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The obvious objective of package printing is to create visibly identifable images, consistently, for a large number of impressions.
There are many types of printing methods that are widely used on plastic packaging materials, such as flexographic printing, rotogravure printing, and screen printing etc.
Flexographic printing or rotogravure printing has a broad range of application in plastic films and bags, while screen printing has a predominant application in plastic bottles and tubes. Here, we’ll describe rotogravure printing.
Principle
The rotogravure process, also called gravure, or intaglio, uses an engraved chromium-plated or copperplate roller that is also called high speed consecutive printing roller, electronic engraving roller, seamless printing roller, etc.
The plates can be made by chemical etching, electronic engraving, or direct digital laser engraving. The gravure image areas are etched onto the roller, while the non-imageareas remain smooth.
The depth of the etching has a close relationship with the intensity of the images and determines the thickness of the ink film.
Heavy ink applications can produce exceptionally bright glossy colors. The printing plate of rotogravure printing has two categories: rotogravure and engraved intaglio.
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Procedure
In rotogravure printing, the gravure roller rotates directly in the ink fountain, and the excess ink is wiped from the flat surface, the non-image area, with a flexible doctor blade.
The ink remaining in the etching, the image areas, is then transferred directly to the plastic material on the pressure of printing as it passes between the plate and the impression cylinders.
Characteristics
The advantages of rotogravure printing include: high printing quality, massiveness of ink layer, vivid color, high color saturation, distinctness of image gradation, high degree of automation, higher print speed and pressrun than any other types of printing, etc.
But rotogravure printing has a relatively high cost of plate-making and a longer cycle.
Therefore,rotogravure printing has an extensive application on the printing of luxury goods, and takes up a large market share in package printing.
Rotogravure printing is mainly used in three fields overseas. First is magazine and product catalogues printing industry. The second is the packaging printing industry. The third is the special service fields such as paper money printing, stamp printing, and decorating materials printing etc.
While in our county, rotogravure printing is mainly used in packaging printing and special service printing.
The familiar forming methods of plastic containers include extrusion, extrusion blow molding, injection molding, compression molding, thermoforming, and rotational forming etc.
Extrusion
In extrusion, materials are melted by heat and pushed forward by the action of the extruder barrel and the screw.
The action of the screw forces the molten plastic through a shaped orifice in a die placed over the end of the extruder. The shape of the die determines the profile of product produced.
Placing a suitable torpedo-shape in the die exit permits extrusion of a hollow pipe or tube.
Extruded profiles are cut to length, and when hollow profiles are fitted with end closures, they can be made into versatile, inexpensive packages
Extrusion blow molding
As the name implies, extrusion blow molding combines extrusion with a blowing step.
In this process, parison, a hollow plastic pipe or tube made by extrusion or injection molding, is placed into the blowing mold in a soft and formable state.
And then, air is blown into the hollow parison and expand the deformable parison to conform the blowing mold walls. The newly formed product is held in the blowing mold until it cools sufficiently to retain the profile of the product.
Injection molding
Injection molding is the leading method of manufacturing closures, wide-mouthed thermoplastic tubs, jewel boxes, and other complex dimensional shapes.
In injection molding, granular or powdery plastics are fed from the food hopper into the extruder barrel where they are melted by heat.
The molten plastics are injected into the closed mold through the nozzle at the front end of the extruder barrel by the thrust of the screw or piston. A cooling system in the mold causes the plastic to solidify.
Compression molding
Compression molding, one of the earliest shaping techniques developed to use for plastics, is primarily used to manufacture thermosetting plastics.
In this process, powdery, granular or fibroid plastic is fed into the cavity of the mold at the forming temperature.
Then, the mold is closed and the plastic is forced to fill the space in the mold by the closing pressure. The heat from the mold cures the plastic.
Thermoforming
Thermoforming method, belonging to post forming, is one of the manufacture techniques of plastic containers.
Thermoplastic sheet is completely widely used as the raw material in this process. Compared with injection molding and blow molding, thermoforming requires lower pressures and the machinery can be consequently less rigid and therefore less expensive to produce.
In thermoforming, thethermoplastic sheet material is heated to the point when it becomes soft and flexible, but below the temperature at which melt flow might occur.
The most common application of thermoforming is for various types of blister or clamshell display and product packaging, of which the wall is relatively thin.
Some of our familiar household plastic products are also manufactured by thermoforming, such as various cups, bowls, trays and plates etc.
Rotational forming
Rotational forming, also called rotational molding technique, does not require an extruder.
In this process, some measure of the liquid, or mushy, or floury polymer is fed into the mold that is then heated while being rotated in two axes.
The molten polymer flows and evenly coats the inside surfaces of the cavity of the mold. While still being rotated, the plastic is cooled and formed.
After opening the mold and removing the part, we can find the hollow container. Consequently, very large bins and bulk containers are completely made by rotational forming.
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