Screw and plastic

With practically all machines, only the cylinder temperature is directly controlled (see Chapter 1). The actual heat of the melt, within the screw and as it is ejected from the nozzle, can vary considerably, depending on the efficiency of the screw design and the method of operation. Factors affecting melt heat include the time plastic remains in the cylinder the internal surface heating area of the cylinder and the screw, per volume of material being heated the thermal conductivity of cylinder, screw, and plastic (Table 1-6) the heat differential between the cylinder and the melt and the amount of melt turbulence in the cylinder. In designing the screw, a balance must be maintained between the need to provide adequate time for heat exposure and the need to maximize output most economically. 

Material is transported upstream in the inner screw and plasticated. At the end of the inner screw, the polymer, which is now molten, is pumped back into the channel of the main screw. A modified version of the SDS screw for use in a molding machine was patented on September 22, 1981 [22]. Another modification of the SDS screw involves using a barrier type main screw to improve the solids draining process. This barrier SDS screw was patented on June 14, 1983 [23]. The SDS screw is claimed to give higher output and lower energy consumption. However, from a functional analysis it is difficult to see why recirculation of a fraction of the polymer flow would increase output or reduce power consumption. 

Few publications are available that provide data on screw and plastic temperatures along an extruder. The publication by Marshall et al. [149] provides some interesting experimental data. They confirm that the screw temperature in the metering section is higher than the barrel temperature. Further, they measured screw temperatures in the feed section and found temperatures in the range of 115 to 127°C (240 to 260°F) with barrel temperatures of 190°C (375°F). When barrel temperatures are around 371 °C (700°F), it can be expected that the feed section of the screw will be in the range of 204 to 260°C (400 to 500°F) if not higher. 

The Dynamit-Nobel extmsion process (252) utilizes a volatile plasticizer such as acetone which is injected into the decompression section of a two-stage screw and is uniformly dispersed in the vinyl resin containing a stabilizer. The resulting PVC foam has low density and closed cells. 

An iajection mol ding machine is operated by hydrauHc power and equipped with an electric motor and hydrauHc pump. The maximum hydrauHc oil pressure is ca 14 MPa (2000 psi). A hydrauHc cylinder opens and closes the mold and holds the mold closed duriag iajection another cylinder forces the screw forward, thereby injecting the melt iato the mold. A separate hydrauHc motor turns the screw to plasticate, homogenize, and pressurize the melt. Control of these movements is a combined function of the hydrauHc and electrical systems (35—37). 

Some of the more obvious sources of contamination of solvents arise from storage in metal drums and plastic containers, and from contact with grease and screw caps. Many solvents contain water. Others have traces of acidic materials such as hydrochloric acid in chloroform. In both cases this leads to corrosion of the drum and contamination of the solvent by traces of metal ions, especially Fe. Grease, for example on stopcocks of separating funnels and other apparatus, e.g. greased ground joints, is also likely to contaminate solvents during extractions and chemical manipulation. 

Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

If a self-threading screw can be substituted, it will not only appreciably decrease mold maintenance and mold cost but most likely, with proper type selection, also give better holding power. A screw that has a thin thread with relatively deep flights can give high holding power. If the screw or plastic is preheated... 

Time, pressure, and temperature controls indicate whether the performance requirements of a molded product are being met. The time factors include the rate of injection, duration of ram pressure, time of cooling, time of piastication, and screw RPM. Pressure requirement factors relate to injection high and low pressure cycles, back pressure on the extruder screw, and pressure loss before the plastic enters the cavity which can be caused by a variety of restrictions in the mold. The temperature control factors are in the mold (cavity and core), barrel, and nozzle, as well as the melt temperature from back pressure, screw speed, frictional heat, and so on in the plasticator. 

Plasticator A very important component in a melting process is the plasticator with its usual specialty designed screw and barrel used that is used in different machines (extruders, injection molding, blow molding, etc.). If the proper screw design is not used products may not meet or maximize their performance and meet their cost requirements. The hard steel shaft screws have helical flights, which rotates within a barrel to mechanically process and advance (pump) the plastic. There are general purposes and dedicated screws used. The type of screw used is dependent on the plastic material to be processed. 

A machine for masticating rubber. The principle is that of an extruder, the rubber being softened by working between the threads of the screw and the surface of the barrel. The best known type is the Gordon Plasticator . Now obsolete. Plasticiser... 

Autotapping screws the screw type, the diameter of the hole that receives the screw and the tightening torque depend on the plastic used. Repeated screwing and unscrewing is not advised. To a first approximation, and as examples, it is sometimes stated that ... 

The plasticator on an injection-molding machine is a specialized plasticating single-screw extruder. The plasticator has two main differences there is a nonreturn valve on the tip of the screw, and the screw retracts as molten material accumulates between the nonreturn valve and the end of the barrel. Pressure is maintained on the accumulated material by a constant force applied to the shank of the screw via the drive system. This force is typically measured as a pressure applied to the shank and is referred to as the back pressure. During the injection step of the process, the screw is forced forward, the nonreturn valve closes, and the material is injected into the mold. Additional information on the injection-molding process can be obtained elsewhere [Ij. 

The availability of power and torque to the shank of the screw is extremely important to the success of an extrusion process, especially if modifications to the line are planned to increase the rate. That is, the proper level of power must be available at the proper screw speed. Thus, the motor size and speed, belt sheaving if used, and gearbox reduction must be specified to provide enough power to the screw to plasticate and pump the resin at a high rate. 

Several mechanisms could cause the specific rate of the screw to be considerably less than the calculated specific rotational flow rate for the screw. These mechanisms include (1) normal operation for a screw with a very short metering section and a low-viscosity resin, (2) the screw is rate-limited by solids conveying, causing the downstream sections of the screw to operate partially filled, and (3) the entry to the barrier section is restricting flow (see Section 11.10.1) to the downstream sections of the screw and causing the downstream sections to operate partially filled. The goal was to determine which of the above mechanisms was responsible for the low specific rates for the plasticator. 

The press was started back up using the high-performance screw and natural HIPS resin with 2 % of a light gray color concentrate masterbatch. For this startup the same 2.69 kg part was produced and the barrel set point temperatures were the same as before. After steady operation was obtained (about 10 parts), the 2.69 kg part and gating were plasticated in 33.5 s at a screw speed of 52 rpm and 9.5 MPa... 

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