Die design and optimisation
	Extrusion  process
	Simulation of extrusion
	Tire manufacturing

 

Extrusion  process

The extrusion process consists of pushing a rubber blend by means of screw extruder through feeding channels and extrusion dies of relatively complex geometry. The channels are used to condition the rubber flow parameters (velocity, temperature) and to distribute the flowrate of different blends in the case of co-extrusion. The extrusion die orifice has to be designed to produce profile with the required geometry. The process involves several complex phenomena: complex rheological behaviour, fluid flow with free surfaces, etc.

Typical tire tread extrusion machine

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Die design and optimisation

Set-up of a successful extrusion line is a long and costly trial and error procedure. It includes design and manufacture of several parts which will then be adjusted as a function of the extrudate shape and distortion in a trial and error manner. Since many different parameters control the rubber flow, this is a very complex task. Often the flow pattern within the die leads to flows too unbalanced to enable the practical and efficient extrusion of the desired product. In other cases the swelling of the rubber after exiting the die is unacceptable. The role of extrusion process designer is to find the die geometric configuration and the process conditions (flow rate, temperature etc.) which permit a stable flow of high precision and high quality profile. When rubber extrusion is considered, there is an additional ecological aspect of the die design problem: the wasted rubber cannot be easily recycled.

Non-equilibrated extrudat	Equilibrated extrudat

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Simulation of extrusion

In extrusion simulations, three dimensional Navier-Stockes equations must be solved. The flow domain is discretized by finite elements. Then boundary conditions are applied. Typical boundary conditions are: inlet pressure or flow rate, slip conditions on the die walls, free surface definitions. The polymer behaivior is characterised by some constitutive law. Several laws could be used, which can include general non-Newtonian or visco-elastic effects. In the extrusion simulation of rubber blends, the so called "power law" is often used. Non-isothermal effects can be also taken into account.

The associated problems are very big and require an application of modern HPCN technologies to perform the solution in reasonable time. Polyflow finite element code from Polyflow SA is the leading program for simulation of rubber and polymer processing. It was successfully ported on parallel platforms in the framework of Europort project.  Direct extrusion simulations of the flow parameters, i.e. computing the flow  velocity, pressure, extrudat shape for a given die geometry and process conditions can be performed, as well as  inverse extrusion simulations where the required die orifice shape is predicted by the software.

Flow domain of a seal extrusion die orifice	Velocity distribution: non-equilibrated die

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Tire manufacturing

The manufacturing process of a typical tire consists on 5 major steps:

1. Compound preparation - the rubber blends are prepared consisting of rubber and exact quantities of the chemical agents in order to ensure the appropriate material characteristics for each task.
2. Calendering - in this stage the fabrics or steel wires are covered by rubber to form a thin and flexible composite tissue.
3. The tread extrusion - in this stage the exact profile of the tread is prepared.
4. Assembly of all components of the tire to one structure. This is done by puting the components on a drum having the dimensions of a specific tire. The critical factor in this stage is to ensure good adhesion between the layers and the axi-symmetry of the components.
5. Moulding - the assembly is loaded into a mould having the final dimensions of the tire. It is then subjected to high pressure and high temperature which accelerate the curing process of the rubber blends.

 

Adding the tread on top of the assembly

Assembly loading into the mould

Double screw extruder

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