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A Productive Rant About Fine Mesh Straining Of Rubber Compound In The Millroom

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.


Representative Image From Web

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.

Image of UTH Strainer Extruder

Image of UTH Strainer Extruder

*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

Contact me if you seek more details. Or if  you are looking for New or Used Rubber Machinery?

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A Dummies Guide to Rubber Extruders

Rubber extruders have a varied field of application. So, when you come across a rubber profile, strip, hose, cable, wire, cord coating, tire tread, v-belt, tube, or blank remember that they are only a few handful of products manufactured using extrusion process. 

And your main goal of extrusion is to get the highest output at good quality of product.

Based on the convenience and usage history, you need to know of the two types of rubber extruders viz. Hot Feed Extruders (HFE) and Cold Feed Extruders (CFE).

And further, you have variants within Cold Feed Extruders viz. Plain Type, Pin Barrel Type and Vent Type Extruders. The recent most advanced adoption that you will come across is that of co-extrusion lines.

When you choose a rubber extruder, you should deliberate and discuss extensively upon few key things i.e the design, material technology and manufacturing accuracy of screw, barrel and die-head. While, I would cover more on them in a later post, I would like you to remember that very few manufacturers world-wide can guarantee you a well-designed and precisely manufactured rubber extruder.

Hot Feed Extruder (HFE)

Rubber industry only had Hot Feed Extruders until 1950’s. HFE’s extrude your rubber compounds at reduced temperatures.

Hot Feed Extruder with Dual Head

Bainite Machines Make Hot Feed Extruder

The screw depth of a HFE is relatively larger and you get a consistent output due to its short screw design. L/D ratio is mostly in the range of 4:1 to 6:1 which keeps your rubber compound dwell time and its temperature increase to a minimum.

Each HFE has an hopper and feed roller section with spiral undercut liner that allows your compound to enter the extruder easily. The feed roller on a hot feed extruder allows your compound to pass the scraper knife, directed around the roll and then fed back into the hopper. The feed roll bearings are placed in positions to prevent contamination. You can vary the output by changing the screw speed using variable speed drives.

Despite these advantages, the HFE’s are getting outdated in many applications.  Because the rubber that is fed into a Hot Feed Extruder needs to be pre-heated or warmed using two-roll mills to achieve the required degree of viscosity and temperature that facilitates smooth flow of rubber, its compaction and extrusion through the die.

And that made experts introduce Cold Feed Extruders.

Cold Feed Extruder (CFE)

Cold Feed Extruders are designed and manufactured with specially designed screws best suited for cold feeding of rubber. You can discuss with your manufacturer and avail various options of screws for a wide range of compound and extrusion applications.

While manufacturers offer L/D ratio up to 24:1, the most preferred by end-users is generally in the range 12:1 to 18:1.

Pin Type CFE

Pin Type CFE Image from Web

For feeding the cold rubber, it is recommended that you use a feeding conveyor with metal detector to remove metal particles. This avoids damage to the screw or barrel. In some plants, I find the sensitivity of the metal detector calibrated to a low value that it virtually renders the detector useless.

Every CFE comes with a Temperature Controller Unit (TCU) that controls the barrel temperature so that the shape and size of the extruded products are uniform.

Variants in cold feed extruders along with their uses are

  1. Plain Barrel Type Cold Feed Extruder – These CFE’s as the name suggests have a plan barrel and used in manufacturing of hoses, blanks, fluorocarbon rubber, butyl rubber, etc.
  2. Pin Barrel Type Cold Feed Extruder – These CFE’s have around 80-100 pins protruding out of the barrel towards the screw center. These pins enhance the mixing and dispersion of your rubber as it is kneaded between the barrel and screw. And the result is processed rubber with outstanding homogeneity and extrudate quality. This flexibility in Pin Type Cold Feed Extruders endears to all making it a universal extruder for many rubber compound formulations involving varied applications. Hard rubber compounds also can be processed because of high extruder torque.
  3. Vent Type Cold Feed Extruder – Vent type or vacuum type extruders were developed for production of non-porous profiles and hoses. These CFE’s have a custom-built screw, and a degassing barrel with a vacuum pump attached to vent bubbles out of extruded compounds.
Triplex Extruder

Nakata Make Triplex Extruder

Co-extrusion: Customer-specific customization and usage complexity demands led to the introduction of co-extrusion for manufacturing of various profiles. And so you today have Simplex, Multiplex (Duplex, Triplex, Quadruplex, and Quintuplex) and Roller Head technology. Multiplex lines of piggy-back type of 2,3,4, and 5 layers have a compact construction.


Berstoff Make Roller Head Extruder

Roller Head Technology involves a combination of extruder with preform head and two-roll calender. They offer twin advantages of – high uniformity of the material thickness over the entire sheet width with absence of air traps even at higher thicknesses (~ 20mm thick as against conventional calender lines that give max 3mm thick sheets with or without air traps) and excellent homogeneity of the material produced.

Both these characteristics are important for high-quality rubber products such as tire components, V-belts, conveyor belts, tank linings, cover sheets, blank sheets and roofing sheets, etc. For even thickness across the entire sheet width of the roller head, there are three options that can be used alone or in combination with one another – roll crowning, roll crossing or roll bending, that will compensate for the elastic deflection of rolls. (I will cover more on these in an another post)

In today’s world you will see that usages of these technologies are overlapped. For example, in tire industry, you can notice that
Tread & Sidewall are extruded using Simplex, Duplex, Triplex, Quadruplex Lines
Apex are extruded using through Simple & Duplex
Inner Liner are extruded using Simple, Duplex & Roller Head Technology

Rubber extrusion is in itself a vast subject, however if you know the above terms and types of rubber extruders, you have made a good beginning.

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