Screw-barrel plasticator

Figure 3.7 Assembled screw-barrel plasticator for injection molding (top) and extruding, (courtesy of Plastics FALLO)...

A practical difficulty which arises during injection moulding of reinforced plastics is the increased wear of the moulding machine and mould due to the abrasive nature of the fibres. However, if hardened tool steels are used in the manufacture of screws, barrels and mould cavities then the problem may be negligible. 

With PBAs the compressed gases often used are nitrogen or carbon dioxide. These gases are injected into a plastic melt in the screw barrel under pressure (higher than the melt pressure) and form a cellular structure when the melt is released to atmospheric pressure or low pressure. The volatile liquids are usually aliphatic hydrocarbons, which may be halogenated, and include materials such as carbon dioxide, pentane, hexane, methyl chloride, etc. Polychlorofluoro-carbons were formerly used but they have now been phased out due to environment problems. 

Pf pressure at the tip of the screw for an injection-molding plasticator (Eq. 10.22) Q extrusion rate in kg/h O specific rate in kg/(h-rpm) for Eq. 10.21 R compression rate in the helical direction Pjs belt sheave reduction Rg speed reduction for the gearbox S support distance from the fixed end of the barrel shear strength of the material used for the screw t plasticating time in seconds (Eq. 10.21)... 

The action that pushes solid particles forward in the feed section of a single screw extruder, blow, or injection machine has always been a potential for one of the weakest features of these machines. This forward feeding force near the feed hopper is often weak and erratic and is classified as non-positive. It can be so tenuous that a specific screw/barrel combination will feed virgin but little or no additions of regrind, or one feedstock shape but not another, and often one family of plastics but not another. This action results in non-uniform feed that will in turn result in poor production rates, non-uniform output (surging), and poor product quality.147... 

The barrel, also called cylinder or pipe, is used to enclose the screw (Figure 3.6). This combination provides the control mechanism that targets to produce a uniform plasticized plastic melt of constant composition, at the required, controllable rate. To achieve this, the barrel must be made very accurately the total out-of-alignment error, after all machining, must be less than one half of the screw/barrel clearance. The screw in the barrel provides the bearing surface where shear is imparted to the plastic material. Heating and with certain types of cooling media are housed around it to keep the melt at the desired temperature profile. 

Colby, P. N., Plasticating Components Technology Screw Barrel Technology, Spirex Bulletin, Spirex Corp., 2000. 

Using the screw /barrel system as an example, the link between input, effective and loss, respectively perturbances, variables has been examined for extruders in plastics processing by adopting a stringent systems analysis approach. As the stress parameters are highly location-dependent, it is necessary to differentiate the tribosystem of smaller barrel and screw-dependent subsystems. An exemplary representation of a section of the barrel surface and corresponding material to be extruded for the area of the solids zone is shown in Fig. 3. The limitations of the system are set in such a maimer that any modification of the parameters (Ap, AT etc.) still possible may be disregarded. 

The process of IM is used for reinforced TPs (RTPs) however, it is principally used for (URTPs) and some TSs. IM machines (IMMs) represent over half of all the primary plastic fabricating machines in the world processing all types of plastics and plastic compounds representing just a USA multibillion-dollar business. Figure 5.29 is a simplified version of IM machine (IMM) where plastic flows from hopper, through plasticator (screw-barrel), to mold cavity. 

For maximum solids conveying Stick to the barrel and slip on the screw . Most plastics normally slip on the root of the screw as long as the feed temperature... 

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