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.

 

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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.

 

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 screw Throughput rate
Extruder size—small or large Downstream process
Particular screw design being used Resin type and formulation
Die design Additives in the resin
Resin melt flow Environmental 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.

 

Crosshead Extrusion Process (Wire & Cable Coating)

Crosshead extrusion process is widely used to coat wires and cables with a polymer. The basic procedure includes pulling of the wire / cable to be coated at a uniform rate via a crosshead die, where it is covered with the molten plastic. This extrusion process for coating is used in most wires and cables that find usage in telecommunication and electrical applications along with electronic industry. For more coatings, two extruders can also be used in tandem.
Development Of Standard Two-Wire-Plus-Ground Cable By Crosshead Extrusion

Consider the example of the development of standard two-wire-plus-ground cable, which is commonly used in home wiring. In the beginning, white insulation is used to coat one copper wire and black insulation is used for the other. In a secondary process, following procedures are followed:

  • A paper-wrapped copper wire is combined with the black & white insulated wires. This third wire is used for the ground.
  • All the wires are fed through die
  • In the die, final insulating jacket is applied for protecting all the wires together. The insulating jacket also assists in holding the three wires in a common plastic sleeve, which is used home wiring
Another approach to produce this product, is via the continuous process of production. In this, two extruders are used to individually apply white as well as back coating on the two conductive wires. The successive processes are as follows:
  • These two products brought together by using a third copper ground wire
  • The three wires are sent via a third crosshead die, where the addition of exterior jacket takes place
As all the three extruders are running at similar speeds, the end product is developed with minimal handling. A negative point of this process is the amount of production loss occurred, in case any of the three extruders is not running efficiently or there is some problem with only one of the extruders.
Wire coating is generally done by the use of single screw extruders, in which the crosshead extrusion process is carried out. The job of the extruder is to melt the resin & forward it to the die at an even & constant melt pressure and temperature. The crosshead extrusion process is carried out by using a general equipment in the line, which includes following basic pieces:
  • Unwind station or some other wire / cable source for feeding the line
  • Pretensioning station for setting the tension throughout the process
  • Preheat station for preparing the wire for coating
  • Crosshead die
  • Cooling trough for solidifying the polymeric coating
  • Test stations for assuring that the wire is suitably coated
  • Puller for providing constant tension through out the process
  • Winder for collecting the product
Typical Polymers Used In Wire Coating Applications

There are various polymers that are used in wire coating applications by the crosshead extrusion process. The characteristics of these polymers, which make them ideal for this purpose are their flexibility, electrical properties, ability to withstand abuse, and durability. Typical polymers are as follows:

  • Polyethylene
  • Polyvinylchloride
  • Polyamide
  • Polybutylene terephthalate
  • Thermoplastic elastomers
  • Ethylene propylene copolymers
  • Polypropylene
  • Fluoropolymers
In the wire coating operation, cross linked polyethylene is used. The wire is extrusion coated with this polymer in this process. At the end of coating operation, polyethylene is cross-linked on to the wire.

 

A vertical press machine for PTFE Tube

Vertical press machine for PTFE tube;Using for Pre-sintering PTFE material;Suit for new PTFE material or Recycle PTFE material;Rangeof diameter PTFE Extrusion 20-500 mm, Heating zones 5 zones; Automatic.
vertical press machine
Vertical press machiner 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.
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.

 

Fluoric polymers (PTFE/PVDF)

Fluoropolymers have excellent, almost universal chemical resistance. They can be used both at high and very low temperatures (-260 to +260°C). They also possess outstanding resistance to weathering (UV-stability).
Due to the low coefficient of friction, they are often used as sliding materials or as corresponding additives in other high-performance plastics.

TECAFLON PVDF (PVDF)

Polyvinylidenefluoride (PVDF) – TECAFLON PVDF – is an opaque, semi-crystalline, thermoplastic fluoropolymer. PVDF is characterized by excellent chemical stability, without having the disadvantages of low-mechanical values and/or processing difficulties which can be experienced with other fluoroplastics.
  • low density compared to other fluoropolymers
    good mechanical strength compared to other fluoropolymers
    high permanent operating temperature (140°C)
    practically no moisture absorption
    good dimensional stability
    high chemical resistance
    good resistance to hydrolysis
    weather-proof
    radiation resistant
    good electrical insulator
    high abrasion resistance

TECAFLON PTFE natural (PTFE)

Polytetrafluoroethylene (PTFE) – TECAFLON PTFE natural – is a semi-crystalline fluoropolymer with an unusually high chemical and thermal resistance (-200 to +260°C, temporarily up to 300°C). In addition, this thermoplastic material has excellent sliding properties, a non-stick surface and the best insulating properties. This is in contrast, however, to low mechanical strength and a high specific gravity, compared to other plastics. In order to improve the mechanical properties, PTFE can be used as a compound reinforced with additives such as glass fibre, carbon or bronze. Due to its structure, this material is formed into semi-finished products using a compression processes and machined afterwards with cutting/machining tools.
  • extremely high chemical resistance
    very good temperature resistance (-200-+260°C, temporarily up to 300°C)
    very low coefficient of friction (static friction = sliding friction)
    extremely low surface tension (practically no materials stick to PTFE -> difficult to adhere to or weld)
    high coefficient of thermal expansion
    relatively low strength/rigidity
    low dielectric constant
    non-combustible

 

Multi-hole Ram Extrusion Process

Multi-hole ram extrusion is the process in which the raw material is pushed through a die having more than one hole. This process is highly productive for producing parts of smaller length and cross-section. For the given billet and final product size, the requirement of the ram force is lesser in the multi-hole extrusion than in the single-hole extrusion. The process has great importance for producing micron-size parts.
For the design of a multi-hole die extrusion machine, the modeling of the extrusion process is required. Although the finite element method can be effectively employed for this purpose, it requires a large amount of computational time besides the requirement of appropriate software to do the mesh generation and finite element processing. After the tentative or final specifications are decided, the design process usually consists of three stages: the conceptual design, the embodiment or preliminary design and the detailed design. At the conceptual design stage, the design concepts are generated. At the preliminary design stage, the chosen concept is given bodily form. Finally, at the detailed design stage, the detailed design calculations are carried out and the manufacturing drawings are generated. At the preliminary design stage, one needs computationally faster analysis for generating an optimum design. The generated design at this stage can be further fine tuned with rigorous finite element analysis of the process.
The estimation of the ram force is carried out using upper bound method by considering the process as a single-hole extrusion. This leads to an overestimation of the ram force, which results in a safer design of the die and ram. Die pressure distribution along the die face has been calculated using the slab method. A finite element analysis of the die has been carried out to estimate the distribution of the von-Mises stresses across the die volume. The experimental investigations have been carried out by extruding the billets made of lead through single-hole and multi-hole dies. The experimental results are compared with the analytical results. To carry out the analysis, the value of the friction factor is found experimentally. It is observed that the ram force calculated by the proposed methodology is about 25% greater than the experimental force for a single-hole extrusion. It is found experimentally that the material encounters more resistance to flow in the central hole than in the holes away from the center. This leads to differences in the lengths of the extruded wires, if the hole-sizes are same. In the case of the multihole extrusion, ram force is always lesser (about two-third) than in the case of the single-hole extrusion. Thus, the multi-hole extrusion process can become a productive process for the mass production of small sized components.