Sheet/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 (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 they've 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 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
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.

  Coextrusion
Coextrusion 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, 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.

 Compound extrusions
A compound extrusion is an extrusion of composite materials. Usually twin-screw extruders are used because they give better conveyance characteristics and production rates. They also give a more uniform extrudate. When reinforcing fibers are mixed in a twin screw reduces screw wear because the fibers can be introduced later into the melt. Single-screw extruders are used for simple extrusions that have little variance in material formulation and viscosity.

About First-rate Mold Solution Co., Ltd.
First-rate Mold Solution Co., Ltd. in China, provide industry design,mold design,mold manufacturing,molding analysis,project management service.

 About www.firstratemold.com
our website has a lot of professional knowledges in mold industry in blog and news, also we are interested in CAD/CAM/CAE, such as AUTOCAD,UGNX, PRO/E,CATIA, CIMATRON, SOLIDWORKS, MOLDFLOW and so on,we welcome professional engineer,mold maker and someone who are interested in injection molding,plastic product,injection mold and mold industry to our section of blog and news,we would like to share with them