Dialing in the wall thickness of "Bump, bubble or Taper" tubing- the smart way

This months extrusion technology column, elaborates on a previous "Dialing in the wall thickness of extruded tubing" article published in the Spearhead's June 1998 issue. A more complex situation is that of sizing a tube that has two or more sections with different O.D. (outside diameter) and I.D. (inside diameter). This type of tube has become synonymous with the terms; "bump tubing", "bubble tubing" and "tapered tubing". In the simplest case, this is a tube with a large section, followed by a small section, followed by a large section and so on.

For bump tubing made using two or more puller speeds, then we can assume that a constant volume of resin exits the die, this allows us to derive an equation to help automate the sizing process.

Step 1

Size the large section of the tube and note the puller speed. (For a faster result, use the tube wall thickness sizing equations given in the June article)

Step 2

Calculate the puller speed for the small section using the following formula:

Puller Speed (small) = Puller Speed (large) X (O.D.2 - I.D.2 )(large)

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(O.D.2 - I.D.2 )(small)= Puller Speed (large) X K (dimensional constant)