Rubber Extruders - A Special Supplement from Rubber & Tyre Machinery World

 

Embed or link this publication

Description

In this Knowledge On-The-Go Special Supplement, we bring to you a ‘collector’s edition’ on Extruders and Extrusion incorporating the fundamentals.

Popular Pages


p. 1

Rubber & Tyre Knowledge On-The-Go Special Supplement Machinery http://rubbermachineryworld.com / http://tyremachineryworld.com World EXTRUDERS

[close]

p. 2



[close]

p. 3

| Editor’s Note | Extruders And Extrusion 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. Rubber industry only had Hot Feed Extruders until 1950’s. Then came the Cold Feed Extruders and other variations as we see them today. Life today is mostly spent trying to catch up with latest developments. However, in the process the basics and fundamentals are sometimes overlooked. In this issue, we bring to you a ‘collector’s edition’ on Extruders and Extrusion incorporating the fundamentals, that I hope will find space in your digital library. My sincere thanks to Dr. S. N. Chakravarty, President of Elastomer Technology Development Society, for his inputs with the material for this edition. As always, I welcome your feedback on rubbermachineryworld@gmail.com. Best Regards Prasanth Warrier Extruders and the process of Extrusion has different applications in the rubber and tyre industry. 3 rubbermachineri rubbermachineryworld +Rubbermachineryworld1 grp/home?gid=8252803 rubbermachineri (Disclaimer: All the pictures and statements in this special supplement are sourced from web or shared by respective companies. All copyrights belong to actual owner. Rubber Machinery World does not independently verify them nor will vouch for their genuineness, hence will not be liable for any misrepresented data. The images are used here for representation purpose only.) Rubber & Tyre Machinery World | FEB 2016

[close]

p. 4

| EXTRUDERS | 4 Image of A Strainer Extruder - Pelmar Engineering Ltd. Rubber & Tyre Machinery World | FEB 2016

[close]

p. 5

| EXTRUDERS | Your Extruders And Its Extrusion Process Prasanth Warrier With Inputs of Dr. S.N. Chakravarty 5 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. Introduction Extruders are machines, which shape rubber to a profiled strip by forcing it through a die. In the simplest form an extruder consists of four basic components viz. Rubber & Tyre Machinery World | FEB 2016

[close]

p. 6

| EXTRUDERS | Image of A Ram Extrusion Line - Barwell 6 (a) a device system (b) a barrel (c) a ram or screw for forcing the rubber through the barrel and (d) a head holding the die which ultimately gives the desired shape. The fundamental distinction between two types of extruders is in their mode of operation: 1. Continuous: Delivers rubber in a continuous manner and has a rotating member or screw; here the pressure is produced by a screw. 2. Discontinuous: Delivers rubber in an intermittent fashion and has a reciprocating ram or screw. These type of extruders are ideally suited for batch type processes such as injection moulding. Screw Type And Ram Type Extruders In Ram Extruders, a quantity of warm compound is placed into the cylinder, the Ram extruders are not frequently used and are good for very specialized extrusions. die is attached to cylinder and ram pushes the compound through the die to form a profiled section. Advantages: • Extrusion can be carried out at lower temperatures. • Difficult compounds can be extruded. • Easy to clean. • Useful to short runs. • Useful for compounds which need to be strained through gauge for quality products requiring completely contamination-free material. Ram extruders are not frequently used and are good for very specialized extrusions. Rubber & Tyre Machinery World | FEB 2016

[close]

p. 7



[close]

p. 8

| EXTRUDERS | The basic principle is that the A Screw Extruder consists of five components viz. (a) Drive system (b) A feed hopper (c) A screw rotating within (d) A barrel (e) A head and die. The basic principle is that the screw carries material from the feed hopper by acting as a conveyor or a hump providing pressure to extrude or force the material along the machine barrel through the head and the die. The extruder drive system comprises of an AC or DC motor along with a reduction gear screw carries material from the feed hopper by acting as a conveyor. unit for power transmission. The extruder drive has to turn the screw at the desired speed. It should be able to maintain a constant screw speed because variations in screw speed will result in throughput fluctuation, which in turn will vary the dimensions of the extrudate. The purpose of the feed hopper is to receive material and pass it down to the flights of the screw. It is mostly supplied in the form of strip. 8 Image of A Screw Type Cold Feed Extruder - Pelmar Engineering Ltd. Rubber & Tyre Machinery World | FEB 2016

[close]

p. 9

| EXTRUDERS | A conventional extruder screw Extruder Screw The screw rotated within the Barrel, the usual clearance is approximately 0.40 mm. Normally, the barrel is fitted with a detachable liner in the form of a sleeve which is highly wear and corrosion resistant, made of hardened steel. The barrel is made double walled for steam or water circulation so that a constant temperature is maintained in the extruder head. A conventional extruder screw has three geometrically different sections. 1. Feed section (closest to the feed opening) generally has deep flights and consists of approximately 1/5th of the length of the screw. The material in this section will mostly be in solid state. 2. Metering section (closest to the die) usually has shallow flights and consists of approximately 2/5th of the length of the screw . The material in this section will mostly be in molten state. 3. This section connects the feed section and the metering section, is called as has three geometrically different sections - Feed, Metering and Compression. compression section. In going from feed section towards the metering section, a compression of the material in the screw channel takes place, which is essential for the proper functioning of extruders. The compression streamlines flow, helps to eliminate air and also ensures a constant pressure in the head. Screws are made of steel alloy forging, heat treated to a machinable hardness and hard chrome plated. The flight lands are surface hardened to approximately 600 Brinnel. The screw is internally bored to facilitate water-cooling. The extrusion process has to be effective. What does this mean to you? Your extruded profile temperature must be within required limits to prevent scorching. 9 Rubber & Tyre Machinery World | FEB 2016

[close]

p. 10

| EXTRUDERS | Reputed manufacturers offer The key screw design elements are Pitch, Flights and Core Diameter. Flights: The built up part in between which the rubber compound traverses. Pitch: Distance between two consecutive flights Core Diameter: Diameter of the Core Shaft Each manufacture has their own design of screw. However, the basic function remain same. An adjustment can be made for volume occupied by the flight lands. Screw design has a direct bearing on out put and quality of Extrudate. Output rate depends upon the screw design and geometry of extrudate. different screw designs. In general, you could increase output by either or a combination of the following - Decrease the Flights - Increase the Pitch - Increase the Core Diameter Reputed manufacturers offer different screw designs to process your specific rubber compound requirements. One of the basic characteristics of all the extrusion process is the clearance between Barrel and screw. This is generally permitted to a limit in order to control •  porosity • extrusion temperature 10 Schematic of a screw inside barrel and the different terms. Image from Web. Core Diameter Rubber & Tyre Machinery World | FEB 2016

[close]

p. 11



[close]

p. 12

When you choose a extruder, you should discuss the design, Extruders are usually designated by the diameter of the extruder barrel. The ratio of relative output of extruders varies as the square of the screw diameter . Thus the output of a 60 mm extruder will be 2.2 times that of a 40 mm extruder. The extruder is operated in such a manner so that temperature is gradually more from feed to discharge, the die being the hottest part. When you choose a rubber extruder, you should deliberate and discuss extensively upon the design, material technology and manufacturing accuracy of screw, barrel and die-head. Very few manufacturers world-wide can guarantee you a well-designed and precisely manufactured rubber extruder. An additional designation often used is the length to diameter (L/D) ratio . This is an important factor in the selection of extruders to match process requirements. material technology and manufacturing accuracy. The length of a rubber extruder depends on whether it is a hot feed or cold feed type. Hot feed extruders are usually very short about 3D to 5D while cold feed ones range from 12D to 24D. Hot Feed Extruder (HFE) Rubber industry only had Hot Feed Extruders until 1950’s. HFE’s extrude your rubber compounds at reduced temperatures. 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. In many cases the screw has increasing pitch. Rubber & Tyre Machinery World | FEB 2016 12 Image: VMI | EXTRUDERS |

[close]

p. 13



[close]

p. 14

| EXTRUDERS | 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 consider 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. 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. Image of Hot Feed & Cold Feed Extruder Screws In most cases, the screw has decreasing pitch. The screw has increasing root diameter with more flight depth at feed end which increase from feed to discharge. And of course, there are various other designs available. You can discuss with your manufacturer and avail various options of screws for a wide range of compound and extrusion applications. The output of an extruder is dependent on size of extruder i.e. the ratio of L/D. First estimate of out put is expressed as : Output = A (L/2) x R x d   A=Cross sectional area of extruder L=Lead Length R=Revolution per minute d=density of rubber Generally these exists a liner relationship between RPM & output 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, the 14 Rubber & Tyre Machinery World | FEB 2016

[close]

p. 15

| EXTRUDERS | 15 Image from Web: Feeding conveyor with Metal Detector For feeding the cold rubber, it is recommended that you use a feeding conveyor with metal detector to remove metal particles. sensitivity of the metal detector is found to be 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. Image from Web: A Set of Compact TCU’s below Cold Feed Extruder. Rubber & Tyre Machinery World | FEB 2016

[close]

Comments

no comments yet