Tyre Asia – May-June 2018

96 Tyre Asia April/May 2018

If we consider pure remill stages - mixing

stages in which the compound viscosity

is merely reduced and in which the

dispersion can be significantly improved

by the addition of cold compound sheets

• it must be assumed that, theoretically
  speaking, the process can also be divided
  exactly in the middle (Image 12). Again,
  an increase in throughput of 100 per cent
  would be achieved.

In the case of final compounds,

compound sheets must first be added and

viscosities reduced so that the material

can absorb cross-linking chemicals. The

chemicals can then be placed in the

upper mixer just before the transfer into

the lower mixer or directly into the lower

mixer. Using a purely internal mixer

process as a basis, it can be estimated

that up to 40 per cent of the cycle time

can be saved using the tandem process

(Image 13). Thus it can be said that the

tandem process in the different mixing

stages, and certainly in the further

detailing, results in different potential

savings depending on the mixing.

As mentioned at the outset, tyre mixtures

are often produced in several stages

due to the high viscosities. Roughly, the

average number of stages for motorcar

mixtures is 2.3 and for truck tyres 3. The

2.3 number of stages means, for example,

that one base compound and one rubber

compound respectively is needed, and

30 per cent of the compound spectrum

requires a remill stage. As a result of the

fact that the relative savings potential

for silica mixtures and remill stages is

the greatest, the consideration of the

required compound quantities leads to

the conclusion that the tandem process

is of interest for all stages. Silica mixtures

account for only 10 per cent of the

necessary quantity and remill stages for

car tires on average account for only 30

per cent, based on the base compound

quantity. however, final mixing stages

are always required if the possibility

of single-stage compound production

should first be ignored.

Based on these considerations, a sample

calculation is used to illustrate the

economic significance of this aspect.

The production of 10 million motorcar

tyres with a unit weight of 10 kg and a

silica tread shall be assumed. For 2.3

mixing stages, 230,000 tons of mixture

would be required per year. For the

individual compound types (carbon

black and silica) as well as the mixing

stages, the cycle times given in Table

1 for conventional internal mixer and

tandem lines are defined according to

the potential savings shown in Images

10 to 13. The proportion of the required

compound quantity is estimated in Table

1 and the required absolute compound

quantity is calculated in tons per year.

A mixing density of 1.15 kg/dm3 is

specified for the calculation. The

intermeshing lines are calculated with

a fill factor of 0.65 for PeS5 and PeS6

rotors, and with 0.7 for tangential lines.

The effective production capacity is set

at 6,000 hours per year, assuming a 100%

degree of utilization.

Table 2 shows that the required

compound quantities can be produced

either with 7 conventional lines or with

4 tandem lines. It is assumed that, in the

case of the conventional lines in option 1,

the remill quantity can also be produced

on the carbon black line, and any residual

quantities can be redistributed to the

other lines. option 2 (tandem) assumes

that the silica, carbon black and remill

compound quantities are produced on

the same big tandem lines.

The investment costs differ by

approximately eUR 5 million; this

being only the investment costs for

the machines.

As four tandem lines certainly require

less space or hall volume, despite the

additionally required building height,

there are further cost benefits for this

option. As mixing stages can be saved

by the tandem process, the possibility of

reducing the necessary mixture storage

is created.

Calculation of mixing costs

The mixing costs are calculated below.

The lines shall be depreciated over 10

years and an interest rate of 4 percent

shall be assumed.

The interest charge is based on a

simplified calculation on half the

investment costs. In addition, 5%

maintenance costs are applied based

on the acquisition value of all lines and

the energy costs are left constant for

both options as it is to be assumed that

the same energy is required to produce

a specific mixing quality. Based on the

large quantities of raw material to be

moved, 3 to 4 persons per line and shift

at an average Western european salary

level of eUR 50,000 are assumed to

calculate the personnel costs.

The large mixing lines require 2 persons

at the feeding belt and one person at the

bottom, while the smaller lines, and in

particular the rubber compound lines,

should operate with one person each at

the top and bottom. In addition, half a

logistics expert for the transport of the

raw materials and half a person for the

small component weighing are calculated

per line. This brings the total number of

persons to 72 for the conventional lines

and 42 for the tandem lines.

From the simplified calculation it can

be concluded that the specific mixing

costs for the selected line configurations

differ by almost 15 per cent in favour of

the tandem process. In absolute terms,

this comes to an annual cost difference

of eUR 1.888 million. other monetary

benefits shall be analysed and evaluated

below.

If the larger space requirement for the

conventional lines is selected according

to option 1, the following estimation can

be made: With an approximate surface

requirement of 20 x 50 metres for one

mixing line, which should only be slightly

different in the lines considered, and

thus negligible, there are differences

in the building heights for option 1 and

2. An additional 5 metres can roughly
   be assumed for the tandem option, i.e.
   approximately 25 metres building height
   for option 1 and 30 metres for option 2.
   This results in reconstructed volumes per

Image 12: Schematically process illustration of

a remill stage without adding further filler or

chemicals

Image 13: Schematically process illustration of a

final batch

Image 11: Schematically process illustration of a

reactive batch

MIXInG