Chapter 30 Processes Used to Condition Plastic Materials

Objectives Producing lightweight cellular plastics. Annealing, to relieving stresses caused by processing. Providing more thermal conductivity.

Annealing Annealing is used with plastics to relieve the internal stress of the material and to improve workability. Plastic materials are annealed by subjecting them to prolonged heating at a temperature slightly above their molding temperature, then slowly cooling them under controlled conditions. Annealing is recommended when the plastic part or product needs to have superior visual appearance, good stability, or exhibit high resistance to solvents.

Ionized Radiation Processing Electron beam acceleration is desired over the other processes. It improves thermal, impact, wear, and chemical resistance of commodity thermoplastics. With thermosets it improves stability, ability to withstand higher temperatures in use, resistance to chemicals and water, and abrasion resistance. Most popular uses are cold sterilization of surgical supplies and medical disposables and cold pasteurization of food.

Non ionizing Radiation Processing Induction heating is a common non ionizing radiation process. When used with non conducting material, a conductive metal attachment is used to transfer the heat to the work piece. Typical RF (radio frequency) power supplies range from 1 to 5 kW (kilo watts). The radiation can improve temperature and environmental characteristics.

Conductive Plastics Conductive plastic coatings are used in Electromagnetic Interference (EMI) shielding. Plastics are also used as thermal insulators to dissipate heat. Prior to the invention of thermal plastics, plastic parts exposed to heat often failed prematurely.

Plastics as Electrical Conductors Plastics are made conductive by adding metal to the plastic base material, resulting in a material referred to as synthetic material. Metal fibers have been used as conductive elements in antistatic fibers and yarns. Research is being conducted in using a conductive gel as a replacement for conductive solder in some industrial electronic applications.

EMI/RFI Shielding Plastic has become the dominant choice for construction of enclosures for computers, appliances, medical instrumentation, and telecommunication devices. One important reason for the popularity is the ability to add conductive metals to the polymer as a means of controlling the EMI/RFI (electro magnetic interference/radio frequency interference). These products can be made with standard injection molding and blow molding equipment.

Electroless Plating Electroless plating produces a coating of metal on plastic to achieve EMI shielding of plastic cases used on electronic products. Most paints are made by adding metal flakes of nickel or silver. The technology does not use current. Instead a chemical process is used.

Plastics as Thermal Conductors Thermal conductors are objects that transfer heat quickly. Thermally conductive plastics weigh about 40% less than aluminum. It is used by manufacturers of stepper motors and computer manufacturers to carry heat away from critical parts in confined spaces.

Plastic Additives Various additives may be used to provide plastics with the characteristics desired for a finished product. Chemical preservatives are added to products such as shower curtains, boat covers, pool liners that are subject to attack by mildew, fungus, mold and bacteria.

Summary Annealing is used with plastics to relieve the internal stress of the material and to improve workability. Electron beam acceleration is desired over the other processes. It improves thermal, impact, wear, and chemical resistance of commodity thermoplastics. Induction heating is a common non ionizing radiation process. Plastics are made conductive by adding metal to the plastic base material, resulting in a material referred to as synthetic material. Metal fibers have been used as conductive elements in antistatic fibers and yarns. Chemical preservatives are added to products such as shower curtains, boat covers, pool liners that are subject to attack by mildew, fungus, mold and bacteria.

Home Work 1. What is a common ionized radiation process? 2. What is a common non ionized radiation process? 3. How are plastics made conductive?