WHAT IS EXTRUSION ?

 WHAT IS EXTRUSION?

                             DO WE KNOW? LET US TRY TO KNOW.

Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters comprehensive and shear stresses. It also forms finished parts with an excellent surface finish.

Extrusion may be continuous theoretically producing indefinitely long material or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.

Commonly extruded materials include metals, polymers, ceramics, concrete and foodstuffs.

Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. 

The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die.

In 1797, Joseph Bramah patented the first extrusion process for making lead pipe. It involved preheating the metal and then forcing it through a die via a hand driven plunger. 

The process wasn't developed until 1820 when Thomas Burr constructed the first hydraulic powered press. At this time the process was called squirting. In 1894, Alexander Dick expanded the extrusion process to copper and brass alloys.

The process begins by heating the stock material (for hot or warm extrusion). It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterward the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.

The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts. The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded. There is a crossover point where roll forming becomes more economical.

Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "indirect extrusion”. The position of the press, either vertical or horizontal. The type of drive, either hydraulic or mechanical. The type of load applied, either conventional (variable) or hydro-static.

There are many different variations of extrusion equipment. They vary by four major characteristics:

A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (often used for steel and titanium alloys), oil pressure (for aluminium), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.

There are several methods for forming internal cavities in extrusions. One way is to use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel, also known as a German type, means it is integrated into the dummy block and stem. 

A floating mandrel, also known as a French type, floats in slots in the dummy block and aligns itself in the die when extruding. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram. The solid billet could also be used with a spider die, porthole die or bridge dies. 

All of these types of dies incorporate the mandrel in the die and have "legs" that hold the mandrel in place. During extrusion the metal divides and flows around the legs, leaving weld lines in the final product. 

Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated.

The major disadvantage of this process is that the force required to extrude the billet is greater than that need in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Because of this the greatest force required is at the beginning of process and slowly decreases as the billet is used up. At the end of the billet the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet (called the butt end) is not used for this reason.

In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a "stem" which has to be longer than the container length. The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container the frictional forces are eliminated. This leads to the following advantages:

A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections.

There is less of a tendency for extrusions to crack because there is no heat formed from friction the container liner will last longer due to less wear.

The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.

The disadvantages are:

Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this the billets may be wire brushed, machined or chemically cleaned before being used.

This process isn't as versatile as direct extrusions because the cross-sectional area is limited by the maximum size of the stem.

In the hydro-static extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydro-static medium. 

The fluid can be pressurized two ways:

Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container.

Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.

The advantages of this process include:

No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures.

Usually the ductility of the material increases when high pressures are applied.

An even flow of material.

Large billets and large cross-sections can be extruded.

No billet residue is left on the container walls.

The disadvantages are:

The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects.

Containing the fluid under high pressures can be difficult.

Most modern direct or indirect extrusion presses are hydraulically driven, but there are some small mechanical presses still used. Of the hydraulic presses there are two types: direct-drive oil presses and accumulator water drives.

Direct-drive oil presses are the most common because they are reliable and robust. They can deliver over 35 MP a (5000 psi). They supply a constant pressure throughout the whole billet. The disadvantage is that they are slow, between 50 and 200 mm/s 

Accumulator water drives are more expensive and larger than direct-drive oil presses, and they lose about 10% of their pressure over the stroke, but they are much faster, up to 380 mm/s . 

Because of this they are used when extruding steel. They are also used on materials that must be heated to very hot temperatures for safety reasons.

Hydro-static extrusion presses usually use castor oil at pressure up to 1400 MP a. Castor oil is used because it has good lubricity and high pressure properties.

Surface cracking - When the surface of an extrusion splits. This is often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die.

Pipe - A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often caused by high friction or cooling of the outer regions of the billet.

Internal cracking - When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydro-static tensile stress at the center-line in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen)

Surface lines - When there are lines visible on the surface of the extruded profile. This depends heavily on the quality of the die production and how well the die is maintained, as some residues of the material extruded can stick to the die surface and produce the embossed lines.

Plastics extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fiber-reinforced tubes) the extrude is pulled through a very long die, in a process called protrusion.

A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films.

The design of an extrusion profile has a large impact on how readily it can be extruded. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section; this is called the circumscribing circle. This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle 60 cm diameter circumscribing circles for aluminum and 55 cm . diameter circles for steel and titanium.

The complexity of an extruded profile can be roughly quantified by calculating the shape factor, which is the amount of surface area generated per unit mass of extrusion. This affects the cost of tooling as well as the rate of production.

Thicker sections generally need an increased section size. In order for the material to flow properly legs should not be more than ten times longer than their thickness. If the cross-section is asymmetrical, adjacent sections should be as close to the same size as possible. Sharp corners should be avoided; for aluminum and magnesium the minimum radius should be 0.4 mm and for steel corners should be 0.75 mm  and fillets should be 3 mm . 

The following table lists the minimum cross-section and thickness for various materials.

Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. 

Extrusion produces items such as pipe/tubing, weather stripping, fence, deck railing, window frames, adhesive tape and wire insulation.

In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder.

 Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.

The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200 °C  to 275 °C depending on the polymer). 

In most processes, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.

Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in heater jackets are employed, and they generally use a closed loop of distilled water in heat exchange with tower or city water.

At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MP a). 

The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be 'tweaked' by varying screen pack composition (the number of screens, their wire weave size, and other parameters). This breaker plate and screen pack combination also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory".

After passing through the breaker plate molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.

The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.

Sometimes on the same line a secondary process may occur before the product has finished its run. 

In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.

There are five possible zones in a thermoplastic screw. Since terminology is not standardized in the industry, different names may refer to these zones. Different types of polymer will have differing screw designs, some not incorporating all of the possible zones.

A simple plastic extrusion screw

Most screws have these three zones:

Feed zone. Also called solids conveying. This zone feeds the resin into the extruder, and the channel depth is usually the same throughout the zone.

Melting zone. Also called the transition or compression zone. Most of the resin is melted in this section, and the channel depth gets progressively smaller.

Metering zone. Also called melt conveying. This zone, in which channel depth is again the same throughout the zone, melts the last particles and mixes to a uniform temperature and composition.

In addition, a vented (two-stage) screw will have:

Decompression zone. In this zone, about two-thirds down the screw, the channel suddenly gets deeper, which relieves the pressure and allows any trapped gases (usually moisture or air) to be drawn out by vacuum.

Second metering zone. This zone is like the first metering zone, but with greater channel depth, and repressurizes the melt to get it through the resistance of the screens and the die.

Often screw length is referenced to its diameter as L:D ratio. For instance, a 6-inch (150 mm) diameter screw at 24:1 will be 144 inches (12 ft) long, and at 32:1 it is 192 inches (16 ft) long. An L:D ratio of 24:1 is common, but some machines go up to 32:1 for more mixing and more output at the same screw diameter. Two-stage (vented) screws are typically 36:1 to account for the two extra zones.

Each zone is equipped with one or more thermocouples or RTDs in the barrel wall for temperature control.

Geometrical possibilities.

There are many geometrical possibilities when using extrusion. Thin film (flat or tubular) is the most common product. Other extruded products include pipe and tubing, coated paper or foil, monofilaments and textile fibers, flat sheet (anything over 0.010 inch (0.25 mm)), wire and cable covering, and a great variety of profiles such as window frames, gaskets and channels, and house siding. The products can be cut to length or rolled up as needed.

Typical extrusion materials.

Typical plastic materials that are used in extrusion include but are not limited to: polyethylene (PE), polypropylene, acetal, acrylic, nylon (polyamides), polystyrene, polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS) and polycarbonate.

Heet/film extrusion

Blow extrusion of plastic film

For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calendar or "chill" rolls), usually 3 or 4 in number. Running too fast creates an undesirable condition called "nerve"- basically, inadequate contact time is allowed to dissipate the heat present in the extruded plastic. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.....

Often co-extrusion is used to apply one or more layers on top of a base material to obtain specific properties such as UV-absorption, soft touch or "grip", matte surface, or energy reflection, where it is needed : on the surface.

A common post-extrusion process for plastic sheet stock is nonperforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Orientation (i.e. ability/ available density of the sheet to be drawn to the mold which can vary in depths from 1 to 36 inches typically) is highly important and greatly affects forming cycle times for most plastics.

Thermoforming can go from line bended pieces to complex shapes, which often look like they have been injection moulded, thanks to the various possibilities in thermoforming, such as inserts, undercuts, divided moulds.

Plastic extrusion onto paper is the basis of the liquid packaging industry (juice cartons, wine boxes...); usually an aluminum layer is present as well. In food packaging plastic film is sometimes metallised, see metallised film.

Blown film extrusion

The manufacture of plastic film for products such as shopping bags is achieved using a blown film line.

This process is the same as a regular extrusion process up until the die. The die is an upright cylinder with a circular opening similar to a pipe die. The diameter can be a few centimetres to more than three metres across. The molten plastic is pulled upwards from the die by a pair of nip rolls high above the die (4 meters to 20 meters or more depending on the amount of cooling required). 

Changing the speed of these nip rollers will change the gauge (wall thickness) of the film. Around the die sits an air-ring. The air-ring cools the film as it travels upwards. In the centre of the die is an air outlet from which compressed air can be forced into the centre of the extruded circular profile, creating a bubble.

This expands the extruded circular cross section by some ratio (a multiple of the die diameter). This ratio, called the “blow-up ratio” can be just a few percent to more than 200 percent of the original diameter. The nip rolls flatten the bubble into a double layer of film whose width (called the “lay flat”) is equal to ½ the circumference of the bubble. This film can then be spooled or printed on, cut into shapes, and heat sealed into bags or other items.

An advantage of blown film extrusion over traditional film extrusion is that in the latter there are edges where there can be quality (thickness,..) variations.

Over jacketing extrusion In a wire coating process, bare wire (or bundles of jacketed wires, filaments, etc.) is pulled through the center of a die similar to a tubing die. Many different materials are used for this purpose depending on the application. Essentially, an insulated wire is a thin walled tube which has been formed around a bare wire.

There are two different types of extrusion tooling used for coating over a wire. They are referred to as either "pressure" or "jacketing" tooling. The selection criteria for choosing which type of tooling to use is based on whether the particular application requires intimate contact or adhesion of the polymer to the wire or not. If intimate contact or adhesion is required, pressure tooling is used. If it is not desired, jacketing tooling is chosen.

The main difference in jacketing and pressure tooling is the position of the pin with respect to the die. For jacketing tooling, the pin will extend all the way flush with the die. When the bare wire is fed through the pin, it does not come in direct contact with the molten polymer until it leaves the die. For pressure tooling, the end of the pin is retracted inside the crosshead, where it comes in contact with the polymer at a much higher pressure.

Tubing extrusion

Extruded tubing process, such as drinking straws and medical tubing, is manufactured the same as a regular extrusion process up until the die. Hollow sections are usually extruded by placing a pin or mandrel inside of the die and in most cases positive pressure is applied to the internal cavities through the pin.

Tubing with multiple lumens (holes) must be made for specialty applications. For these applications, the tooling is made by placing more than one pin in the center of the die, to produce the number of lumens necessary. In most cases, these pins are supplied with air pressure from different sources. In this way, the individual lumen sizes can be adjusted by adjusting the pressure to the individual pins.

Co-extrusion

Co extrusion is the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which will extrude the materials in the desired form. This technology is used on any of the processes described above (blown film, overjacketing, tubing, and sheet). The layer thicknesses are controlled by the relative speeds and sizes of the individual extruders delivering the materials.

There are a variety of reasons a manufacturer may choose coextrusion over single layer extrusion. One example is in the vinyl fencing industry, where coextrusion is used to tailor the layers based on whether they are exposed to the weather or not. Usually a thin layer of compound that contains expensive weather resistant additives are extruded on the outside while the inside has an additive package that is more suited for impact resistance and structural performance.

Extrusion coating

Extrusion coating is using a blown or cast film process to coat an additional layer onto an existing roll stock of paper, foil or film. For example, this process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is tetrapak.

Compound extrusions

Compounding extrusion is a process that mixes one or more polymers with additives to give plastic compounds. The feeds may be pellets, powder and/or liquids, but the product is usually in pellet form, to be used in other plastic-forming processes such as extrusion and injection molding. Machine size varies from tiny lab machines to the biggest extruders in the industry, running as much as 20 tons per hour, as used by the chemical companies that make the base resins. Usually twin-screw extruders are preferred because they give better mixing at lower melt temperatures. Most of these have screws and barrels made up of smaller segments (mixing, conveying, venting and additive feeding) so that the design can be changed to meet the production and product needs. 

Single-screw extruders can be used for compounding as well, especially with appropriate screw design and static mixers after the screw. Selection of the components to be mixed (viscosities, additive carriers) is as important as the equipment. In the next edition we will meet with more information about extrusion technology.