Pin Extruders

The expander extruder is a relatively simple machine. The function of the pins is twofold to provide a more stable throughput and to improve mixing. Because of the stationary pins, slip at the wall is prevented and the material cannot rotate with the screw. This provides a more stable throughput. The mixing is increased 

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A Uniroyal patent originated the principle of using pins protruding into the extruder barrel to disrupt the laminar flow of compound, and this principle was further developed by Troester in their QSM (cross-flow mixing) machines. The protrusion of pins into the barrel necessitated a modification to the screw design to accommodate them. Many papers can be found in the literature dealing with this type of pin extruder. 

An exceptional type of single-screw extrader is the pin extruder. In this extruder the screw has interrupted flights and it rotates in a barrel with stationary pins. The effect is twofold because of the rotation of the screw a good mixing action can be achieved, and because of the increased resistance against slip the throughput is more stable than in ordinary single-screw extruders. However, the pressure built up in this type of machine is rather poor. In some special types the screw not only rotates but also oscillates in an axial direction. 

FIG. 18-48 Mixing enhancers for single-screw extruders a) Maddock (straight), (h) Maddock (tapered), (c) pineapple, (d) gear, (e) pin,... 

Basically the BM lines have an extruder with a die or an injection machine with a mold to form the parison or preform, respectively. In turn the hot parison or preform is located in a mold. Air pressure through a tubular pin-type device located usually at the parting line of the mold will expand the parison or preform to fit snugly inside their respective mold... 

The massive quadroplex extmsion head is designed to handle four extmders with two each 10 in. (250 mm) and 8 in. (200 mm) pin-barrel extruders. 

In extrusion of solid rubber sections, the shaped metal plate fitted in the head of the extruder to produce the desired contour in extrusion of hollow tubing the outer circular part which fits over the pin, pencil or nozzle to give the hollow section. 

The gear pump can be fitted to a normal cold-feed, pin or vacuum extruder and enhances the accuracy and quality of the product. 

Extruders with retractable pins (hydraulically inserted or withdrawn) can be used to vary the degree of mixing that takes place as the compound travels the length of the barrel. 

A further development is the combination of the pin barrel extruder system with that of the Transfermix (see under EXTRUDERS, MIXERS). The combined system is known as Pinconvert and is available with or without vacuum extraction. 

Vacuum extruders can be single- or multi-vented. Versions also exist which have an added pin barrel processing extension. 

Flight width and flight clearance were 13.5 and 0.12 mm, respectively, in all sections of the screw. The extruder had a smooth-bore feed section. Three rings of pin mixers were evenly positioned in the middle portion of the metering section of the screw. All pins in a single ring had the same axial position as the type shown by Fig. 8.25. The specific rotational flow rate for the metering section was calculated at 5.0 kg/(h rpm). The compression ratio was 2.7. 

Although the full history of this screw is unknown, the mixing pins appeared to have been added to the screw after the screw failed to mix and homogenize the extrudate. Pin mixers have been retrofitted to screws for many years as a way to improve the distributive mixing for an extruder. Here the pin mixers would distribute the solid polymer fragments evenly in the extrudate, hut the mixers lacked the ability to trap and disperse them. Pin mixers were clearly the wrong type of mixer to use for this application. Instead, a dispersive mixer such as a Maddock-style mixer would be preferred see Section 8.5.1. 

Fig. 23. Screw sections for enhancing distributive mixing in single-screw extruders (a) Dul-madge mixer, (b) Saxton mixer, (c) pin mixer, (d) pineapple mixer, (e) cavity transfer mixer, (f) slotted screw flight, (g) Kenics and Ross ISG static mixers [148]...

Fig. 25. Ko-kneader intensive compounding extruder [Courtesy Buss. AG] (a) Schematic diagrams of slotted screw and barrel pins, (b) ko-kneader with barrel in open position...

The molten filaments are extruded through the spinnerette down a vertical chimney where they are air-quenched. The filaments are then converged to form the threadline in the V shape formed by crossed ceramic pins or other similar devices. The threadline passes to the floor below where finish is applied and is wound up on the spin bobbins. 

Mixing in single screw extruders. Mixing caused by the cross-channel flow component can be further enhanced by introducing pins in the flow channel. These pins can either sit on the screw as shown in Fig. 3.29 [9] or on the barrel as shown in Fig. 3.30 [15]. 

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