When you buy metal for injection mold making, it is either in ingot or semifinished form. In making the injection mold, the metal is either melted and cast, or mechanically worked or machined to form the finished shape. Consequently, there is little, if any, difference in the composition and properties of the metal as-purchased and the metal in the completed tool. The producer of the metal has predetermined its properties by alloying or processing it to its as-purchased form.
When you buy a tooling plastic, it is usually in the form of a liquid (or paste)resin (a partially polymerized monomer) —or, basically, a chemical compound. In making the tool, the resin is mixed with other chemicals and often solid materials such as fillers or reinforcements, and is cast or laminated to the finished shape. Chemical reactions within the material actually change the material, producing its final performance capabilities. From one standpoint, you are actually making the material, along with the tool.
This is an important difference between metals and plastics, because the mixing, formulating and curing of the plastics determine to a great extent the properties of the finished material—just as the alloying of the metal in the producers plant has a determining effect on properties of the alloy. Consequently the engineer must know a great deal more about the basic chemical technology of plastics than he had to know about the alloying of metals. The subsequent chapters on materials and fabricating techniques attempt to clarify this area of technology. For more detailed information references at the ends of appropriate chapters should be consulted.
From the behavior standpoint, one of the important differences between plastics and metals is the sensitivity of plastics to the effects of time. Consequently they are much more sensitive than metals to rate of stressing, and aging under various environmental conditions. In spite of the newness of plastics, a substantial amount of design information is available. But long periods of time required for testing to determine effects of long-time use have limited the amount of data on creep, fatigue and long-time exposure to heat and chemicals.
Engineering properties as determined by standard test procedures are the only practical measure we have of predicting the behavior of a material in service. The engineering properties of plastics most pertinent to tooling use include
- mechanical properties (both short-term and longterm )，such as strength, hardness, and wear resistance, and their relationship to physical properties such as density and moisture absorption；
- thermal characteristics, such as heat resistance, thermal conductivity and thermal expansion characteristics； and
- chemical resistance.