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Improved rubber extrusion

A study of the influence of different extrusion screw surface coatings – CrN applied using PVD technology and nitridation – on the rubber extrusion process


In tire manufacturing, there are many ways to improve processes, increase output, improve machine durability and decrease downtime. In processing materials to improve composition, machines must be developed for increased output and they also must be resistant to new materials that can cause abrasion, corrosion and other chemical actions.

According to Konštrukta Industry, several customers are asking for more a durable surface of flow systems such pre-die, flow channels, inserts and screws. CrN applied using physical vapor deposition (PVD) technology is usually used, rather than common nitridation. PVD is typically used as a surface coating for machining tooling or aluminum extrusion dies. This surface coating however, can influence the rubber extrusion process. Konštrukta Industry has conducted trials to investigate this further.

Results indicate an influence of extrusion screw coating on the rubber extrusion process, material homogenization, heat and pressure development and extrusion output. A rubber compound, such as a pseudoplastic or non-newtonian liquid, has specific flow and thermodynamic properties. In regard to flow and different screw surface coatings, wall slip is a very important feature. Abrasion, chemical and corrosion resistance properties of CrN PVD are known – therefore this was not investigated as part of the presented research.

Apparatus and materials

The results provide a direct comparison of mechanically identical extrusion screws, with different surface coatings.
Screw design – decompression developed by Konštrukta Industry;
Surface coating
- PVD of CrN layer; thickness 0.002-0.009mm; hardness 1,750-2,800HV, applied on nitrided base surface. Resistant to wear, abrasion, corrosion, oxidation and chemical solvents;
- Nitridation – gas nitriding process based on penetration of atomic nitrogen into steel layer; thickness 0.5-0.7mm; hardness ~1,000HV; standard screw surface coating.

The properties of the CrN PVD surface, in terms of durability and resistance, have been studied by other researchers, thus they were used as part of this investigation.

Complete trials were conducted at Konštrukta Industry’s facility. The main testing equipment used was an extruder with a screw diameter of 120mm with five tempering zones. Five melt temperature and six melt pressure probes were used. All data from the measuring probes and drive were recorded every second onto a computer and available to export for future evaluation.

The rubber compounds chosen for the trials cover a wide range of properties and composition:

KZ1 – An NR-based compound filled with CB, 68 ML (1+4), 100°C (chosen due to its difficult extrusion processing);
KZ2 – An SBR-based compound with a high silica content, 64 ML(1+4), 100°C (for high temperature generation);
KZ3 – An SBR-based filled with CB, 87 ML(1+4),100°C (with smooth flow and high viscosity Mooney (denoted KZ3)).

Table 1

Testing screw speed: 10, 20, 30, 40, 50rpm
Extrusion die: tread 180 x 11mm
Pin configuration: 100% (60pcs)

Results and discussion
Extrusion output
Figure 3 shows the extrusion output profile temperature dependency for all trials.
In comparing outputs, calculated at the same profile temperature, the screw with PVD gives a lower output than nitridated with all testing compounds. The difference changes in the range of 10% to 18%. The difference between PVD and nitridation with compounds KZ1 and KZ3 remains constant in a whole range of testing speeds, with KZ2 getting larger.

Melt temperature and pressure change
Taking a closer look in the extruder, melt pressure and temperature probes inside the extruder show the melting mechanism of the rubber compound.
Figure 4 shows melt pressure development at 50rpm. It is not possible to find out exactly how screw surface influences pressure development inside the extruder. A screw with PVD coating shows during compounds KZ1 and KZ3, filled with CB, a higher processing pressure during the whole process, but KZ2, filled with silica, shows a lower pressure. KZ3 shows higher pressure due to higher viscosity Mooney.

In comparing heat development at 50rpm screw speed (Figure 5), behavior on the base of rubber compound flow properties can be observed. Rubber compounds KZ1 and KZ3 during PVD screw processing have higher melt temperatures (Figure 4 a, c). The melt pressure difference in the head between screws is very small, to influence profile shaping stability.

Drive output
From an energy costs point of view, extruder drive output was also recorded during trials (Figure 6). If we calculate drive output at the same reference temperature, the difference between PVD and nitridation varies in the range of 11-21% depending on the rubber compound. PVD coating shows lower drive outputs and this positively affects input energy consumption.

Based on the findings from these trials, the nitridated screw surface offers a higher screw output than PVD at the same reference temperature, up to 18%. Comparing drive output, PVD reduces energy consumption by up to 21% depending on the rubber compound. Comparing costs per unit, drive output and extrusion output, extrusion output is more valuable due to price per compound unit. The wear resistance of PVD surface coating of extrusion screws has been studied by different authors and is available in current literature, thus was not included in this investigation.

Figure 1: Screw surface coating PVD (left), nitridation (right)

Figure 2: Testing extruder with 120mm screw diameter

Figure 3:
Extrusion output: a) compound KZ1; b) compound KZ2; c) compound KZ3




Figure 4: Compound KZ3 melt pressure change inside extruder at 50rpm screw speed: a) compound KZ1; b) compound KZ2; c) compound KZ3






Figure 5:
Melt temperature change inside extruder at 50rpm screw speed: a) compound KZ1; b) compound KZ2; c) compound KZ3




Figure 6: Drive output with different compounds: a) compound KZ1; b) compound KZ2; c) compound KZ3






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