twin-screw extrusion | Computer modelling
account complex thermal and mechanical interactions in twin-screw extruders. Other institutions that are doing related work include
the University of Brunel in the UK, and Tomas Bata University of Zlín, Czech Republic.
The challenge of acceptance It is surprising that modelling software is not more widely used in the optimisation of compounding processes. As noted, versions have been commercial for at least 25 years, which is almost as long as mould optimization software has been available to injection moulders and mould-makers. “In moulding, no one cuts metal before running a simulation,” says Century’s Dreiblatt. “The mould optimization software has been validated and yields substantial cost savings for moulders.” In compounding, though, old ways seem to persist.
Most compounders appear comfortable with running materials and tweaking formulations and machine parameters to get the right production balance. Moreover, the prices quoted to buy or license
modelling software range from US$15,000 to US$20,000 (€12,000 to €16,000) on average to as much as US$80,000 (€60,000) for large compounding operations. That size of investment might be off-putting to many
Polyflow software from Ansys can model complex thermal and mechanical interactions in twin-screw extruders
companies. However, vendors explain that the price of acquiring the software is far less than the ongoing cost of tweaking formulations and machine set-up by less-automated means. “The alternative to simulation is running a machine under what you believe are correct conditions,” Dreiblatt says. “A compounder can be lucky and come close to optimum operating conditions without too many adjustments, or not.” What does this mean in per-job expenditures? If a
compounding line runs only 5,000 lb per hour, and it takes eight hours for a compounder to fine-tune a formulation and optimize machine settings for maxi-
mum process efficiency, 40,000 lb of material could easily be consumed, all or most of it scrap, before the best formulation and machine set-up data is derived. At a material cost of only $1 per pound (including com- modity resin, additives, fillers, reinforcements and colorants, if any), this comes to an initial expenditure for one job of US$40,000 (€30,000). Added to this are the costs of machine time for the equivalent of one shift, labour, electricity and related expenses, disposal of scrap, and cleaning and maintenance of the line. Compounders doing this once a month are looking at an annual investment of almost US$500,000 (€400,000) for material alone. Should a compounder be involved in processing more expensive materials, such as engi- neering resins, bioplastics, or more expensive additives, then costs would be even higher. Some vendors claim that the alternative of a one-time investment of say US$20,000 for modelling software means that a compounder could determine the optimum formulation and machine configuration in as little as 30 minutes. Even if it takes far longer to develop the correct compound and machine param- eters, no material is consumed by the software, and no compounding lines are diverted to testing until the simulation data needs to be proven in a trial run. “Both bigger and smaller compounders have a need
for this software,” Dreiblatt claims. “The value is you can experiment with the processing effect of different screw configurations without running one pellet through the extruder.” While he acknowledges that modelling software is
not always 100% accurate, 80 to 85% accuracy is attainable and this is “better than not using it at all.” SCC’s Chassagnolle cites additional benefits. One
advantage, he says, is the ability of modelling software to reduce a compound’s time to market. Compounders must still test every formulation and machine set-up developed with the software, of course, “but they can drastically reduce the number of trials necessary to prove material performance.” The simulation software doesn’t eliminate the need for physical testing; rather, it complements the process. Based on customer feedback, Chassagnolle says
compounders could run as much as 50% fewer physical trials and see 30 to 50% reductions in time to market with modelling software, though he adds these numbers will vary by end-user. The different versions of modelling software have
features that are unique to each program, of course. In general, though, all require similar input to simulate compounding and generate analytical data. Important components here are material specifications and machine data files. The latter include detailed specifica-
34 COMPOUNDING WORLD | February 2015
www.compoundingworld.com
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