What is the Draw down ratio (DDR) and draw ratio balance (DRB) calculations?

Draw down ratio (DDR) and draw ratio balance (DRB) calculation need to be made to insure that correct product sizes are produced. These calculations are available in standard extrusion texts.
Without performing calculations for tip and dies sizes, based on the final product dimensions and the consideration for the polymer being processed, and the quenching method being used, the
achievement of the correct final product dimensions will be difficult.

The draw down ratio (DDR) is simply the ratio of two areas. The first area is the annular exit area of the die assembly, produced by the tip outer diameter and the die inner diameter. The
second area is the cross-sectional area of the final annular extruded product. This calculation dies not take into account the die (extrudate) swell as the melt emerges from the head assembly
exit annulus. The draw down ratio can be a low as 1:1 in the case of certain profile extrusion processes, to as high as 100:1, as in the case of small diameter fluoropolymer tube extrusion.
More often, ratios of 1.5:1 to 5:1 are found.




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The draw down ratio (DDR) is simply the ratio of two areas. The first area is the annular exit area of the die assembly, produced by the tip outer diameter and the die inner diameter. The second area is the cross-sectional area of the final annular extruded product. This calculation does not take into account the die (extrudate) swell as the melt emerges from the head assembly exit annulus. The draw down ratio can be as low as 1:1 in the case of certain profile extrusion processes, to as high as 100:1, as in the case of small diameter fluoropolymer tube extrusion. More often, ratios of 1.5:1 to 5:1 are found.

DDR = (Dt + Dd) / (Do + Di)
The draw ratio balance (DRB) is a ratio of ratios. The first ratio is the comparison between the inner diameter of the die, and the width of the annular gap between the die inner diameter and the tip outer diameter. The second ratio is the comparison between the outer diameter of the final product, and the wall thickness of the final product. Draw ratio balance is a description of how the wall thickness of the extrudate is going to need to change during its time in the air gap, between the annular die exit, and the final extruded product.

A DRB value of 1 indicates that the wall thickness of the extrudate as it exits the die orifice, compared to the die inner diameter, is exactly the same as the wall thickness of the final product, compared to the outer diameter of the final product. No additional drawing of the extrudate wall is taking place in the air gap.

Usually, a DRB value slightly greater than 1 is used, for example: 1.04, or 1.06. This means that as the extrudate is drawn through the air gap, not only will the outer diameter of the extrudate be decreasing, but the wall thickness will need to draw down a little more. This calculation again does not take into account the die (extrudate) swell.

It is possible, in rare occasions, to have a DRB value calculated as being slightly less than 1, if there is sufficient die swell taking place at the die exit.

DRB = (Dd / Dt) / (Do / Di)

DDR and DRB tools

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UNDERSTANDING & COMMUNICATING DRAW DOWN RATIO BALANCE
In a previous issue of the Extruder newsletter, we explained Draw Down
Ratio (DDR) and its impact on tooling design for plastic tubing, hose and
wire insulation jacketing. This article complements that discussion by
explaining Draw Ratio Balance (DRB). The need to understand the
relationship between the dimensions of the tooling and those of the
final product is met by understanding both Draw Down Ratio and Draw
Ratio Balance. It is required that both the tooling designer and extruded
product manufacturer understand and are able to communicate these
relationships to each other.

Draw Down Ratio is the ratio of the cross sectional area of the extruded
plastic melt to the cross sectional area of the plastic in its final product form,
be it a tube, hose or insulation on a core, such as a wire or cable. It is the
extent to which the plastic has been reduced in size to make the part.
A larger DDR enables faster production rates, while a lower DDR facilitates
more precise control of the product dimensions. A low draw down ratio
process is more stable than a high one.

Success in specifying DDR lies in the skill and experience of the
manufacturer.  B&H Tool Company, with many years experience in
the design and manufacturing extrusion tooling,  is well able to help
in this regard.

Draw Ratio Balance describes the balance between the rate the outside
of the cone draws down, and the rate the inside of the cone draws down.
Most products made by drawing a plastic melt are smaller in cross sectional
area than the tooling gap.

B&H Tool Company has made available on its website a Draw Down
Calculator where you can input dimensional data and calculate draw down
ratios and draw ratio balance. You can determine the current draw down
ratio of your existing tooling and product and determine the balance
between them. You ca also use the calculator to deter-mine the tooling
sizes required to extrude a product with known target dimensions
and acceptable draw down ratios.

As you work with the calculator and make adjustments to the data,
the results and adjustments will be recorded and printable for your review.

Niall McKee, Senior Technical Consultant with
DuPont Fluoropolymers offers these insights:

Drawing down the cone creates different angles inside and outside the
cone, which differentiates the distance and speed the plastic melt needs
to travel on the outside of the cone to that traveling on the inside.
(See figure 1) This is very obvious with large Draw Down Ratios.
It is important that the plastic melt flows smoothly down to its final
size. When the tooling is out of balance, one of the cone's surfaces is
trying to feed more plastic than the other surface can handle. When the
cone's inside and outside lengths are equal, the Draw Ratio Balance is 1.00.

In most tubing set-ups, this is the desired condition. In jacketing
applications where the plastic ID is supported by a core, production
rates are usually faster. Experience has taught us that a slight positive
DRB (1.03 to 1.08) is preferable in that case. At all times, a negative DRB,
i.e. less than 1.00, should be avoided.

As the sides of the draw down cone cannot be measured, we again
use geometric relationships to determine the DRB. If the ratio of the
die diameter to the product outside diameter is Ratio 1(R1), and, the
ratio of the tip diameter to the product inside diameter is Ratio 2 (R2),
then, the ratio of R1 to R2 is the Draw Ratio Balance.

Stated another way, the die diameter divided by the product OD,
all divided by the tip diameter divided by the product ID is the DRB.

It sounds complicated but it is not really. (D/O)/(T/I) = DRB
Where: D = Die Diameter T = Tip Diameter O = Product OD I = Product ID

Example: Determine Draw Ratio Balance Die Diameter = .500″
Tip Diameter = .250″
Product OD = .250″
Product ID = .125″ (.5/.25)/(.25/.125) = 2/2 = 1.00 DRB

To increase the DRB, increase the Die Gap by either increasing
the Die Diameter, or decreasing the Tip Diameter or a smaller
adjustment of both.

Example: Increase Die Diameter by .020 Die Diameter = .520″
Tip Diameter = .250″
Product OD = .250″
Product ID = .125″ (.52/.25)/(.25/.125) = 2.08/2 = 1.04 DRB

Example: Decrease Tip Diameter by .010 Die Diameter = .500″
Tip Diameter = .240″
Product OD = .250″
Product ID = .125″ (.50/.25)/(.24/.125) = 2/1.92 = 1.04 DRB

To decrease the DRB, decrease the Die Gap by either decreasing the
Die Diameter or increasing the Tip Diameter or a smaller adjustment
of both. Out of balance tooling will cause disruptions to the final product
dimensions. Tubing may fold in on itself trying to reach a balanced
condition. Wire insulation tooling imbalances may break the cone or
form teardrops. Understanding the need for and achieving a Draw
Ratio Balance will prevent many product quality headaches.

Use the easy to understand "fill-in-the-blank" calculator to determine
your tooling DDR and DRB. Or contact B&H Tool Company for assistance.

Understanding and Communicating Draw Down Ratio Balance

For the example above:

Understanding and Communicating Draw Down Ratio Balance

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 prevent the inaccuracies
of guessing or being led into doing some quick divisions based on
diameters rather than areas. If you remember high school geometry,
you might wonder what happened to Pi in figuring areas. With this
formula, you can dispense with it; the results will be the same.

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Drawdown Ratio (DDR)
The drawdown ratio (DDR) characterizes a tubular die and is defined as the ratio of the cross-sectional area of the annular die to the cross-sectional area of the finished insulation.

Formulas: 
DDR = (A_D - A_T) / (A_cw - A_bw) 
or in simplified form: 
DDR = (D_D^2 - D_T^2) / (d_cw^2 - d_bw^2)
Where:
- A_D, D_D = Die cylinder cross-sectional area/diameter 
- A_T, D_T = Guide tube (mandrel) cross-sectional area/diameter 
- A_cw, d_cw = Wire and insulation cross-sectional area/diameter 
- A_bw, d_bw = Wire (conductor) cross-sectional area/diameter 

Typical Ranges:
- Fluoropolymers (FEP, PFA, ETFE) have DDR values between 10 and 250.
- Larger diameter cables may have DDR between 2:1 and 3:1.

Draw Ratio Balance (DRB)
The draw ratio balance (DRB) ensures that the interior and exterior surfaces of the tube are drawn evenly.

Formula: 
DRB = (D_D / d_cw) / (D_T / d_bw)
Ideal Value: 1.00 (meaning no additional wall drawing in the air gap). 
Typical Ranges:
- FEP & PFA: 0.9–1.15 
- ETFE: 1.04–1.07 
- Values below 0.9 cause concentricity issues, while values above 1.1 may cause material tearing.

Drawdown Ratios for Different Polymers

Resin	Drawdown Ratio	Maximum Drawdown Ratio
FEP	50–150	200
PFA	50–250	—
ETFE	10–50	100
Polyethylene	3–4	10
PVDF	2–3	4
PVC	1.5–3	5
Polyurethane	1.5–3	5

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Effect of Drawdown Ratio (DDR) in Plastic Extrusion

Drawdown Ratio (DDR) plays a crucial role in plastic extrusion, affecting product quality, material efficiency, and processing stability.

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Effects of Low DDR (Small Drawdown Ratio)
- DDR close to 1:1 → The extrudate retains nearly the same shape as it exits the die.
- Characteristics: 
  - Low material stress 
  - Minimal mechanical stretching 
  - Better material homogeneity 
  - Thicker extrudate walls 
  - Higher die swell (material expansion after exiting the die)
- Advantages: 
  - Higher material stability 
  - Suitable for fragile or sensitive materials 
  - Ideal for thick-walled tubes and low-speed extrusion
- Disadvantages: 
  - Lower production speed 
  - Higher material consumption

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Effects of High DDR (Large Drawdown Ratio)
- DDR of 50:1 or higher → The extrudate undergoes significant stretching after exiting the die.
- Characteristics: 
  - Increased material stress and stretching 
  - Thinner wall thickness 
  - Higher production speed 
  - Reduced die swell effect
- Advantages: 
  - Reduced material usage 
  - Lightweight products 
  - Higher processing speeds
- Disadvantages: 
  - Increased risk of tearing or deformation 
  - Difficulties in maintaining concentricity and wall thickness uniformity 
  - Requires high tensile strength materials

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Ideal DDR in Plastic Extrusion
The optimal DDR depends on the material and extrusion process:

Material	Typical DDR Range	Maximum DDR
FEP	50–150	200
PFA	50–250	—
ETFE	10–50	100
Polyethylene	3–4	10
PVDF	2–3	4
PVC	1.5–3	5
Polyurethane	1.5–3	5

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Conclusion
- Low DDR → Best for thick-walled, slow-speed extrusion. 
- High DDR → Ideal for thin-walled, high-speed extrusion but requires strong materials. 
- Too high DDR → May cause tearing or instability. 
- Too low DDR → Reduces efficiency and increases die swell.

Understanding Drawdown Ratio (DDR) Scale

What Do 1:1 and 50:1 DDR Mean?

DDR (**Drawdown Ratio**) is a scale that represents the reduction in cross-sectional area of extruded material from the die exit to the final product.

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DDR Scale and Meaning

- DDR = 1:1 → The cross-section of the material remains the same after exiting the die. No stretching occurs.
- DDR = 2:1 → The material is reduced to half of its original cross-section.
- DDR = 10:1 → The material is drawn down to one-tenth of its original cross-section.
- DDR = 50:1 → The material is significantly stretched and reduced to 1/50th of its original size.

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How is DDR Calculated?
DDR is determined using cross-sectional areas or diameters.

Using Cross-Sectional Area:
DDR = (Die Exit Cross-Section) / (Final Product Cross-Section)

Using Diameters:
DDR = (D_d² - D_t²) / (d_cw² - d_bw²)
Where:
- D_d → Die exit diameter
- D_t → Guide tube diameter
- d_cw → Wire and insulation diameter
- d_bw → Conductor (wire) diameter

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DDR Scale and Application

DDR Range	Description
1:1 – 3:1	Minimal drawdown, suitable for thick-walled products.
3:1 – 10:1	Moderate drawdown, standard for many plastic extrusion applications.
10:1 – 50:1	High drawdown, used for lightweight, thin-walled products.
50:1+	Extreme drawdown, requires precise control to prevent material failure.

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Conclusion
- DDR = 1:1 → No reduction in size.
- DDR = 50:1 → Highly stretched and thinned material.
- The ideal DDR depends on the material type, extrusion speed, and final product requirements.