EXTRUSION MOULDING
Extrusion is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods,and fiber. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape.The shape of the die determines the shape of the tube.The extrusion is then cooled and forms a solid shape.The tube may be printed upon,and cut at equal intervals.The pieces may be rolled for storage or packed together.Shapes that can result from extrusion include T-sections,U-sections,square sections,I-sections,L-sections and circular sections.One of the most famous products of extrusion moulding is the optical fiber cable.
Everyday examples of extrusion can be seen when toothpaste is squeezed out of a tube, icing is pushed out of an icing bag and "Play do" shapes are made. Extrusion moulding of plastics is used to make any long shape that has a constant cross section. Pipes, gutters, window sections and decorative trims can all be made using the process.,
Typical extrusion material-
• PVC (polyvinylchloride),
• LDPE (low-density polyethylene, or polythene),
• HDPE (high density polyethylene) and
• PP (polypropylene)
Thermoplastics are pliable when they are heated but form a rigid shape once cooled. Perhaps most common plastic extruded is PVC. Hard, "unplasticised" PVC can be used in pipes for water, sewage and gas.
Working principle
The extrusion process begins with the plastic polymer in a granule or powder form. A funnel, or hopper, feeds this into the heating chamber of the extruder. The polymer melts as it is heated to temperatures typically above 200ºC
Once molten, a screw-mechanism pushes the melted polymer through the shaped "die head" which forms the plastic into shape. As it leaves the extruder, the plastic is cooled by water or air to give the final form. Flexible products, like tubing, may be coiled whilst rigid plastics, such as window-frame sections, will be cut to length.
Parts of plastic moulding machine-
Hopper
The hopper is a large funnel that holds the raw plastic in either granular or pellet form. The hopper sends a continuous stream of raw plastic into the heating barrel of the extrusion machine.
Heated Barrel
The heated barrel of a plastic extrusion machine is a long, horizontal cylinder. The cylinder is heated internally through the use of a heating jacket. As the plastic is fed into the cylinder by the hopper, the plastic melts from the heat
Die and Mandrel
A water-cooled die is located at the end of the heated barrel. The intended shape of the plastic is cut into the die. Water passes along the metal die to cool the plastic as it passes and harden it into shape. A mandrel may also be located after the die to produce a hollow shape.
Screw design:-
Most screws have these three zones:
• Feed zone (also called the solids conveying zone): this zone feeds the resin into the extruder, and the channel depth is usually the same throughout the zone.
• Melting zone (also called the transition or compression zone): most of the polymer is melted in this section, and the channel depth gets progressively smaller.
• Metering zone (also called the melt conveying zone): this zone melts the last particles and mixes to a uniform temperature and composition. Like the feed zone, the channel depth is constant throughout this zone.
Compound extrusions
Compounding extrusion is a process that mixes one or more polymers with additives to give plastic compounds. The feeds may be pellets, powder and/or liquids, but the product is usually in pellet form, to be used in other plastic-forming processes such as extrusion and injection molding. As with traditional extrusion, there is a wide range in machine sizes depending on application and desired throughput. While either single- or double-screw extruders may be used in traditional extrusion, the necessity of adequate mixing in compounding extrusion makes twin-screw extruders all but mandatory
Extrusion is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods,and fiber. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape.The shape of the die determines the shape of the tube.The extrusion is then cooled and forms a solid shape.The tube may be printed upon,and cut at equal intervals.The pieces may be rolled for storage or packed together.Shapes that can result from extrusion include T-sections,U-sections,square sections,I-sections,L-sections and circular sections.One of the most famous products of extrusion moulding is the optical fiber cable.
Everyday examples of extrusion can be seen when toothpaste is squeezed out of a tube, icing is pushed out of an icing bag and "Play do" shapes are made. Extrusion moulding of plastics is used to make any long shape that has a constant cross section. Pipes, gutters, window sections and decorative trims can all be made using the process.,
Typical extrusion material-
• PVC (polyvinylchloride),
• LDPE (low-density polyethylene, or polythene),
• HDPE (high density polyethylene) and
• PP (polypropylene)
Thermoplastics are pliable when they are heated but form a rigid shape once cooled. Perhaps most common plastic extruded is PVC. Hard, "unplasticised" PVC can be used in pipes for water, sewage and gas.
Working principle
The extrusion process begins with the plastic polymer in a granule or powder form. A funnel, or hopper, feeds this into the heating chamber of the extruder. The polymer melts as it is heated to temperatures typically above 200ºC
Once molten, a screw-mechanism pushes the melted polymer through the shaped "die head" which forms the plastic into shape. As it leaves the extruder, the plastic is cooled by water or air to give the final form. Flexible products, like tubing, may be coiled whilst rigid plastics, such as window-frame sections, will be cut to length.
Parts of plastic moulding machine-
Hopper
The hopper is a large funnel that holds the raw plastic in either granular or pellet form. The hopper sends a continuous stream of raw plastic into the heating barrel of the extrusion machine.
Heated Barrel
The heated barrel of a plastic extrusion machine is a long, horizontal cylinder. The cylinder is heated internally through the use of a heating jacket. As the plastic is fed into the cylinder by the hopper, the plastic melts from the heat
Die and Mandrel
A water-cooled die is located at the end of the heated barrel. The intended shape of the plastic is cut into the die. Water passes along the metal die to cool the plastic as it passes and harden it into shape. A mandrel may also be located after the die to produce a hollow shape.
Screw design:-
Most screws have these three zones:
• Feed zone (also called the solids conveying zone): this zone feeds the resin into the extruder, and the channel depth is usually the same throughout the zone.
• Melting zone (also called the transition or compression zone): most of the polymer is melted in this section, and the channel depth gets progressively smaller.
• Metering zone (also called the melt conveying zone): this zone melts the last particles and mixes to a uniform temperature and composition. Like the feed zone, the channel depth is constant throughout this zone.
Compound extrusions
Compounding extrusion is a process that mixes one or more polymers with additives to give plastic compounds. The feeds may be pellets, powder and/or liquids, but the product is usually in pellet form, to be used in other plastic-forming processes such as extrusion and injection molding. As with traditional extrusion, there is a wide range in machine sizes depending on application and desired throughput. While either single- or double-screw extruders may be used in traditional extrusion, the necessity of adequate mixing in compounding extrusion makes twin-screw extruders all but mandatory
Application:-
• Insulated coating for wires and cables.
• Monofilament: for rope, bristles and synthetic textile fibres.
• Flat plastic sheets for signs, refrigerator interiors, glazing and lighting applications.
• Pipes and tubing used for hoses, water, gas, sewers and drains.
Advantages of Plastic Extrusion
3 of the major advantages of plastic extrusion are as follows.
• Low Cost: Compared to other molding processes, plastic extrusion molding has a low cost and is more efficient. The extrusion molding process uses thermoplastics and they undergo repeatedly melting and hardening, this allows the waste to be reused rather than be discarded. Raw material and disposal costs are lowered.
• Better Flexibility: Extrusion molding will provide considerable flexibility in the products being manufactured with a consistent cross section. As long as the cross section stays the same, the extrusion molding can produce complex shapes
• After Extrusion Alterations: Plastics remain hot when they are removed from the extruder and this allows for post-extrusion manipulations. Many manufacturers will take advantages of this and use a variety of roller, shoes and dies to change the shape of the plastic as needed.
Disadvantages of Plastic Extrusion
Plastic extrusion has also some disadvantages.
• Expansion: Once the hot plastic is removed from the extruder it will many times expand. This is called die swell. It is problematic to try and predict the exact degree of expansion. Because it is hard to predict the exact expansion, manufacturers often accept significant levels of deviation from the dimensions of the product.
• Product Limitations: Extrusion plastic molding does place limits of the types of products that can be manufactured. There are alternatives that require an investment in a different type of extrusion equipment.
