The cost of extrusion machinery is moderate compared to other processes and offers good flexibility for product changes without the need to make major investments.The main restriction is that the products obtained by extraction must have a constant cross section at any point along their length, excluding all those with irregular or non-uniform shapes.
Most of the products obtained in an extrusion line require subsequent processes in order to properly enable the article, such as in the case of sealing and cutting, to obtain bags from tubular film or the formation of the union. .Thermoplastic sheets and films are produced by numerous processes, the most important of which are those based on the extrusion process. Roof Sheet Making Machine
It is responsible for laminating plastic raw materials, leaving them at the right point to be transformed.Depending on the materials, their configurations, the amount of material to be delivered, etc., appropriate extruders must be used for such purposes.
The plasticized material passes through a system of metal filters in order to eliminate solid contaminants that the raw material may contain.The filter system may be part of the extruder or a separate unit placed at the end of the extruder.This process is used to make plastic sheets or films that are too thick to be blown.
The head or flat die is considered the most important piece of the flat sheet extrusion line, since it has the function of taking the flow of plasticized materials coming from the extruder, and forming the sheet in fractions of a second, giving the main characteristics to the formed material such as the desired width, thickness range, uniformity in the product profile, homogeneity in the distribution of the plasticized material, and others.
The most important thing to highlight is that if the head or die is not efficient in the effective formation of the product, it is almost impossible to correct the deficiencies originating in this head.There are three types of sockets used: T-shaped, clothespin and fishtail.The purpose of these dies is to reorient and guide the flow of polymer melt from a single circular extruder outlet to a smooth, flat flow.
In both types of dies a constant and uniform flow must be ensured throughout the entire cross-sectional area of the die.Sheets and films are produced in various thicknesses by conventional extrusion, using a die whose opening is in the shape of a thin slit.The slit can be up to 3 m long with a width close to 0.04 mm.
A difficulty with the extrusion method is the uniformity of thickness across the width of the material.This is due to the drastic change in shape that the polymer melt undergoes during its passage through the die and to variations in temperature and pressure in the die.
The die includes a distributor conduit that spreads the polymer melt laterally before it flows through the slit.A difficulty with the extrusion method is the uniformity of thickness across the width of the material.To achieve high production speeds, it is necessary to incorporate efficient cooling and film collection methods into the extrusion process;This is achieved by immediately driving the extrusion into a water quenching bath or onto cooled rollers.
A typical sheet extrusion line is schematized below.
Co-extrusion is often used to apply one or more layers on top of a base material to obtain specific properties, such as UV absorption, texture, oxygen barrier or energy reflection.
As technologies evolved and markets demanded better materials, co-extrusion was found as a way to “unite” different types of plastic materials (each with different characteristics) in such a way as to be able to add these characteristics to give it to the final product qualities never before achieved.
Sheets or sheets are materials with a thickness between 0.5 mm to about 12.5 mm and are used for products and material for thermoforming.The film refers to thicknesses below 0.5 mm.Thin films are used for packaging material, bags…
The flat die extrusion process offers some variants with respect to blown film extrusion, being the method to obtain sheet for applications such as thermos and sheets of various thicknesses for various uses.
More than half of the films produced today are made of polyethylene, mostly low density.Polypropylene, polyvinyl chloride and regenerated cellulose (cellophane) are other widely used materials, all of which are thermoplastic polymers.
The chilled roller method appears to be the most commercially important.The low temperatures of the rollers cause rapid cooling and solidification in extrusion, in fact, the extruder serves as a feeding device for the cooling rollers, but these are the ones that actually form the film. The process is notable for its high speeds of production up to 5 m/s.
Cooling is normally by pulling through a set of cooling rollers (or calender).In sheet extrusion, these rollers not only provide the necessary cooling, but also determine the sheet thickness and surface texture.
In addition, close tolerances in film thickness can be achieved.Due to the cooling method used in this process, it is known as coolant roll extrusion.When the material comes out with the main characteristics, it is necessary to finish forming it definitively to give the final surface and body characteristics to the product.
For this, the grille has an important role.The calender is a set of several tempered rollers.Depending on the type of process and material, these rollers can form figures in their arrangement, for example they can be arranged vertically, horizontally, inclined, forming an L, etc.
The flat sheet passes in the spaces between the rollers and rests on their surfaces to achieve a final formation of the surfaces at the same time that said product finishes cooling to also obtain the final dimensional characteristics.
Depending on the thickness of the material, the last step of this process may be:
Normally bioriented polypropylene is produced in flat die extruders, where biorientation can be achieved, which is a stretching of the film during the process in both directions (transverse and longitudinal).
Biorientation is what, to a certain extent, gives the properties to the material.
Stretch Oriented in Machine Direction.It consists of a preheating roller, stretching and tempering roller.Stretching is achieved by passing the film through this series of rollers rotating at different speeds.
Orientation Stretch in Transverse Direction.It consists of a parallel preheating zone, divergent stretching zone, convergent fixation zone, neutral zone and cooling zone.
In BOPP extrusion lines, the formed film after being stretched longitudinally is held by clips or pressure jaws from the edges and forced to transverse stretch, as it advances, with the addition of heat.
Another way to lower production costs is by using recovered materials in the central layer of a co-extrusion.The center layer can become the layer that makes up the majority of the thickness of the material (if the final product allows it) so the final cost of the material could be substantially lower.
A layer of recovered material within a co-extrusion must meet certain requirements such as the recovery process must be efficient, the origins of the recovered materials must be known and they must also be properly classified, in many countries food legislation does not allows the recovered materials to be the outer layers of a material to avoid contact with food products, etc.
The products found on the market, transformed by the extrusion process, are: Tubular film
The sheets and films obtained by extrusion in flat die have a large number of applications: plastic films in general, stretch film for use in packaging, shower curtains, barrier materials for packaging of food products, sheets for thermoforming of disposable products, thermoform surfaces, dressings and covering elements for medical uses, plastics for coating on others, magnetic tapes, packaging for electronics, sheets for coating on plasmas and LCDs, exterior coverings of constructions, covering sheets for windshields, blister packs for medicines and products in general, advertising panels, corrugated alveolar plastic, sheets for bookstore products, plastic identification cards, exhibition panels, etc.
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