Types of Plastic

A resin compound of polystyrene and styrene copolymers, blowing agent, and other additives is known generically as expandable polystyrene or EPS. Pentane is frequently used as the blowing agent. EPS can be used to produce low density foam products. The largest applications for EPS are thermal insulation, disposable drinking cups, and cushioned packaging materials.

EPS is typically supplied as beads having a diameter from 0.01 – 0.1 inches. These beads are mostly made by suspension polymerization, but are sometimes larger particles are produced using pelletization. The choice of particle size is based on the desired characteristics of the foam product. The wall thickness of the resulting foam will be equal to the diameter of the starting particle. The starting particle size can influence the final product in other ways as well. For instance, lower densities foam is easier to obtain with larger particles. Small particles, on the other hand, make it easier to uniform fill a part.

Many properties of foam are strongly influenced by the density of the product. Some of the properties affected by foam density include energy absorption, resistance to heat flow, stiffness to weight ratio, buoyancy, and cost per volume. Because these properties functions of product density, manufactures have fine control of the final product simply by adjusting the particle size of the input resin and some processing changes. Often, there is no need for any retooling or other equipment changes.

Polyethylene terephthalate or PET, which is polymerized using a high purity antimony trioxide catalyst, is a very common resin used in the manufacture of beverage bottles. However, to be used in other applications, PET needs to be reinforced to increase strength, stiffness and heat-resistant properties. Engineering-grade PET has all of these characteristics.

Engineering-grade PET resin is typically reinforced with glass fibers, glass flakes, minerals or micas. One very common formulation is a 30% glass-reinforced PET. These upgraded PET resins have superior strength and stiffness as well as excellent dimensional stability. In addition they have better chemical and heat resistance than their unreinforced counterparts and have good electrical properties.

These enhanced properties make engineering-grade PET appropriate for a variety of industries including the automotive industry, electronics, appliances and furniture. Basically, engineered PET resins can be used in any application that needs and very sturdy plastic that will be under a lot of stress.

Polyurethane foam is one of the most versatile plastics used today. To manufacture polyurethane foam you need the following basic equipment: material supply containers, metering units, mix heads, temperature control systems, and process control systems. Depending on the complexity of the line, other specialty equipment may be needed, but these basic components can get the job done.

Material supply containers are used in many phases of the manufacturing process. First, the raw materials are delivered in some kind of container like a tote bin or drum. Alternatively, if you receive your raw materials via tank car or railcar then you will need some kind of container to transfer the materials to. Materials can also be stored in blending containers or intermediate storage tanks.

Metering units are needed to deliver all raw materials to the machine in varying speeds and manners. Metering units are considered either low-pressure, high-pressure, or some combination of the two. Whatever type you use, materials need to be delivered to the mix head with plus or minus 1 percent accuracy. Sometimes materials are used at rates of ounces per second and other times in quantities as large as 1000 lb/min.

Mix heads are where all the action happens. Materials are mixed either mechanically in a low-pressure system or with impingement mixing in high-pressure systems. Low-pressure systems give excellent mixing but need to be purged quite often, which can slow down production. High-pressure systems do not need purging or flushing but you need to perfect the diameter in order to prevent poor mixing or splashing.

Finally, your process needs to be maintained by temperature and process control systems. The temperature control system will make sure your product is consistent by accounting for any heat generated by the mechanical aspects of production. The process control system can be as simple as an on and off switch or very complicated because they monitor, control and provide data for statistical analysis. These parts of the manufacturing line will account for the bulk of the cost of the entire system but are critical to producing quality material in a timely manner.

Styrene-acrylonitrile (SAN) copolymers are transparent plastics with excellent chemical resistance and high heat resistance. They are usually hard and rigid and as a result have good dimensional stability and load-bearing capability. For all the benefits you can reap with SAN resins, they are most often chosen for clear applications because of their transparency.

As with other polymers, properties like tensile strength and resistance to impact are adjusted by changes in molecular weight. Changes in the styrene/acrylonitrile ratio can also contribute to these variations. Tensile strength can range from 10,000 and 12,000 psi and notched Izod impact values range from 0.2 to 0.5 ft.-lb./in.

SAN is not the only styrenic plastic commonly used. ABS or acrylonitrile-butadiene-styrene is used in even larger volumes than SAN. SAN is basically ABS without the rubber. Because it has no rubber, the impact resistance suffers, but heat and chemical resistance, tensile strength and flexural strength are superior.