Extrusion

In the extrusion process, molten plastic is pushed through a die with a cross section that shapes the plastic. The plastic is then drawn through a cooling apparatus, and then cut into sections or rolled up. Extrusion can only produce parts that have a continuous cross section. Some common parts that are extruded are pipe, vinyl siding, weed trimmer line, sheet, and film. Extrusion is a continuous process and can produce parts of any length.

How Extrusion Works

First, raw material in the form of plastic pellets is placed in the hopper (refer to Figure 1 throughout this discussion). An auger-like screw is rotated inside of a heated steel barrel, feeding the cold plastic pellets forward. As the unmelted pellets are conveyed forward by the screw, they heat up from shear heat and heat from the barrel. By the time the plastic reaches the front of the screw, it is completely melted. The screw's pumping action forces it through an orifice with a specially designed cross section, called a die. The die shapes the plastic into the desired form. As soon as the melted plastic exits the die, it enters a cooling apparatus, which usually uses air or water to solidify the plastic. A puller is placed after the cooling fixture to draw the plastic out of the die. At the end of the line, there is a removal system that either cuts the parts or rolls it up. Other secondary operations can be placed anywhere in the line

Materials

Most thermoplastic and a few thermoset materials (mostly rubber) can be extruded. As long as the material can be melted without degradation (excluding thermosets), and it has sufficient melt strength, it can be extruded. Melt strength is a measure of material's ability to be pulled when melted. Since the plastic is melted when it exits the die, melt strength is required for the plastic to retain its shape until it enters the cooling apparatus. As an example, water has no melt strength, whereas honey does. In general, high molecular weight polymers exhibit high melt strength. When purchasing a plastic for extrusion, plastic manufacturers will usually have "extrusion grade" polymers, whose viscosity is too high for injection molding, but has excellent melt strength for extrusion.

Types of Extrusion

Profile Extrusion: Solid parts with cross-sectional shapes other than round rods or flat sheets are called profiles. Figure 2 shows some examples of profile extrusion parts, along with the corresponding die shape. Because of the way the plastic shrinks, the way it flows through a die, and the way it shapes when it is pulled, the shape of a profile part can be significantly different than the die's shape.

Pipe and Tubing Extrusion: Pipe and tubing is produced by extruding a circular cross section. This time, the die is made of two pieces: the die and the mandrel. The mandrel forms the hollow center, and is supported as shown in Figure 3. The die and mandrel pictured in Figure 3 is called a spider die, because of the way the supports look.

Cable and Wire Coating Extrusion: Wire coatings are manufactured with extrusion. As shown in Figure 4, the bare wire passes through the back of the die and is drawn forward through a stream of melted plastic. The plastic coats the wire and then exits the die as insulated wire or cable.

Sheet and Film Extrusion: Sheet and films are also manufactured with extrusion. Thickness differentiates sheet from film. Sheet is 0.004" or thicker, whereas film is 0.004" or thinner. Figure 5 shows a die for extruding sheet. It is called a clothes hanger die and is used to equalize flow across the die as the sheet is extruded.

Blown Film Extrusion: This process produces extremely thin films used in garbage bags and plastic wraps. Plastic is extruded vertically through a tubular die that has an air channel passing though out. Air is blown into the tube, expanding the plastic against the sizing basket. The blown film is then rolled up.

Summary

There are some advantages to extrusion over other processes.

· Parts of infinite length can be produced

· Some parts, like blown film, can only be made with extrusion

· Extrusion is a continuous process capable of very large production volumes

· Most thermoplastics, and some thermosets can be extruded

Some disadvantages are listed below:

· Since the dies contain no moving parts, only continuous cross sections can be manufactured

· Complex geometry cannot be produced

· Complicated feature can be very hard to produce because of difficulties in producing a balanced die