B&H Tool Company Advisor
Understanding and Communicating Draw Down Ratios
Tooling design for plastic tubing, hose and wire insulation jacketing requires an understanding of the relationship between the dimensions of the tooling and the dimensions of the final product. It is required that both the extruding manufacturer and the tooling designer or builder need to understand and be able to communicate that relationship. Draw Down Diagram

Generally, manufacturers will desire that a set of tooling be designed for each particular product. Once the desired productís dimensions have been determined, a tooling designer will usually ask what has been a successful draw down ratio based on the manufacturerís experience with previously produced products. This is where communications become unclear. A designer is looking for a ratio that will represent the relationship between the die and tip dimensions and the finished product. This is usually suggested by the compound manufacturer or has been determined by calculations of current tooling and successfully produced products. However, many inquires are answered with information that does not represent the true relationship between the tooling and product. Statements of draw down such as "50%" or "2 Times" or "1.5" do not accurately or correctly state the relationship.

Letís look at a simple example: A tubing manufacturer desires to produce a 1/4" tube with an ID of 1/8". They have successfully produced a 1/4" tube with a similar polymer with a die having the ID of 1/2" and a tip with the OD of 1/4". Many manufacturers would report to the tooling designer that his draw down ratio is 1/2 or 50% or 2:1 because of the relationships between the diameters of the product and the tooling. He may even short cut the whole confusion by simply stating that his tooling is twice as large as his final product. All this can lead to confusing communication and improperly designed and manufactured tooling. To solve this communication problem both the extrusion manufacturer and the tooling designer need to understand and use the proper determination of the draw down ratio.

The correct draw down ratio is obtained by using the following formulas:

DDRatio =      
Where: D = Die Diameter
T = Tip Diameter
O = Product OD
I = Product ID

For the example above:

If the tooling designer has the properly expressed ratio and the desired dimensions of the final product, he will use the following formula to determine the size of die and tip to manufacture in order achieve the desired results.

      For the example above:

Some manufacturers and designers will simply state that they are using a multiplier on the finished product diameter dimensions of OD and ID. In the example, the multiplier would be two.

This may seem obvious with the example given; however, when using odd dimensions the proper formulas save on the inaccuracies of guessing or being lead into doing some quick divisions based on diameters and not on areas. If you remember high school geometry, you might wonder what happened to Pi in figuring areas. Well, in this formula you can dispense with it, the results will be the same.

If all this is too much trouble you can visit a new page on our web site at www.bhtool.com any time you want to figure draw down ratioís and draw down balances. There, you will find a fill-in-the-blank calculator that will help in all your tooling dimension problems.


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