Back Casting

The industry should commit itself long term to a closed-loop system of PVC waste management (system conditions 1 to 4 - in relation to a back-casting perspective, today s use of PVC in society is highly wasteful, with around 50 percent ending up on land deposits). 

Natural mbber latex is also used in adhesives for tape, packaging, envelopes, and in the footwear industry. It is used in the carpet industry as a binder for backing compounds, but this is another area in which synthetic SBR latex has competed effectively. There are a number of relatively small and specialized appHcations for natural mbber latex including mbberized coir or hair and cast products such as toys. Latex sheeting which is used in dental dams and for numerous other purposes can be made by dipping or casting onto a continuous belt. 

After leaving the casting machine, the slab is cooled by water sprays to 180—240°C and fed into the mill, which is generally of the four-high roU type, where a 60% reduction is taken in one pass. The strip is then coiled and, when the casting mn has been completed, is fed back through the mill and roUed at a temperature of 80—90°C, with a reduction up to 50%, to give the required final thickness, finish, and properties. 

With binary copper—lead, the continuous copper phase provides the primary load support while pockets of 20—50% lead supply a continuous lead surface film. Tin content of 3—5% is commonly incorporated with the lead to minimi2e corrosion. Copper—lead alloys, either cast or sintered on a steel back, provide good fatigue resistance for heavy-duty main and connecting rod bearings for auto, tmck, diesel, and aircraft engines. 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

Defects in finished components can sometimes be traced as far back as casting problems in the ingot or billet from which the component was eventually produced. Defects in welded tubes may be linked to defective metal or to the welding process itself Defects in seamless tubes can originate in extrusion or drawing processes. 

Early records also indicate that cast mouldings were prepared from shellac by the ancient Indians. In Europe the use of sealing wax based on shellac can be traced back to the Middle Ages. The first patents for shellac mouldings were taken out in 1868. 

Another version of the dewatering screw that has been used successfully in a number of installations (Figure 40) consists of an inclined screw with a box-like back end and an internal overflow weir. The drive is usually a variable-speed motor reducer. Rotary air lock fitted with a variable-speed drive is typically used as shown in Figure 37 in connection with the plain receiving tank and an inclined screw conveyor. Materials of construction can be chrome-plated cast iron or nickel-hard casing. The rotary valve is made of hard faced tips or stainless steel replaceable blades. 

The old dry process enamelling of cast iron (baths etc.) is no longer widely used. The method consisted of sieving finely powdered frit onto the preheated casting and inserting the casting back into a furnace at about 900°C to produce the smooth finish. 

The softening point of conventional cast and sheet iron enamels is about 500°C, but special compositions are obtainable which operate successfully at 600°C. Other more specialised enamels withstand service conditions ranging from being in excess of dull red heat, e.g. as obtained in fire backs, to those capable of enduring short exposure to temperatures of around 1 000°C, e.g. in jet tubes, after burners, etc. 

---