Plastics forming techniques

Sintered silicon carbides are formed by all the traditional methods as well as standard plastic forming techniques such as injection molding. These compacts are sintered with small amounts of additives at very high temperatures in inert atmospheres to form essentially a single phase silicon carbide structure. 

Three carbon-plastic compositions were also included as materials for investigation. These compositions were molded by commercial plastic forming techniques. The PTFE and nylon had carbonaceous fillers present as discrete particles in the plastic matrix. The third type of material had continuous phases of both graphitic carbon and plastic (phenol-furfural),... 

Some ductile plastics, such as PC and ABS, can be fabricated like metals with punching and cold-forming techniques. These processing techniques are analogous to the hardness tests in that a rigid indentor is pressed into a sheet of a less-rigid plastic. 

Over the past few years, however, techniques have been developed to enable continuous reinforcement of thermoplastics. The simplest way is to put a cloth and a plastic sheet on top of each other in a heated press and to carry out impregnation under pressure. More difficult is the forming of an end-product from the sheet produced with conventional sheet-forming techniques the position of the fibres will be distorted in an unacceptable way. As in nearly all processing techniques, the modern finite-element methods with advanced computers are able to present solutions to this problem in principle they can predict the position of the fibres in the sheet-forming operation, so that optimum reinforcement is realised in the end product. 

Vacuum forming Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mould. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary moulds. 

The plastic mass is usually homogenized and degassed in pug extruders or augers, from which it is obtained as a continuous strip from which the final shape is cut. This forming technique is called extrusion and is used for instance in the manufacture of bricks, tubes, rods, etc., from both clay-based and non-plastic mixes. The former usually contain 15—25% water, the latter have to be plasticized with organic substances (see below). 

Vitreous silica is high purity Si02 glass that can withstand service temperatures above 1,000°C. As a metastable phase of silica, vitreous silica can be readily obtained in nature and synthetically. Silica glass can be produced in a pure and stable form, displaying useful properties, but is rigid and difficult to shape even at 2,000°C. Hence, it is not accessible to mass production plastic-forming methods. However, techniques have been developed to produce vitreous silica in various shapes and sizes [36-39],... 

Forming techniques used for clay-based ceramics require control of water content in the batch. Water content, in turn, affects the response of the clay during forming [27], As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, generally decreases [26], However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14], One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14], The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most. 

Failure of parts, irrespective of plastic t5 e, is an inevitable fact of the operation of chemical plants. Fluoropolymers are no exception in spite of their excellent chemical, thermal, and mechanical properties. These plastics form the processing surfaces of equipment where they are exposed to the most aggressive and corrosive chemicals. The repeated exposure of fluoroplastics to these chemicals, in addition to other factors, can affect the integrity and surface quality of the parts. The chapters dealing with properties and part fabrication techniques of fluoropolymers should be consulted extensively. An understanding of the limitations of fluoropolymers and flaws created by fabrication methods is required for successful failure analysis of parts. 

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