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Plastic extrusion is a high-volume process. Plastic granules are fed to the hopper of the extruder. Continuous screw rotation feeds the material down a heated barrel which converts it into molten polymer. The molten material is then forced through a metal die that contains the cross section of the profile of the extrusion required. The specific cross section of the die forms the design of plastic profile, film, tube or sheet in a continuous length. The lengths are pulled though cooling and sizing baths using caterpillar haul-off machines. The lengths can then be cut, sawn and coiled depending on the final product required.
More complex products can be manufactured using a process called co-extrusion. Co-extrusion involves multiple materials being processed by adjacent extruders and combined while still in a semi-molten state.
The tools contain no moving components and are therefore cheaper to manufacture than some other processes.
Example products are window profiles, pipes, tubes, and films for packaging applications.
A horizontal extruder passes molten material through a die mounted at its output end. The material is partially cooled as it passes through the die head, enough to retain its shape. The continuously extruded material is pulled through elongated water baths or similar device that completely cures the plastic before it is cut to length or wound into a coil.
Example products are window profiles, pipes, guttering, electrical conduit and tubes.
The extruded material is fed through in-line dies that convert the tube of solid molten plastic into a flat and thin planar flow of material. This material is then passed through a series of water-cooled rollers called calendar or chill rolls. As well as removing heat, the rollers are used to determine the thickness of the sheet or film. A film is often measured in microns of thickness, whereas a sheet is measured in mm.
Plastic sheet stock is commonly used to feed thermoforming lines. In this process the sheet is heated and then formed in a mould into the desired shape.
In cases where a more technical solution is needed, e.g. for microwaveable food containers, several layers of different material grades are co-extruded and combined.
On exiting the extruder, the material is usually channelled vertically through a circular die that forms a tube of semi-molten material. The tube is then inflated with air while being simultaneously drawn upwards by rollers, which stretches the film and produces the required wall thickness. As the film continues to cool, it is drawn through several sets of nip rollers to flatten it into ‘lay-flat’ tubing, which can then be spooled or slit into two or more rolls of sheeting.
Plastic bags are an example of products made from this process.
In compounding, various materials are added, combined, melted and fed through a twin screw extruder. The resulting material blend is then passed through a die to form long filaments of plastic that are then pelletized. In this way, a bespoke formula can be produced. This often consists of one or two primary material grades in combination with additives that enhance the appearance and/or processing properties and/or post processing characteristics of the final product.
Downstream equipment are auxiliary devices that manage the extrusions as they exit the die. Common types of this equipment include:
Haul-offs ensure the extrusions are pulled steadily, consistently and accurately from the die. This process is essential to avoid out of tolerance dimensions and maintain quality control.
Variations include open cooling, spray and vacuum baths.
Mostly used for hollow and complicated extrusions. These units are also used if you want to avoid stressing the parts by raid cooling.
There are various types of cutters including rotary, wheeled and guillotine. This process is used for accurate and consistent cutting of tube and profiles.
These are used for tubes and profiles. In this situation it is critical to control the tension and speed. There are a big range from simple units to fully automatic multiple units.
New Screws & Barrels for extrusion-based processes can be produced from existing drawings or by using sample components. Prices are usually more competitive than those of the OEM's, and it is sometimes possible to improve design in terms of durability, and/or the ability to process a specific material type.
For example, some OEM's supply machines with Nitrided barrels fitted. Although the internal bore is extremely hard wearing, the layer of hardened material is also extremely thin. By contrast, the Bi-metallic barrels available from our UK & IE based specialists have a much thicker layer of hardened material, and offer a considerably longer and more consistent service life.
Refurbishment of Screws & Barrels relies on the refurbishment provider retaining the components for a more extended period of time, but costs will be lower.
In the case of Screws, worn areas of the flights and/or mixing sections are removed and replaced, usually with a material that will provide superior wear characteristics to the original item when new.
Nitrided barrels can sometimes be 'honed' and re-hardened, the screw being refurbished so as to match the slightly larger barrel I.D. Alternatively, if wear is predominantly at the 'output' end of the barrel, a 'pocket' can be created, and a Bi-metallic sleeve inserted.
The primary working part of an extruder is the screw and barrel which can be replaced or refurbished. As such the efficient working life of a plastics extruder can be very long.
This section provides details of pre-owned or surplus extrusion and down-stream equipment that is currently available to buy in the UK & IE. Most of the listed equipment is located at a UK or IE based processing company. As such you may be able to view it in production.
This section provides details of specialist engineering services relating to extrusion equipment. This can involve inspection, maintenance, repair, and bespoke machinery refurbishment and development.