management feature | Safe working practices
These strands then need to be cut to be able to fi t within the pelletizer infeed chute; however, the linear speed of the line requires this task to be performed while walking quickly to keep the strands from dropping out of the die. In many cases, this procedure requires two people – one to pull the strands and the other to cut the strands. Cutting the strands must be done quickly, requiring extremely sharp cutting tools. Effi cient and safe start-up of strand lines requires good communica- tion between these two people to avoid serious injury. The linear speed the strands need to be manually
A properly designed fume extraction and dust collection system ensure safe operation without the need for operators to wear respirators or dust masks. Insulated barrel covers are another example of engineering controls for protection from hot surfaces. Photo courtesy Apex Engineering
Proper ventilation of material handling systems,
extruder feed hoppers and atmospheric vents to eliminate airborne dust is necessary. When operators are required to remove such controls – for example during product changeovers and the clean-out of feeders – safe working practices can require operating staff to wear proper PPE for these short-term events. There is no reason for the environment surrounding the compounding extruder to be covered in dust, and this is a clear signal that engineering controls are not designed or functioning correctly.
Touch – every aspect of twin-screw compounding involves high temperature, hot metal surfaces and molten plastic. Once again, engineering controls in terms of machine guarding can be successfully applied up to a point. Where machine guarding and other engineering controls cannot be implemented to protect personnel from these hazards, procedures must be in-place to minimize worker exposure. One example would be start-up of a strand pelletiz-
ing system where operators are required to manually pull the molten strands through a water bath and insert them into the pelletizer. Safely executing this task requires both appropriate work- place practices and proper PPE, since the operators are required to use their hands to grab the partially-cooled strands. Operating procedures, for example, would require long sleeves and a specifi c type of thermally protective glove to avoid burns from molten polymer.
50 COMPOUNDING WORLD | November 2013
pulled through the water bath is related to the produc- tion rate – more capacity translates to higher line speed. Operating personnel are usually pressed to start-up the line as fast as possible, since material consumed up to this point is scrap. These production pressures must be weighed against safe operation. Another potentially hazardous operation associated with twin-screw compounding extruders that cannot be mitigated with engineering controls is the removal and disassembly of screws, dies and die plates for cleaning and/or replacement. This procedure could be per- formed every few days at some compounders who change the screw confi guration from one product to another, while at other facilities the screw/element removal may be done once every few years when screws are worn and require replacement. In either case, personnel are required to grasp
heated extruder components that are covered in molten plastic. Removal of the shafts or dies from the extruder barrel and disassembly of the elements from the screw shafts must be done with the machinery at or above the normal processing temperature which can exceed 300°C. On larger diameter extruders, these compo- nents are relatively heavy, which presents additional hazards requiring safety shoes as part of the work practice controls. Engineering controls cannot be provided to guard
against possible burns from heated surfaces or molten plastic when performing this routine operation. It is only procedures and proper PPE that can be applied to safely execute these tasks. In the case of PPE, gloves with good thermal protection do not typically have the dexterity required to manipulate the tools necessary to perform these tasks; the molten plastic additionally will stick to the gloves reducing their effective- ness. It is the responsibility of the employer to provide personnel with the proper PPE to safely perform the tasks
required.
www.compoundingworld.com
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