Access to compound extrudability by using a 5-finger die – Application of standardized test formulations

Body sealing profiles are highly sophisticated in terms of lip forming and overall extrusion process. Especially the introduction of electrical isolating compounds has increased the demands in this area. The work presented in this paper proposes and describes a lab method to get access to the extrudability of compounds by means of lip shaping: the use of a 5-finger die, where the five lips have an increasing degree of difficulty, and also represent the variety of thin profile lips in sealing profiles. The sensitivity of the methodology is demonstrated by applying model compounds derived from ISO 4097 EPDM polymer testing standard. The standard is slightly modified for the curatives and more relevantly for the flow properties in adding a variation of type and amount of white filler into the recipe. On one hand this represents the need of the market for cost effective recipes. But also, the necessity to compound with low carbon black loadings in order to meet electrical isolating properties requires smart compounding with specific fillers. In total, that compounding approach leads to flow characteristics substantially different from conventional compounds, worth to be analyzed. In order to mirror the derived 5-finger die results to other existing methods, frequency sweep investigations by using an RPA on the model compound, a DoE study is conducted. If the 5-finger die results are summarized into a single number (like a cost function) a general correlation to delta-delta or shear thinning of the complex viscosity curve can be demonstrated.

Results from manual lip forming evaluation with the 5-finger die

D. Schramm, T. Schmid

Extruded profiles are widely used in many applications, such sealing profiles for windows in housing or other civil engineering applications. Automotive sealing systems, for example, have very complex cross sections and very narrow cross section tolerances down to ±0.2 mm are applied in some areas. Apart from the processing technology, the general compounding “for processing” plays an important role in the achievement of production processes capable of delivering this kind of precision. This has become even more important with the introduction of non-conductive compounds to avoid white corrosion in contact with aluminum or aluminumalloy body parts.

One important part for any description of processability is a rheological description of the material. Rheological descriptions start with typical rubber tests like Mooney viscosity and MDR vulcameter testing and can be refined to shear viscosity or strain viscosity behavior as well as to linear and non-linear viscoelastic behavior. Ideally it includes or corrects effects also for pre-shear, wall slip and shear heating. This listing makes it evident that a comprehensive assessment of the rheological behavior of rubber compounds is very tedious. Besides, effects at the die orifice or post-die are hardly included and not predicted. At this position there is an overlay of die swell, take-away forces and the velocity changes from low velocities close to the die wall (including wall slip) at the inner die to a no-flow condition going along with the line speed at every position in the cross section. Consequently, at the area of the die exit an acceleration occurs which also creates inner tensions. Depending on the cohesive capabilities of the compound the imposed tensions can create very slight roughness up to cracks or so-called mousebites at lip-tips. Of course, a balanced flow situation with sufficient material disposal is helping to avoid such profile ruptures.

The situation explained above has led to the idea to set up a lab scale extrusion system consisting of lab extruder and a die representing the situation of profile extrusion. The concept includes the same approach other lab scale processability testing methods have used. The aim of this publication is to propose a die geometry and methodology to access compound extrudability for profile extrusion – maybe as a new industry standard. The predictive capability will be shown by using model recipes and comparison to RPA frequency sweep data respectively. This is an arbitrary choice in a way and many other comparisons can be made. However, the analysis of tan δ data is often discussed in the context of processing predictions coming from the pure polymer perspective.

Citation:
D. Schramm, T. Schmid, RFP Rubber Fibres Plastics, 04 2020, 192-196.

https://www.gupta-verlag.com/magazines/rfp-rubber-fibres-plastics-international/04-2020