Relaxed extrusion: A very important for high process capability
Since the introduction of tire labeling in Europe, and many other countries of the world, it is very easy to recognize the various aspects of the quality of a tire. Noise generation, rolling resistance and the wet skid of a tire can be influenced by the tread construction and by the compounds which are applied in the tread. To fulfill the requirements tread constructions may contain 5 or even 6 different compounds. Each compound is prepared by its own extruder in a tread extrusion line. It means that 5 (even 6) extruders are mounted into the same co-extrusion head.
One of the most demanding compounds in a tread is the cap compound. It forms the contact between the vehicle and the road surface. The formulations of cap compounds are completely different compared with a cap compound of 1 or 2 decades ago. New polymers and new filler systems are being applied to fulfill the requirements of its vulcanized properties. Also base compound may be based on solution SBR polymers and silica-silane filler systems. The processing of these kinds of compounds often is more difficult than with traditional compounds and is still not completely understood.
Process capability limiting factors.
There are 2 limiting factors to achieving process capability in tread extrusion.
Homogeneity problems can be easily recognized by surface roughness or even surface defects and are caused by temperature differences in the profile due to an insufficient plastication in the extruder. The root cause of shrinkage can be found in the microstructure of the compound. It is well known that a rubber compound consists of polymers, fillers, process oils and chemicals. During deformation the polymer chains will orient however and, because of thermodynamics, the molecules tend to disorient up to a stage of maximum chaos. This leads to extrudate swell. If the profile is stretched after leaving the final die (to keep the dimensions within specification) the natural process of disorientation is hindered and, once released after cutting, the tread will shrink when put into the leaf truck or put on the tread server of the tire building machine. Silica containing compounds tend to shrink more than carbon black compounds so that the shrinkage problem is more pronounced in modern compounds.
The answer to improving the process capability of a tread extrusion line is relaxed extrusion. In fact an extrusion process is relaxed if the profile stretching is minimized. It is a clear indication that the natural behavior of the compound is being adhered to. Stretching of the compound can only being minimized with a proper preformer and die design. But also other preconditions need to be fulfilled:
Preconditions for a relaxed extrusion process
• Thermally homogeneous extrudate.
• Balanced flow channels which means that velocity gradient across the extrusion head outlet openings should be zero
• No starvation in extruder feeding to avoid any pulsation of the extrusion process
• Preformer and final die design with front or back relief to reduce differences in flow resistance across the die opening
• Optimal settings of the shrinkage conveyor
• Dancer controllers which do not put an additional load on the profile
A homogeneous extrudate is a matter of correct screw design. The choice of the correct screw design depends on 3 process parameters: The maximum allowable extrudate temperature, the homogeneity and the required output. Optimizing one of the three parameters often leads to a compromise in one or both of the others. Depending on the material to be processed, various screw geometries can be applied. Screw designs which are optimized for high output can be used for easily processable compounds. Scorch-sensitive compounds or compounds which may get porous need screw designs which are optimized for lower extrudate temperatures.
Furthermore the quality of the extrusion process very much depends on the shaping of the profile. Fig 4 shows the principle of the shaping of a triplex-profile. In a cassette a splice bar (or preformer) and a final die are assembled and mounted as a complete set to the extruder head. In the performer, the three different rubber compounds flow together so the preformer determines the junction lines in the profile. The final die determines the outer geometry of the profile. Flow channels should be designed in such a way that the velocity at the outlet of such a flow channel does not vary across the opening. Computational fluid dynamics is an important tool for designing flow channels. Figure 5 shows the result of such a CFD calculation. It can be clearly seen that even outlet speed condition has been fulfilled.
HF: Highest extrusion quality
HF-Tiretech Group can assist you in achieving the highest process capability in your tread co-extrusion line. A new extrusion line is designed in close cooperation with the customer. Especially the flow channels exits out of the extrusion head should be matched with the tread profile portfolio which the customer plans to run on the extrusion line. In many cases flow channels can be changed by exchanging flow channel inserts so that each family of tread profiles has its own set of optimized flow channels.
Furthermore HF has got the competence and the methods to run in new profiles efficiently and according to the relaxed extrusion principle. It is not only a matter of correct die-design but also the setting of line speed and dancer settings which need to be optimized for each profile.
HF can also offer service to customers to assess and to optimize the process on existing extrusion lines and to train process technologists on customers’ site.