Die-casting metals

The plunger is then withdrawn and the dies are closed ready for the next cycle. The cycle time is not as fast as for the hot-chamber machine. 

In this machine there is no sprue, since the die is mounted at the end of the cylinder. Instead, the metal not used in filling the cavity remains in the cylinder and is attached to the casting when the spray is removed from the die. This excess metal attached to the runner is known as the slug.  

Although techniques for pressure die-casting in steel have been developed, the metals most widely used are certain non-ferrous alloys which have relatively low melting points. 

The designation of zinc alloys is set out in BS EN 1774. The alloy (a number) comprises the letters ZL denoting zinc alloy, followed by four numbers. The first two numbers denote the aluminium content, the third number the copper content and the fourth number any other main element (if less than 1 %, the number is 0). The alloys are also given a short designation. 

Cold-chamber machines are more commonly used with the higher melting-point aluminium alloys poured at about 650 °C to 670 °C and with magnesium alloys poured at about 680 °C. 

Alloys ZL3 and ZL5 are most suitable, the compositions being similar except for the copper content in ZL5. 

ZL3 is a general-purpose alloy used in hot-chamber high-pressure die-casting for the engineering and automotive industry, household equipment and utensils, office equipment, locks and toys. It is easily machined, polished, lacquered and electroplated for decorative and functional purposes. 

ZL5 is used for castings which require moderately higher strength and hardness than the ZL3 alloy. The copper content makes this alloy more expensive and its use is therefore restricted to applications requiring the higher properties. 

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Conventional machining operations are used preferably from the same plastic to be used in the product (Chapter 8, SECONDARY EQUIPMENT). Different casting techniques are used that provide low cost even though they are usually labor intensive. The casting of unfilled or filled/reinforced plastic used include TS polyurethane, epoxy, structural foam, and RTV silicone. Also used are die cast metals. 

Small single crystals, such as those of potassium titanatc, are being used at an annual rate of over 10,000 tons for the reinforcement of nylon and other thermoplastics. These composites are replacing die-cast metals in many applications. Another microfiber, sodium hydroxycarbonate (Dawsonite), also improves the physical properties and flame resistance of many polymers. Many other single crystals, called whiskers, such as alumina, chromia, and boron carbide, have been used for making high-performance composites. 

Properties most often improved are modulus, strength, and dimensional stability, approaching the properties of die-cast metals. 

Fig. 6.2. Internal-vane, external-vane, and plunger-type rotary vacuum pumps. It will be noted that the internal-vane pump involves a rotor concentric with the drive shaft, but which is off-center with respect to the stator. By contrast, the external-vane and rotary-plunger pumps have a rotor which is asymmetric with respect to the shaft however, the shaft is centered in the stator. All three involve close tolerances, so the high-vacuum performance is impaired by particles of dirt or corrosive gases. Some pumps are partially constructed from soft die-cast metal, which is eroded by mercury. Special corrosion-resistant pumps and inert flourinated pumps are used in the semiconductor industry.

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

NSCEsbrld. [Thermofil] Ceramic fiber/ glass fiber-reinfnoed nylon for replacement for die cast metal parts used in business machines, home tqipliances, and automotive qqilics. 

As an example LNP Engineering Plastics, USA has been publishing results of tests that include a selection of die-cast metals compared with a 60 wt% long glass fiber reinforced nylon 6/6 compound (Verton RF700-12EM). The conclusion was that (for moisture-conditioned samples) with a density lower than metals, with the exception of certain... 

When rt mgning to replace a product previously made from other materials, such as iieel, bass, alununum, zinc or magnesium die cast metals, or wood, etc.. 

Sodium dodecyl diphenyl ether disulfonate cleaner, tile crystal growth control Sodium isopropyl naphthalene sulfonate cleaner, toilet bowl Methoxy PEG-40 Methoxy PEG-100 cleaner, waterless hand Cetoleth-6 cleaner, window lsotrideceth-7 cleaner, wood Potassium acid oxalate cleaner, zinc-base die-cast metals Ammonium thiosulfate cleaning agent, metals Hydrochloric acid cleaning compound mfg. 

Kehrer, H. P. (1974). The effect of small impurities on grain boundary corrosion of zinc die castings. Metall, 28(9), 883-889 (in German). 

The revolutionary solution was to think fundamentally about the best way to make the new shapes - simple rectangles at first, but later more sophisticated shapes. The alternatives were pressed steel die cast metal plastics. The pressed steel approach is the conventional one, and it is not as good, even at its best, as a moulding approach for accurate, complex shapes. 

Table 5.10 Long-fibre plastics compared with die-cast metals <23°C)...

Based on industry data, substitution of the metals segment of die-cast metals, hot rolled steel and structural steel by engineering plastics is currently about 12.5 percent on a one-for-one volumetric basis. This segment currently accounts for 40 percent of all metals. 

Molded ABS parts are used in both decorative and protective applications in communications equipment like telephones (see Fig. 6-3), and for appliances, business-machine housings, safety helmets, camper tops, pipe fittings, shoe heels, and so on. Chrome-plated ABS has replaced die-cast metals in plumbing hardware and auto grills, wheel covers, and mirror housings. Some typical products vacuum-formed from extruded ABS sheet are refrigerator liners, luggage shells, tote trays, mower shrouds, boat hulls, and recreational vehicle parts. The possible extruded shapes include weather seals, conduit, pipe for drain-waste-vent (DWV) systems, and the like. 

Figure 6-58. Example of strength versus stiffiiess for unreinforced and reinforced TP composites and die-cast metal.

Acetal homopolymers and copolymers are crystalline polymers displaying physical properties that compare with die-cast metal. They are strong, stiff, tough, hard, and have excellent frictional properties over a wide range of temperature, humidity, and solvent exposures. 

Of the GRTP only ABS, polypropylene, poly-sulfone, and modified phenylene oxide are electroplated. Of these ABS represents about 85% of the market. Since the introduction of platable plastics, new formulations have been developed to give superior physical properties, platability, and appearance. For example, 25% glass-filled ABS has a shrinkage nearly equal to that of die-cast metals the increased stiffness reduces distortion during the plating operation and a reduction of 50% in the coefficient of linear thermal expansion permits it to pass the standard thermal cycling test of 82 to -40 C (190 to - 40 F) (Ref. 29). 

The potential use of ABS both inside and outside the car has almost reached saturation as the material has almost completely replaced die-cast metal in key applications such as light and mirror surrounds. PC/ABS blends however remain a popular choice for instrument panels in some luxury models and demand will continue to grow. PC/ABS should also see good growth prospects in navigation systems housing. 

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