Processing of polymer or plastic:

Processing of polymer or plastic:

1. Forming technique for plastic

Different techniques are employed in the forming of polymeric material. The method used depends on several factors

There are numerous similarities with the fabrication of metal and polymer.

• Fabrication of polymeric material normally occurs at high temperature by the application of pressure.
• Thermoplastic are formed above their glass transition temperature. If the material are formed above their melting temperature.
• The economic benefit of temperature is that they can be recycle, they can be removed and reformed into new shape.
• Fabrication of thermosetting polymer is obtained in two stages:

1. There is the preparation of linear polymer also called pre-polymer in a liquid having low molecular weight.
2. This material is converted into hard and stiff product which is carried out in a mold having desired shape. This stage is termed as ‘curing’. Curing may occur during heating and addition of catalyst. There is chemical and structural change on a molecular live.

For example: a crossed linked or network structure forms.

• Molding is the most common technique for forming plastic polymer. The several molding technique used include compression, transfer, blow and injection and extrusion. For each finally a pelletized of granulized plastics are formed and forced at high temperature and pressure to flow into fill and assume the shape of a mold cavity.

Compression and transfer molding

• In compression molding the appropriate amount of polymer with necessary additives are placed between male and female molds members. The mold is closed and heat and pressure are applied causing the plastic to become viscous and to flow to confirm the shape of mold.
• In transfer molding, the solid ingredients are first melted in a heated transferred chambers. As the molten material is injected into the mold chamber pressure is disturbed uniformly all over the surface. This process is used for thermosetting polymer and plastic having complex geometry.

Injection molding

It is similar to die casting for metal and widely used technique for metal and widely used technique for fabricating thermoplastic material. The correct amount of pelletized material is forced into a cylinder by the motion of ‘plunger’ or RAM. Thermoplastic material melts into the heating chamber to form a viscous liquid. This material is forced into mold cavity and pressure is maintained, finally the piece is ejected in the form of solid.

Extrusion

In extrusion process molding of a viscous thermoplastic under pressure is passed in an open ended die similar to metal extrusion. A mechanical screw or ‘auger’ ‘propels’ through a chamber having pelletized material. This method is successively compacted, melted and forced into a continuous charge of viscous fluid shown if figure. Extrusion take place as molten mass is forced through die orifice. This technique is usually used to make rod, tubes, filament, nose channels.

Blow molding

It is similar to that blow molding for glass bottle in ceramic .1) parision of length of polymer tubing is extruded 2) the hollow pieces form by blow air or stem under pressure, forcing the tube wall in desired shape.

Casting

Like metal polymeric material may be caste into a mold and allowed to solidified for thermoplastic solidification occurs upon cooling from molten state, whereas for thermosetting plastic hardening is a consequences of polymerization at high temperature.

References

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Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.