Screw/ram machine

Horizontal reciprocating screw machines (Fig. 20). Vertical screw/ram machines (Fig. 21). 

To ensure consistency, the two critical areas for control specific to injection molding are shot size control and temperature control on the injection system, i.e., screw portion for the reciprocating screw machines, and both the screw and ram sections for the screw/ram machines. As mentioned previously, for temperature and cure control, the use of microprocessor controls has significantly improved the performance of injection molding machines, including control of the shot size. 

Injection Molding. Any standard design plunger or reciprocating screw injection machine can be used for PEA 340, although a reciprocating screw machine is preferred (32). Slow injection into mold cavities avoids surface or internal melt fracture, and control of ram speed is important at low... 

In recent years, the trend has been toward an increasing use of the reciprocating screw machines. One of the most common reasons given for the change is that the first in-last out principle applies for screw/ram... 

Simple ram machines are cheaper than screw machines having equivalent shot volumes. Although the simple ram injection moulding machine was once quite widely used it was generally obsolescent by 1977. Plasticisation by screw is quicker and more controllable than by heat conduction in simple ram machines. A higher and more uniform compound temperature and thus a more uniform viscosity is achieved. 

Screw machines with a separate ram combine the advantages of both screw and simple ram machines. 

Fluoropolymers can be injection-molded in plunger or ram-type equipment but a screw machine works best with this family of plastics. The screw-type injection molding machines have a number of advantages including those listed in Table 6.23. Various components of screw-type machines for processing fluoropolymers are described in this section. 

Rotating spreader n. A type of torpedo used in plunger-type injection machines that consisted of a finned torpedo rotated by a shaft extending through the cored injection ram behind it. This was soon superseded by the screw-injection machine. 

For fast injection and consistency of moulding, the melts are momentarily ( 0.1s) pressurized by the screw rams before injection into the mould - this feature is programmed into the moulding cycle. Fast injection also helps avoid weld lines due the less time available for the melt to cool before welding takes place. Ten-stage programmable injection speed controls for both cylinders is incorporated on the machine together with closed-loop facilities. 

In 1958 Du Font s Experimental Station obtained its first screw molding machine. This type of machine was recommended and promoted for use with Delrin . Favorable customer test results with various polymers aided in industrial acceptance of this new type of equipment. By the middle of the next decade virtually all new molding machines sold in the U.S. were screw machines rather than the old ram type [3]. The melting efficiency, good streamlining, and uniformity of melt temperature of screw machines greatly improved molding performance with polyformaldehyde polymer. 

Injection machines are classified generally into three types—reciprocating screw, ram or plunger, and two-stage or preplasticizers— described in the paragraphs that follow. Most injection... 

There are two types of injection moulding equipment plunger-type (also called ram or piston) and screw-type machines. Linear low-density PE (LLDPE) requires more heat to be moulded than LDPE. These materials are more easily moulded in a screw machine in which the mechanical work of the screw adds considerably to the heat delivered by external electric heaters. 

In injection molding, a reciprocating ram is used rather than a screw. Ram pressures up to 30,000 psi are developed in the chamber of the machine, which is heated to a range of 300-700°F. Injection molding is a repetitions process rather than a continuous process (like extrusion). Figure 10.7 shows an injection molding machine. There are three basic components that make up an injection mold the cavity, the core, and the... 

The rubber compound is fed into the machine and the screw loads a known volume of rubber into the cylinder. When the cylinder is completely filled, the screw then acts as a ram to force the rubber compound into the runner system and the mold. 

A machine for carrying out the processing operation of extrusion also historically termed tubing machine and forcing machine. The majority of extruders use an electrically-driven screw to force the rubber to the head of the machine, but a piston-operated type has recently been introduced. See Barwell Preformer/Ram Extruder. 

Machines differ in their mode of heating, plasticisation of the mix and delivery of the mix to the mould cavity. This may be by simple ram (plunger or piston) or by screw. 

A ram system allowing introduction under high pressure of the dosed material into the mould. On some types of injection machine, the screw also acts as the ram. 

Because of its high melt viscosity it has no useful melt flow index. Conventional screw plasticizing extrusion and injection molding can noy process them. The processing methods used are compression molding, ram extrusion, ram injection, and warm forming of extruded slugs from powdered plastic. In turn many components are machined from semifinished products. 

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