Extruder improvements

Considering the relative dryness of the body, it would be practically impossible to extrude good-quality cellular bricks at any reasonable pressure, if the insides (= feed contours) of the pressure head and die are not lubricated. The bricks would almost certainly have dragon s teeth and torn surfaces, and any penetrometric value in excess of roughly 3 would make some compounds stop up the pressure head and die. If, however, the compaction surfaces in the pressure head and die are lubricated, less extrusion pressure is needed, wear is minimized, the throughput capacity increases, and the surface quality of the extrudate improves. Large-format cellular bricks, e.g., 25 x 25 cm, are difficult, if not impossible, to stiff-extrude, because the resistance inside the corebar is simply too high. Smaller formats with up to about 40 % perforations are extrudable if the body has the appropriate properties. 

Polyurethane, PVC, and extruded polystyrene provide the bulk of the cellular plastics used for low and cryogenic temperature appHcations. In some cases, eg, the insulation of Hquid hydrogen tanks on space systems, foams have been reinforced with continuous glass fibers throughout the matrix. This improves strength without affecting thermal performance significantly. 

Considerable amounts of polyethylene film are produced using coaxial extrusion processes in which two or more melt streams are combined in the die to produce extruded film of two or more layers of plastics materials. Layers in such a composite may be included, for example, to improve barrier properties, to enhance sealability or even simply to act as an adhesive between dissimilar layers. 

Dispersion polymer, which leads to products with improved tensile strength and flex life, is not easily fabricated by the above techniques. It has, however, been found possible to produce preforms by mixing with 15-25% of a lubricant, extruding and then removing the lubricant and sintering. Because of the need to remove the lubricant it is possible to produce only thin-section extrudates by this method. 

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

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. 

To improve the end use performance and make the processability easy, control of MW and MWD as well as the use of.more than one comonomer has been reported for LLDPE [28]. Union Carbide s high MW-LLDPE with broad MWD is a 1-hexene-based resin, and its film provides superior (about 30-50% higher) tensile strength, puncture resistance, and dart impact strength than conventional 1-hexene-based resin, but with lower tear resistance in the transverse direction. The broad MWD makes the resin processability easy on the conventional extruder. 

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

For long lengths of anode it is sometimes necessary to extrude one material over another to improve a particular characteristic. Thus titanium may be extruded over a copper rod to improve the longitudinal conductivity and current attenuation characteristics of the former lead alloys may be treated similarly to compensate for their poor mechanical properties. It should he noted that these anodes have the disadvantage that, should the core metal be exposed to the electrolyte by damage to the surrounding metal, rapid corrosion of the former will occur. 

Platinised-titanium installations have now been in use for 30 years for jetties, ships and submarines and for internal protection, particularly of cooling-water systems . For the protection of heat exchangers an extruded anode of approximately 6 mm in diameter (copper-cored titanium-platinum) has shown a reduction in current requirement (together with improved longitudinal current spread) over cantilever anodes of some 30% . This continuous or coaxial anode is usually fitted around the water box periphery a few centimetres away from the tubeplate. 

As the general standard of protective coatings has improved (materials, application and construction standards) the use of sacrificial anodes has extended to soil of ca. 10 000 ohm cm or more for buried pipelines. By using anodes in extruded ribbon form and placing them as close as possible to the structure to be protected they can be used in soils of high resistivity. 

The designer should be aware of the fact that this is to be considered in designing with extruded products. The designer can exercise little control over this pull back condition except to be guided by the experience of the extrusion processor to indicate which materials are particularly susceptible to this problem and what the recommended wall thicknesses are to minimize the effect. In general, one of the best ways to improve the condition is to slow down the rate of extrusion. As a result, products have a tendency to pull back. They also will be more costly to produce. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include... 

Reinforcing fillers in general and high-structure carbon blacks in particular improve the extrusion characteristics of elastomers by decreasing extrudate swell. The extrudate swell decreases with increasing carbon black content [33]. 

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

Reactive extrusion is the chemical modification of polymer while it is being transported in an extruder. In this work, polypropylene is intentionally degraded by the addition of a free radical initiator (a peroxide) during extrusion. The product has improved flow properties because of the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. 

---