Die Extrusion - Polymer Extrusion Process

Once a polymer has been melted, mixed and pressurised in an extruder, it is pumped through an extrusion die for continuous forming (after cooling and solidification) into a final product. The most common die types are flat, annular and prole. Products made by extrusion include pipe, tubing, coating of wire, plastic bottles, plastic films and sheets, plastic bags, coating for paper and foil, bres, laments, yarns, tapes and a wide array of profiles.
Die Extrusion
Polymer extrusion through dies has certain similarities to the hot extrusion of metals.However, there are also signi-ficant differences. In metal extrusion the material is pushed forward by a ram, while in polymer extrusion the material is continuously supplied by a rotating screw. In hot metal extrusion the temperatures range from 340°C for magnesium to 1325°C for steel, and the corresponding pressures range from 35 to over 700 MPa.1 5 In polymer extrusion the temperatures seldom exceed 350°C, and pressures usually do not go much above 50 MPa at the screw tip. Solid phase extrusion of polymers has been developed for the production of certain high strength products.At low temperatures, the molecular orientation imparted by the forcing of the material through the shaping die remains in the extrudate. Solid state polymer extrusion has certain similaritiesto the cold extrusion of metals.
Blown film extrusion is the most important process for the production of thin plastic films from polyethylenes.The molten polymer is extruded through an annular die (normally of spiral mandrel construction) to form a thin walled tube which is simultaneously axially drawn and radially expanded. In most cases the blown film bubble is formed vertically upwards. The maximum bubble diameteris usually 1.2 – 4 times larger than the die diameter. The hot melt is cooled by annular streams of high speed air from external air rings, and occasionally also from internal air distributors. The solidi ed film passes through a frame which pinches the top of the bubble and is taken up by rollers. Coextruded films with 3 – 8 layers (sometimes up to 11) are also produced by this process, for use in food packaging.
Cast film and sheet extrusion involves extruding a poly- mer through a at die with a wide opening (up to 10 m), onto a chilled steel roller or rollers which quench and solidify the molten material. Film is generally defined as a product thinner than 0.25 mm, while sheet is thicker than this.The cast film process is used for very tight tolerances of thin film, or for low viscosity resins. Most flat dies are of T slot or coathanger designs, which contain a manifold to spread the flowing polymer across the width of the die, followed downstream by alternating narrow and open slits to create the desired flow distribution and pressure drop. Most cast film lines manufactured today are coextrusion lines, com- bining layers from as many as seven extruders into the product through multimanifold dies, or single manifold dies with the aid of feedblocks. 
In film extrusion, the shear rates at the die lips are usually ~103 s21. When the wall shear stress exceeds a certain value (usually 0.14 MPa in research papers, higher in industry with the help of additives),the extrudate surface loses its gloss owing to the sharkskin melt fracture phenomenon.Sharkskin can be described as a sequence of ridges visible to the naked eye, perpendicular to the flow direction.
Pipe and tubing extrusion involves pumping a molten polymer throughanannulardie,followingwhich the extruded product, while being pulled, passes through a vacuum sizer where it attains its final dimensions. This is followed by spray or immersion cooling and cutting to fixed lengths. Pipe of diameter up to 2 m or greater is made by this process, and tubing with diameters from 10 mm down to below 1 mm. The annular dies are normally of spider or spiral mandrel design.
In wire and cable coating processes, individual wires or wire assemblies are pulled at very high speed through a crosshead die, at right angles to the extruder axis. In high pressure extrusion, the polymer melt meets the wire or cable before the die exit, for example insulating of individual wires. In low pressure extrusion, the melt meets the cable after the die exit, for example jacketing of assemblies of insulated cables. Very high shear rates are frequently encountered in this process(up to 106 s2 1 ) and low viscosity resins are used.
Profile extrusion is a manufacturing process used for products of constant cross-section. These can range from simple shapes to very complex profiles with multiple chambers and fingers. Examplesrange from picture frame mouldings, to automotive trim, to edging for tabletops,to window lineals. The extruded materials are classi ed (roughly) as rigid or fiexible. The typical prole extrusion line consists of an extruder pumping a polymer through a prole die, followed by a sizing tank or calibrator, additional cooling troughs, a puller and a cutoff device. The design of profile dies requires considerable experience and patience.Output limitations in profile extrusion are encountered owing to either sharkskin (for thin products produced from high viscosity polymers) or the ability to cool thick walled products. Polymer pipe and profile extrusion is similar to the hot extrusion of metals for the production of continuous hollow shapes of barlike objects. However, the mathematicalmodelling of these processesfor metals is based mainly on elastic – plastic flow hypotheses.
In melt spinning, the molten polymer ows through numerous capillaries in a spinneret (up to 1000). The poly- mer is delivered under pressure by a gear pump for accurate metering, after passing through a lter which follows the extruder. On exiting the capillaries, the laments are attenuated to the desired diameter.For the production of very thin fibres, the melt blowing process is used. In this process the bres are attenuated by the drag force exerted by a high velocity air jet.

Plastics Extrusion Working Principle & 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.
Extrusion of Plastics
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. 

Plastic Extrusion Types of Extrusion Process

Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation.

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 the pressure 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.

TYPES OF EXTRUSION PROCESS

The extrusion process is broadly classified into seven different types depending upon the specific applications.
(a) Sheet/Film Extrusion
In this extrusion process, the molten plastic material is extruded through a flat die. The cooling rolls are used to determine the thickness of sheet/film and its surface texture. The thickness of sheet can be obtained in the range of 0.2 to 15 mm. The thin flat sheet or film of plastic material can be made. Generally, polystyrene plastic is used as a raw material in the sheet extrusion process.
(b) Blown Film Extrusion
In the blown film process, the die is like a vertical cylinder with a circular profile. The molten plastic is pulled upwards from the die by a pair of nip rollers. The compressed air is used to inflating the tube. Around the die, an air-ring is fitted. The purpose of an air-ring is to cool the film as it travel upwards. In the center of the die, there is an air inlet from which compressed air can be forced into the centre of the circular profile, and creating a bubble. The extruded circular cross section may be increased 2-3 times of the die diameter. The bubbles are collapsed with the help of collapsing plate. The nip rolls flatten the bubble into double layer of film which is called layflat. The wall thickness of the film can be controlled by changing the speed of the nip rollers.
The layflat can be spooled in the form of roll or cut into desired shapes. Bottom side of the layflat is sealed with the application of heat, and cut across further up to form opening; hence it can be used to make a plastic bag. The die diameter may vary from 1 to 300 centimeters.Generally, polyurethane plastic is used in this process.
Blown Film Extrusion​
(c) Over Jacketing Extrusion
This is also called wire coating process. In this process, a bare wire is pulled through the center of a die. There are two different types of extrusion tooling used for coating over a wire i.e.pressure or jacketing tooling. If intimate contact or adhesion is required between the wire and coating, pressure tooling is used. If adhesion is not desired, jacketing tooling is used. For pressure tooling, the wire is retracted inside the die, where it comes in contact with the molten plastic at a much higher pressure. For jacketing tooling, the wire will extend and molten plastic will make a cover on the wire after die. The bare wire is fed through the die and it does not come in direct contact with the molten plastic until it leaves the die. The main difference between the jacketing and pressure tooling is the position of the wire with respect to the die.
Over Jacketing Extrusion​
(d) Tubing Extrusion
In this process, the molten plastic is extruded through a die and hollow cross sections are formed by placing a mandrel inside the die. Tube with multiple holes can also be made for specific applications, by placing a number of mandrels in the center of the die.
(e) Coextrusion
Coextrusion is the extrusion process of making multiple layers of material simultaneously. It is used to apply one or more layers on top of base material to obtain specific properties such as ultraviolet absorption, grip, matte surface, and energy reflection, while base material is more suitable for other applications, e.g. impact resistance and structural performance. It may be used on any of the processes such as blown film, overjacketing, tubing, sheet/film extrusion. In this process, two or more extruders are used to deliver materials which are combined into a single die that extrudes the materials in the desired shape. The layer thickness is controlled by the speed and size of the individual extruders delivering the materials.
(f) Extrusion Coating
Extrusion coating is used to make an additional layer onto an existing rollstock of paper, foil or film. For example, to improve the water resistant of paper polyethylene coating is used. The applications of extrusion coating are liquid packaging, photographic paper, envelopes, sacks lining for fertilizers packaging and medical packaging. Generally, polyethylene and polypropylene are used.

Blow Moulding & Compression Moulding

Blow moulding is the process by which articles are formed by in ation of a molten resin to fill a mould cavity having the desired shape and dimensions.Compression moulding is the oldest technique for the production of polymer products,and is mainly used for thermosets. 
Blow Moulding & Compression Moulding​
 

BLOW MOULDING

Blow moulding is the process by which articles are formed by in ation of a molten resin to fill a mould cavity having the desired shape and dimensions. Bottles for soft drinks and other liquids are blow moulded. The two most important process variants are extrusion blow moulding and injection blow moulding.In extrusion blow moulding, an extruder pumps the melt through an annular die to form a molten tube or parison with well de ned and controlled dimensions. The parison is clamped between the two mould halves and is in ated by internal air pressure to take the shape of the mould cavity, which is usually cooled. Finally, the formed article solidi es as a result of cooling and the mould is opened to eject the article without damage. Depending on the thermal stability of the material, the extrusion process can be continuous or intermittent. Intermittent systems can use reciprocating screws which operate as rams, in the same manner as in injection moulding.
Injection blow moulding is a two stage process. In the first stage, the plastic is injected into a cavity where the preform is moulded. The preform is then transferred to the blow mould for infiation. Injection blow moulding offers the advantages of accurate dimensional control, elimination of scrap and the moulding in of threads before blowing. Extrusion blow moulding is preferred for contain- ers with high length/diameter ratios and for products with handles.
Polyethylene terephthalate or PET is the polymer most widely used in injection blow moulding, for carbonated drinks and water bottles. Stretch blow moulding is used to produce PET bottles of enhanced physical properties. In this process, stretching induces the formation of small lamellar crystals. These crystals result in more transparent and tougher products than those produced without stretch blowing, which have spherulitic crystals.

COMPRESSION MOULDING

Compression moulding is the oldest technique for the production of polymer products,and is mainly used for thermosets. In this process the compound is pressed in the mould by the heated platens of a hydraulic press. This pro- cess is to some extent analogous to sheet metal stamping. Injection moulding of polymers has replaced compression moulding for some polymers, because of the advantages in materials handling and automation. However, compression moulding has an advantage in the processing of reinforced polymers.Owing to modest levels of deformation and stress involved in compression moulding, the reinforcing fibres are not damaged. Very high fibre concentrations and longer fibres can be included in compression moulded products.

Types of Hot Melt Extrusion

There are two types of extruders: single and twin screw extruders. Single screw extruders are primarily used for melting and conveying polymers to extrude them into continuous shapes, whereas twin screw extruders are used for melt-mixing polymers with additional materials (pigments, fillers, reinforcers, and APIs), and for devolatilization.  In the production of pharmaceutical formulations, which require homo-geneous and consistent mixing of multiple formulation ingredients, a twin screw extruder is preferred because the rotation of the inter-meshing screws provides better mixing to produce a homogeneous solid containing finely dispersed API particles, or a solid-solution of API in polymer. This can improve the dissolution rate and bioavail-ability of poorly-water soluble API formulations. Uniformly distrib-uted API is also a pre-requisite for the production of drug-eluting devices with intra- and inter-batch reproducibility of drug-release ki-netics.
Melting is accomplished by fric-tional heating within the barrel, and for twin-screw extruders, as the materials undergo shearing between the rotating screws and between the screws and the wall of the barrel as they are conveyed. The barrel is also heated with heaters mounted on the barrel, or cooled with water. The barrel sec-tion temperatures are usually opti-mized so that the viscosity of the melt is low enough to allow con-veying down the barrel and proper mixing, while keeping tempera-tures low enough to avoid thermal degradation of the materials. 
The screws of a twin screw extruder are usually to provide different types of mixing and conveying conditions at various zones in the barrel. During product development, modular screws with multiple ele-ments  fitted on a common shaft, allow the tailoring and optimization of the screw design for each product. Sections of the screw can be designed to perform particle size reduction, mixing and conveying functions. The length of the screw in relation to the barrel diameter (the L/D ratio) is chosen to optimize the degree of mixing and the number of zones required to achieve the final product char-acteristics. An example of a complete modular screw. Single piece production screws may be built to the same design as the development screws, but are easier to clean for cGMP compliance.

HME (Hot Melt Extrusion) Related Features

Rotation of the screws,There are two families of twin screw extruders,material selection is critical in the development of a successful product...
Hot Melt Extrusion
Rotation of the screws creates distributive and dispersive mixing. Distributive mixing maximizes the division and
recombination of the materials while minimizing energy input by mixing with low extensional and planar-shear effects. This uniformly blends the materials but does not significantly reduce dispersed material particle size and yields minimal thermal and shear degradation of sensitive materials.
Dispersive mixing applies extensional and planar shear fields to break the dispersed materials to smaller size, ideally using energy at or slightly above the threshold  level needed to break them down.
The use of different mixing elements allows the twin screw extruder to perform both particle size reduction and mixing so that the APIs can be incorporated into
the polymer in dispersed form or, if the API solubility in the polymer is high enough, in dissolved form. Since the extrudate cools rapidly on exiting the extruder, any API  that is dissolved in the polymer at the mixing temperature may be unable to recrystallize on cooling, leading to supersaturated solid solutions. In such cases stability of the product must be closely followed as recrystallization of the API over long time-scales is possible, especially at elevated storage temperatures and high API loadings, and may impact the shelf life of the final product.
There are two families of twin screw extruders: high-speed energy input (HSEI) twin-screw extruders, which are primarily used for compounding, reactive processing and/or devolatilization, and low-speed late fusion (LSLF) twin-screw extruders, designed to mix at low shear and pump at uniform pressures.  Screws may be co-rotating (self-wiping), or counter rotating (calendar gap),with most extruders used for mixing being co-rotating.
Different types of exit dies are used to shape the extrudate to the desired profile.  These dies include sheet and film dies used in transdermal film applications, strand dies used for medical tubing and some drug-eluting devices, shape dies used in blow moulding, and co-extrusion dies used in reservoir device designs.  Different downstream auxiliary components are also used in the finishing process, including water baths and air knives for cooling, conveyor belts for moving the extruded product from the die to the end of the line, strand-cutters for cutting the extrudate into tubing or rods, and spoolers for extrudate collection. Pelletizers are used for cutting the extrudate into smaller pieces for direct capsule filling and in the case of some devices for injection molding to form the final product.  
As with any dosage form, material selection is critical in the development of a successful product. For most applications, the polymer should be thermoplastic, stable at the temperatures used in the process, and chemically compatible with the API during extrusion. For solid oral dosage forms, water soluble polymers are usually chosen from among polymers already used in pharmaceutical products such as poly(ethylene glycol) and poly(vinylpyrrolidinone). With the increased interest in using HME for pharmaceutical products, major polymer suppliers are also beginning to offer polymers specifically designed for pharmaceutical applications. For drug-eluting devices, the polymers are generally water-insoluble, and the majority of products under development use either ethylene vinyl acetate copolymers (EVAs) or polyurethanes.
HME allows the API to be mixed with the polymer under the minimum of shear and thermal stresses and hence with the formation of minimal process-related API degradants. Antioxidants are often included within the formulation, and the short residence time in the barrel (typically on the order of minutes) also helps to minimize thermal degradation especially compared to batch mixing and other compounding processes. 
One strategy for controlling drug elution kinetics from devices such as intravaginal rings involves an extension of the simple extrusion technique. Simultaneous extrusion of a drug-loaded core strand with a release-controlling polymer sheath that encapsulates the core in a single co-extrusion process produces a two-layer core-sheath strand. A specially designed extrusion head is fed by two perpendicular extruders – one supplying the core composition, the other supplying the sheath material. The core-sheath strand is cut and the ends connected to make the final device.
HME provides product developers of medical devices, dissolving oral dosage forms and drug-eluting devices with a process option that maximizes API mixing with polymer, while minimizing API degradation, and even opens the door to products that cannot be prepared by other means.

Tube Plastic Extrusion Machine Process

Plastic extrusion of tube, hose and pipe is a steady-state process for converting a thermoplastic raw material to a finished or near-finished annular product. The raw material is usually in the form of plastic pellets or powder. The conversion takes place by forming a homogeneous molten mass in the extruder and forcing it under pressure through an extrusion die orifice that defines the shape of the product's cross section. The formed material, or extrudate, is cooled and drawn away from the die exit at a controlled rate. The extrudate can then be wound on a spool, cut to a specified length, or directed into another in-line process.
Plastic Extrusion Machine Process
By contrast with injection molding or blow molding, which are cyclic processes, extrusion is a steady-state process. This steady-state characteristic produces some unique benefits and challenges as a manufacturing process. Extruded products are very long and continuous, and have a cross section that is usually constant with respect to the axis or direction of production. Injection-molded products are discrete items with varying cross sections in each axis. The fact that the extrusion of tube, hose and pipe is a steady-state process will be discussed in many areas of this article.The terminology used for describing tube, hose and pipe varies from industry to industry.
Equipment
The major components of a tube, hose, or pipe extrusion system are often divided into components that are upstream of the extruder, the extruder itself, and those that are downstream of the extruder. Upstream components include resin handling equipment for conveying the raw materials to the extruder, and resin conditioning equipment that will pre-heat the resin, or control the moisture content of the resin before it enters the extruder feed throat. The downstream components include a molten resin filter or screen changer, the extrusion die assembly, the quench or vacuum sizing tank, any product measurement instruments, the extrusion line speed controller in the form of a belt puller or nip roll set, and finally a product cut-off device or winding equipment. There are additional extrusion process methods that will take the extruded product directly into another in-line process for the addition of a reinforcement braiding or for the addition of other coatings or resin layers. 
Materials
Extrusion techniques can be used to process most thermoplastics and some thermoset plastics. The resins most commonly extruded for tube, hose and pipe products include high and low density polyethylene, polypropylene, polyurethane, polystyrene, fluoropolymers, PTFE, polyester, and flexible and rigid PVC. A characteristic that often differentiates extruded resins from injection-molded resins is the melt viscosity of the plastic at normal processing temperatures. Extruded plastics often have a higher melt viscosity, which allows the extrudate to retain the shape imparted to it by the die while the extrudate is in the quenching stages.
Combinations of various resins can be used to gain special physical, biological, or chemical properties. Many additives can be used during the extrusion process to enhance processing characteristics of the polymer or to alter product properties. Such additives include lubricants, thermal stabilizers, antioxidants, radiopacifying agents, and colorants. 
Processing Parameters
The parameters important to extrusion processing are similar to those of injection molding processes. Resin temperature, resin pressure, resin moisture content, screw speed, and screw motor amperage are usually controlled or monitored to provide a homogeneous melt at a controlled volumetric rate. Quenching temperature and the rate at which the extrudate is drawn are controlled or monitored to provide a controlled product size. Dimension measurements, using a variety of gauging methods, can be taken of the extrudate as it is produced. In contrast to injection molding, extrusion can vary the size of the final product without changing the die tooling. Common extrusion production tolerances or process consistency are held to within 1% of the nominal measured value. As these manufacturing processes involve steady state conditions, any action that can stabilize any parameter or condition is beneficial to the process.