EXTRUSION TECHNOLOGY

DRAW DOWN RATIOS - What you need to know to make tubing and cable

This month's Extrusion Technology column explores die design for tubing and cable sheathing, specifically in regard to the I.D. of the Die and O.D. of the Mandrel (also known as tip). Usually for thermoplastics, the size of this tooling is chosen larger than the product, so that the melt is "drawn down". It is this process of drawing that has become synonymous with "Draw Down Ratio" equations.

The equations presented here today, are simple proven equations which find great acceptance 'out on the shop floor'. In addition to being simple, the same equations apply to tubing formed by free extrusion or vacuum sizing and the sheathing of core when tube forming tooling is used (not pressure jacketing).

DRAW DOWN RATIOS
DIRECT DRAW DOWN = Die I.D. or Mandrel O.D ~ 1.5 to 3.0

Tube O.D. Tube I.D. (most common)

 

DRAW BALANCE = (Die I.D./Tube O.D.) = 1.05 to 1.15

(Mandrel O.D./ Tube I.D.)

 

The first equation is simply the ratio of the tooling divided by the tube dimensions. So if the die is twice as large as the tube O.D., the Direct Draw Down would be 2.0. This should not be confused with Area Draw Down (not considered here) which would be approximately this value squared.

The most important equation is Draw Balance. This must be kept between 1.05 to 1.15. The first thing that should be noted is that because Draw Balance is almost 1.0, Direct Draw Down for the Die and the Mandrel are almost the same. For example, if the Mandrel direct draw down is 2.0, the Die Direct Draw down will be about 2.2, or about 5% to 15% more.

Why must the Draw Balance be between 1.05 and 1.15?

Free Extruded Tube ( with internal air pressure trim)

For tubing which is not vacuum calibrated, but uses the application of internal air pressure to trim size, the inherent I.D. created by the tooling must be slightly small. This facilitates the application of internal air pressure to trim size. If the draw balance is less than 1.0, the I.D. will be inherently slightly large and it will be impossible to achieve correct size.

Vacuum Sized Tube

When a calibration device and vacuum are used to trim size, two situations may be employed. A draw balance of 1.0 is commonly practiced, but because of the friction with the calibrator, a bulging draw region results, increasing surface contact and friction with the calibrator. More resin flows in the areas of least resistance creating thicker walls there. As the frictional characteristic of the calibrator changes, so does the concentricity of the tube formed. This becomes increasingly pronounced with; increased line speed, larger tube O.D. and reduced melt viscosity.

A better situation results if a Draw Balance of 1.05 to 1.15 is employed. Bulging in the draw region is reduced, minimizing surface contact and the effect of frictional differences around the calibrator. It should be noted however that as the Draw Balance is increased, it becomes more difficult to maintain a vacuum at the calibrator, especially during start up.

Sheathing

The alternative to pressure Jacketing is sheathing, using tube tooling, and drawing the sheath down on to the core. If a draw balance less than 1.0 is used, a fold results in the sheath since the sheath circumference is inherently too big. With a Draw Balance of 1.05 to 1.15, the sheath circumference will be inherently small and stretch snugly onto the core.

Direct Draw Downs between 1.5 and 3.0 are suitable for most resins and tube sizes. Some resins accommodate significant drawing while others do not possess enough melt strength. In general, Direct Draw Down of about 2.0 is a good starting point.

Mark Carter

Principal Extrusion Engineer

Carter & Assoc., Contracting Extrusion Engineers