Low Production Processes

Unlike the high volume production process discussed earlier, the low volume production processes do not adapt well to automation and mass production. With the exception of dip molded gloves, these processes require an operator to load and unload material and parts.
 

Compression Molding

Compression molding was developed in 1909 by Leo Bakeland to produce phenolic radio cabinets. Phenolic was originally named after Bakeland and called Bakelite. In the compression molding process, illustrated in Figure 7-6, a thermoset plastics, phenolic, melamine, or polyester, is molded in an enclosed mold between two heated platens. The plastics, in powder, pellets, liquid, or performs, is introduced into the mold in a partially cured condition. A perform is the exact amount of material required to mold the part pressed into a large pellet or plug. The thermoset materials are introduce into the mold just prior to molding. The platens that hold the two halves of the mold are heated to approximately 350 degree F. Pressure in the range of 1,500 PSI is exerted on the material. The shearing action of the material being compressed together and the heat from the platens cause the plastic to become soft. The soft plastic fills the cavity and is compressed by the pressure. The contributory strength of the temperature and the pressure accelerates the curing of the plastic in approximately four minutes. The compression molding process produces a heavy and dense product. Thermoset products have the highest electrical, heat and chemical resistance of any plastics material.

Compression molds are machined from steel. Their cavities are designed to be full positive, semi-positive, landed positive or flash molds shown in Figure 7-7. The fully positive mold requires that the exact amount of material be placed in the mold. The fully positive mold does not allow excess material to flow out between the parting line. Semi- positive molds allows for flash (material that flows out of the mold at the parting line) of excess materials just prior to the final closing of the mold. Landed positive molds allow for flashing in a prescribed area which forms a land or tab which can be removed and subsequently machine polished after molding. Flash molding allow for the elimination of excess materials during the final moments of the compression cycle. Products are more easily produced from flash mold since they do not require the exact amount of material. But, they must be machined and polished in post molding operations.

Handles for cooking pots, housings for hot plates, electrical receptacles, and high voltage switch housing are common products. Melamine, which enjoys lighter colors, is used in inexpensive dinner ware, kitchen utensils, and high pressure laminated counter tops. Liquid thermoset polyesters resins are molded into cafeteria trays, banquet hot food covers, and electrical boxes.
 

Transfer Molding

The thermoset plastics used in compression molding are ideal for electrical connector products. However, the high pressures produced in the compression mold cavity distort and crush any small metal conductors placed in the mold. Transfer molding, illustrated in Figure 7-8, heats and compression the material in a pot built over an enclosed mold cavity. When the phenolic, melamine, or polyester becomes semi-soft it flows under reduced pressure through a sprue shaped channels and fills the enclosed mold. Multiple cavities can be feed by runners from the sprue opening.

The low pressures of the transfer process allows for the molding of thin inserts without deformation from the molding process. Multi-pin connecting busses for telephones, electrical, and computer cables are produced through transfer molding. Both compression and transfer molded parts are rigid and dimensionally stable. The molded part's properties are not affected by elevated temperatures. Unlike the characteristics of the injection molded thermoplastics, these thermoset parts withstand long-term stress.
 

Rotational Molding

Rotational molding, illustrated in Figure 7-9, is a three station process. An enclosed mold rotates biaxally as it is moved from a loading, heating, and cooling station. The mold is attached to the end of a three arm turn table. The mechanism at the end of each arm is capable of holding the mold and rotating it throughout the three stage process. A pre-measured amount of powder of liquid plastic is placed inside the mold at the first station. At the same time, a second arm is located at the second stage, inside an oven, one third the distance around the circumstance. As the mold rotates in its second stage, the oven heats the mold and the plastic. As the mold rotates, the plastic falls against the wall melting and fusing plastic together. Once all the plastic has coated, melted, and fused together, the mold is moved to the cooling stage. In the cooling phase, the mold continues to rotate as water is sprayed over the mold surface. The mold and plastic cool and then are rotated for removal. The mold is continuously biaxally rotated through out the second and third stage. When it returns to its initial spot, the mold stops rotating to remove the part and to recharge the mold.

Rotational molding is a relatively slow and low pressure process. Plastic parts that are enclosed, such as children toys, barrels, surf boards, tanks, and watering pitchers, are rotationally molded. Molds can be easily fabricated from aluminum sheet or castings. Molds are thin walled for good heat transfer. The wall thickness of the part is controlled by the amount of material placed in the mold. The interior surfaces of all rotationally molded part are smooth with a high gloss finish. The exterior appearance of the part duplicates the mold surface finish. Generally, these products have thick walls and are heavy. Small rotational molding machines have multiple cavities for the production of enclosed hollow toys and balls. Large 40,000 gallon acid storage tanks have been successfully molded. Liquid storage tanks with complex shapes are easily and economically produced by rotational molding.
 

Dip and Slush Plastisol Molding

Plastisol molding produces flexible parts that are used as covers or coating. The process requires a metal mold capable of withstanding the 300 to 400 degree F molding temperatures and liquid plastisol. Plastisol is a solution of powdered polyvinyl chloride (PVC) and plasticizer. The mold is preheated to 350 degrees F. It is then dipped into (for as in dip molding) or filled with (for open slush molding) plastisol. The heat from the mold moves into the plastisol heating and swelling the PVC particles. The mold remains in contact with the liquid Plastisol until the heats penetrates to 1/16 or 1/8" depth. The heated PVC particles swell, absorb the plasticizer, and then come in contact with each other. From there, the dip mold is then removed from the plastisol. In slush molding the excess Plastisol is poured out of the mold. The coating on the surface remains. The thickness of the coating is determined by the amount of time the mold stays in contact with the liquid plastisol. The mold is then reheated to 350 degrees F. The reheating fuses the PVC particles together and forces off any excess plasticizer converting the remaining plastisol into an homogenous mass. The plastisol fusion process is illustrated in Figure 7-10.

The ratio of PVC to plasticizer determines the hardness of the coating. Rigid plastisol coating has PVC to plasticizer ratios of 100 part of PVC to 25 parts plasticizer. which results in Shore A hardness readings (Rockwell Hardness Test) of 8 duramoter. Plastisol with 1:5 ratios are much softer, with Shore A hardness reading of 98. Plastisols are also used in rotational molding, spray coatings. Common dip molded products are: gloves, tool handle coatings, lip on grips, cushioning for kitchen implements, covering for sharp objects, insulation and the oval shaped coin purses. Common slush molded products are arm rests, head rests, road safety cones, doll parts, and toys. The resiliency of plastisol parts is seen kitchen gloves which can be pulled and stretched without tearing and returned to their original shape.
 

Casting, Potting and Encapsulation

Thermoset plastics and thermoplastic acrylics in liquid form can be poured into molds or coated on surfaces. The plastics then hardens at room temperature or cured in an oven. Typical products produced include art sculptures, decorative objects, simulated wood frames, figurines, skate board wheels, enclosures for electrical devices, or other thick complex shapes.

The most used plastic resins include: thermoset polyurethane, polyester, epoxy, silicone and thermoplastic acrylic. These resins are formulated to chemically cure and crosslink at room temperature. The chemical reaction is exothermic; heat is given off form the center. The resins and the reaction have toxic by-products such as solvents and fumes. These by-products have limited their use in consumer products for various reasons.

Molds for casting are made of materials resistant to the chemical attack of the resins. Flexible polyurethane and silicone molds are used for intricate shapes and detailed parts. Polyethylene molds are the most inexpensive. The polyethylene has limited flexibility. A part cast in polyethylene molds resists details.
 

Reinforced Plastic - Composite Processes

A reinforced plastic composite is a two part component of fiber and plastic resin. The fiber can be in the form of powder filler, particulate, or laminate mat and cloth. The flexible fibers easily form to the contours of a shaping device and provide high strength to the part. The plastic resin acts as the glue to bind and hold the fibers in their molded shape. The strength and integrity of the composite part is dependent upon the bond between the reinforcing fiber and the plastic material. Plastic materials include both thermoset polyester and epoxy. Fiber reinforcements include glass, carbon, stainless steel, and nylon fibers. There are five major reinforced plastic processes: hand lay-up, spray-up, pultrusion, filament winding, and matched die molding.
 

Hand Lay-up

The largest number of reinforced plastics composite products are produced by the hand lay-up process, illustrated in Figure 7-11. These include larger structural parts such as: boats, portable toilets, picnic tables, car bodies, diesel truck cabs, hard shell truck bed covers and high performance air craft skins and interiors. The hand lay-up process is labor intensive plus the plastic resins produce toxic fumes requiring well ventilated facilities and protective equipment for workers.

The hand lay-up process produces parts from an open, glass reinforced, mold. The mold surface is treated with several layers of release wax and then spray coated with a pigmented polyester resin called a gel coat. The surface of the mold will be duplicated by the gel coat. Over the gel coat, glass fiber (in the form of chopped strand mat or glass cloth) is layered. Each layer is saturated with polyester resin that is specifically formulated to cure at room temperature. The polyester resin must be catalyzed 2% by weight. The catalyst is a toxic and corrosive peroxide. Each glass layer is pressed by hand with rollers to work the polyester resin into the glass fiber. Three to nine layers are added and allowed to cure depending on the desire strength of the part. Once the polyester is cured, the part is removed from the mold. The part requires trimming and post mold surface preparation. In the production of the high performance airplane, both carbon fiber and epoxy plastic are used.

 Hand lay-up parts can consist of any size or configuration. Moreover, the process does not require any special tools. Parts can be easily fabricated as a weekend backyard project. Molds can be easily modified, cut into part for prefabrication and applied to create various surface textures. The rigid properties of the final product require that under cuts and straight wall be eliminated. Any openings must be machined in post molding operations. All corners must have generous radii. The structure is often stiffened with the additions of honeycomb, balsa or rigid foam blocks.
 

Spray-up

The spray-up process, illustrated in Figure 7-12, is similar to hand lay-up except that a mechanical spray system is used to apply both chopped glass strands and polyester resin. The spray system, often called a chopper gun, is connected to resin, catalyst, and glass strands. Air pressure is used to spray both chopped glass into a catalyzed resin stream. The steam is directed toward the mold surface. The ratio and amount of glass and resin is controlled by the air pressure and gun. The thickness of resin and glass is controlled by the speed of the gun moving across the surface. Compared to hand lay-up, the spray-up process requires more skill but it's faster, and can be automated more easily. Spray-up systems are often used to back up thermoformed acrylic plastic sheet for bathtubs, shower stalls, hot tubs, camper tops, cycle fairing, and pool accessories.
 

Pultrusion

Pultrusion, illustrated in Figure 7-13, produces a part that looks similar to an extruded product. In pultrusion, continuous glass strands are pulled through and completely saturated with a catalyzed plastic resin. Once through the resin the strands are pulled through a forming die. The dye is heated partially sets the strands and resin into the continuous profile die shape. The pultrusion is then oven cured after leaving the die. Pipe, tubing, structural beams and special shapes are pultruded products. The products are heavy with high strengths. They are often used to replace metal structural parts when a corrosive resistant or non-magnetic material is required.
 

Filament Winding

Filament winding, illustrated in Figure 7-14, is similar to pultrusion. A glass strand is drawn through a catalyzed resin bath. After the strand is saturated with resin, it is wound on a mandrel. The mandrel is turned and the glass wraps around the mandrel. The mandrel determines the shape of the final product. Poles for fishing and golf cubs are made by winding glass or carbon fiber strands onto steel rods. Large service station gasoline storage tanks are produced by winding glass strands around large rubber inflatable bags. Once the resin has cured to a rigid form, the rubber bag is deflated and removed. Glass tape is often used rather than glass filament. The tape is wound around a cone-shaped mandrel. Once the resins cures, the hardened part is removed and used as exhaust cones on the space shuttle and launch vehicles.
 

Matched Die Molding

Matched die molding, illustrated in Figure 7-15, is similar to low temperature compression molding. A matched metal die mold is used to shape the material. A premix of polyester resin and glass fibers, called a batch, is placed in the mold. The two halves of the mold close and shape the part. The mold is heated to the curing temperature of the resin varying from 325 degrees to 400 degrees F. The process produces parts with uniform thickness and fully cured resin. Pre-impregnated glass cloth with polyester resin, called sheet molding compound (SMC) is used to produce luggage, automobile body parts, and outboard motor covers. The pre-impregnated sheet is passed between metal stamping dies that shape and cure the resin. The process works similarly to the metal stamping process. Matched die molded parts are hard and have a uniform wall thickness. Ordinary uses for this process include: cafeteria trays and automotive fender liners.