Right:
Compounders have proved
more resistant to adopting
custom-devel- oped purging compounds, such as this Asaclean PF grade illus- trated in an injection moulding purging
application
processing | Purging compounds There are two basic types of commercial purging
compounds, chemical and mechanical. Chemical purge compounds work at the molecular level to break the bonds of the polymers, which results in a reduction in melt viscosity and an increase in turbulent flow. Mechanical purge compounds work by scrubbing the machine clean. “A good analogy for the two types of purge compound is that a chemical purge is similar to using soap and water to wash dirt from a person’s hands, while conversely a mechanical purge is compa- rable to cleaning a person’s hands with a brush,” Reeder says. Most manufacturers of purge compounds have multiple grades that are designed to purge specific types of resin and machinery. Recently introduced Ultra Purge 5150 and the Ultra
Purge 5160 grades have been designed for fast and efficiently cleaning of screws and dies in compounding machines. They incorporate Mould Plus’s “Ultra-X” technology, which uses a proprietary mineral filled material that adds a mechanical component to cleaning the machine. “Additionally, Ultra-X is a better alterna- tive to a glass filled carrier because it allows for the Ultra Purge active part to expand more in order to get to the dead spots of the machine,” Reeder claims. In order to limit the amount of purging grades to
have on hand, suppliers recommend having a good multi-purpose purging compound capable of working with many different resins. Running a purge compound through the extruder before pulling the screw will also save the compounder time as commercial purge compounds are typically much easier to clean off the screw compared to standard thermoplastic resins, Reeder claims. After the screw has been pulled, the remnants of the commercial purge compound can be
Figure 1: Colour changeover cost analysis based on black to yellow HDPE running on a 4-inch (100mm) compounding extruder
Colour change/Material Change: Total Scrap Resin Cost of Resin
Quantity of Purging Compound Total Purging Material Cost Time required for purging
Units Lbs
USD/lb Lbs
USD Min
USD USD
HDPE Ultra Purge 196 0.89 0
40
0.89 15
174.44 92
Down-Time/Lost Production Hourly Cost USD/hr 100.00 Total Down-Time Cost Total Cost per Purging
Total amount in USD saved per colour change using Ultra Purge
Reduction Purging Material Reduction Down-Time
Source: MouldsPlus International 54 COMPOUNDING WORLD | January 2016 USD
202.32 54% 71%
153.33 327.77
80.45 27
100.00 45.00 125.45
easily unwrapped from the screw. Another commercial purging compound supplier, Shuman Plastics’ Dyna-Purge division, says its newest grade, D2, is well suited to the compounding market. Technical products manager Nancy Mitchell attributes this to its “enhanced purging efficiency and versatility with a wide range of resins.” One proprietary compo- nent in the D2 grade is formulated to flow to the boundary layers to flush out the resident resin, while another acts as a pressurising agent allowing for the purge to reach stagnation points. In addition, an encapsulated fine component targets degraded material adhering to the surface. This “triple action” is said to thoroughly clean and remove material from the screw and barrel as well as the die. D2 has an operating temperature range of 177- 329°C. Mitchell says the amount of purge needed is roughly equivalent to between one and two times barrel capacity, depending on machine conditions. Chem-Trend has a long history in purging com- pounds, particularly with its Lusin brand. “Purging is an odd science we are always trying to demystify, says Laurent Saleur, Account Manager Thermoplastics with Chem-Trend France. “In the early 90’s we were among the few offering good performing products, but nowadays we are in an environment that is a lot more innovative,” he says. Even so, Saleur says the compounding sector is still lacking a truly high quality purging solution. “Com- pounding has its own processing issues that nobody has fully answered yet. Most compounders are using their own solution, low melt, high specific gravity, abrasive purges,” he says. Chem-Trend has several compound- ing customers, but he admits that due to materials costs, they are using its products mostly for technical polymers application. “But we know that there is a demand.” Chem-Trend’s Lusin Clean 1010 purges machines
processing polymethylmethacrylate (PMMA), solving process related problems with colour changes and
www.compoundingworld.com
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