Tubing tip and die are also calculated taking into account the shrinkage of the polymer because this physical phenomenon has a direct incidence on the DRB factor and also to some extend on the DRB, draw ratio balance.
The DDR Draw Down Ratio is the ratio between the die opening S1 and the insulation cross section S2.
Typical DDR values:
• FEP 80 – 150
• PA 12 4 – 10
• HDPE 1.4 – 4
• PVC 2 - 10
The DRB Draw Ratio Balance compares the die and tip ratio and the DOD and COD ratio. When the two ratios are equal i.e. Die/Tip is equal to DOD/COD, the DRB must be equal to 1. The DOD is the hot overal diameter.
Note: the DRB calculated from the cold DOD will be larger than 1.
Tooling calculation with hot overall diameter
Core diameter COD 1 mm
Overall diameter hot DOD 2 mm
The formula developed to calculate the die diameter of compression tools
can be used to calculate the hot diameter
If the DRB = 1 the ratios DIE/DOD and TIP/COD are equal to the square root of the DDR !!
Die diameter = sqrt (16) x DOD = 4 x 2 = 8 mm
Tip diameter = sqrt (16) x COD = 4 x 1 = 4 mm
This calculation method is simple and accurate it is particularly useful for the calculation of tooling for thick insulation and large DRB ratio.
For years, extrusion tooling has been calculated using simple thumb rules and it worked well, the choice of compound was limited, the machines speed low and the productivity was not as important as it is nowadays.
For instance in the case of telecom or installation wire we used to define the dies diameter equal to the DOD for PVC insulation, 10% larger in the case of HDPE etc.
With the extension of the polymer choice and the variety of the grades, the simple thumb rules are no longer satisfactory, it is necessary to consider the variation of the melt specific volume to define the tooling size.
Shrinkage during cooling down
Between the exit of the cross head and the end of the cooling trough, the specific volume of the compound decreases as its temperature goes down from process temperature to room temperature.
The specific volume of the compounds doesn’t decrease in a linear way but changes abruptly during the transition from the fluid to the solid state.
The diagram shows the specific weight variation of LDPE and HDPE.
Compression die calculation
On an extrusion line this variation results in a reduction of the outer diameter of the insulated core.
When the insulation thickness is small against the core diameter, the overall diameter reduction is insignificant but when the insulation thickness is important, this reduction must be taken in account to define the extrusion die diameter.
over insulation cold mm
over insulation hot mm
difference cold -> hot %
The ideal compression die diameter should be equal to the hot over insulation diameter.
Table 1: Typical insulation section variation of common polymers:
K = Cross section
Variation cold > hot
Die calculation example :
COD = core overall diameter
DOD = Diameter over dielectric (cold)
K = shrinking factor from table 1
The advantages of dies calculated with this method are numerous.
• Better insulation surface quality.
• Less die drool.
• Less shear.
• Higher production speed.
• Good adhesion of the insulation.
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