Processing, thermoplastics injection molding

The diversity of release products and the wide range of release problems make classification difficult. One approach is by product form, with subdivisions such as emulsions, films, powders, reactive or inert sprays, reactive coatings, and so on. Another approach is by application, eg, metal casting, mbber processing, thermoplastic injection molding, and food preparation and packaging. 

Properties shown also apply to some polyesters formulated for thermoplastic processing by injection molding... 

Important are behaviors associated and interrelated with plastic materials (thermoplastics, thermosets, elastomers, reinforced plastics, etc.) and fabricating processes (extrusion, injection molding, blow molding, forming, foaming, reaction injection molding, etc.). They are presented so that the technical or non-technical reader can readily understand the interrelationships. 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Recendy, storm doors have been constmcted of advanced thermoplastic composites. Stampable, glass-mat reinforced polypropylene sheet is used to create a high strength outer skin. These compression molded skins are welded together using a friction or ultrasonic process then injection molded with a polyurethane foam core to produce an insulated structure. New technology for window frames incorporate the pultrusion of frame channels to produce a thermoset composite channel that can be filled with fiber glass for further insulation enhancement (12,31,33,34,48,49,54—56,60—67). 

Another proposed process employed injection molding in which starch and limited amounts of plasticizing water are heated under pressure to temperatures above the Tg and Tm to transform the native starch into a homogenous, destructured, thermoplastic melt. The process melt is then cooled to below the Tg of the system before pressure release to maintain the moisture content. Additives include natural and synthetic polymers, plasticizers and lubricants.136-139 159 160 The technology has been used to prepare pharmaceutical capsules and shaped objects, such as disposable cutlery, straws and pens. 

Thermoplastic polyurethane elastomers are processed by injection molding and other processes used for thermoplastics. Small gears, seals, and even automotive fender extensions can be produced by this means. 

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... 

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. 

Other developments for RIM in the future include Internal Maid Release - the successful application of this technology should bring the overall cycle-time for a RIM process down bo that of thermoplastic injection molding. 

Polyphenylene Sulfide Sulfone - Thermoplastic with good thermal stability, chemical resistance, and dielectric properties, and low flammability and creep. Processed by injection molding. Used in electrical components. Also called polyphenylene sulfide sulfone. 

Limera is a family of styrenic thermoplastic/ rubber compositions. The compositions are suitable for compounding with a variety of additives, fillers and polymers such as PS, PVC, PMMA, ABS and PPE. They are processable by injection molding, extrusion and blow molding. 

From the perspective of a life cycle analysis (LCA), the RIM process presents two winning cards with respect to thermoplastic injection molding the use of liquids instead of thermoplastic granules and the necessity to use much less clamping force during the molding process. 

Because of the low pressure (0.3 MPa) in the mold as compared to those encountered in thermoplastic injection molding (150 MPa) this process is suitable for the production of thin part with large surface areas. A significant application for this is in panel formation in the automotive industry, including fascias, door panels, spoilers, grills, and bumpers. While a rigid foam part with a flexural strength of 700 MPa would require a thickness of 7 mm, a RRIM part, because of the reinforcement, would allow a much smaller thickness (< 3 mm) by virtue of the increase in flexural modulus. 

Table 5.10 RTM, SRIM and thermoplastic injection molding processes compared...

A thermoplastic processed by injection molding, with properties similar to ABS, but higher elongation at yield. Also called ABS Nylon Alloy. 

Nylon 6/10 Thermoplastic polymer of hexameth-ylenediamine and sebacic acid. Has decreased melting point and water absorption and good retention of mechanical properties. Processed by injection molding and extrusion. Used in fibers and machine parts. 

Nylon 6/66 Thermoplastic polymer of adipic acid, caprolactam, and hexamethylenediamine. Has good strength, toughness, abrasion, and fatigue resistance, and low fiiction, but high moisture absorption and low dimensional stability. Processed by injection molding and extrusion. Used in electrical devices and auto and mechanical parts. 

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