Spiral mandrel

Tq eliminate spiders in the die and the inherent film weakness, the spiral mandrel die is used (Figure 17.11). This design usually is computer calculated since the flows and pressure drops are complicated. 

Processing can use spider dies, spiral mandrel dies, or basket-type dies that support the inner mandrel with a perforated sleeve through which the melt flows. Figure 17.12 provides examples of different die designs. 

Coextrusion can be performed with flat, tubular, and different shaped dies. The simplest application is to nest mandrels and support them with spiders or supply the plastic through circular manifolds and/or multiple ports. Up to 8-layer spiral mandrel blown film dies have been built that require eight separate spiral flow passages with the attendant problem of structural rigidity, interlayer temperature control, gauge control, and cleaning. Many techniques are available for coextrusion, some of them patented and available under license (Chapter 5). 

Fig. 3 Schematic of spiral mandrel blown film die operation (1) ring-shaped melt distribution (2) die body (3) spiral flow mandrel (4) sizing ring (5) spreader (6) film bubble (7) frost line (8) solidified film (9) bubble collapsing rollers (10) nip rollers (11) external bubble cooling air (12) internal bubble cooling air inlet (13) internal bubble cooling pipe and (14) heated internal bubble air return.

To produce multilayer and multicolor extrudates, for example for cable sheathing, packaging films with barrier layers and/or recycled layers, two to seven extruders can be fitted with multiple-layer blown film fittings with the same number of nesting spiral mandrels to distribute the melt flows (Fig. 20). 

US4201532 A, Extrusion dies of spiral mandrel type... 

Several types of blown film dies are available, varying in cost, complexity, and purpose. Side fed, bottom fed, and spiral mandrel dies are used for producing monolayer films. 

Figure 3.5 Spiral mandrel die produces excellent melt quality...

Concentric dies (Fig. 3.6) are comprised of a series of hollow spiral mandrels nested inside one another. A narrow gap remains between each mandrel and the one directly inside it. Inside these gaps is where polymer flows. The different gaps separate the various polymer layers. Just prior to the die exit, the different polymers flow together into a single flow channel. Their laminar flow behavior mainly keeps the polymers in separate layers through the final product. These types of dies have been used for many years and perform very well. A couple of possible concerns to consider when designing... 

This process has been studied for some time and the effect of viscoelasticity on bubble shape, velocity, and stress are still being explored. Some simulation of the flow in the spiral mandrel has also been performed. ... 

On the other hand, a spiral mandrel manifold creates flow in the helical direction, resulting in some degree in circumferential orientation. This combined with the near absence of weld lines make spiral mandrel dies capable of producing a tube or pipe with better mechanical properties simply by modifying the flow inside the die. 

Another method of improving circumferential orientation in tubing and pipe dies is by inducing relative motion between the tip and the die. This can be done by rotating the tip relative to a stationary die or by rotating the die relative to a stationary tip. It is even possible to rotate both the tip and the die separately and in different directions. With rotation of the tip and/or die there is a greater degree of control over the orientation of the extruded tube than with a spiral mandrel section. Obviously, this is at the expense of increased mechanical complexity. 

The most common die used in blown film extrusion is the spiral mandrel die. In this die, the polymer is divided into a number of spiraling channels with the depth of the channels reducing in the direction of flow. The popularity of the spiral mandrel die is due to its relatively low pressure requirement and its excellent melt distribution characteristics. Spiral mandrel dies can be used with a wide range of materials over a wide range of operating conditions. 

The distribution characteristics of conventional crosshead dies may not be good enough for application in blown film extrusion, where wall thicknesses are generally quite small (the typical range is 0.005 mm to 0.25 mm). Spiral mandrel dies can achieve good flow distribution and largely eliminate weld lines. As a result, spiral mandrel dies are widely used in blown film extrusion. 

Rauwendaal [41] used the following assumptions to analyze the flow in the spiral mandrel die ... 

The flow will be analyzed by unrolling the spiral mandrel onto a flat plane as shown in Fig. 9.26. 

Within the first AZc of helical distance the flow in the spiral mandrel channel is not affected by the leakage flow from a channel below it. For this section of the die the following mass balance is valid, assuming the melt density to be constant ... 

With these expressions a complete analysis of the flow process in the spiral mandrel section can be made. 

Coextrusion is practiced on a wide scale in blown film. There are many five-layer blown film coextrusion dies used in the industry five-layer films are now considered a commodity [41], Even seven-layer dies are not unusual. Some coextrusion dies use as many as 8 to 10 layers. Most of these multi-layer dies are used in high-barrier packaging for food. Conventional blown film coextrusion dies have a concentric arrangement of spiral mandrel manifolds. In some cases conical spiral mandrel sections are used, while in other cases the spiral mandrel section is machined into a flat horizontal surface. The latter arrangement is referred to as a pancake coextrusion die, because the different sections of the die are stacked like pancakes. A schematic of a pancake coextrusion system is shown in Fig. 9.39. 

The residence time in the die can be increased by reducing the flow rate (extruder throughput) or by changing the die geometry. The flow splitter has to be located as far away from the die exit as possible. Some die geometries reduce weld line problems. For instance, spiral mandrel dies for pipe, tubing, and blown film spread out... 

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