Processing, thermosets machining

While injection molding machines designed specifically for processing thermoset materials are available, a number of manufacturers offer replacement screw and barrel assemblies and stuffer hoppers to convert conventional thermoplastic injection moulding machines to process thermosets. 

Advantages of processing thermosets on injection moulding machines ... 

For injection machines to be used for processing thermosets, the following equipment is required ... 

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

Machining practically all the thermosets can be machined to some degree by almost all the metal machining methods after adaptation of the tools and processes to a greater or lesser extent ... 

In addition to the advantages of the composite plate over the traditional graphite plate mentioned before, the carbon/carbon composite plates have the advantage of lower densify (about 30% lower than the thermoset- or thermoplastic-based composite plates [16]) and higher manufacturing efficiency. This offers the potential of continuous production in comparison with the machining process for graphife plates. 

Engineers like to design machines that take plastics and inject them into molds, extrude them into filament, sheets, rods, and film, or blow-mold them into shapes. They want these plastics to have a terrific balance of mechanical, electrical, and chemical properties. That s why they have attached their name to engineering resins, a set of plastics unlike thermosets that they can remelt and further process mechanically. Nylon, polycarbonates, and polyesters are the three most popular engineering resins. 

Reaction injection molding (RIM) is a fast, low-pressure, low-temperature, low-cost process for one-step conversion of reactive liquids into large finished solid plastic products. Liquid polyol and liquid diisocyanate are mixed by impingement, pumped instantly to fill a large mold cavity, and polymerize/ cure rapidly to form a thermoset polyurethane product. The cured polymer may be a stiffly flexible product such as automotive bumper covers, front ends, and trim or a rigid foamed product such as furniture and housings (cabinets) for computers, business machines, TY and radio. 

Thermoplasts, after being combined with energy carriers, are processed on hot rollers into a plastic material. The subsequent shaping is achieved by means of hydraulic mold presses and cutting machines. Depending on the binder type, the resulting powder kernels are cured (thermoset material), cooled (thermoplast), or dried by the removal of solvents (gelatin). 

Until the 1960s TSs (thermosets) were primarily molded using compression or transfer presses (Chapter 14). At that time screw injection machines with modifications were developed to process TSs. These modifications included low to zero compression for screw depths, deeper channel depths, short length to diameter screws (L/Ds), tool steel construction, barrel cooling with heat transfer fluids, and spiral down discharge ends in place of non-return valves.3... 

The process of injection molding (IM) is used principally for processing unreinforced or glass fiber reinforced thermoplastics (TPs) and thermosets (TSs) (Figure 4.1). Up to at least 90wt% of all plastics processed are TPs. There are many different types or designs of IM machines (IMMs) that permit molding many different products based on factors such as quantities, sizes (such as auto bumpers to medical micro products), shapes (simple to complex), product performances, and/or economics.1,150>157,173 176 476... 

Wet layup consists of placing a layer of dry reinforcement inside a mold and then applying an uncured, low-viscosity thermoset resin as shown in Fig. 2. Woven glass fibers are the prevalent reinforcing preform utilized in layup processes, although carbon and aramid fibers are also used to a lesser extent. Typical fiber volume fraction of composites manufactured via wet layup range between 30% and 50%. The resin can be poured, sprayed, or brushed on top of the preform layer either by hand or by machine. The fiber preform layer is rolled on or pressed after the application of resin to evenly distribute the resin and remove air pockets. Resin is applied on top after each layer of fiber mats is properly placed. This process is repeated until the desired thickness is reached. To provide a smooth surface finish on the mold side, a thin layer of mold release is often applied prior to starting the layup. Thereafter, pressure and heat are applied to allow the composite to cure. Pressure can be applied... 

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