Tooling Corner: Die design for plastic extrusion, Part 2
Published: March 5th, 2003
Part 1 of this article (PM&A Nov./Dec. 2002) discussed how the size and shape of an extruded product changes from that of the die flow channel and how to control the size and shape of the product. Part 2 describes basic extrusion dies for sheet, flat film, tubing, and pipe dies.
Dies for Sheet and Flat Film
Dies for flat film extrusion are essentially the same as dies for sheet. The difference between sheet and film is primarily the thickness. Webs with a thickness of .5 mm or less are generally referred to as film; webs with a thickness of more than .5 mm are generally referred to as sheet. The simplest film die is the T-shaped manifold die.
This flow channel geometry is simple and easy to machine. The T-die is often used in extrusion coating applications. However, the distribution of the polymer melt is not very uniform and the flow channel geometry is not well streamlined. As a result, this die is not suitable for polymers with limited thermal stability.
|Even well-made dies produce variable thickness, since dies can make sheet 30 ft or more in width. Built-in adjustment capabilities eliminate variations.|
Even when a sheet die is well designed, it is possible for thickness variations to occur in the extruded sheet or film. This is not surprising when you consider that some dies produce sheet with a width of 30 ft or more. For this reason most sheet and film dies are designed to allow adjustments so that thickness variations can be largely eliminated.
in Sheet and Film Dies
Most commonly, a mechanical flow adjustment changes the actual geometry of the flow channel. The flex lip and choker bar adjustments (Figure 1) are two common mechanical adjustments in sheet dies. The flex lip mechanism allows adjustment of the final die gap by turning die lip bolts, which are generally spaced about 25 mm apart. For example, a die 1m wide has 40 flex lip adjustment bolts. Some sheet dies have a sliding lip adjustment rather than a flex lip. The flex lip allows for fine adjustment of the thickness of the extruded sheet or film. In some automated extrusion lines the gap of the sheet die is adjusted automatically. This is done with heat-expandable die lip bolts, first developed by Welex (Blue Bell, PA) in the early 1970s.
The choker bar is located upstream of the flex lip and downstream of the manifold. The choker bar adjustment works similarly to the flex lip adjustment. The choker bar can be locally deformed by a number of bolts located along the width of the die. This deformation causes a change in the flow channel height and thus allows adjustment of the flow distribution in the die. The choker bar is generally used for coarse thickness adjustment while the flex lip is used for fine adjustment.
Figure 1 shows a sheet die with a removable lower die lip. Such a die can be used, for instance, for both 1- and 4-mm sheet extrusion by changing only the lower die lip. Without the removable die lip, another sheet die would have to be used because the flex lip can only adjust over a limited range. Extrusion Dies Inc. (Chippewa Falls, WI) recently developed a method that allows the opening or closing of the die lip with a single screw located at the end of the die. This device is called Fast-Gap and it allows the die gap to be changed ±2.5 mm while the line is running.
An elegant approach to thickness adjustment in sheet and film dies was developed by Heinz Gross (Rossdorf, Germany). Gross has developed a system with a flexible membrane in the die to allow rather simple thickness adjustment using low-force actuators to create a smoothly curved surface without sharp angles or dead spots. This is shown in Figure 2. Several extrusion die manufacturers have licensed the membrane die. More recently, a similar system has been developed for the extrusion of tubing and pipe.
Pipe and Tubing Dies
The difference between pipe and tubing is mainly determined by size. Products of 10 mm diameter or less are generally referred to as tubing, while larger products are generally referred to as pipe. Annular products can be extruded on inline and crosshead dies. In the crosshead die the polymer melt makes a 90 turn and simultaneously splits over the core tube. The plastic melt then recombines below the core tube; this is where a weldline forms. After the 90 turn, the plastic melt flows through the annular flow channel, where it adopts the approximate shape of the final land region.
Weldlines are usually unavoidable in hollow extruded products. The plastic has to be given sufficient opportunity to ?heal? along the knitlines. This healing process is essentially a re-entanglement of the plastic molecules. Important parameters in this process are time, temperature, and pressure. The healing time increases with molecular weight, and decreases as temperature rises. To enable the plastic to heal sufficiently, the point of weldline formation has to be a reasonably far distance upstream from the die exit.
The crosshead die is also used for wire coating. In wire coating, a conductor passes through the hollow center of the core tube and becomes coated with polymer melt close to the die exit. The conductor may be bare or it may already have been coated with one or more layers of polymer.
In most crosshead dies, the location of the die relative to the tip can be adjusted by means of centering bolts. This allows adjustment of the wall thickness distribution and concentricity. In some extrusion lines with inline wall thickness measurement, automatic wall thickness control is obtained by using the signal from the wall thickness probe to automatically adjust the position of the die. In some cases, a slight internal air pressure is applied through the center of the core tube in order to maintain the ID of the tubing or to prevent its collapse. This is particularly useful in extrusion of tubing with very small IDs (.1 mm or less). In wire coating, sometimes a vacuum is applied to the center of the core tube to prevent air from being dragged along with the conductor; this can cause imperfect contact between the polymer and the conductor.
Pipes are often extruded with inline dies (Figure 3). 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 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 weldline forms. 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.
A manifold geometry that largely eliminates weldlines is the spiral mandrel die (Figure 4). These dies were originally developed for blown film extrusion. However, it became clear that spiral mandrel dies are equally or even more beneficial in pipe and tubing extrusion. Conventional tubing and pipe dies create a weldline (actually a weld region) that runs along the length of the extruded product and extends from the ID to the OD of the tube. Such a weldline reduces the hoop strength of a tube because the weldline/region has the least favorable orientation relative to the stresses in the tube caused by internal pressure.
On the other hand, a spiral mandrel manifold creates flow in the helical path, producing some degree of circumferential orientation. This, combined with the near absence of weldlines, make spiral mandrel dies capable of producing a tube or pipe with better mechanical properties by modifying the flow inside the die.
Another method of improving circumferential orientation in tubing and pipe dies is to induce 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 one obtains a greater degree of control over the orientation of the extruded tube than is available with a spiral mandrel section. The trade-off is increased mechanical complexity.
Rauwendaal Extrusion Engineering
Los Altos Hills, CA