Technical Info South Africa

Plastic Extrusion Process

Plastic Extrusion Process Information

Plastic Extrusion Process

In the extrusion of plastics the thermoplastic raw material in the form of small pellets is gravity fed from a top mounted hopper into the barrel of the plastic extrusion machine. Plastic additives such as colourants and UV stabilizers (in either liquid or pellet form) can be mixed into the plastic raw material before being fed into the extruder.

The raw material enters through the feed throat and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic pellets forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200 °C/400 °F to 275 °C/530 °F 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 to the front. This allows the plastic pellets 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 MPa). 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.

Screw design

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
Feed zone. Also called solids conveying. This zone feeds the resin into the extruder.
Melt zone. Also called the transition zone. The resin is melted in this section.
Pressurizing zone. Also called metering or melt conveying. This zone gives the plastic uniform pressure and flow characteristics.
Decompression zone. In this zone, the melt is unpressurized, allowing trapped gases (hydrochloride) to escape and be vented out.
Mixing zone. There are two types of mixing zone. They either distribute small particles evenly, or break large particles into small ones which can then be mixed.
Often screw length is referenced to its diameter in terms of an L:D ratio. For instance, a 6-inch (150 mm) diameter screw at 24:1 will be 144 inches (12 ft) long, and 192 inches (16 ft) at 32:1. In years past screw ratios of 24:1 were fairly common, but modern machines use 32:1, or higher ratios, which allow better mixing at higher throughput.

Each zone will be equipped with a one or more thermocouples or RTDs for temperature control.

Geometrical Possibilities
There are many geometrical possibilities when using extrusion as a method to process plastics. Simple round pipes and square shapes are common. More complex shapes, such as tracks and profiles, are also commonly produced by extrusion molding. Solid extrusions can be produced with a large thickness. Wall thicknesses of hollow extrusions typically vary from 0.005 in. to around 0.3 in. The Length is typically anywhere between 10 ft and 100 ft., although theoretically if you had enough room and material you could produce an extrusion with an infinitely long length.

Sheet Extrusion and Film Extrusion

For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender 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 to obtain specific properties such as UV-absorption, soft touch or "grip", matte surface, or energy reflection.

A common post-extrusion process for plastic sheet stock is thermoforming, 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.

Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like being 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 metres to 20 metres 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 “layflat”) 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.

Overjacketing 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

Plastic tubing, such as drinking straws and medical tubing, is manufactured by extruding molten polymer through a die of the desired profile shape (square, round, triangular). 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.

Sometimes 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

Coextrusion refers to 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, 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 rollstock 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.
Typical workpiece materials
Typical materials that are used in extrusion molding include but are not limited to: Acetal, acrylic, nylon, polystyrene (which are the best materials) and acrylonitrile butadiene styrene (ABS) and polycarbonate(which aren’t as moldable)

Info Courtesy Wikipedia

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