How to Calculate a Draw Down Ratio

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In manufacturing, the draw down ratio is a measure of the reduction in size of an extruded product from the die to its final size. When plastic or other materials are pushed through the die to shape them into a tube or other useful shape, it is under compression. If nothing is done about this, the material will swell up to 200 percent when it exits the die and is released from compression. To prevent this and make the product dimensions more precisely controllable, some manufacturing processes involve pulling on, or "drawing down" the material as it emerges from the die so that the tension counterbalances part or all of the compression.

  • Determine the cross-sectional area of the die. Count only those areas through which the material is extruded; if the product is hollow, do not count the empty regions of the die where no material goes. Measure from the inside edge of the die so that the thickness of the material of the die itself does not affect the calculation.

  • Measure the cross-sectional area of the final product. Again, do not count any hollow regions.

  • Take a ratio (fraction) of the two measurements. Note that the units of area cancel out, leaving the draw down ratio as a dimensionless number. A larger draw down ratio is associated with faster production rates, and a lower one with greater precision and a more stable process.

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