Using Laboratory-Scaled Extruders in the Testing of Unvulcanized Rubber

Editor’s Note: This is the last article in a 3-part series on How to Relate Test Results of a Torque Rheometer to Problems in Elastomer Processing

To widen the variety of process relevant test methods for rubber applications, laboratory-scaled rubber extruders (with roll feeders) are available for torque rheometers. These measuring extruders are specially designed to simulate extrusion processes and simultaneously to control and evaluate the process parameters.

The measuring extruders can easily be equipped with a large number of dies which, according to their geometry are suitable for a variety of applications. Three different methods are presented below, which may be helpful in solving problems in the processing of unvulcanized rubber.

1. Automatic Extrusion-Capillary Measurement Capillary measurements show their advantage where information about the flow behaviour of an unvulcanized compound is required as an absolute value.

   Injection moulding processes (e.g. for seals and O-rings) can be mentioned as an application, where rheological factors influence the success of the process to a great extent. Due to the narrow clearances and high rubber compound melt flow rate, extremely high shear rates can occur in the injection channels. On the other hand, the final filling of the mould is slow and the applied shear is low.

   As a rule, rubber compounds exhibit pseudo-plastic flow behaviour. Various shear stresses can be exerted on a rubber compound during processing. Because the viscosity is highly dependent on the shear rates, it is imperative that the viscosity be characterized as a function of the relevant shear rates. The processing engineer’s main goal is the determination of these relationships for the injection moulding process.

   In the case of extrusion capillary measurements the compound is forced through a slit capillary and a rod capillary die. The slit capillary die covers a shear rate range that usually accurse in extrusion process. The material flow in the smaller rod capillary reaches much higher shear rates, which correlate more with the injection moulding process. The rheological factors are calculated from the differential pressure and the volumetric flow rate.

   The great advantage of extrusion-capillary measurements is that the sample is made to flow under process conditions.

   As this test can be run fully automatically, it is also suitable for fast and very accurate quality control differentiation of different batches.

2. Die Swell Behavior Apart from the viscosity, the elasticity of the polymer melt greatly influences the processing and the end product. Tire components like tread profiles have to be extruded in very narrow tolerances. The success of this process depends on the evenness of the raw rubber polymer which is used for the compound. If this basic material differs slightly in its elastic behaviour, the given tolerances for the end product, such uniformity, cannot be reached. To ensure that your process does not run out of tolerance, the die-swell test can be a useful quality control instrument.

   For this test, which can be directly linked to the above described capillary measurement, a laser sensor simultaneously measures the swell of the extruded strand. The viscosity and die-swell, which is a measure of the elasticity, is evaluated as a function of the applied shear rate. The tests show very clearly, that the viscosity behaviour of both compounds show shear thinning and scarcely differ. On the other hand the elastic behaviour of both samples is extremely different. With this information, the processing of a compound can be easily optimized. Time and money-consuming empirical tests for e.g. designing a new die on production-scaled machines can be minimized.

3. Qualitative TestingExtreme shapes like a tire tread with wings, a sidewall or an apex are very common in tire manufacturing. They require a great deal of experience in die-design and rubber extrusion to attain acceptable surfaces, sharp edges and equal dimensions for these complicated end products.

   The Garvey test described in ASTM 2230 is an approved method which offers an easy alternative to test the extrudability of compounds according to the above mentioned parameters. For the Garvey test the measuring extruder is equipped with a standardized die, the shape of which is a much scaled-down version of half of a tire tread.  Problems concerning the extrudability of a compound can be found out very easily, without carrying out expensive empirical tests on production-scaled machines. (Image below:  Garvey die profile)

   

The increasing demand for high quality products at a low price level increases the need for meaningful, accurate and simple test methods as a development and quality control instrument. The importance of a linkage between the test results and the process experience is vital.

You can see more specifics, including charts, spectra, and instruments used, in the application note How to Relate Test Results of a Torque Rheometer to Problems in Elastomer Processing.