Die spider line

Spider lines are essential elements in extruded composite hollow deck boards. Spiders in an extrusion die interrupt the melt flow, causing it to separate and then rejoin (see Figs. 15.6 and 15.7). This welding of the profiles creates the so-called spider lines along hollow composite deck board profiles. 

Die, weld, and spider lines Damaged die ring Repair or replace die tool-... 

Die spider n. In extrusion, the legs or webs supporting the die core within the head of an in-line pipe, tubing, or blown-film die. In many pipe dies, the spider legs are cored to permit application of air or water for cooling the mandrel. 

Section of poorly mixed extrudate with spider lines (two arrowed) from die head. 

Pipe and Tubing. A typical die for extmding tubular products is shown in Figure 4. It is an in-line design, ie, the center of the extmded pipe is concentric with the extmder barrel. The extmdate is formed into a tube by the male and female die parts. The male die part is supported in the center by a spider mandrel. Melt flows around legs of the mandrel and meets on the downstream side. The position of the female die part can be adjusted with bolts adjustment is requited to obtain a tube with a uniform wall thickness. 

Fig. 4. Pipe or tubing die for in-line extmsion A, die body B, mandrel, pin, and male die part C, die, die bushing, and female part D, die-retaining ring E, die-retaining bolt F, die-centered bolt G, spider leg H, air hole I, seat for breaker plate J, ring for attachment to extmder and K, die land (15).

Plastic tube and tubular films are formed continuously by extruding a polymer through an annular die. The annular flow channel is formed by the outer die body and the die mandrel. A number of annular die designs are currently employed. In the first, the mandrel is supported mechanically onto the outer die body by a number of fins called spider legs Fig. 12.41 illustrates this type of die. The flow is axisymmetric, and the only serious problem encountered in the cross-machine direction uniformity of the extruded product is that of weld lines and streaks caused by the presence of the spider legs, which split the flow. 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Both pipe and tubing are made in dies with an annular die exit. A pipe product is defined as being greater than 1 in. in outer diameter and a tube less than 1 in. Dies for these products are made in two styles 1) in-line dies (also called spider dies) shown in Fig. 4A and... 

Thin film is inflated from extruded tube consequently to the die exit. The blown film dies could be in general divided to spider and spiral types, as presented in Fig. 4.3. Since spiral dies provide more uniform melt distribution and eliminate weld or knit lines caused by the spider holding the mandrel, they almost replaced... 

Figure 4.2). These die forms create a more tortuous flow path which avoids localised orientation by introducing changes in flow direction until the pipe extrudate achieves a uniform, isotropic, microstructure in the pipe wall. PVC and other more amorphous polymers such as ABS, PMMA and PC can still be extruded with the more free-flowing spider mounted dies without creating lines of weakness. 

Blown film lines use three major die designs side fed, spider arm, and spiral flow. In side-fed dies (Fig. 5.39a ), the melt is fed into one side of the die body while air is introduced into the bottom of the mandrel. Since this produces a relatively low pressure drop, such dies are good for high-viscosity materials. However, the melt encircles the mandrel and joins in a single, relatively weak weld line. The pressure drop around mandrel deflects the mandrel to produce nonuniform flow. 

With a spider-arm die (Fig. 5.396 ), polsuner melt is fed through the bottom of the die while air is introduced through three or more spider arms that extend from the sides of the die to support the mandrel. This creates a relatively low pressure drop, and allows the die to be used with high-viscosity materials. The weld lines are stronger than the single-weld line produced with a side-fed die, but can result in weak points in the film. Spider-arm dies are used for poly(vinyl chloride) and other high-viscosity, heat-sensitive materials. 

In these dies, the center line of the die is in line with the center line of the extruder. The central torpedo is supported by a number of spider legs, usually three or more. The spider legs are relatively thin and streamlined to minimize the disruption of the velocity profile. Of course, as the polymer recombines after the spider leg, a weld line will form. Thus, the location of the spider support should be far enough from the die exit to enable the polymer to heal. The location of the die is generally adjustable relative to the pin, just as in the crosshead die. 

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