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Using mineral fillers in a polymer matrix can improve productivity and reduce costs in raffia and woven packaging applications, but processors must use suitable extrusion equipment to achieve the best results.

Volker Schöppner

July 28, 2017

6 Min Read
Choosing the best extrusion process for raffia and woven packaging applications

Over the past few years, many major developments have taken place in the raffia and woven packaging sector, including advances in resins, increased capabilities of extrusion lines and improvements in process speed. Raw materials and energy costs remain areas of concern, however. The use of mineral fillers, such as a calcium carbonate masterbatch (CCMB), can improve productivity and reduce costs.

A major dilemma for processors is choosing suitable equipment—either a smooth or grooved bush extruder—when using a high percentage of CCMB. An important factor in extruder selection is understanding the effect of adding the filler/CCMB to a polymer matrix, which significantly changes processing properties.

Before discussing the effect of CCMB dosing levels when using smooth and grooved bush extruders, it’s important to define some terms:

  • Thermal conductivity indicates the heat transfer rate. The thermal conductivity of calcium carbonate is approximately 2.7 W/(m*K), which is much higher than polyolefin resins (0.5 W/(m*K)). This means that heat transfer will be easier with a higher percentage of CCMB.

  • Specific heat is the amount of heat (kilo-Joules) required to increase temperature of unit mass (1 kg) material by 1°K. The specific heat of calcium carbonate is 0.9 kJ/(Kg*K), whereas it varies from 1.8 to 2.4 kJ/(Kg*K) for various polyolefins. In other words, a higher percentage of CCMB in a polymer will reduce the amount of energy needed to heat the mixture.

Adding CCMB also affects the density and viscosity of the resulting polymer matrix.

To summarize, adding CCMB to the polymer affects the thermal conductivity, specific heat, density and viscosity of the resultant polymer matrix.

The resultant polymer matrix of polyolefin and CCMB has reduced viscosity, as the increased internal friction from the CaCO3 particles in the polymer matrix raises shear, reducing melt viscosity. Changes in viscosity increase shear stress in the extruder, which is the main basis of mechanical-to-thermal energy conversion in a single-screw extruder. CaCO3 in PP accelerates heat transfer and reduces the amount of energy needed to heat up or cool down the mixture. Thus, heat will spread faster in a polymer matrix with a higher percentage of CCMB.

To summarize, a higher filler/CCMB in the polymer matrix:

  • Improves thermal conductivity 


  • Increases the blend density 


  • Changes viscosity, leading to a higher shear rate 


  • Decreases the internal volume of the screw, as CaCO3 particles do not expand

Single-screw extruders convey polymer pellets via friction. By virtue of its contact with the pellets, the barrel drags the material over the screw against the helix, advancing the pellets. In an extruder equipped with a smooth bush, pellets act like ball bearings: As contact between the barrel and pellet surface is minimal, a significant amount of slippage occurs, causing the pellets to tumble and slide. Since the compression ratio is mainly responsible for transporting the pellets, it must be kept on the higher side. The pellets follow a helical path in the solids conveying section of the smooth bush screw and are pushed along a helix of the screw, taking a slow, winding path.

Furthermore, there is very low pressure in the solids conveying zone of a smooth bush extruder. Typically, pressure rises through the melting section of the screw and reaches its highest point at the end of the transition. Thus, there is a limitation on pressure buildup in smooth bush extruders.

It is well known that increasing screw speed using a smooth bush barrel tends to increase melt temperature. The compression section of the screw is largely responsible for the increase in melt temperature. Looking to the low specific output of the smooth bush machine, melt temperature becomes a limitation when using high screw speeds.

In a grooved bush, multiple grooves are added to the bore of the barrel under the hopper and into the solids conveying zone. The pellets become trapped in the grooves and advance against the helix of the screw. This substantially increases pellet transport, as they advance forward in the grooves by the push of the screw helix. Even with typically low compression ratios, a grooved bush extruder achieves higher specific melt conveying compared with a smooth bush extruder.

Moreover, the grooved feed section generates high pumping pressure because of the increase in conveying capacity. So, in a grooved bush extruder, pressure generation is no longer a function of the metering section of the screw. The grooved section also produces a large amount of friction-based heat/energy for melting, resulting in intense water cooling and excess heat removal. This high specific output related to screw speed helps achieve a low melt temperature. Consequently, varying percentages of calcium filler masterbatches in different base polymers [PE or PP or any other elastomeric carrier resin] in PP or HDPE blends for extrusion result in better process consistency and quality of output melt using a grooved feed bush.

When the high filler/CCMB material is processed, the CaCO3 and other additives/pigments rub off on the screw over time, accumulating like chalk on a blackboard. In a smooth bush, as the pressure in the initial section is very low, it is difficult to scrape off the accumulated filler content and residue, much like gently wiping chalk off a blackboard. This may result in machine downtime. With a grooved bush, the high initial pressure buildup can scrape off the accumulated CaCO3 content and residue, leading to a more stable process.

Design challenges in grooved bush extruder design

The major challenge is to minimize wear in the grooves and screw in the pellet conveying section. Screw design provides a solution: A specially designed feeding section can lower the pressure and reduce wear, since abrasive wear is proportional to the pressure that is acting on the surface.

A second challenge involves achieving suitable melt quality, which requires good distribution of the CaCO3 in the melt. Here again, screw geometry is important. Modern screws for grooved bush machines with high output are barrier screws for good melting—they are equipped with mixing sections for improved temperature homogeneity.

Conclusion

Smooth bush extruders have limitations in pressure buildup and a higher melt temperature under high output rates. Grooved bush extruders have distinct advantages in terms of higher pressure generation and lower melt temperatures. Greater process capabilities enable handling of changes in material properties, such as increased thermal conductivity and variations in viscosity with the addition of higher CaCO3 filler masterbatch to polyolefin materials. Modern grooved bush machines use specially designed screws to lower pressure in the feeding section and reduce wear.

Prof. Dr.-Ing. Volker Schöppner, Kunststofftechnik Paderborn,
Fakultät für Maschinenbau,
Universität Paderborn. Schöppner received his engineering degree at Paderborn University in Germany. After holding several positions in industry, he returned to the university in 2007, where he is a professor of polymer processing. He is working on extrusion, with a specific focus on extrusion simulation. He can be reached by e-mail at [email protected].

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