DESIGN FOR MANUFACTURING:DESIGN FOR BASIC PROCESSES—PLASTICS

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DESIGN FOR BASIC PROCESSES—PLASTICS

There are more than 30 distinct families of plastic, from which evolve thousands of types and formulations that are available to the functional designer. However, in the fabrication of plastics, either thermoplastic or thermosetting, only a limited number of basic processes are available. These processes include compression molding, transfer molding, injection molding, extrusion, casting, cold molding, thermoforming, calendering, rotational molding, and blow molding. The functional designer usually gives little thought to how the part will be made. He or she is usually concerned primarily with the specific gravity, hardness, water absorption, outdoor weathering, coefficient of linear thermal expansion, elongation, flexural modulus, izod impact, deflect temperature under load, and flexural yield, tensile, shear, and compressive strengths.

Compression Molding

In compression molding, an appropriate amount of plastic compound (usually in powder form) is introduced into a heated mold, which is subsequently closed under pressure. The molding material, either thermoplastic or thermosetting, is softened by the heat and formed into a continuous mass having the geometric configuration of the mold cavity. If the material is thermoplastic, hardening is accomplished by cooling the mold. If the material is thermosetting, further heating will result in the hardening of the material.

Compression molding offers the following desirable features:

1. Thin-walled parts (less than 1.5 mm) are readily molded with this process with little warpage or dimensional deviation.

2. There will be no gate markings, which is of particular importance on small parts.

3. Less shrinkage, and more uniform, is characteristic of this molding process.

4. It is especially economical for larger parts (those weighing more than 1 kg).

5. Initial costs are less since it usually costs less to design and make a compression mold than a transfer or injection mold.

6. Reinforcing fibers are not broken up as they are in closed-mold methods such as transfer and injection. Therefore, the fabricated parts under compression molding may be both stronger and tougher.

Transfer Molding

Under transfer molding, the mold is first closed. The plastic material is then conveyed into the mold cavity under pressure from an auxiliary chamber. The molding compound is placed in the hot auxiliary chamber and subsequently forced in a plastic state through an orifice into the mold cavities by pressure. The molded part and the residue (cull) are ejected upon opening the mold after the part has hardened. Under transfer molding, there is no flash to trim; only the runner needs to be removed.

Injection Molding

In injection molding, the raw material (pellets, grains, etc.) is placed into a hopper, called the barrel, above a heated cylinder. The material is metered into the barrel every cycle so as to replenish the system for what has been forced into the mold. Pressure up to 1750 kg / cm2 forces the plastic molding compound through the heating cylinder and into the mold cavities. Although this process is used primarily for the molding of thermoplastic materials, it can also be used for thermosetting polymers. When molding thermosets, such as phenolic resins, low barrel temperatures should be used (65– 120 C). Thermoplastic barrel temperatures are much higher, usually in the range of 175–315 C.

Extrusion

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding.

Casting

Much like the casting of metals, the casting of plastics involves introducing plastic materials in the liquid form into a mold that has been shaped to contour of the piece to be formed. The material that is used for making the mold is often flexible, such as rubber latex. Molds may also be made of nonflexible materials such as plaster. Epoxies, phenolics, and polyesters are plastics that are frequently fabricated by the casting process.

Cold Molding

Cold molding takes place when thermosetting compounds are introduced into a room-temperature steel mold that is closed under pressure. The mold is subsequently opened, and the formed article is transferred to a heating oven, where it is baked until it becomes hard.

Thermoforming

Thermoforming is restricted to thermoplastic materials. Here sheets of the plastic material are heated and drawn over a mold contour so that the work takes the shape of the mold. Thermosetting may also be done by passing the stock between a sequence of rolls that produce the desired contour. Most thermoplastic materials become soft enough for thermoforming between 135 and 220 C. The plastic sheet that was obtained by calendering or extrusion can be brought to the correct thermoforming temperature by infrared radiant heat, electrical resistance heating, or ovens using gas or fuel oil.

Calendering

Calendering is the continuous production of a thin sheet by passing thermoplastic compounds between a series of heated rolls. The thickness of the sheet is determined by adjusting the distance between the rolls. After passing between the final set of rolls, the thin plastic sheet is cooled before being wound into large rolls for storage.

Blow Molding

In blow molding, a tube of molten plastic material, the parison, is extruded over an apparatus called the blow pipe and is then encased in a split mold. Air is injected into this hot section of extruded stock through the blow pipe. The stock is then blown outward, where it follows the contour of the mold. The part is then cooled, the mold opened, and the molded part ejected. In very heavy sections, carbon dioxide or liquid nitrogen may be used to hasten the cooling. This process is widely used in molding high- and low-density polyethylene, nylon, polyvinyl chloride (PVC), polypropylene, pol- ystyrene, and polycarbonates.

Parameters Affecting the Selection of the Optimum Basic Process

Selecting the optimum basic process in the production of a given plastic design will have a significant bearing on the success of that design. The principal parameters that should be considered in the selection decision include the plastic material to be used, the geometry or configuration of the part, the quantity to be produced, and the cost.

If the functional designer cannot identify the exact plastic material that is to be used, he or she should be able to indicate whether a thermoplastic or thermosetting resin is being considered. This information alone will be most helpful. Certainly both thermoforming and blow molding are largely restricted to thermosetting resins, as is transfer molding. Injection molding is used primarily for producing large-volume thermoplastic moldings, and extrusion for large-volume thermoplastic con- tinuous shapes.

Geometry or shape also has a major impact on process selection. Unless a part has a continuous cross-section, it would not be extruded; unless it were relatively thin walled and bottle shaped, it would not be blow molded. Again, calendering is restricted to flat sheet or strip designs, and the use of inserts is restricted to the molding processes.

The quantity to be produced also has a major role in the selection decision. Most designs can be made by simple compression molding, yet this method would not be economical if the quantity were large and material were suitable for injection molding.

The following design for manufacturing points apply to the processing of plastics:

1. Holes less than 1.5 mm diameter should not be molded but should be drilled after molding.

2. Depth of blind holes should be limited to twice their diameter.

3. Holes should be located perpendicular to the parting line to permit easy material removal from the mold.

4. Undercuts should be avoided in molded parts since they require either a split mold or a removable core section.

5. The section thickness between any two holes should be greater than 3 mm.

6. Boss heights should not be more than twice their diameter.

7. Bosses should be designed with at least a 5 taper on each side for easy withdrawal from the mold.

8. Bosses should be designed with radii at both the top and the base.

9. Ribs should be designed with at least a 2–5 taper on each side.

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10. Ribs should be designed with radii at both the top and the base.

11. Ribs should be designed at a height of 1.5 times the wall thickness. The rib width of the base should be half the wall thickness.

12. Outside edges at the parting line should be designed without a radius. Fillets should be specified at the base ribs and bosses and on corners and should be not less than 0.8 mm.

13. Inserts should be at right angles to the parting line and of a design that allows both ends to be supported in the mold.

14. A draft or taper of 1–2 should be specified on the vertical surfaces or walls parallel with the direction of mold pressure.

15. Cavity numbers should be engraved in the mold. The letters should be 2.4 mm high and 0.18 mm deep.

16. Threading below 8 mm diameter should be cut after molding.

Table 5 identifies the major parameters associated with basic processes used to fabricate ther- moplastic and thermosetting resins.

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