Case study:- Sheet Extrusion
Description

A Belgian company extrudes PMMA, PET, and PS in sheet. There are a number of extruders, with screw diameters ranging from 120-150 mm, and dies from 1,5 to 2 m. With some of the material changes it is not easy to clear the extruder from the previous material. Purging times of up to 8 hours are no exception to clean the extruder from black specks.

Problem analysis

All polymers burn, and especially materials with a different melting range create burned particles at a material change. The burning of polymers is foremost at those places in an extruder where the flow and the pressure of material is low, like on the backside of the flights of the screw. At a material change where the change-over is to a material with a relatively higher melting range, the burning of polymer goes only faster. A gel-layer forms on the metal surface, which carbonises over time. Small particles break off from this carbonised layer, which are visible as black specks. Additives like pigments can easily get stuck in this rough surface. Flushing won't remove this layer.

Target

Our target is two-fold: to get to the source of the problem, and to get control over the source of the problem in future. All polymers burn, but this burning process should be controlled before the residues get a chance to carbonise. In other words: the forming of layers of gel should be broken off at an early stage, when this is relatively easy to do, and with minimal loss of (machine-)time and cleaning material. Preventive maintenance is important !

Solution

The test was carried out at a material change from PMMA to PS. Based on the melting range of the materials, the test was carried out with Econoclean 200-c, with PS as carrier resin, because that is the material where we are changing to. The vacuumhole was closed, and the compound processed according to the manual. Within 1 hour the PMMA was removed from the extruder, and the white foamy appearance of Econoclean was clear of black specks.


Costcalculation

1.  A. current cleaning material:PS
     B. Used quantity (kgs)2.400
     C. Price/kg:1,23 Euro
                Total material costs:2.952,00 Euro
     D. Cleaning time (hours):8
     E. Costs/hour:180,00 Euro
                Total costs machine time:1.440,00 Euro

      TOTAL COSTS CLEANING:
4.392,00 Euro

2.       A. ECONOCLEAN blend 
     B. Used quantity (kgs)100
     C. price/kg of blend:5,67 Euro
                Total material costs:567,00 Euro
     D. Cleaning time (hours):1
     E. Costs/hour:180,00 Euro
                Total costs machine time:180,00 Euro
      TOTAL COSTS CLEANING:747,00 Euro

      Difference between 1 and 2:       3.645,00 = 83% cost reduction!

Note:

By applying preventive maintenance this way, not only are costs reduced, but also production hours gained, flexibility increased, and pulling the screw has been reduced to an absolute minimum !