Pressure molding

Molding and injection Compression ratio Compression molding pressure (lbf/in. ) Compression molding temperature (°C) Injection molding pressure (Ibf/in. ) Injection molding temperature (°C) Molding qualities Mold (linear) shrinkage (in./in.) Specific volume (Ib )... 

TSs has experienced an extremely low total growth rate, whereas TPs have expanded at an unbelievably high rate. Regardless of the present situation, CM and TM are still important, particularly in the production of certain low-cost products as well as heat-resistant and dimensionally precise products. CM and TM are classified as high-pressure processes, requiring 13.8-69 MPa (2,000 to 10,000 psi) molding pressures. Some TSs, however, require only lower pressures of down to 345 kPa (50 psi) or even just contact (zero pressure). 

Compression molding Filament winding Hand lay-up Mat molding Pressure bag molding Continuous pultrusion Injection molding Spray-up... 

Rotolining ETEE Rotationally molded Pressure ok Max size Visual By hot patching... 

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under molding pressure to fill the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Molding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Use a higher temperature on the mold side, where high surface finish is required combined with internal mold pressure during cure... 

Thermoset molding compounds, when contained within a hardened steel mold, require heat and pressure to be polymerized into a solid mass. Molds may be heated by steam, electricity, or hot oil to temperatures of 280° to 425°F, depending entirely on the type of material and method of molding. Molding pressures may vary from a low of 50 p.s.i. to 15,000 p.s.i. Epoxy materials will mold at 50 p.s.i. whereas, phenolic fabric-filled material may require excessive pressures. Again, the method of molding dictates molding pressures. 

Processing. Polycarbonates may be fabricated by all conventional thermoplastic processing operations, of which injection molding is the most common. Recommended operating conditions are stock temperatures of 275—325°C and molding pressures of 69—138 MPa (10,000—20,000 psi). 

Fig. 3. Core torque related to channel length versus angular velocity of core rotation (O) at different specific molding pressures (MPa/m) 1-5 2-30 3-45 4-60...

P molding pressure, external pressure Pco, collapsing hydrostatic pressure... 

In order to obtain in practice the theoretical highest apparent density of the syntactic foam, the mixture has to be molded at the pressure at which the microspheres pack the closest. Several methods (mold vibration, high molding pressures, etc.) have been recommended as ways of obtaining a syntactic foam density close or equal to y by helping the microspheres pack more closely and thus approach the foam s maximum strength. It should be remembered that ideal packing is never actually achieved and there is always some disorder 80). 

Fig. 5. Molding pressure (P) versus binder concentration (C) for BV-01 phenol microspheres and binder viscosities of 1.25 (1) and 10.8 (2) Pa s...

The properties of the final material are greatly influenced by the mold pressure, and the strength of a carbonized syntactic material can be doubled in this way (Fig. 8). 

Fig. 8. Effect of molding pressure P on the stress in compression (crc) for a carbonized foamed plastic made from a compound binder (wood resin, furfuryl alcohol, maleic anhydride) and phenolic microspheres 381...

The viscosity of the resin can be maintained at a much higher level in the heated mold. This allows the material to hold molding pressure, which results in much improved surface finish. 

The gas counterpressure IM method uses a sealed mold pressurized to 2.8 to 3.5 MPa (400 to 500 psi) with an inert gas, sufficient pressure to suppress foaming as the plastic mix enters the mold cavity. After the measured shot is injected, the mold pressure is released, allowing the instantaneous foaming to form the core between the already formed... 

With the pressures in the mixing head at between 1,500 to 3,000 psi (10.3 to 20.6 MPa), the in-mold pressures are significantly lower than in many of the other molding processes. When comparing a typical RIM in-mold pressure of 50 to 150 psi (0.4 to 1.1 MPa) with the 5000 to 30,000 psi (34.5 to 206.7 MPa) required for thermoplastic injection molding (Chapter 4), it becomes apparent why RIM is particularly suitable for larger parts. Automotive bumpers are routinely produced on RIM presses with 100 to 150 tons of clamping force, while comparable injection molded parts require presses of 3500 tons or more. 

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