Machinery's Handbook, 31st Edition
MANUFACTURING WITH PLASTICS 587 Reinforced Plastics.— For superior strength and modulus, many items are reinforced with cloths of various fibers and weaves. Felted mats are often cross-layered and then impregnated with liquid resin (e.g., polyester or epoxy) prior to being cured to make final products. Some of these products, if not too large, can be compression molded. But with judicious catalysis, the resin can be cured at room temperature, facilitating manufacture at low pressure, with or without heat, of large structures such as boats and storage tanks for liquids. With layup one can handily distribute the reinforcing web in accordance with the stress distribution expected when the product is placed in service. Fig. 16 shows four increasingly positive (and more costly) ways of curing hand layups. Also, many laminates are produced in sheets, typically 4 ft by 8 ft (122 by 244 cm), for subsequent fabrication into final products. The familiar decorative laminates (Formica), for example, are pro - duced in large presses with multiple steam-heated platens interleaved between laminates, permitting production of dozens of sheets in one pressing. Properties of some reinforced plastics available in sheet form are given in Table 6. Table 6. General Properties of Reinforced Plastics (ASTM Test Procedures)
Tensile
Heat Resistance Continuous ° F
Compressive Strength 10 3 psi
Flexural Strength 10 3 psi 40–90 25–40 50–70 13–28 13–18 70–100 20–26 19–24
Izod Impact Strength ft-lb/in notch
Arc Resistance seconds
Specific Gravity
Strength 10 3 psi
Modulus 10 6 psi
Material
Polyester 25–50 15–50 30–50 20–25 23–24 50–70 30–38 20–28 35–40 Phenolic Epoxy
Glass cloth Glass mat Asbestos
1.5–2.1 30–70 1.6–1.9 30–60 1.3–2.3 20–25 0.5–2 l–3 l–3 1.2–1.5 6–14 0.5–1.5
5–30 2–10 2–8 l–2 1–4
300–350 60–120 300–350 120–180 300–450 l00–140 220–250 28–75 230–250 70–85 330–500 100–110 330–500 110–125 260–300 30–100 350–500 20–130 350–500 40–l50 350–600 120–200 225–250 Tracks 225–250 Tracks 150–165 Tracks 400–700 150–250
Paper
Cotton cloth 1.2–1.4 7–9 0.5–l.5
Glass cloth Glass mat
1.9–2.0 20–60 2–4 1.8–2.0 14–30 l–3 1.4–1.5 10–19 0.5–1
11–26 8–15 0.5–1 10–35 8–16 0.3–1 0.5–3 2–4 5–13 6–9 l–6
Paper
Glass cloth Glass mat Asbestos
l.8–2.0 40–60 1.7–1.9 5–20
l–3
65–95 10–60 50–90 10–30 14–30 9–22 10–38 12–20
17–26 45–55 20–40 30–44
1.7–1.9 40–65 2–5
Paper
1.3–1.4 8–20
l–2
Cotton cloth 1.3–1.4 7–16 0.5–1.5 Nylon cloth 1.1–1.2 5–10 0.3–0.5
28–36 Silicone 25–46 40–50
Glass cloth
1.6–1.9 10–35
l–2 l–2
Asbestos cloth 1.7–1.8 10–25 450–730 150–300 Injection Molding.— This process produces the majority of plastics consumer and indus trial products. The modern injection molding machine consists of an extruder, much like the one described previously, but with the added feature that the screw can be driven for - ward to quickly inject the stored-up hot melt into the second main part, the mold. A nozzle at the end of the extruder presses tightly into a recess ( sprue) in the stationary half of the mold. The two halves of the mold, with a vertical parting surface, are made from heavy steel and mounted on thick steel tie-bars. Within the mold is a network of runners distrib- uting melt from the sprue to the mold cavities that determine the dimensions of the molded product. The mold halves are clamped together with high pressure by a hydraulic piston or toggle mechanism driven by hydraulic or electric means. Coolant channels passing near the cavities keep the mold cold, well below the freezing range of the plastic. The screw moves forward, forcing the melt through the runners and into the cavities, and maintains melt pressure (5,000–15,000 psi, 34–103 MPa) while the moldings cool and freeze. Then the far half of the mold is withdrawn, activating ejection pins that push the moldings and runners out of the mold. Meanwhile the extruder rotates the screw to generate more hot melt, while retracting the screw. Typically the process is highly automated with computer controls. Injection machines are rated by their melting capacity, shot size, platen size,
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