Extrusion produces plastic products such as drainage and irrigation pipe, medical fluid and IV tubing, weather-stripping, fencing, deck railings, vinyl siding, window frames, home décor and furniture trim, automotive components, appliance trims and seals, filtration parts, drinking straws, the “zip” strip on re-sealable bags, food processing tubing and rails, plastic films and sheeting, thermoplastic coatings, electrical conduit and cable protectors and wire insulation.
Extrusion is a process where a material is pushed through a tool with a specialized shape called a die, producing continuous objects of a fixed cross sectional profile.
Extruded plastics are similar to extruded metals, however, the difference, as the name implies, is in the material used. Plastics extrusion is a normally high volume manufacturing process where a polymer material, enriched with the desired additives, is melted and formed in a continuous process.
Plastics Extrusion in advanced manufacturing is not simply choosing different colors to push through several basic shapes… Custom Plastic Extrusion can combine the properties of advanced materials to enable specific performance characteristics for plastic tubing or plastic parts, and can form multi-lumen tubing or complex profiles to fit a wide range of applications for commercial, industrial, food processing, filtration, automotive, military and medical/pharmaceutical industries
The extrusion process starts by feeding raw material polymer (pellets, granules, flakes or powders), is gravity fed into the hopper and through the feed throat, drops on a rotating screw. Screw rotation is provided by an electric motor. The screw design varies and is dependent on the material and final product design. The rotation of the screw forces the plastic forward through a heated barrel. As the plastic is conveyed through the barrel the channel or thread of the screw decreases, thus compressing the plastic. Three or more independent Proportional Integral Derivative PID controllers, creating zones of gradually increasing temperature, heat the barrel. The plastic melt temperature is normally higher than the set temperature for the controllers. This additional heat is generated through a combination of compressive force and shear friction (shear heat). When the plastic melt reaches the end of the screw the plastic melt is well mixed and pushed through a screen pack, supported by a breaker plate, filtering contaminants and removing the materials rotational memory. Finally the filtered melt is then pushed through the die. The die gives the final product the desired profile and shape. After exiting the extruder the extrudate is pulled and cooled. The cooling method is dependent on the profile and shape of extrudate.
Plastic extrusion is continuous and capable of high volume output, making it a more economical plastics manufacturing method than other processes. In addition, tooling costs are relatively low.
Plastic extrusion works particularly well for production of plastic parts that have consistent wall thicknesses, and for profiles that are wider and longer than those produced by other types of manufacturing. Plastic films, bags, piping, tubing, rods, weather stripping, and deck railing are just a few of the many products that are successfully and economically produced by plastic extrusion.
For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender or “chill” rolls), usually 3 or 4 in number. Running too fast creates an undesirable condition called “nerve”- basically, inadequate contact time is allowed to dissipate the heat present in the extruded plastic.
In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.).
Often co-extrusion is used to apply one or more layers to obtain specific properties such as UV-absorption, soft touch or “grip”, matte surface, or energy reflection.
A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape. When vacuum is used, this is often described as vacuum forming.
Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer
In a wire coating process, bare wire (or bundles of jacketed wires, filaments, etc) is pulled through the center of a die similar to a tubing die. Many different materials are used for this purpose depending on the application. Essentially, an insulated wire is a thin walled tube which has been formed around a bare wire.
There are two different types of extrusion tooling used for coating over a wire. They are referred to as either “pressure” or “jacketing” tooling.
Extruded tubing process, such as drinking straws and medical tubing, is manufactured the same as a regular extrusion process up until the die. Hollow sections are usually extruded by placing a pin or mandrel inside of the die, and in most cases positive pressure is applied to the internal cavities through the pin.
Coextrusion is the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which will extrude the materials in the desired form. This technology is used on any of the processes described above (blown film, overjacketing, tubing, sheet). The layer thicknesses are controlled by the relative speeds and sizes of the individual extruders delivering the materials.
Extrusion coating is using a blown or cast film process to coat an additional layer onto an existing rollstock of paper, foil or film. For example, this process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is tetrapak.
A compound extrusion is an extrusion of composite materials. Usually twin-screw extruders are used because they give better conveyance characteristics and production rates. They also give a more uniform extrudate. When reinforcing fibers are mixed in a twin screw reduces screw wear because the fibers can be introduced later into the melt. Single-screw extruders are used for simple extrusions that have little variance in material formulation and viscosity.
Filler Masterbatch and Color masterbatch are also the products of the compound extrusion processing. These below pictures are the products and machine of plastic extrusion in Us Masterbatch factory