COST REDUCTION BY SUITABLY MODIFYING THE BULK DENSITY OF THE PVC DRY BLEND COMPOUND

Introduction:

It is well known that the PVC dry blend enters the extruder from the hopper on the basis of “Bulk density”.

Appropriate bulk density of compound gives a best balance between the output (efficiency) as well as quality (effectiveness) of PVC products. This aspect is hardly considered by PVC processors.

In an industry, output from one process becomes input for the next process. Visualising, identifying and implementing areas for improvement provides value addition for the immediate customer and in turn cost reduction. Changes made in PVC compound considering this aspect, improves the cost.

By value addition, I mean that the immediate customer should be able to reduce his cost of production without sacrificing quality, rather improving the quality.

This requires the two process owners, the immediate supplier (mixing) and immediate customer (extrusion) to meet and review each other’s operations, with the intention to add value to their respective processes.

There can be innumerable areas for improvement.

I believe that a customer is satisfied when his customer is satisfied. Internal customer satisfaction is worth measuring as it forms a basis for value addition.

The bulk density of PVC compound depends upon:

At compounding and mixing level:

1.      The average particle size of PVC and other ingredients,

2.      The average porosity of PVC particles (K - value),

3.      The density of additives,

4.      The level of CaCO3 added (its density is almost double the density of PVC),

5.      The dispersion and distribution during mixing process to ensure free flowing nature, and

6.      The duration of high speed mixing (more the duration, more the bulk density).

At hopper level:

1.      Maintaining the compound temperature 40-45 deg C to ensure smooth flow at the entry point.

2.      Blending recycled PVC crush at different particle size levels to manipulate bulk density.

A case study:

Problem statement: Formulation working on conical twin screw extruder was not giving the same output on parallel screw extruder of same capacity for the same profile, using the same die, in terms of m/min line speed, resulting in increased cost of production due to less production and in turn delays in dispatch.

The quality was acceptable.

Results of Brainstorming:

During brainstorming session, questions were raised as to why this happens?

During discussions it came to the surface that the output in twin screw extruder depends on inputs.

In twin screw extruder, which acts as a positive displacement pump, the input of the PVC dry blend compound is on the basis of “bulk density” of the compound in the hopper. More the bulk density more is the output. On the other hand, the output from the twin screw extruder is on the basis of melt density. Whatever goes in from hopper comes out from the die in twin screw extruder.

It follows that if bulk density is increased without compromising the quality of PVC profile, the output in terms of m/min would increase. The temperature of the dry blend at hopper should be between 40 – 45 deg C, to keep the flow smooth and variation in bulk density to minimum.

Bulk density and output of twin screw extruder:

By increasing the Phr of heavier ingredients in the compound, e.g. calcium carbonate, the bulk density can be increased.

Since there was no problem about processing as well as profile properties except the line speed, it was decided not to change the lubricant and stabilizer level in the formulation, to begin with.

The only ingredient that would contribute to increase in bulk density of the compound was the filler.

The filler used was precipitated calcium carbonate (0.5-0.7 g/cc). One way to increase the bulk density was to use finer particles of ground calcium carbonate (0.8-1.3 g/cc).

Every parallel twin screw extruder gives best output and quality at a particular bulk density of the compound. On the other hand, in case of conical twin screw extruder due to more volume in the feed section, some variation in the bulk density can be accommodated.

By taking advantage of this fact, it was thought to make an attempt to increase and match the bulk density of the compound with the bulk density requirement of the parallel twin screw extruder; the best possible production rate and quality could be achieved.

The results:

Without making any changes in the formulation, it was decided to replace precipitated calcium carbonate with finer particle size ground calcium carbonate, though it was costly, keeping its Phr same.

Thus the bulk density was increased without changing the Phr of calcium carbonate.

With the increase in bulk density and adjusting the process parameters, the line speed was increased by 20% without sacrificing the quality.

Cost of production:

By using the costly finer particle size ground calcium carbonate, the cost of compound actually increased by Rs. 0.11 per Kg.

But due to increased bulk density of the compound at input point, the line speed of PVC profile increased by 20%, reducing the cost of production by Rs. 3.60/ Kg.

The properties of the profile were excellent.

Thus, even though the compound cost was increased by Rs. 0.11/Kg, with reduction in cost of production by Rs. 3.60/kg, the net cost saving was Rs. 3.49/kg.

Looking at the production rate of 250 Kg/hr, assuming 22 hrs of working and 26 days a month, the potential annual saving would be Rs. 3.49 x 22 x 26 x 12 per annum for one extruder.

= Rs 5,988,840 on one extruder.

On horizontal deployment to other machines, one can get an idea of possible annual saving.

It’s a good “Kaizen”.