Die: The die is the shaping tool that is mounted on the end of the extruder, usually onto a ring called the adapter. The purpose of the die is to give shape to the melt so that after leaving the die the melt can be cooled into the shape desired.
The key die features can be seen in showing a die used for making a rod. In solid extruded parts, such as a rod, the inside of the die gives the desired shape. These are the simplest of the shapes commonly extruded. Some important shapes, including the die, other equipment, and any particular operating conditions unique to that shape that will be discussed are pipes, profiles (non-regular solid and hollow shapes), wire and cable coating, paper and fabric coating, sheets, films and fibres. The shapes of dies are as varied as the shapes of the parts that are made. In all dies, however, some general considerations are constant. Dies are usually made of tool steel and can be chrome plated to achieve very smooth, corrosion-resistant surfaces. For short runs, uncoated carbon steel or even aluminum dies can be used but these wear quickly and tolerances cannot be kept over long extrusion runs. The flow pattern inside the die is extremely important. The flow should be streamlined with no opportunities of stagnation, which could cause the material to sit for long periods and degrade. Streamlined flow is less likely to cause shear stresses in the melt which might cause unwanted deformations and defects in the final part. The exit orifice of the die is usually proceeded by a short section where the walls are parallel. This section is called the land. The purpose of the land is to build a uniform pressure in the melt so that the material is stabilized and flows evenly before exiting the die. The land is usually the zone of maximum pressure in the entire extruder and can have a strong influence on the back pressure on the screw. Therefore, if the land is too long, excessive back pressure could reduce extruder output and cause internal wear on the thrust bearing and other extruder parts. On the other hand, if the land is too short, the resin flow can be erratic and the part will be less uniform. Hence, some experimentation in determining the land distance is often required in building a new die. Experience in practice dictates making the land slightly longer than is thought necessary and then removing material from the face of the die, thereby shortening the land.
Solid cross – sections: A Typical Die Design for extruding a solid rod is shown in fig.
1. In the figure, DD is the diameter of die orifice, DB is the diameter of bore of extruder barrel, ( is the lead-in angle, and P is the die land.
2. Because of the screen pack and breaker plate assembly, the pressure in the extruder (PE) is reduced by the pressure loss across the assembly (PL).
3. Since the die outlet is at atmospheric pressure, the working pressure is the die pressure (PD) given by the difference: PD = PE – PL.
Hollow sections: Hollow products like pipe or tubes are produced using the die design shown in Fig.
1. The outer diameter of tube is determined by the diameter of the outer die ring orifice.
2. The inner diameter is determined by the mandrel diameter
3. To make the mandrel and outer die ring orifice concentric, centering screws are provided.
4. The mandrel is held in position by a spider. In the centre of the spider a hole is drilled to supply air down the mandrel.
5. To provide a smooth glossy extrudate, the die head is heated. A cold die may cause blockage of the die.
Straight – through dies: Those dies whose axes are arranged to be in line with the direction of supply of melt.
1. Spider, Mandrel is needed for tubes
2. Used for the extrusion of pipe, rod, profiles and sheet
Crosshead dies:
1. Arranged with their axes at an angle of 908 (458 and 308 are also used) to the melt feed.
2. No need for spider assembly.
3. Used for the production of insulated wires, cables
Offset dies:
1. Combination of both straight – through die and off-set die.
2. Used for the production of pipe.
Extrudate swell:
1. Extrudate is contraction in the direction of extrusion and expansion in the cross-section while emerging from the die is called Extrudate Swell.
2. The phenomenon (previously called die swell) is illustrated in fig.
3. Numerically, the extrudate swell is defined as the ratio of the outer extrudate diameter (DE) to the other diameter of the die exit (DD), i.e., B = DE / DD
4. When the melt emerges out of the die lips, there will be expansion in the direction perpendicular to flow and contraction in the direction parallel to flow.
5. A constrained molecule tends to relax at the die outlet. This leads to die swelling.
6. This is nullified by higher take off speed.
Extrudate Swell may be reduced by:
1. Decreasing the extrusion rate
2. Increasing the melt temperature
3. Increasing the die land
4. Increasing the draw-down ratio.
Die entry effect and exit instabilities:
Melt fracture:
1. It is a die-entry effect
2. In any converging flow there are tensile and shear forces
3. If tensile stresses become large and if they exceed the tensile strength of melt, the desirable smooth laminar flow is lost completely.
4. The extrudate emerging from die exit will be of irregular shape. This phenomenon is called “Melt fracture”.
If die entrance is tapered:
1. Dead spots are eliminated
2. Minimize development of tensile stresses and hence minimize distortion of stream lines.
Basic Features of an Extruder Die:
1). A specified volume throughput should be achieved at a minimum of extrusion pressure.
2). The melt being fed from the extruder finds sufficient residence-time in the die to achieve relaxation of stresses & equalization of temperature.
3). The melt emerges at the same accurate rate from the whole of the outlet cross-section of the die.
4). The desired extrudate geometry is achieved for semi-finished products.
5). The surface of the extrudate remains smooth even at high volume outputs.
6). There is no stagnation or decomposition of the extruded material.
7). Die parts have adequate mechanical stiffness to withstand high internal pressure.
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