IX Международная студенческая научная конференция Студенческий научный форум - 2017

Introduction

Because thermoset plastics undergo an irreversible chemical reaction (cross-linking or polymerization) involving a time-temperature exposure, processing of such materials consists of: (1) getting the resin formulation to a liquid state, (2) causing the liquid to flow into a cavity of the desired configuration, (3) heating the liquid sufficiently to cause the chemical reaction to advance to the point where the plastic is essentially rigid (conforming to the cavity configuration), and (4) removing the rigid part from the cavity. The other critical variable in most thermoset processing is pressure. Pressure on the molding compound is needed: (1) to create the flow of plastic, during its liquid state, into the interstices of each cavity, prior to the material becoming so viscous from the cross-linking reaction that it flows no more; and (2) to ensure that the plastic, during crosslinking, is kept at maximum density in order to obtain optimum physical properties of the molded part. The several processing techniques described in this chapter represent the current methods for effecting the liquification of the formulation, the timely flow into the cavity, and the required heat and pressure to enable the chemical reaction to proceed to completion as rapidly as practical in order to achieve an acceptably short production cycle with full densification of the plastic.

Casting with liquid resins

For prototyping or for limited production runs, liquid resin casting offers simplicity of process, relatively low investment in equipment, and fast results. Thermoset casting resins may be epoxies, polyesters, phenolics, etc. The resins harden by a polymerization or cross-linking reaction. Such resins are often poured into open molds or cavities. Because pouring is done at atmospheric pressure, molds are simple, often made of soft metals.

An example of liquid casting is in fabrication of design details such as scrolls and floral or leaf patterns for furniture decoration. Such parts are often made from filled polyester resins, and the parts, after curing, are simply glued to the wooden bureau drawer or mirror frame. The parts can readily be finished to look like wood. Molds for such parts are often made by casting an elastomeric material over a wood or plastic model of the part. When the elastomer is removed, it generally yields a cavity enabling very faithful reproduction of the original pattern.

Another widely used industrial application is casting with liquid resins to embed objects, such as electronic components or circuits, in plastic cups or cases or shells, giving the components mechanical protection, electrical insulation, and a uniform package. Such processing, when the case or housing remains with the finished part, is called potting.

When casting applications require fairly high-volume production, machines for mixing and dispensing the liquid plastics may be used for shorter production cycles, and curing ovens, conveyors, and other auxiliary capital equipment may be added. In short, the liquid resin casting operation may be a low-cost manual one, or it may be highly automated, depending on the nature of product desired and the quantities required.

Potting of high reliability products is often done under vacuum to ensure void-free products, and may be followed with positive pressure in the casting chamber to speed the final penetration of casting formulation into the interstices of the component being embedded before the resin system hardens.

Hand lay-up (composites)

When larger plastic parts are required, and often when such parts must be rigid and robust, a process referred to as hand lay-up is closely related to liquid resin casting.

A reinforcing fabric or mat, frequently fiberglass, is placed into an open mold or over a form, and a fairly viscous liquid resin is poured over the fabric to wet it thoroughly and to penetrate into the weave, ideally with little or no air entrapment. When the plastic hardens, the object is removed from the mold or form, trimmed as necessary, and is then ready for use.

Many boats are produced using the hand lay-up process, from small sailing dinghies and bass boats, canoes, and kayaks to large sailboats, commercial fishing boats, and even military landing craft.

This basic process can be automated as required, with proportioning, mixing, and dispensing machines for liquid resin preparation; with matched molds (that is, with two mold halves closed after the reinforcing material has been impregnated with the liquid to produce a smooth uniform surface on both top and bottom of the part); and with conveyors, curing ovens, etc.

Compression molding

Compression molding is a process that is very similar to making waffles. The molding compound, generally a thermosetting material such as phenolic, melamine, or urea, is placed in granular form into the lower half of a hot mold, and the heated upper mold half is then placed on top and forced down until the mold halves essentially come together, forcing the molding compound to flow into all parts of the cavity, where it finally “cures” or hardens under continued heat and pressure. When the mold is opened, the part is removed and the cycle is repeated.

The process can be manual, semi-automated, or fully automated (unattended operation), depending on the equipment. Molds are generally made of through-hardened steel and are highly polished and hard chrome plated, and the two mold halves, with integral electric, steam, or hot oil heating provisions, are mounted against upper and lower platens in a hydraulic press capable of moving the molds open and closed with adequate tonnage to make the plastic flow.

Molds may be single cavity or multiple cavity, and press tonnage must be adequate to provide as much as 300 kg/cm2 for phenolics, less for polyesters, of projected area of the molded part or parts at the mold parting surfaces. Overall cycles depend on molding material, part thickness, and mold temperature, and may be about 1 min for parts of 3-mm thickness to 5 or 6 min for parts of 8-cm thickness.

The process is generally used for high-volume production because the cost of a modern semi-automatic press of modest tonnage, say 50 tons clamp, may be as much as $50,000, and a moderately sophisticated self-contained multicavity mold may also cost $50,000.

Typical applications include melamine dinnerware; phenolic toaster legs; and pot handles, electrical outlets, wall plates, and switches— parts which require the rigidity, dimensional stability, heat resistance, or electrical insulating properties typical of thermosetting compounds.

To simplify feeding material into the mold, the molding compound is often precompacted into “preforms” or “pills” on a specially designed automatic preformer which compacts the molding compound at room temperature into cylindrical or rectangular blocks of desired weight of charge. And to reduce molding cycle time, the preform is often heated with high-frequency electrical energy in a self-contained unit called a preheater, which is arranged beside the press. The preform is manually placed between the electrodes of the preheater before each molding cycle, and heated throughout in as little as 10 to 15 s to about 90°C, at which temperature the plastic holds together but is slightly mushy. It is then manually placed in the bottom mold cavity and the molding cycle is initiated. Cure time may be cut in half through the use of preheating, a step which reduces mold wear and improves part quality.

Transfer molding

A related process for high-volume molding with thermosetting materials is transfer molding, so called because instead of the material being placed between the two halves of an open mold, followed by closing the mold, to make the material flow and fill the cavity, the material is placed into a separate chamber of the upper mold half, called a transfer pot, generally cylindrical, which is connected by small runners and smaller openings called gates to the cavity or cavities.

In operation, the mold is first closed and held under pressure; the preheated preform is dropped into the pot; a plunger comes down into the pot where the material liquefies from the heat of the mold and the pressure of the plunger, and flows (is “transferred”) through the runners and gates into the cavity or cavities. The plunger keeps pushing on the molding compound until the cavities are full and until the material cures. At that point, the mold is opened, the plunger is retracted, and the part or parts, runners, and cull (the material remaining in the pot, generally about 18 in thick and the diameter of the pot and plunger) are removed. Because the gate is small, the runners and cull are readily separated from the molded parts at the surface of the parts, leaving a small and generally unobtrusive but visible “gate scar”.

Transfer molding is often used when inserts are to be “molded in” the finished part, as, for example, contacts in an automobile distributor cap or rotor or solenoid coils and protruding terminals for washing machines. Whereas in compression molding, such inserts might be displaced during a compression molding cycle, in transfer molding the inserts are being surrounded by a liquid flowing into the cavity at controlled rates and pressures, and generally at a relatively low viscosity. The inserts are also generally supported by being firmly clamped at the mold parting line or fitted into close-toleranced holes of the cavity. Also, when dimensions perpendicular to the parting line or parting surfaces of the mold must be held to close tolerances, transfer molding is used because the mold is fully closed prior to being filled with plastic.

With compression molding, parting line flash generally prevents metal-to metal closing of the mold halves, making dimensions perpendicular to the parting line greater by flash thickness—perhaps as much as 0.1 to 0.2 mm.

Transfer presses and molds generally cost 5 to 10% more than compression presses and molds, but preheaters and performers cost the same as for compression molding. Transfer cycle times are often slightly shorter than compression molding cycle times because the motion of the compound flowing through the small runners and gates prior to its entering the cavity raises the compound temperature by frictional and mechanical shear, therefore accelerating the cure.

References

1. Charles A. Harper Modern plastics handbook

2. GOST 11645-73

3. Billmeyer, F. W., Jr., Textbook of Polymer Science, 2d ed., John Wiley and Sons, Inc., New York, 1962, p. 439.

4. Strong, A. B., Plastics: Materials and Processing, Prentice-Hall, Englewood Cliffs, N.J., 1996, p. 193.

5. MacKnight, W. J. and R. D. Lundberg, “Research and ionomeric systems,” in Thermoplastic Elastomers, 2d ed., G. Holden et. al., eds., Hanser Publishers, New York, 1996, p. 279.

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