PLASTICS

PLASTICS

Plastics are non-metallic, polymeric engineering materials that can be formed by many methods. They have taken the place of traditional materials like woods and metals. Plastics differ from other materials largely because of the size of their molecules. Most materials have molecules made up of less than 300 atoms, whereas plastics contain thousands of atoms. We call them Macromolecules. There are some plastics derived from naturally occurring materials like wood, animals and insects.

Plastics are normally man made synthetic resins. Most synthetic plastics come from crude oil but coal and natural gas is also used. When crude oil is refined, gases are given off. The gases are broken down into Monomers. These are chemical substances consisting of a single molecule. Thousands of these are linked together in a process called Polymerization to form new compounds called Polymers.

In addition, various additives may be added to injection molding compounds to accomplish various purposes. The table below summarizes some of them.

Additive	Function	Examples
Filler	Increase bulk density.	Calcium carbonate, talc, limestone
Plasticizer	Improve processability, reduce product       Brittleness.	Phthalate esters, phosphate esters
Antioxidant	Prevent polymer oxidation.	Phenols, aromatic amines
Colorant	Provide desired part application color.	Oil-soluble dyes, organic pigments
Flame retardant	Reduce polymer flammability.	Antimony trioxide
Stabilizer	Stabilize polymer against heat or UV  Light.	Carbon black,  Hydroxybenzophenone
Reinforcement	Improve strength.	Glass fibers, carbon, Kevlar fibers

 Additives their functions and examples

The properties common to most of the plastics are:

      They are light in weight.

They have resistance to deterioration by moisture.

They have low electrical and thermal conductivity.

They have good color range.

Contrary to the above properties, plastics are inexpensive and usually are competitive price wise with other engineering materials, since

  They can be molded to any shape, as it is impossible for other engineering materials. 

     Machining is not necessary hence it reduces machinery cost.

Material selection for molding can be a complicated task. It usually depends to a large extent on the functional constraints of the part. The development of a variety of polymers combined with the ability to blend or mix different polymers has made available a wide selection of plastics. This range of choices allows product designers to select the materials they use to possess specific properties, processing characteristics and costs. The physical properties of the materials (density, thermal conductivity, melting temperature, etc.) must be considered to obtain the required mechanical properties on the finished part (strength, stiffness, hardness, etc.). Both amorphous and crystalline thermoplastic resins are used in injection molding.

2.1.1    PLASTIC MATERIAL CHARACTERISTICS

Important characteristics of plastic materials to be considered while designing and fabricating are:

1.      Dimensional Stability: Dimensional stability is defined as the ability of a material to maintain its size and shape under various temperatures and stresses, which is necessary for satisfactory part performance in many applications. The complexity, size of the mold cavity and the tendency of the material being molded to shrink as it cools in the mold determine the final dimensions of a molded part.

2.      Drying: Under adverse high humidity conditions or large temperature fluctuations from cold temperatures to hot temperatures, moisture pickup may occur and cause splay marks or bubble formation in formed parts. Drying the resin for about two hours at 71 - 82°C will eliminate condensed moisture on the granules and assure introduction of constant temperature granules to the fabrication equipment.

3.      Compatibility: Equipment should be thoroughly purged with respective resins while fabricating. Few resins are physically compatible with themselves. Delaminating, streaking or haze will occur if incompatible resins are mixed with the material to be fabricated.

4.      Outdoor Weatherability:      Most of Plastics are not considered to be weather resistance plastics. Continuous long-term outdoor exposure results in both discoloration and reduction in strength and toughness properties. Weatherability can be improved by the addition of certain pigments or additives. Best results are obtained with finely dispersed carbon black or UV stabilizers.

5.      Use of Regrind:         Many thermoplastic resins can be reground for use as 100% regrind or blends of regrind resins with virgin resin. When you are fabricating with regrind resins, you need to use experienced judgment and screening. The use of degraded or contaminated regrind product may result in lower quality parts and performance.

     CLASSIFICATION OF PLASTICS

The important classifications of plastics are:

1. Thermoplastics:  Thermoplastic is linear or branched polymeric materials, which soften on heating and harden on cooling. The molecules of thermoplastics are in lines or long chains with very few entanglements. When heat is applied the molecules move apart, which increases the distance between them, causing them to become untangled. This allows them to become soft when heated so that they can be bent into all sorts of shapes. When they are left to cool the chains of molecules cool, take their former position and the plastic becomes stiff and hard again. The process of heating, shaping, reheating and reforming can be repeated many times. Thus the processing of the material involves only physical changes. Re-using materials by adding regrind (ground up runners and scrap parts) to virgin materials generates even more cost savings.

Examples: Polypropylene, Polystyrene, Polyethylene, etc.

Based on molecular structure, thermoplastics can be classified into two groups:

Amorphous and Crystalline: Amorphous materials never really melt during processing. They just soften and are formed under pressure. These materials posses close dimensional tolerances on the part compared to crystalline materials. ABS, PS, PC, etc. are classified under this group. Crystalline materials have a specific melt temperature and remain solid until this temperature is reached. After the melt temperature is achieved, the materials flow very easily with very low viscosity. When the material is cooled to a temperature below the melt temperature, the material hardens to a solid form. Nylon, Polyethylene, PP, etc.  Are classified under this group.

The common methods of molding thermoplastic materials are Injection molding, Blow molding, Extrusion molding, etc.,

2. Thermosetting plastics: Thermosetting materials are those, which set solid on curing after being melted to the liquid state by heating. The thermoset polymeric material can be formed by the application of heat and pressure, but cannot be reformed upon further application of heat and pressure. These materials chemically react to form cross-link polymer chain molecules during processing. Most of these materials posses heavy impact strength with good resistance against heat and chemicals.

Examples: Phenol formaldehyde, Melamine formaldehyde, urea formaldehyde, etc.

The most common method of molding thermosetting a material is Compression molding and Transfer molding.