Reworked plastic

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Here, the harder partner penetrates the surface of the softer partner resulting in a microcutting process. This problem is particularly pronounced when processing plastics because the fillers and reinforcements that cause wear are added in their pure form, i. e., not coated in melt. The problem applies both for the plastification zone (not in the case of rework materials) as well as for solid fillers. The amount of abrasive wear is primarily determined by ... 

Regrind consists of reworked material, i.e. either natural plastic or plastic plus other constituents as per material being moulded. Regrind is usually used to specific levels, e.g. 10%, 15%, 20%, 25% maximum. However, continuous use of regrind may lead to some degradation of either the basic polymer or the other constituents. If regrind is not permitted a specific instruction has to be included in the specification. Some countries do not permit the use of regrind for certain types of product (e.g. for IV, injectibles.)... 

The decision to rework electronic devices or assemblies depends on the cost of reworking versus the value of the part. Consumer electronics, due to their low cost, are considered throwaways and seldom reworked. For some, such as plastic-encapsulated microcircuits (PEMs) that are produced in high volumes, rework is impractical because of the difficulty in removing the plastic encapsulant without destroying the components. However, for high-value assemblies, such as densely populated PWBs and MCMs, where the final value may be as high as 10,000 or more, the ability to rework is essential. In such cases, rework may be necessary to meet schedules when small numbers of assemblies are produced. 

Preconsumer plastic Plastic material diverted from the waste stream following an industrial process, but excluding reutilization of material such as rework, regrind, or scrap generated in a process and capable of being reclaimed within the same process. ... 

Moreover, PP/LDPE blends are the products of plastic recycling. Since separation of PP and PE in sohd domestic waste (SDW) is very expensive, many recycling companies sell mixtures of reworked PE and PP ( polymer mix ) [13]. Thus, the study of the stmcture of blends of these materials will increase the field of application of recycled plastics. This is the reason for why PP-LDPE blends are the main subject considered in this chapter. 

Much has been written about the endless application possibilities for mixed plastic waste. Fence posts, floor tiles, storage pallets, road cones and other large volume, low technology items are cited, and there have indeed been some successes. The fact is, however, that randomly mixed polymers can be difficult (and possibly dangerous) to process, with a variability in performance which could be unacceptable even in these undemanding applications. Furthermore, the size of this market sink is not unlimited, and it is arguably better served by second grade but unmixed bulk polymers, especially in various reworked and somewhat contaminated manifestations, at very low prices. 

Laser is a recent innovation in rework and repair. The fundamentals of laser for initial soldering or for repair have already been discussed in Chap. 47. In the commercial incarnation of this technique, a laser beam is quickly scanned around component leads or, package surface, in the case of area-array devices such as EGAs and chip-scale packages (CSPs). It is most effective for plastic packaged components rather than the thermally massive CBGAs, CCGAs, and so on. In this technique, the component body is heated. Otherwise, there is little difference between this and alternative rework or repair techniques. 

The press-fit operation is inherently high yielding, but problems with the process or a bad press-fit connector may necessitate the removal and replacement of a connector. As a result of the press cycle, the compliant pins of a press-fit connector are plastically deformed.Therefore, once used, the same connector cannot be removed and reinserted in a circuit board. However, most press-fit connectors are designed to be reworked—that is, repaired or replaced. In some cases, individual leads or contacts can be replaced. In other cases, banks of leads or contacts can be substituted. Some press-fit connectors require complete removal of the damaged connector for replacement with a new one. Many different press-fit connectors are on the market, and each has its own manufacturer-recommended repair strategy. 

Reworked Material n A thermoplastic from a processor s own production that has been reground, pelletized, or solvated, after having been previously processed by molding, extrusion, etc. NOTE — In many specifications the use of reworked material is limited to clean plastic that meets the requirements specified for the virgin material, and yields a product essentially equal in quality to one made from only virgin material (ASTM D 883). See also Regrind. 

If metal models were used for the electroplating, no reworking of the mold contour is required other than cleaning. When using plastic models, an electrically conductive silver layer has to be applied, which initially remains adhered to the contour surface and needs to be removed chemically or by polishing. 

Prototype Tooling and Indirect Tooling Master models for molding processes can be manufactured with sintering processes. This rarely happens due to poor surface and therefore required extensive rework. For prototype molds, plastic molds can be produced which are then preferably casted. 

Due to the process, generative components have steps that can be easily leveled manually with relatively soft materials such as plastics. In this way, excellent surface qualities are manually achieved. But dimensional deviations are also generated. This can be problematic if a batch of very precise components is manually reworked by different employees. 

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