase for further thickening
- A cooling phase
P and FP powder grades are normally used in compression molding. If granulates were used, the air inclusion would be too large, and with finer powders the molds are difficult to fill. Here, bridging can also cause air pockets, which results in porous components.
By adding PTFE, graphite, nanoscale titanium oxide or silicon oxide, glass or carbon fibers, for example, certain properties such as surface hardness, frictional coefficient and abrasion resistance can be selectively improved.
To prevent bubble formation through moisture, the powder should be dried for 3 hours at 150 °C or overnight at 120 °C, either in the drying cabinet or in the mold prior to processing.
For easier demolding, small amounts (2–3%) of PTFE can be added to the powder. A silicon-free mold-release agent can also be used, provided its temperature-resistance is higher than 400 °C.
Before the mold is heated, the powder is thickened to allow air to escape. Processing parameters such as pressure, temperature, holding time, etc. depend on the quantity of powder, the surface area and geometry of the form, and must be determined case by case.
To prevent stresses in the mold, cooling should be slow and controlled and not exceed 40 K/hour. Beginning at approximately 140 – 150 °C, the pressed article can be demolded.