Definition: 1) plastic, 2) any of a group of synthetic or natural organic materials that may be shaped when soft and than hardened. 3) A macro molecule which has or is made as describe in 2.

In the absence of going into the carbon and hydrogen formations of material make up our definition of plastics are based upon materials that can be molded or shaped by a heat forming process. They than can be broken down to a few basic groups of materials. These would be characterizing by the macromolecule (very large chains) which give them their characteristics. These materials can be broken down further into Thermoset, Thermoplastics and Elastomer category.

When taken as a group Thermoset can be explained as thermo meaning heat and set which means to take a set. Basically when discussing these materials some people will refer to this group as an egg in that once the shell is broken and the egg is cooked there is no way to put the egg back to its original form, or to change it to another form. These are plastics and can be molded with equipment designed for it. Some materials will come in 2 parts or components which the machine will mix before injecting or dispersing into a mold or form. The mold or form may or may not apply heat to cause a chemical reaction which cures the material.

Once this group is cured or more appropriately cross linked the material cannot be reshaped or reused as originally done. These material do not melt or soften, thus they have attributes when high heat is desired. Silicone is one of these materials, as is Polyurethane, and Phenolic. Further by the addition of peroxide in polyethylene there now a cross linked PE which is used in rotational molding.

Another group of materials are thermoplastics; basically this group will soften or melt upon the application of heat. In basic terms these materials can be explained as similar to chocolate which can be melted, shaped/formed and if need be remelted shaped and formed again. This group can be broken down into two major groups amorphous and semi-crystalline.  A newer third group would be thermoplastic elastomers, not to be confused with their counterpart elastomers which are cross linked, (rubbers).

Amorphous materials have long tangle macromolecule chains which never really untangle and only soften when heat is applied. They can be characterized by having low shrinkage, poor solvent resistance, and fair chemical resistance and can be clear in some cases. Also by some they are referred to as easy processing. Some materials which are amorphous are: ABS, Acrylic, Polycarbonate and SAN.

Semi-crystalline materials can be characterized as having order. They have both crystalline regions and amorphous regions within the macromolecule itself. They can be characterized as having high shrinkage, good solvent resistance and good chemical resistance. Clarity is not one of their attributes but with new chemistry that is changing all the time. Semi crystalline materials can be on the difficult side to process dependant on opinion. Some semi-crystalline materials are: HDPE, Polypropylene, Nylon, and Acetal.

Processing effects:

The differences in processing the amorphous and semi-crystalline materials can also be related to the shrinkage of the polymers with amorphous materials shrinking less than 0.007 in/in and the semi-crystalline materials shrinking above 0.018 in. /in. This is basically due to the fact that one melts (semi-crystalline) and one only softens (amorphous).  When this aspect is taken further and a processor understands the basic difference in materials than the conditions to process can be adjusted accordingly.  This difference can account for frustration when a processor whom only runs nylon has issues when processing ABS, the difference is one is semi-crystalline and the other is amorphous. What happens is that conditions and processing steps that work well with Nylon do not necessary work well with ABS.

A few of the various process differences:

Amorphous:

1-     Soften upon heating

a.       Adding more heat results in a softer flow material as flowabilty increases

2-     Less shrinkage thus less packing pressure needed in relation to fill pressure

3-     Stringing is taken care of by lower temperatures

Semi-Crystalline:

1-     Melts upon heating (there is also some softening due to amorphous regions)

a.       Once you have reached mid point on the melt range any additional heat only retards cooling rate,

b.      Flow can be enhanced by raising heats but this is due to the material now being hotter and taking longer to cool.

                                                                           i.      This is not a decrease in viscosity by heat

2-     High shrinkage thus a higher packing pressure is required in relation to fill pressure.

a.       It is not uncommon to use higher pressure to pack a part than to fill it

3-     Stringing is a major issue that is not always taken care of by lowering heats as this result's in gate freeze or blockage.

4-     Part size can be drastically changed by mold temperature setting which affects rate of cooling.

On the whole there are many thousands of material variations out there, each with its own characteristics. Not only that but alloys and blends, which each have their own special variations. Understanding the basic groups well help in the processing as the large families have similar properties in how they react to process conditions.