Land length

In addition to the tip and die diameter, the land length and the cone angle are important design parameters. In many situations a long land length is desired because a long land tends to  

The gap between the tip and the die, H, is half the die diameter minus half the tip diameter, or H = 0.5D,i-0.5D,. The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials. In many cases, therefore, the pressure drop will determine what land length is practical. 

A special extrusion technique that is occasionally used is the extended mandrel. This is particularly used for thin wall tubing. In this technique the tip extends beyond the die by a considerable distance. The purpose of the extended mandrel is to obtain better shaper definition. Another use of the extended mandrel is to provide localized heating of the tip using an induction coil at the die exit. This is a variation of the G-Process discussed under special features. It is a suitable method to eliminate internal melt fracture or internal die drool. The mandrel extension should be made of a ferro-magnetic material to obtain an efficient temperature increase under the influence of the alternating magnetic field of the induction coil. 

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The land of the hinge should be at least 1.5 mm (0.06 in.) wide for a proper flow pattern and at least wide enough so that when the product is bent in service it will not develop strains. Too short a land length will cause the hinge to have limited flex life. 

You are to extrude a polystyrene tube at an average speed of 0.1 m/s. The relaxation time, A, of the polystyrene, at the processing temperature, is 1 second. The die land length is 0.02m. Will elasticity play a significant role in your process. 

Using the nomenclature presented in Fig. 6.12 and assuming a land thickness of h we can assume the land length to be described by slit flow and the manifold by the Hagen-Poiseuille flow with a variable radius as... 

Figure 6.14 Schematic diagram of die with two different die land lengths and thicknesses.

The die land thickness differences can be compensated by using different land lengths such that the speed of the emerging melt is constant, resulting in a uniform product. If we assume a power-law viscosity model, a uniform pressure in the manifold and an isothermal die and melt, the average speed of the melt emerging from the die is... 

Die design with two die land thicknesses. Determine the die land length ratios, L2/L1 for a die land thickness ratio, /i2/ i of 3, for various power-law indeces. Using eqn. (6.54), we can easily solve for the land length ratios for several power-law... 

Figure 6.15 Land length ratios as a function of power law index for a die with a land height ratio of 3.

Using the geometry and notation given in Fig. 6.14, relate the die land length ratio, L1/L2 for a die land thickness h //i2 = 2 to the power law index of a shear thinning polymer. 

The required land length depends not only on the type and temperature of the TP melt, but also on the flow rate. The deformation of the melt in the entry section of the die invariably causes strains that only gradually decrease with time (relaxation). Usually the target is to allow the melt to relax before leaving the die. Otherwise the product dimensions and the mechanical properties may vary, particularly with rapid cooling. 

Fig. 2 Comparing designs of T-type die (A) [(1) constant cross-section manifold (2) constant land length] to coat hanger-type die (B) [(1) manifold cross-section decreases as distance from centerline increases (2) land length becomes shorter farther from the centerline of the die].

Use modular design with stacked plates for manufacturability, convenient assembly, and disassembly, as well as convenient modifications and cleaning. Die land length should be at least 10 times the product thickness (or gap) to facilitate the polymer melt stress relaxation within the die. 

The melt parison is extruded from an annular die. The wall thickness of the parison depends both on the annular gap setting and on the shear rate of the melt in the die. The melt will swell after the parison exits from the die and the die swell Increases with increasing shear rate. The die swell is also a function of the temperature, the type of polymer, and its elastic melt properties. Usually the shear rate in the die varies from 10 to as high as 700 s . As a rule of thumb one uses a die land length 8 times the annular gap. The parison should normally not be blown up beyond 3 1 (i.e., bottle parison diameter). 

Resin Type Reduction Ratio Lubricant Content Lubricant Type Die Cone Angle Die Land Length Extrusion Speed Temperature... 

An alternative to the check ring valve is a smear head (Fig. 6.33), which is based on the use of a small clearance in the diameter over significant land length. This design restricts the back flow of the melt during... 

Film blowing die designs generally involve a multiple threaded helical mandrel that discharges the melt into a smooth cylindrical land. Depending on the desired film thickness, the land length is 10 to 15 times the annular gap. Cooling is... 

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