What's That Process? Part 5
Plastic processing is a major part of the process heating world. Continuing with my “What's That Process” series look at plastic processing, this month I will look at thermoforming, compression molding and roto-molding.
ThermoformingThermoforming is another plastic-forming process. Here, a sheet, as produced on a sheet-extrusion line, is the initial product. Again, the mold for thermoforming is the negative of the final desired part and may be internal (vacuum formed), external (pressure formed), or both.
The sheet is fed into the thermoforming machine, where it is heated by means of a platen or in a circulating hot-air oven to a temperature that will permit forming. This temperature differs with the plastic, the thickness and the complexity of the part.
Once at temperature, the part is sealed on the mold, and either a vacuum is drawn that “pulls” the plastic into the mold, so forming it, or the mold is pushed into the sheet, so forming the part. The mold may be cooled, and the plastic is permitted to cool before being ejected from the mold.
Again, thermoformed products are vast and include items such as plastics sinks and baths.
Compression MoldingIn compression molding, the plastic, which typically is preheated, is placed into a heated open-mold cavity. Once the correct volume of material has been applied, the mold is closed under pressure and the material is forced into the mold cavity. Pressure and temperature is maintained until all of the interstices of the mold are filled. If the resin is a thermoset, the resin is permitted to cure. If the resin is a thermoplastic, the mold will have cooling that will permit the resin to crystallize. The mold is opened and the part is ejected.
Compression molding is suited to thermoset resins. Handles for cooking pots, housings for hot plates, electrical receptacles and high-voltage switch housings, trays and pallets are common compression-molded products.
Roto-MoldingRotational molding, or roto-molding, as it is endearingly called, uses molds that are mounted on a structure that indexes or revolves. These machines can be very large depending on the part being formed. Typically, a roto-molding machine has four stations and revolves through each station to complete a cycle.
The first station is loading, where granulated or powdered resin is placed into the mold. The mold then moves into the baking or melting station, where heat is applied to the mold while the mold rotates simultaneously on two vertical axes. The resin melts and, by the rotating action, coats the internal surface of the mold evenly and uniformly. The mold then moves to the third station, where it is cooled by air or water as it continues to rotate. In the last station, the part is extracted from the mold.
Roto-molding can produce very large parts that are hollow. These include items such as furniture, sheds, toys, containers and outdoor play equipment. Double walled parts such as coolers also can be produced using rotational molding technology.
You will note that some of these plastics processing technologies require multiple process heating steps to enable a part to be made. For example, petrochemicals are heated in a reactor to make a polymer. The polymer is heated in an extruder to make a pellet. The pellet is heated to be injection molded and make a preform. The preform is heated to be blow molded into a bottle. The polymer is also cooled during each one of the steps.
You may wonder how a person that is involved in drying has knowledge about various different plastic processing technologies. I will share the secret with you. Many of the plastics and other commodities that are used in these processes require drying before they can be used. Examples of these are nylon, ABS and PET. These are commonly dried using desiccant air dryers because they need to get to extremely low final moistures. Wood flour and kindred products are emerging commodities that require drying for use in the wood plastics composite industry that is growing steadily.
There are very few things that do not require drying at some stage of their production. PH