Molding methods complex shapes

For several basic reasons, the extrusion process does not have the large number of possible process product interactions that the preceding molding methods presented. Due to this situation it can not fabricate the complex shapes and tighter tolerances obtained from molding. The process is a steady-state continuous production operation that can be brought to a condition of control. However it has its share of potential problems (Chapter 8, EXTRUSION). 

Injection molding is one of the most widespread methods used to make polymer products. A cursory look around our homes, schools, work places or cars suffices to find an abundance of injection molded products. One of the reasons that this conversion method is so popular is that it enables us to create complex shapes in a single fabrication process. 

Fluorosilicone compounds can be processed by the same methods used for silicone elastomers based on PDMS. They can be milled, calendered, extruded, and molded. A large proportion of fluorosilicone compounds is used in compression molding. Molded parts produced in large series are made by injection molding, and parts with complex shapes are produced by transfer molding. Calendering is used to produce thin sheets and for coating of textiles and other substrates. 

Extrusion and injection molding are the two leading methods of converting plastic materials into solid products. Extrusion produces continuous products with a fixed cross-section injection molding produces discrete products with more complex shapes. Modern injection molding is a very fast, automated process for large-scale manufacture of complex products at minimum cost. An injection molding machine can be used for many different jobs and for many years, so the amortization per job can be very low. On the other hand, it requires a different mold to make each product, so the cost of the mold must be amortized over the life of its individual production run. 

Injection molding is a method to make complex shapes rapidly, normally fitment or closure. The main function of a closure is to keep the container sealed until the... 

Compression molding is the method by which massive (700 kg), cylindrical (billet), rectangular, and sheet shapes of PTFE are produced. The blocks and cylinders can be solid or annular and are by far the heaviest objects produced from any fluoropolymer. The height of a cylinder can exceed 1.5 m. These billets are cut (skived) into wide thin films (<0.5 mm thick) or sheets (7 mm thick). Sheets, blocks, and cylinders are utilized as stock shapes for machining more complex shapes. The same principles are applied to mold any shape. 

In matched mold thermoforming (Fig. 10.9), matched sets of negative and positive molds are used to form the part from the softened sheet. Vacuum is generally applied through the negative cavity, as well. The result is excellent dimensional control, and the ability to form very complex shapes. This method is most often used in thermoforming of foams, due to their tendency to deform if not restrained. 

There are reinforcement preform constructions. A preform is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes (hand layup, injection, etc.). Oriented fiber patterns can be incorporated in the preforms (Chapter 5). 

A technique suggested to fabricate complex shaped articles is the matrix transfer molding technique [69]. Woven stmctures used as reinforcement are arranged inside a mold cavity. Fluid matrix is transferred at high temperature into the mold cavity to fill the void space around the reinforcement stmcture. In this way, for example, thin-walled cylinders can be fabricated. Another proposed method for obtaining structures of complex shapes uses a... 

This process is an economical press molding method for manufacturing an intermediate number of parts, such as 200 to 2,000. It uses low-pressure, room-temperature curing resins, and inexpensive molds. Cold press is similar to compression molding except that the resin curing action occurs via its own exothermic heat of reaction (after resins are mixed with catalyst, etc.). Pressures are moderate, usually 20 to 50 psi. Thus molds can be made of relatively inexpensive metals, plaster, or reinforced plastics. The edges need not be trimmed. Ribs, bosses, and other fairly complex shapes are not easily produced. Two good mold surfaces are obtained. 

The requirement for assembly of castings is limited since casting can make complex shapes. The most common methods are adhesives and fasteners. Self-tapping screws can be used with most of the materials used for casting. Threaded inserts can be molded in place, however they are difficult to salvage in the event of a reject moldment because the material is a thermoset and the inserts cannot be melted out. Threaded inserts can also be glued or tapped in place. 

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