Plastic Extrusion Working Principle and Process Parameters

Extrusion is a high volume manufacturing process. The plastic material is melted with the application of heat and extruded through die into a desired shape. A cylindrical rotating screw is placed inside the barrel which forces out molten plastic material through a die. The extruded material takes shape according to the cross-section of die.

Working Principle

In this process, plastic material in the form of pellets or granules is gravity fed from a top mounted hopper into the barrel. Additives such as colorants and ultraviolet inhibitors (liquid or pellet form) can be mixed in the hopper. The plastic material enters through the feed throat and comes into contact with the rotating screw. The rotating screw pushes the plastic beads forward into the barrel. The barrel is heated using the heating elements up to the melting temperature of the plastic. The heating elements are used in such ways that gradually increase the temperature of the barrel from the rear to the front.
There are three possible zones in a rotating screw i.e. feed zone, melting zone, and metering zone. In the feed zone, the plastic beads melt gradually as they are pushed through the barrel. The plastic material is completely melted in the melting zone. A thermostat is used to maintain the inside temperature of the barrel. The overheating of plastics should be minimized which may cause degradation in the material properties. A cooling fan or water cooling system is used to maintain the temperature of the barrel during the process.
At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the molten plastic. The screens are reinforced by a breaker plate. The breaker plate assembly also serves to create back pressure in the barrel. The back pressure gives uniform melting and proper mixing of the molten plastic material into the barrel. After passing through the breaker plate, molten plastic enters into die. The die gives the desired shape of plastic product. An uneven flow of molten plastic would produce unwanted stresses in the plastic product. These stresses can cause warping after solidification of molten plastic. Plastics are very good thermal insulators and therefore it is very difficult to cool quickly. The plastic product is cooled by pulling through a set of cooling rolls.

Extrusion Process Parameters

There are five important process parameters to be considered before extrusion process:

• Melting temperature of plastic
• Speed of the screw
• Extrusion pressure required
• Types of die used
• Cooling medium 

Screw Design:

The design of screw is important for plastic processing. It has mainly three different functions:namely, feeding mechanism; uniform melting and mixing of plastic and finally it generates thepress ure to push the molten material through die. A screw length (L) is referenced to its diameter (D) as L/D ratio. Generally, L/D ratio is used as 24:1, but for more mixing and output, it may increase up to 32:1. There are three possible zones in a screw length i.e. feed zone, melting zone,and metering zone.
(a) Feed zone: In this zone, the resin is inserted from hopper into the barrel, and the channel depth is constant.
(b) Melting zone: The plastic material is melted and the channel depth gets progressively smaller. It is also called the transition or compression zone.
(c) Metering zone: The molten plastic is mixed at uniform temperature and pressure and forwarded through the die. The channel depth is constant throughout this zone. 

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What is Skiving ?

Skived PTFE Tape is made with Teflon™ fluoroplastic and a pressure sensitive silicone adhesive on one side. Skived PTFE tape is highly conformable with 300% elongation. PTFE tape features superior insulating strength, is non-stick, flexible, high temperature, as well as chemical, heat & moisture resistant. Other properties include no porosity, excellent release, and of course it has a super-low coefficient of friction. Also used as an insulating and masking tape. 

Skived PTFE (Polytetrafluoroethylene) film is sold by the pound, ft, or meter in slit width or full width rolls and can also be supplied in sheet form. It is used in electrical applications where high temperature service rating and superior electrical properties are desired. Examples of electrical applications include capacitor films, harnesses for electrical wiring in automotive and aerospace applications, spacers for transformers and other electrical insulation applications where high dielectric strength and high temperature resistance are requisite.

What is Skiving ?

Skiving or scarfing machines cut material off in slices, usually metal, but also leather or laminates. The process is used instead of rolling the material to shape when the material must not be work hardened, or must not shed minute slivers of metal later which is common in cold rolling processes.

The skiving process, meaning "to slice", can be applied to a variety of applications and materials. In leather, skiving knives trim the thickness of the leather, often around the edges, to thin the material and make it easier to work with. In metal working, skiving can be used to remove a thin dimension of material or to create thin slices in an existing material, such as heat sinks where a large amount of surface area is required relative to the volume of the piece of metal.

The process involves moving the strip past precision-profiled slotted tools made to an exact shape, or past plain cutting tools. The tools are all usually made of tungsten carbide-based compounds. In early machines, it was necessary to precisely position the strip relative to the cutting tools, but newer machines use a floating suspension technology which enables tools to locate by material contact. This allows mutual initial positioning differences up to approximately 12 mm (0.47 in) followed by resilient automatic engagement. Products using this technology directly are automotive seatbelt springs, large power transformer winding strip, rotogravure plates, cable and hose clamps, gas tank straps, and window counterbalance springs. Products using the process indirectly are tubes and pipe mills where the edge of the strip is accurately beveled prior to being folded into tubular form and seam welded. The finished edges enable pinhole free welds.

For lines which use low speed welding processes, such as laser welding, the skiving tools cannot normally cut - for example at speeds below metal planing speeds or about 10 meters/minute . In these cases the tools can be vibrated at high frequency to artificially increase the relative speed between the tools and strip.

Another metal skiving application is for hydraulic cylinders, where a round and smooth cylinder inside is required for proper actuation. Several skiving knives on a round tool pass through a bore to create a perfectly round hole. Often, a second operation of roller burnishing follows to cold-work the surface for mirror-finish. This process is common among manufacturers of hydraulic and pneumatic cylinders.

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Film and Sheet Extrusion

In extrusion, thermoplastics are melted to a viscous mass in a screw and then pressed into shape through a shaping die.

Machine parameters for the success with Effect pigments 

Masturbates or compounds are usually used to color the molten mass with effect pigments. For a satisfactory result in plastic extrusion with effect pigments, a balanced ratio must be maintained between the mixture energy and pigments which are as undamaged as possible. Excessive shear from mixing sections or inappropriate screws or filters destroy effect pigments and dramatically decrease the pearl luster effect.

The orientation of the pigments is a requirement for an even effect. This has to be ensured in the process through an corresponding engineering and design of the machinery.

Special possibilities in Co-extrusion 

Co-extrusion is used to combine different materials or the same materials in different colors or effects. The two materials are combined into one flow in the co-extrusion die. If an effect pigments is used in the surface layer, it is possible to increase the effect strength and by the same time saving cost due to a thin layer and the perfect orientation of the effect pigment on the visible surface However, it must be taken into account that, due to the much thinner co-extrusion layer, a higher concentration of effect pigment must be used than in solid layers in regular extrusion – this could mean up to ten percent in the plastic, depending on the layer thickness and desired effect. Less pigment is needed when the entire layer mass is colored.

The inner layer in co-extrusion usually uses high coverage. It is also called the substrate layer because of its much higher layer thickness. Functional materials can be used in this, such as to improve the barrier or other properties, or internal recycling materials can be used to save raw materials and boost waste reduction and economy. Effect pigments are seldom used in this layer.

This technique can be used in all extrusion processes, including extrusion blow molding.

Factors Affecting Extrusion

Shape is a determining factor in the part's cost and ease with which it can be extruded. In extrusion a wide variety of shapes can be extruded, but there are limiting factors to be considered. These include size, shape, alloy, extrusion ratio, tongue ratio, tolerance, finish, factor, and scrap ratio. If a part is beyond the limits of these factors, it cannot be extruded successfully.

The size, shape, alloy, extrusion ratio, tongue ratio, tolerance, finish, and scrap ratio are interrelated in the extrusion process as are extrusion speed, temperature of the billet, extrusion pressure and the alloy being extruded.

In general, extrusion speed varies directly with metal temperature and pressure developed within the container. Temperature and pressure are limited by the alloy used and the shape being extruded. For example, lower extrusion temperatures will usually produce shapes with better quality surfaces and more accurate dimensions. Lower temperatures require higher pressures. Sometimes, because of pressure limitations, a point is reached where it is impossible to extrude a shape through a given press.

The preferred billet temperature is that which provides acceptable surface and tolerance conditions and, at the same time, allows the shortest possible cycle time. The ideal is billet extrusion at the lowest temperature which the process will permit. An exception to this is the so-called press-quench alloys, most of which are in the 6000 series. With these alloys, solution heat-treat temperatures within a range of 930°-980° F must be attained at the die exit to develop optimum mechanical properties.

At excessively high billet temperatures and extrusion speeds, metal flow becomes more fluid. The metal, seeking the path of least resistance, tends to fill the larger voids in the die face, and resists entry into constricted areas. Under those conditions, shape dimensions tend to fall below allowable tolerances, particularly those of thin projections or ribs.

Another result of excessive extrusion temperatures and speeds is tearing of metal at thin edges or sharp corners. This results from the metal's decrease in tensile strength at excessively high-generated temperatures. At such speeds and temperatures, contact between the metal and the die bearing surfaces is likely to be incomplete and uneven, and any tendency toward waves and twists in the shape is intensified.

As a rule, an alloy's higher mechanical properties means a lower extrusion rate. Greater friction between the billet and the liner wall results in a longer time required to start the billet extruding. The extrusion ratio of a shape is a clear indication of the amount of mechanical working that will occur as the shape is extruded.

Extrusion Ratio = area of billet/area of shape.

When the extrusion ratio of a section is low, portions of the shape involving the largest mass of metal will have little mechanical work performed on it. This is particularly true on approximately the first ten feet of extruded metal. Its metallurgical structure will approach the as-cast (coarse grain) condition. This structure is mechanically weak and shapes with an extrusion ratio of less than 10:1 may not be guaranteed as to mechanical properties.

As might be expected, the situation is opposite when the extrusion ratio is high. Greater pressure is required to force metal through the smaller openings in the die and extreme mechanical working will occur. Normally acceptable extrusion ratios for hard alloys are limited to 35:1 and for soft alloys, it is 100:1. The normal extrusion ratio range for hard alloys is from 10:1 to 35:1, and for soft alloys is 10:1 to 100:1. These limits should not be considered absolute since the actual shape of the extrusion can affect results. The higher the extrusion ratio, the harder the part is to extrude which is the result of the increased resistance to metal flow. Hard alloys require maximum pressure for extrusion and are even more difficult because of their poor surface characteristics which demand the lowest possible billet temperature.

Difficulty factor is also used to determine a part's extrusion performance.

Factor = Perimeter of Shape/ Weight per Foot.

Weight per foot is of primary importance because of the consideration for profitable press operation. As might seem obvious, a lighter section normally requires a smaller press to extrude it. However, other factors may demand a press of greater capacity such as a large, thin wall hollow shape. Though it has low weight per foot it may take more press tonnage to extrude it. The same reasoning applies to the factor as with the extrusion ratio. A higher factor makes the part more difficult to extrude consequently affecting press production.

The tongue ratio also plays an important role in determining a part's extrusion performance. The tongue ratio of an extrusion is determined as follows: square the smallest opening to the void, calculate the total area of the shape, and then divide the opening squared by the area.. The higher the ratio, the more difficult the part will be to extrude.

In order to help us understand your needs and requirements and service you better, the following is a check list of things to consider when submitting items to an extruder for quoting or new business:

1. Description or drawings of the part- talk to the extruder early before the design is finalized.
2. Specifications to be met; Federal specs, military, ASTM, etc.
3. Alloy and temper; if unknown, indicate requirements for strength, corrosion resistance, machinability, finish, weldability, to aid the extruder in making a recommendation.
4. End use length and purchase length.
5. Tolerances; commercial, per drawing, other.
6. Surface Finish; mill, anodize, paint, exposed surfaces, etc.
7. Packaging; acceptable maximum and minimum weight per package and shipping and handling requirements.
8. Secondary fabrication requirements-mitering, punching, bending, anodizing, drilling, etc.
9. Product end-use.
10. Quantity needed; this order and on an annual basis.
11. Shipping date.
12. Special quality considerations.

Plastic Co-Extrusion

Coextrusion is the process of pressing two or more materials through the same die to produce a single piece. When multiple plastics are combined, the result can yield properties distinct from those of a single material. Coextrusion has opened up new frontiers in material engineering and addressed several previously difficult manufacturing needs.

Coextruding a stripe of radiopaque plastic into a catheter, for example, improves x-ray quality as the catheter moves through a vein without compromising the effectiveness of the catheter itself. Coextrusion can also reduce costs by using recycled and reground scrap inside virgin material for handrails, fences and other applications. The process can be seen in projects as diverse as tubing and structural components or air blown food containers.

The Coextrusion Process

In standard extrusion, solid plastic pellets are gravity fed into a forming mechanism, where jacketed compression screws melt and feed the materials into a die. By contrast, coextrusion involves multiple extruders forming layered or encapsulated parts. Sometimes five or more materials are used in a single cycle, with each extruder delivering the precise amount of molten plastic needed for the operation..

Unlike ordinary plastic mixing, each individual plastic retains its original properties, but is combined into a compound-material part. If mixed prior to extrusion, the characteristics of the individual materials may be altered, but the end result is a homogeneous product.

Not all plastics are suitable for coextrusion because some polymers will not adhere to others, although introducing a conductive middle layer can often solve this problem. Plastics with drastically different melting temperatures are also unsuitable for the process, as degradation will occur in the lower melting material. Finally, PVC and acetals should never be coextruded together because of the potentially violent reactions that can occur when they are joined.

Coextruded Tubing

The multi-colored drinking straw is a good example of coextruded tubing’s design features. Striped tubing also serves many purposes in the medical field, in which stripes and colors can denote different chemicals. Coextrusion can produce internally hardened tubes through which a cable can be run while retaining the tube’s flexibility. Other tubes benefit from a high performance liner impervious to corrosives, or an inexpensive coating to add bulk and stability. In addition, plastic fiber optic cables are composed of a coextruded cable and jacket.

Coextruded Structural Units

Plastic is sometimes used as a substitute for wood. Manufacturers can create decking, boat docks, fences and dimensional components with “plastic lumber,” which has some advantages over natural wood. Coextrusion is a cost-effective method of fabricating many of these artificial materials. It can add titanium dioxide, a weather resistant material, to exterior structural plastics or produce decking with an inner layer of recycled plastic.

 

Polymer Particle Structure

The particles of polymer produced in the dispersion polymerisation process are of the order of 0.2µm in size, whilst those from a granular polymerisation are hundreds of µm in size, built up from smaller particles.They are both highly crystalline - about 90 to 95%.

The dispersion particles can be studied directly in a conventional transmission electron microscope, providing the electron intensity is kept low. On raising the electron beam intensity the particles change rapidly in appearance to become transparent with a crumpled texture. At this stage the crystallinity has disappeared and the ‘particles’ probably consist of a shell of carbon. Electron diffraction patterns and dark field micrographs suggest that the particles are composed of a pile of small single crystals with the molecular axis along the axis of the brick-shaped particles . The particles also appear to have a striated surface structure generally parallel to the long axis. A replica of some dispersion particles. During coagulation, the dispersion particles aggregate to form a larger particle, made up of a loose structure of agglomerates of the primary particles. During cold, lubricated extrusion the agglomerated particles are highly distorted, with their primary particles becoming aligned and also drawn into fibrous material.

During the early stages of polymerisation granular particles form as aggregates of smaller particles. This process continues and large irregular fibrous structures are produced.This material is then modified mechanically to reduce it to the familiar form suitable for processing.

Radiation and Industrial Polymers

With the advancement of industrialisation, pollution is a crucial problem for mankind. In the Green drive, i.e tomake the world pollution-free, radiation technology takes an important position. Nuclear radiation has made itsentry into many chemical processes. ‘Polymerisation’, ‘grafting’ and ‘curing’, all-important chemical processes inthe polymer field, can proceed through radiation techniques. The radiation technology is preferred over the otherconventional energy resources due to some reasons, e.g. large reactions as well as product quality can becontrolled, saving energy as well as resources, clean processes, automation and saving of human resources etc.Apart from this, radiation is also a good sterilising technique over other conventional sterilising techniques. Theirradiation of polymers can be applied in various sectors. In this review, the attention has focussed primarily to foursectors, i.e. biomedical, textile, electrical and membrane technology. 

From the age of stone and metals, we have come to the age of nuclear energy and polymers. Indeed,we live in the world of polymers. That is why scientists and technologists have termed this era as the‘polymeric age’. In every step of our daily lives, we come across things, which are the fruits of polymerresearch. The ever widening application of polymers in everyday life over the last several decades hasgenerally been acknowledged as a mixed blessing by scientists and technologists. Though started in themiddle of the last century, work in this field of chemistry has been so rapid and the application so usefuland versatile, that the number of polymer systems are enormous.

The last three decades have also witnessed the emergence of nuclear radiation as a powerful source ofenergy for chemical processing applications. Thus, it can be applied in different industrial areas. The factthat radiation can initiate chemical reactions or destroy micro-organisms has led to the large-scale use ofradiation for various industrial processes. Nuclear radiation is ionising, which on passage throughmatter, gives positive ions, free electrons, free radicals and excited molecules. The capture of electronsby molecules can also give rise to anions. Thus, a whole range of reactive species becomes available forthe chemist to play with.

Radiation-based processes have many advantages over other conventional methods. For initiationprocesses, radiation differs from chemical initiation. In radiation processing, no catalyst or additives arerequired to initiate the reaction. Generally with the radiation technique, absorption of energy by thebackbone polymer initiates a free radical process. With chemical initiation, free radicals are broughtforth by the decomposition of the initiator into fragments which then attack the base polymer leading tofree radicals. Sakurada [1] compared the efficiency of the two processes and estimated that the samenumber of initiating radicals are produced in unit time with a radiation dose of 1 rad/s or a chemicalinitiator, e.g. benzoyl peroxide, at a concentration of.01 M is used. Chemical initiation is howeverlimited by the concentration and purity of the initiators. However, in the case of radiation processing,the dose rate of the radiation can be varied widely and thus the reaction can be better controlled. Unlikethe chemical initiation method, the radiation-induced process is also free from contamination. Chemicalinitiation often brings about problems arising from local overheating of the initiator. But in the radiation-induced process, the formation of free radical sites on the polymer is not dependent on temperature but isonly dependent on the absorption of the penetrating high-energy radiation by the polymer matrix,Therefore, radiation processing is temperature independent or, in other words, we may say it is a zeroactivation energy process for initiation.

As no catalyst or additives are required, the purity of the processed products can be maintained. Withradiation processing, the molecular weights of the products can be better regulated. Radiation techniquesalso have the capability of initiation in solid substrates. The finished products can also be modifying bythe radiation technique.

Nuclear radiation energy, however, is expensive though very efficient in bringing about chemicalreactions. The unit cost of installed radiation energy is much higher than that of conventional heat orelectrical energy. Despite this fact, the application of nuclear radiation energy has proved its superiorityand its cost effectiveness in a number of chemical processes over that of other forms of energy such asheat or electrical energy. Radiation techniques have good efficiencies with regard to power and needsonly a small space to be set up.

The application of radiation on polymers can be employed in various industrial sectors, i.e. bio-medical, textile, electrical, membrane, cement, coatings, rubber goods, tires and wheels, foam, footwear,printing rolls, aerospace and pharmaceutical industries. In this review, attention is focused primarily onfour sectors: biomedical, textile, electrical and membrane technologies.

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