Sunday, December 10, 2017

Polymer Screw Extrusion

Most polymeric materials are extruded at least twice in their lifetime, Ž rst through a pelletising die after the reactor and then for Ž nal shaping.In simplest terms, screw extruders are polymer pumps with the capacity to melt the material which they are fed. Screw extruders comprise one or two Archimedean screws rotating in a heated barrel.
Polymer Screw Extrusion
The single screw extruder (SSE) is the workhorse of the plastics industry. Polymer resins in the form of pellets, powders or fakes from a hopper to the gap between a rotating screw and a heated barrel. The depth of the con- veying channel in the screw is contoured from large to small in the  ow direction, to account for the density change from the particulate solid feed to the molten polymer extrudate, and for pressure development. The SSEs normally have diameters between 25 and 250 mm, and length/diameter ratios between 20 and 36. Usual rotation speeds range from 20 to 150 rev min . A 60 mm diameter machine may deliver up to 200 kg , while a 150 mm diameter machine can exceed 1000 kg.
In the first, or solids conveying, zone of the extruder, the solid polymer particles are compacted together in the screw channel by the rotating action of the screw to form a solid bed of material.At the start of the next extrudersection, the plastication (melting) zone, barrel heaters cause a thin Žfilm of molten polymer to form in the gap between the solid bed and the barrel wall. The melt Ž lm is subjected to intense shearing in the thin gap, and because of the extremely high viscosities of molten polymers, high rates of viscous dissipation result. The generated heat melts the solid bed within a short distance of the start of melting. In the last zone of the extruder, the metering section, the polymer melt flow is stabilised in the shallow screw channels, and Ž nally the material passes out through the die at the end of the machine.
Pressurebuildupowing to frictionalforcesin the solids con- veying section of the screw has been modelled by Darnell and Mol (along with some more recent research ) on the basis of force and torque balances, which result in an exponential expression. For forward motion of the solid bed, the friction coeffcient must be larger on the barrel than on the screw. Screws have polished surfaces. The barrel surfaces are sometimes intentionally roughened (as in grooved barrels).
To improve the mixing capabilities of single screw extruders, various types of mixing devices have been developed. Most prominent among these is the Maddock (or Union Carbide) mixer, in both straight and spiral variations.These devices are normally located at the downstream end of a screw, and improve melt quality by reducing temperature non-homogeneities in the polymer stream and increasing the dispersion and distribution of additives. Used increasingly in the extrusion industry are barrier screws and grooved barrels. Barrier screws have a secondary screw ight in the melting section of the screw, which serves to segregate the solid bed from the molten polymer. By independently controlling the dimensions of the solids and melt channels, the melting process can be accelerated and made more stable, there by increasing the extruder output and melt quality. Grooved barrel extruders feature axial
grooves or slots in the part of the barrel immediately following the feed throat. The grooved barrel can signiŽ cantly enhance feeding of resin pellets and flakes, and thereby sharply increase the output of the extruder above the drag ow output of equation.
Twin screw extruders(TSEs) are extruderswith two screws of the same diameter, which turn side by side within the extruder barrel at the same speed. In recent years, twin screw extruders have come into increasingly wider use in applications such as mixing, blending, compounding of thermoplastic polymers with additives, devolatilisation and reactive extrusion.The TSEs offer greater control over residence time distribution (RTD) and mixing than that of single screw extruders, and have superior heat and mass transfer capabilities. The disadvantage of TSEs compared with SSEs is their signiŽ cantly higher capital cost.
There are three major classes of twin screw extruders: corotating intermeshing, counter rotating intermeshing and counter rotating non-intermeshing. The screws are made up of conveying, kneading block and mixing sections. The screw design is frequently modular, which allows for a nearly unlimited number of possible screw conŽ gurations. Kneading blocks comprise several discs staggered at an angle to one another, the motion of which causes intense shearing and has a chopping effect on the material stream. Most of the dispersive mixing (melt homogenisation and solids breakup) and melting that occurs within the extruder takes place in kneading blocks.
Corotating intermeshing twin screw extruders are the most common type of TSE. These machines have the feature that their screws are self-cleaning,owing to their special design.This leadsto narrower RTDs in these extrudersthan in SSEs, making them particularly attractive for use with thermal and shear sensitive materials, as there are no dead spots in which material can collect and degrade. Large twin screw extruders can rotate at more than 1200 rev min RPM with outputs of more than 10 tons/h.

Thursday, December 7, 2017

PTFE tube molding machine

PTFE tube molding machine Features:Save time and money;Advance designed;High output;Long-life;Tube OD Size range:20-500 mm;Production Per Hour in Kg:10+;Automatic;Computerized;

PTFE tube molding machine is to extrude PTFE/UHMWPE Tubes. It is designed using latest technology, fully reliable and easy to operate for customers. Performance in terms of production and working hour capacity is competitive to other machines. Low maintenance is required , giving high production in market.
Benefits:
1. Save time and money, good advantage price offer to customers.
2. Intelligent and easy-to-use operations.
3. Small workplace required due to its compact design and electricity saving machine.
4. Super quality output and the physical properties is adjustable.
5. Precise temperature control, reaches + -1 degree.
6. Long- life, with modern technology and optimized design.

Wednesday, December 6, 2017

Types of injection molding machines & molding machine process

Molding Machines are classified primarily by the type of driving systems they use: hydraulic, mechanical, electric, or hybrid.
Hydraulic
Hydraulic presses have historically been the only option available to molders until Nissei Plastic Industrial Co., LTD introduced the first all-electric injection molding machine in 1983.
Hydraulic machines, although not nearly as precise, are the predominant type in most of the world, with the exception of Japan.
Mechanical
Mechanical type machines use the toggle system for building up tonnage on the clamp side of the machine. Tonnage is required on all machines so that the clamp side of the machine does not open (i.e. tool half mounted on the platen) due to the injection pressure. If the tool half opens up it will create flash in the plastic product.
Electric
The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive.
Hybrid injection (sometimes referred to as “Servo-Hydraulic”) molding machines claim to take advantage of the best features of both hydraulic and electric systems, but in actuality use almost the same amount of electricity to operate as an electric injection molding machine depending on the manufacturer.
A robotic arm is often used to remove the molded components; either by side or top entry, but it is more common for parts to drop out of the mold, through a chute and into a container..

Molding (process)

Molding or moulding is the process of manufacturing by shaping liquid or pliable raw material using a rigid frame called a mold or matrix. This itself may have been made using a pattern or model of the final object.
A mold or mould is a hollowed-out block that is filled with a liquid or pliable material such as plastic, glass, metal, or ceramic raw material.The liquid hardens or sets inside the mold, adopting its shape. A mold is the counterpart to a cast. The very common bi-valve molding process uses two molds, one for each half of the object. Piece-molding uses a number of different molds, each creating a section of a complicated object. This is generally only used for larger and more valuable objects.
The manufacturer who makes the molds is called the moldmaker. A release agent is typically used to make removal of the hardened/set substance from the mold easier. Typical uses for molded plastics include molded furniture, molded household goods, molded cases, and structural materials.

Monday, December 4, 2017

Extrusion Reprocessing

The term extrusion reprocessing refers to the minimizing of the scrap generation and recapturing of the material value in most of the operations. In almost all types of extrusion processes, it is a common practice to grind up the scrap materials by using granulator or allied suitable equipment. With the usage of this equipment, such granules can be made that are fed back in the extruder. In the compounding operations, initial material is granulated & then it is fed back in the extruder at a pre-decided level for the augmentation of raw materials, while the reduction of scrap to be disposed.
Extrusion Reprocessing
There is another compounding option, which is to select the products that are not as per the user’s specifications of color, or appearance. Then grind-up these products and add black concentrate to them. Afterwards, reextrude (by Extrusion Reprocessing) it into a black product. A number of colors may be added with the black concentrate to the extruder for the production of a black product that conforms to every specification of the customers. The mixing is done in a ribbon blender, tumble blender, continuous mixer, etc, for the development of an even mixture which is extruded in black product.
Few Important Points Of Extrusion Reprocessing
  • While the extrusion process is brought to equilibrium, the reprocessed or recycled material may enter from film operations or edge trim in sheet. This material may flash from off-spec products, blow molding, and the start-up material.
  • The scrap is either generated as post-industrial waste, which is created by plastic companies, or as post-consumer waste, which is created by consumer. If the scrap created in extrusion process is used properly, it can be a cost-effective source of raw material. Consequently, the overall cost of extruded product is reduced.

Laminated Products And Extrusion Reprocessing
Laminated products are a challenge, when it comes to their use in regrind or off-specification product. Here, two cases arises; one where the components in the laminate are compatible and other when they are not compatible. In case the components are compatible, then the laminate can be may be first granulated and then fed back in the process for producing its own special layer. Its alternate process includes:
  • The grounded laminate is fed to a compounding extruder.
  • In the extruder, melting and homogenization of the product takes place.
  • Hence, useful plastic is formed which may be sold as pellets or further extruded in another congenial application.
In case the components in the laminate are not compatible, then for the required result the material is incorporated in a center layer in a co-extruded structure. An alternative method is to try and sell the product as it is.
Extrusion reprocessing, re-extrusion or compounding of a recycle for making new product demands additional additives for improving the product’s performance, during the development of a value-added product. Some of these additives include:
  • Heat stabilizers
  • UV stabilizers
  • Flame retardants
  • Impact modifiers, etc.

Single Screw & Twin Screw Extruder Difference

single screw:In single screw extruders, a screw mixer melts the entering plastic materials, pushes the material through the extruder, and forms the plastic into the desired shape.
Twin screw:Twin screw extruders have two intermeshing screws and operate in the same manner as a single screw extruder. The picture below shows the barrels of the extruder exposed. Twin screw extruders depend minimally on the friction of the material against the barrel to move forward. They rely instead on the properties of the extruder and the screws. The two most important features are the meshing characteristics and the type of rotation.

Compared to single screw extruder, double screw extruder has below features:
  • 1) Material flow is stable, uneasy happen cutoff or billow, productive process is reliable.
  • 2) Most heat of double screw extruder mainly come from mechanical transformation during operation, small amount of heat comes from the heating jacket. The single screw extruder often require additional equipment preheat quenched materials.
  • 3) Time distribution is relatively narrow range of material retained in the machine, which is easier to control the temperature of the material, sufficient energy utilization, yield and quality are very stable.
  • 4) Double screw extruder has a screw surface self-cleaning effect, so that the transportation of materials is stable, little residual material in cavity when finish working, and no need stop to cleaning if change the production material.
  • 5) Double screw extruder has large productivity, is appropriate for processing of materials containing high oil (> 17%) and high humidity of materials (more than 30% moisture content)
Double screw extruder has complex structure, request high precision machining, therefore, investment for production facilities is much greater if select this type, so whether select it according to the actual situation.

Thursday, November 30, 2017

Extrusion Materials – Polymeric Materials

Polymeric materials are widely extruded into various products, using sophisticated extrusion machines such as extruder and others. The finished products are used for consumer as well as industrial applications. This extrusion process is an integrated one, in which extruder involves one of the components of the complete line. On the basis of the requirement of the product, some pre-blending or few ingredient mixing may be needed before the process of extrusion is carried out.
Before we proceed ahead, it is important to understand the basic meaning of the term “Polymer”. Polymers consists of various atoms that are connected together to form long chains (polymer backbone). The atoms are generally carbon (C), oxygen (O), sulfur (S), or nitrogen (N) that are combined in a unique configuration which is specific for every polymer. Various other atoms may also be present in a definite combination in the polymers.
There are a number of guidelines for the extrusion processing condition for different resin systems. These conditions are the ideal beginning points for various resin systems and may need an optimization for definite extrusion process for obtaining optimal throughput rate along with meeting or surpassing the mentioned quality criteria. The process of optimization of a specific extrusion operation is dependent on following:
Extruder type—single or twin screwThroughput rate
Extruder size—small or largeDownstream process
Particular screw design being usedResin type and formulation
Die designAdditives in the resin
Resin melt flowEnvironmental factors around the extruder
Some of the important Polymeric Materials are enlisted below:
  • Acrylonitrile Butadiene Styrene (ABS)
  • Polyamides (PA) or Nylons
  • Polyethylene (PE) Grades
  • Polymethyl Methacrylate (Acrylic)
  • Polypropylene (PP)
  • Polycarbonate (PC)
  • Polystyrene (PS)
  • Polyvinyl Chloride (PVC)

Polymer Rheology

Rheology can be defined as the science which deals with the materials’ deformation and flow. In case of polymers, it is important to understand these two factors both in die and in extruder for the optimal operation of the extrusion process. In the process of co-extrusion, it is pivotal to match the viscosities of resin layer at the processing temperatures. It is important for eliminating all the interfacial instabilities which can make the product simply good-for-nothing.

Wednesday, November 29, 2017

Overjacketing And Tubing Extrusion

Overjacketing and tubing extrusion are highly preferred processes for extruding a range of plastic materials. The overjacketing extrusion process is discussed here. In the process of wire coating, the bare wire or some bundles of jacketed wires, filaments, etc. are pulled through the die’s center, which is similar to the tubing die. The different materials that are used here, depends on the application. Basically, an insulated wire is a tube (thin walled) that is formed around bare wire.
For coating a wire, there are mainly two different kinds of extrusion tooling used. These are called “pressure” tooling or “jacketing” tooling. The selection of the type of tooling to be used depends on the fact that whether the specific application needs intimate contact or if it needs polymer adhesion to the wire or not. In case there is the requirement of an intimate contact or adhesion, pressure tooling is used or else, jacketing tooling is selected.
Jacketing Tooling vs Pressure Tooling 
Jacketing tooling differs with pressure tooling on the parameter of pin’s position with respect to the die. In case of jacketing tooling, the pin is extended all the way and flushed with die. Upon feeding the bare wire through the pin, it does not directly makes contact with molten polymer unless it leaves the die. In pressure tooling, the pin’s end is retracted in the crosshead, where it contacts with polymer at a higher pressure.
overjacketing-and-tubing-extrusion
Tubing Extrusion 
The process of tubing extrusion or extruded tubing process is used to manufacture products like medical tubing and drinking straws. This process is exactly similar to the process of regular extrusion, until the stage of die. To extrude the hollow sections, a pin or mandrel is placed inside the die and in various cases, positive pressure is applied through the pin onto the internal cavities.
For specialty applications, it is required to make tubings with a number of lumens or holes. For such applications, tooling is created with the placement of more than one pin in die’s center for the production of required number of lumens. In large number of cases, the pins are available with air pressure from a range of sources. This way, it becomes easy to adjust individual lumen sizes with the adjustment of pressure to the individual pins.