Plastics Processing.....How Plastic Resin Becomes Pipe, Valve, Etc.

Just about everyone knows that plastic valves, pipes, tanks, and fittings are molded, but three's a bit more to it than that. Knowing something about the processes that used to produce these products will help to make you more knowledgeable in selecting and using them in various applications.

Extrusion

Pipe is one of the more common types of plastic product we're familiar with. Pipe is produced by an extrusion process. Basically extrusion can be defined as forcing a material through a die orifice. This die orifice produces the final shape of the finished product. It's probably a bit of over simplification, but extrusion is somewhat analogous to squeezing toothpaste through a tube.

An engineering explanation of plastic pipe extrusion would be something like this: plastic pellets or granules are thermally fluidized, homogenized and continuously formed. Or, in other words, plastic resin granules are heated and melted, then mixed and formed into pipe.

Pipe is not the only product produced by the extrusion process. Plastic tube, sheet, wire and profile shapes are all also manufactured by this process. In fact, the plastic resin granules or pellets used for injection molding are produced by extrusion. A long stand or filament of extruded plastic is chopped or cut into pellet size pieces to produce plastic injection molding materials.

Extrusion produces an inherently strong finished product. More so than a molding process. This is one of the reasons that plastic pipe is rated at higher pressures than injection molded plastic fittings.

Injection Molding

When people talk about molding they're usually referring to injection molding. This is the manufacturing process that is used to produce the plastic parts that valves are manufactured from as well as plastic pipe fittings.

An injection molding machine uses heat to melt plastic pellets. The molten plastic is then force, under very high hydraulic pressure, into a mold. Cold water running inside the mold cools the plastic and it solidifies into the shape of the mold cavity, producing the fitting or valve component part. Molding machines are sized by the amount of clamp pressure (tonnage) required to hold the mold halves in place while the molten plastic is forced into the mold cavity and by the volume (ounces) of plastic the machine can supply to the mold with one continuous injection cycle. A small part, like a one-half inch end connector may be molded on a 250 ton machine while a larger part, say a four inch basket strainer body, might be molded on a 1000 ton machine.

Injection molding is a more complex process than extrusion because more variables come into play such as mold design and cooling times as well as the temperature of the molten plastic and the pressure at which it is injected into the mold. Even the type of plastic being molded makes a difference. This is because of the different molding characteristics of different plastics. In fact, because of one of these characteristics, shrink rate, a mold that easily produces a part from one type of plastic may not be able to be used for another. This is because as different types of plastic cool inside a mold they contract or "shrink" differently. This changes the dimension of the finished part.

The major advantages of injection molding are speed of production, minimal requirements for post molding operations such as machining to final dimensions, and the ability to mold several parts at one time with a multi-cavity mold.

Blow Molding

Blow molding is another common type of plastic molding. In this process a plastic tubular form, produced by extrusion or injection molding is used to form the part. This form, called a parison, is softened inside a mold and then injected with air or other compressed gas. This expands the parison against the sides of the mold cavity forming a hollow object the size and shape of the mold. Blow molding is often used to produce plastic vessels and containers.

Rotational Molding

Rotational molding is the last molding process we'll talk about. In this type of molding, a ground plastic powder is heated in a rotating mold until it become molten. As the mold rotates, the molten plastic uniformly coats the inside of the mold cavity. When it is cooled, a hollow finished part is formed. Rotational molding is used to produce plastic tanks.

Back to Hayward Back to PEP