Hi. This article was sent to me by Dan McAuley*, a regular reader of Rubber Machinery World and avid follower of developments in Rubber Technology.
If you are a producer of technical rubber goods un-dispersed materials or foreign particle contamination can be a significant source of rejects especially in products demanding high show surface quality. Given increasing requirements and the risk of grit, packaging materials etc entering the production process an effective means of ensuring defect free compound for your customers is required.
Extrusion processes have relied on the use of screen packs to provide some measure of protection from such defects and develop back pressure to stabilize output. This stated the extruder screw to barrel interface generates shear heat as it conveys rubber compound to the die in turn reducing its viscosity to promote flow. The use of finer mesh screens will result in greater material resonance time and higher back pressures increasing heat input which can create pre-cure and reduce run duration. Molding processes via injection, compression or transfer are at greater risk as screen use is not possible.
Enter the rubber gear pump or straining device.
Gear pumps are not new technology as they have been used for a multitude of applications ranging from automotive oil pumps to plastic melt pumps and with consideration for its unique and temperature sensitive characteristics are an ideal means of “filtering” your rubber compound.
Unlike extruders, gear pumps offer positive displacement and use two counter rotating gears in place of a screw as a means of conveying material. Shear heat (minimal) in a gear pump is introduced as material enters and is conveyed between the gears and the surrounding housing, both of which are typically temperature controlled. Similar to an extruder the material then passes through the screen pack and supporting breaker plate en-route to the die. As flow is directional and the shear area is much smaller than an extruder the gear pump is capable of gently processing temperature sensitive materials at pressures in excess of 500 bar with screen packs finer than 100 mesh.
Although extruders are self feeding devices, gear pumps rely on being fed with a consistent supply of compound and this may be accomplished in several ways. Selecting the appropriate design depends on your install application however supply via strip fed extruder or two roll cram feeders are typical methods employed.
Regardless of feed design choice, the gear pump entrance is typically pressurized to less than 50 bar and is maintained by varying the feed device speed relative to gear pump speed.
Gear pumps used for rubber straining are unique in the sense that conventional bearing support of the gear shafts and lubrication with oil or grease can present a contamination risk given the high working pressures. Designs vary, however a common solution permits rubber leakage flow between the rotor shaft and the housing effectively making the compound the lubricant highlighting the need to ensure consistent gear fill. The tailings generated due to the rubber leakage may in many cases be recycled back into your process to minimize waste.
Gear pump technology in the mill room – in line
Straining technology has advanced to the point where machines capable of outputs in excess of 10000 kg /hr are possible. The use of a gear pump at the source of compound production can reduce capital and operating cost at end use processes, permit the use of lesser grade raw materials and produce continuous and consistent strip feed for your extrusion or downstream mixing operations.
Mill room operation can also benefit as the straining process generates a steady/stable output which establishes a process ”heartbeat” or pull to which other systems must be optimized.
Key considerations in the selection of an appropriate machine include the design of the pump feeding method, touched on earlier. Options such as a mill fed continuous strip to an extruder or a two roll feed mechanism are available as is a conical twin screw feed which presents an alternative to the roller die at mixer discharge.
Taking advantage of the warm feed output from the mixing process to the strainer permits inline fine mesh straining of a wide variety of compound types and viscosity. To accommodate throughput rates, machines make use of large breaker plate -screen pack configurations and may be equipped with dual heads etc to facilitate quick screen change.
Given the nature of the device, the low impact on material temperature vs working pressure and the ability to achieve extremely fine filtration the technology is suited to both master batch and final compound production.
Your peers in the industry are taking advantage of inline straining to provide end users with clean compound improving their operations by reducing defects at the source. This coupled with added flexibility in raw material selection and the continuous flow output to your mill room’s downstream operations can offer a significant improvement in operating efficiency. A win-win result.
*Dan McAuley is a Mechanical Engineering Technologist with extensive rubber industry experience primarily as a project engineer. He has participated in equipment installations in green field start-up ventures in Brazil and Mexico as well as implementing new processes and supporting programs within existing production facilities. He has worked as project engineer, plant engineer, project engineering manager having worked in the UK, USA, Brazil, Mexico and Canada for various extended assignments. He can be reached at email@example.com.
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