Extrusion

Extrusion
Extrusion is the process that forces metal to flow through a shape-forming die. The metal is plastically deformed under compression in the die cavity. Extrusion produces only compressive and shear forces in the stock without any tensile force, which makes high deformation possible without tearing the metal. It is a hot-working process which, like forging, rolling, etc., uses the good deformability of heated metallic materials for shaping them.
Fig.12Extrusion Processes
In hot extrusion a metal billet heated to the appropriate temperature is fed into the cylindrical container of the extrusion press and is forced by the action of a ram through a steel die whose orifice has the desired shape to produce the solid or hollow section. The metal emerges from the die as a continuous bar, which is cut to the required lengths. Extrusion products are therefore essentially linear in character, in the cross-section only. The process is therefore suitable for the production of bar-like and tubular objects.
The cross-section that can be produced from solid round, rectangular, to L shapes, T shapes, tubes and many other different types. Extrusions, often minimize the need for secondary machining, but are not of the same dimensional accuracy or surface finish as machined parts. However, this process can produce a wide variety of cross-sections that are hard to produce cost-effectively using other methods.
Cold extrusion
Cold extrusion is the process done at room temperature or little increase temperatures. This process can be used for most materials subject to designing strong enough tooling that can withstand the stresses created by extrusion. Cold extrusion can be used with any material that possesses adequate cold work ability–e.g., lead, tin,
aluminum alloys, copper, titanium, molybdenum, vanadium, steel. Typical parts which are cold extruded are collapsible tubes, aluminium cans, cylinders, etc.
The advantages of cold extrusion are:
1. No oxidation takes place.
2. Good mechanical properties.
3. Good surface finish with the use of proper lubricants.
Hot extrusion
Hot extrusion is basically a hot working process. It is done at fairly high temperatures, approximately 50 to 75% of the melting point of the metal. The pressures can range from 35-700 MPa. Due to the high temperatures and pressures and its detrimental effect on the die life as well as other components, good lubrication is necessary. The principal variables, which influence the force required to cause extrusion, are:
(1) The type of extrusion.
(2) The extrusion ratio.
(3) The working temperature.
(4) The speed of deformation.
(5) The frictional conditions at the die and container wall.
Typical parts produced by hot extrusion are trim parts used in automotive and construction applications, window frame members, railings, aircraft structural parts.
Injection moulding
Injection Molding is a process, in which a materials such as polymer is heated to a highly plastic state (no melting) and forced to flow under pressure into a mold cavity, where it solidifies. The part, called a molding, is then removed from the cavity. Injection moulding is a very common process used in the manufacture of the casing of many electronic products such as TV s and radio.
Fig.13 Injection Molding
Injection and pressure keeping
1-Pressure Injection pressure can be considered as the fill pressure (primary pressure) and the hold pressure (secondary pressure). Generally the fill pressure will be set stronger than the hold pressure. When low-temperature solidification, crystalline resin will cause a big shrink, therefore the hold pressure is necessary for filling up and is closely related to the molding shrinkage.
2- Injection speed In the case of thin molded product or multi-cavity molded product which high size precision is required, faster injection speed is better. Also, slower injection speed is better for thick molded product. And the program control of injection speed is effective to resolve the jetting and the flow mark.
3- Injection time setting will differ by the molding machine, but basically should be considered as below. Injection time (filling time + pressure keeping time) is more than gate sealing time. Gate sealing time is the time when resin stops flowing by solidification at the gate part.
Plastic Injection Mould
The mould tool is the key component in the injection moulding of plastic:
1-It provides a passageway for molten plastic to travel from the injection cylinder to the mould cavity.
2-It allows the air which would be trapped inside when the mould closes to escape. If the air could not escape then the moulded component would contain voids and have a poor surface finish.
3-It cools the moulding until it sets. The temperature of the mould is controlled because it is important that the moulding cools at the correct rate to avoid distortion and stress. In most systems water circulates through channels drilled through the mould similar to a car engine cooling system
4-By means of ejectors the finished moulding is pushed from the mould.
Injection operation
Fig. 14Injection operation
1- Guide Pins – fixed to one half of the mould and align the two halves by entering the holes in the other half.
2- Runner – passageways in the mould connecting the cavities to the sprue bush.
3- Gate – Frequently the runner narrows as it enters the mould cavity. This is called a gate and produces a weak point enabling the moulding to be easily broke or cut from the runner.
4- Sprue bush – Tapered hole in the centre of the mould into which the molten plastic is first injected.
5- Locating Ring – Positions the mould on the fixed platen so that the injection nozzle lines up with the sprue bush.
6- Mould Cavity – The space in the mould shaped to produce the finished component.
7- Ejector Pins – These pins push the moulding and sprue/runner out of the mould.
8- The Shot – Total amount of plastic injected into mould.
9- Sprue – Material which sets in the sprue bush.