How does twin screw extruder work?

Extrusion processing aims to physico-chemically transform continuously viscous polymeric media and produce high quality structured products thanks to the accurate control of processing conditions.
 
Twin screw extruders consist of two intermeshing, co-rotating screws mounted on splined shafts in a closed barrel. Due to a wide range of screw and barrel designs, various screw profiles and process functions can be set up according to process requirements. Hence, a twin screw extruder is able to ensure transporting, compressing, mixing, cooking, shearing, heating, cooling, pumping, shaping, etc. with high level of flexibility. The major advantage of intermeshing co-rotating twin screw extruders is their remarkable mixing capability which confers exceptional characteristics to extruded products and adds significant value to processing units.
In twin screw extrusion processing, the raw materials may be solids (powders, granulates, flours), liquids, slurries, and possibly gases. Extruded products are plastics compounds, chemically modified polymers, textured food and feed products, cellulose pulps, etc.
The Advantages of Twin Screw Extrusion:
  • More consistency in production and control of product quality
  • Increased productivity due to continuous processing, faster start up and shut down between product changes, quick changeover and advanced automation
  • Greater flexibility, with the capability to process a wide range of raw materials
  • Optimized footprint thanks to energy and water savings
  • Simple and easy to maintain and clean
Twin screw extrusion is used extensively for mixing, compounding, or reacting polymeric materials. The flexibility of twin screw extrusion equipment allows this operation to be designed specifically for the formulation being processed. For example, the two screws may be corotating or counterrotating, intermeshing or nonintermeshing. In addition, the configurations of the screws themselves may be varied using forward conveying elements, reverse conveying elements, kneading blocks, and other designs in order to achieve particular mixing characteristics.

UHMW Ram Extrusion Process

UHMW powder is gravity fed into a chamber and a hydraulic ram pushes the powder from this chamber into the die. The die is the shape of the desired plastic, a certain diameter rod, a certain OD and ID tube, or a profile shape.
 
Heaters are employed on the outside of the die to heat the plastic and make it form into the shape of the die. The hydraulic ram moves back and forth continuously feeding the powder into the die.
As the material comes out of the die, it travels the length of the conveyor after which it is cut to length. Ram extrusion does not shear the material that is being manufactured as does single screw extrusion which employs a rotating screw to move the material.
It moves the material by hydraulically pushing it through the die which is the desired shape of the end product. UHMW-PE, which becomes gelatinous when it melts instead of molten, can only be extruded with this type or similar type process.

Furnace types

Single-stage:A single-stage furnace has only one stage of operation, it is either on or off.This means that it is relatively noisy, always running at the highest speed, and always pumping out the hottest air at the highest velocity.
One of the benefits to a single-stage furnace is typically the cost for installation. Single-stage furnaces are relatively inexpensive since the technology is rather simple.
Two-stage:A two-stage furnace has two stages of operation, full speed and half (or reduced) speed. Depending on the demanded heat, they can run at a lower speed most of the time. They can be quieter, move the air at less velocity, and will better keep the desired temperature in the house.
Modulating:A modulating furnace can modulate the heat output and air velocity nearly continuously, depending on the demanded heat and outside temperature. This means that it only works as much as necessary and therefore saves energy.
 

General sintering

Sintering is effective when the process reduces the porosity and enhances properties such as strength, electrical conductivity, translucency and thermal conductivity; yet, in other cases, it may be useful to increase its strength but keep its gas absorbency constant as in filters or catalysts. During the firing process, atomic diffusion drives powder surface elimination in different stages, starting from the formation of necks between powders to final elimination of small pores at the end of the process.
 
The driving force for densification is the change in free energy from the decrease in surface area and lowering of the surface free energy by the replacement of solid-vapor interfaces. It forms new but lower-energy solid-solid interfaces with a total decrease in free energy occurring. On a microscopic scale, material transfer is affected by the change in pressure and differences in free energy across the curved surface. If the size of the particle is small (and its curvature is high), these effects become very large in magnitude. The change in energy is much higher when the radius of curvature is less than a few micrometres, which is one of the main reasons why much ceramic technology is based on the use of fine-particle materials.
For properties such as strength and conductivity, the bond area in relation to the particle size is the determining factor. The variables that can be controlled for any given material are the temperature and the initial grain size, because the vapor pressure depends upon temperature. Through time, the particle radius and the vapor pressure are proportional to (p0)2/3 and to (p0)1/3, respectively.
The source of power for solid-state processes is the change in free or chemical potential energy between the neck and the surface of the particle. This energy creates a transfer of material through the fastest means possible; if transfer were to take place from the particle volume or the grain boundary between particles, then there would be particle reduction and pore destruction. The pore elimination occurs faster for a trial with many pores of uniform size and higher porosity where the boundary diffusion distance is smaller. For the latter portions of the process, boundary and lattice diffusion from the boundary become important.
Control of temperature is very important to the sintering process, since grain-boundary diffusion and volume diffusion rely heavily upon temperature, the size and distribution of particles of the material, the materials composition, and often the sintering environment to be controlled.

Sintering mechanisms

Sintering occurs by diffusion of atoms through the microstructure. This diffusion is caused by a gradient of chemical potential – atoms move from an area of higher chemical potential to an area of lower chemical potential. The different paths the atoms take to get from one spot to another are the sintering mechanisms. The six common mechanisms are:
 
  • Surface diffusion – Diffusion of atoms along the surface of a particle
  • Vapor transport – Evaporation of atoms which condense on a different surface
  • Lattice diffusion from surface – atoms from surface diffuse through lattice
  • Lattice diffusion from grain boundary – atom from grain boundary diffuses through lattice
  • Grain boundary diffusion – atoms diffuse along grain boundary
  • Plastic deformation – dislocation motion causes flow of matter
Also one must distinguish between densifying and non-densifying mechanisms. 1–3 above are non-densifying – they take atoms from the surface and rearrange them onto another surface or part of the same surface. These mechanisms simply rearrange matter inside of porosity and do not cause pores to shrink. Mechanisms 4–6 are densifying mechanisms – atoms are moved from the bulk to the surface of pores thereby eliminating porosity and increasing the density of the sample.
Plastic materials are formed by sintering for applications that require materials of specific porosity. Sintered plastic porous components are used in filtration and to control fluid and gas flows. Sintered plastics are used in applications requiring caustic fluid separation processes such as the nibs in whiteboard markers, inhaler filters, and vents for caps and liners on packaging materials. Sintered ultra high molecular weight polyethylene materials are used as ski and snowboard base materials. The porous texture allows wax to be retained within the structure of the base material, thus providing a more durable wax coating.

Types of molding machines

Molding Machines are classified primarily by the type of driving systems they use: hydraulic, mechanical, electrical, or hybrid.
Hydraulic molding machines
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 molding machines
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 molding machines
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.
Pneumatic molding machines
Pneumatic type machines use the air pressure for building up the tonnage on the clamp side of the machine.
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..

PCTFE Differences from PTFE

PCTFE has high tensile strength and good thermal characteristics. It is nonflammable and the heat resistance is up to 175 °C.It has a low coefficient of thermal expansion. The glass transition temperature (Tg) is around 45 °C.
PCTFE has one of the highest limiting oxygen index.It has good chemical resistance. It also exhibits properties like zero moisture absorption and non wetting.
It does not absorb visible light. When subjected to high-energy radiation, it undergoes, like PTFE, degradation. It can be used as a transparent film.
The presence of a chlorine atom, having greater atomic radius than that of fluorine, hinders the close packing possible in PTFE. This results in having a relatively lower melting point among fluoropolymers, around 210–215 °C.
PCTFE is resistant to the attack by most chemicals and oxidizing agents, a property exhibited due to the presence of high fluorine content. However, it swells slightly in halocarbon compounds, ethers, esters and aromatic solvents. PCTFE is resistant to oxidation because it does not have any hydrogen atoms.
PCTFE exhibits a permanent dipole moment due to the molecular asymmetry of its repeating unit. This dipole moment is perpendicular to the carbon-chain axis.
PCTFE is a homopolymer of chlorotrifluoroethylene (CTFE), whereas PTFE is a homopolymer of tetrafluoroethylene. The monomers of the former differs from that of latter structurally by having a chlorine atom replacing one of the fluorine atoms. Hence each repeating unit of PCTFE have a chlorine atom in place of a fluorine atom. This accounts for PCTFE to have less flexibility of chain and hence higher glass transition temperature. PTFE has a higher melting point and is more crystalline than PCTFE, but the latter is stronger and stiffer. Though PCTFE has excellent chemical resistance, it is still less than that of PTFE. PCTFE has lower viscosity, higher tensile strength and creep resistance than PTFE.