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Rubber Extrusion Process

Rubber Extrusion Process

Extruded rubber products will differ from molded rubber products based on the process where extruded parts are forced through a die of the required cross section under pressure of an extruder. Often extruded products are unvulcanized prior to being extruded, leaving the rubber in a soft and pliable state post extrusion. If this is indeed the case, the finished extruded products will normally need to be vulcanized before they are rendered usable.

The extrusion process begins with the unvulcanized rubber compound being fed into the extruder. Next, the flutes of the revolving screw will begin to carry the rubber forward into the die, with an increase in pressure and temperature occurring as the material gets closer to the die itself. Once it reaches the die, the built up pressure forces the material through the openings, where it will consequently swell in various degrees based on the material compound and hardness. Because of this tendency towards swelling, many extruded parts require plus or minus tolerances on their cross sections. During the vulcanization, the extruded rubber will well or shrink in both its cross section and its length depending on the type of rubber compound used. After vulcanization, a length of rubber extrusion will tend to be reduced in dimension more in the center of the length than in the ends.

Extrusion Dies for Extruded Rubber

The extrusion die is a precise and specific tool made by cutting an opening shaped in the form of the finished rubber cross section desired through a blank of steel. Once in place, the rubber material will be forced through this die via the pressure that builds up from the revolving screw of the extruder. Many rubber compounds will tend to swell when passing through the extrusion die, causing them to experience an increase in dimensions. Thus, each die is made according to each particular part and material to ensure that all tolerances are met for the finished extruded rubber part.

steel extrusion dies

RMA Standards for Cross Sectional Tolerance Table — Rubber Extrusions

The closer tolerance classes outlined below should not be specified unless required to do so by the final application and they should be restricted to the critical dimensions. The closer tolerances demanded, the tighter the control must be exercised when the material is being extruded and hence the higher the cost incurred.

Dimensions in millimeters
RMA Class 1 - High Precision 2 - Precision 3 - Commercial
Drawing Designations E1 E2 E3
Above Up to  
0 1.5 ±0.15 ±0.25 ±0.40
1.5 2.5 0.20 0.35 0.50
2.5 4.0 0.25 0.40 0.70
4.0 6.3 0.35 0.50 0.80
6.3 10.0 0.40 0.70 1.00
10.0 16.0 0.50 0.80 1.30
16.0 25.0 0.70 1.00 1.60
25.0 40.0 0.80 1.30 2.00
40.0 63.0 1.00 1.60 2.50
63.0 100.0 1.30 2.00 3.20
Dimensions in inches
RMA Class 1 - High Precision 2 - Precision 3 - Commercial
Drawing Designations E1 E2 E3
Above Up to
0.00 0.06 ±0.006 ±0.010 ±0.015
0.06 0.10 0.008 0.014 0.020
0.10 0.16 0.010 0.016 0.027
0.16 0.25 0.014 0.020 0.031
0.25 0.39 0.016 0.027 0.039
0.39 0.63 0.020 0.031 0.051
0.63 0.98 0.027 0.039 0.063
0.98 1.57 0.031 0.051 0.079
1.57 2.48 0.039 0.063 0.098
2.48 3.94 0.051 0.079 0.126

In general, softer materials and those requiring a post cure will need greater tolerances.