Toolset Calculations

One of the more critical aspects of extrusion die design are the dimensions given to the mandrel tip and die bushing. These dimensions are the die bushing inner diameter, the mandrel tip outer diameter, and the land length of the die bushing. Two important calculations involve the draw down ratio (DDR) and the draw ratio balance (DRB).

The DDR is the ratio of the annular toolset exit area (bushing ID area minus mandrel OD area) divided by the final product area (OD area minus ID area). This can also be referred to as the area draw ratio (ADR). The greater the DDR, the more precise the final product diameter can be. All polymers have different amounts of acceptable drawability before process instability is created. The DRB is the way the proportion of the wall to diameter changes from the toolset exit to the final product. When the polymer exits the toolset, there is usually a small expansion of the polymer that occurs, often called “die swell”. As the material is drawn through the air gap to the quench tank, it is important to have slightly more wall at the die exit than at the final product. The DRB value will usually be slightly greater than one (1), i.e. 1.05. The land length (L/D) is measured by taking the length of the straight portion of the die bushing exit and dividing that value by the annular gap of the toolset (bushing ID – mandrel OD/2). The greater the L/D ratio, the greater the melt flow stability, and less die swell will be produced. With greater L/D ratio, the greater the extruder head pressure will be. In the upper table, enter the desired product OD and ID, then enter the desired DDR and DRB values. The mandrel tip and bushing diameter will be calculated. I

n the lower table the product ID and OD are entered, as well as the mandrel OD and the bushing ID, and the desired land length. The product wall value, the L/D value, the DDR value and the DRB value will be calculated. Sample values are shown in the tables below, replace them with the values for your application.

Non non-shaded cells are values to be entered, the shaded cells are the calculated values.

To determine mandrel tip and bushing diameter using DDR and DRB values.
tube OD tube ID DDR DRB pin OD bush ID
To determine DDR, DRB and land length L/D values using mandrel tip and bushing diameters, and land length value.
tube ID tube OD wall pin OD bush ID lang lg land L/D DDR DRB

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