Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, window frames, adhesive tape and wire insulation.
In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.
The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200 °C (392 °F) to 275 °C (527 °F) depending on the polymer).
In most processes, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in heater jackets are employed, and they generally use a closed loop of distilled water in heat exchange with tower or city water.
At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa).
The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be ‘tweaked’ by varying screen pack composition (the number of screens, their wire weave size, and other parameters). This breaker plate and screen pack combination also does the function of converting “rotational memory” of the molten plastic into “longitudinal memory”.
After passing through the breaker plate molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product’s profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.
The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.
Sometimes on the same line a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.
Extrusion has always been a high-volume manufacturing process. It started in Germany when Paul Troester created the first plastic extruder. Since then the fundamentals of the process haven’t changed much.
However, there is a lot of improvement in the process when it comes to its efficiency and material handling capabilities. In the past, extrusion machines could only handle specific types of plastics and had many requirements. With time, this improved, and today, plastic extrusion is more versatile, flexible, and capable of handling different kinds of plastics together.
Another common problem with extruded plastics is the fact that they don’t possess the right mechanical properties and strength. This was a significant problem in the past but new developments in technology improved things. Today, custom plastic extrusion is the go-to process of choice as it delivers the quality, consistency, and functionality you expect from modern industrial applications.
The plastic extrusion process begins with filling the hopper with smaller plastic pieces that are simpler to process. The feed throat uses gravity to transfer that plastic to the barrel for further processing.
When the material enters the barrel, it starts to be warmed by means of at least three intensity zones, where the temperature intensity increases as you move away from the feed throat.
As the temperature increases, the barrel uses a continuously rotating screw to push the molten plastic towards the next component of the machine. The screw and pressure additionally create heat, so the intensity zones themselves don’t need to be basically as hot as the expected extrusion temperature, thereby saving energy and facilitating the extrusion process.
The liquid plastic leaves the barrel through a screen supported by the breaker plate. This screen eliminates foreign substances from the material and maintains the internal pressure. The material goes through a feed pipe into the uniquely fabricated die, which has the same shape as the extrusion profile you want from the project.
At the point when constrained through the die, the molten material creates the same shape as the die opening, which completes the extrusion process.
When completely through the die, the extrusion profile is cooled in a water shower or by means of a bunch of cooling rolls to ensure that the shape of your thermoplastic extrusion profile becomes permanent.