Polypropylene Boxes

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Printing Process

 

There are three basic forms of printing onto polypropylene:

Silkscreen Printing

Silkscreen printing is the process of printing an image onto a substrate by means of applying ink through a porous mesh. A significant characteristic of silkscreen printing is that the press can print on stocks of any shape, thickness and size.

Block prints (as opposed to toned photographic images) are most suited to the silkscreen printing process. Block prints are images such as company logos or text where solid lines are required rather than gradual tonal differences in colour shades. Half tone images (picture images) are frequently carried out with silk screen printing although there is a compromise with the definition of the print. When a half tone colour image is silk screen printed, the result is a series of different sized dots which come together to form an end result.

Silk screen printing is normally used where anything up to 4 colour block prints are required. Our automatic machines can cover as many colours and volumes as required.

The silk screen printing process has many advantages over other forms of printing such as lithoprinting. Set up times are generally quite short and therefore economical compared with other printing methods. In addition, economical set up times facilitate low production runs as well as large order runs.

The Process

The silkscreen printing process uses a fine nylon porous mesh stretched tightly over a frame made of wood or aluminium. Correct tension is essential for accurate colour registration.

The mesh is coated with a light sensitive emulsion or film, which - when dry - will block the holes in the mesh.

The image that needs to be printed is output to film either by camera or image-setter. This "film positive" and the mesh on the screen are sandwiched together and exposed to ultra-violet light in a lightbox.

The screen is then thoroughly washed down with a water jet which washes away all the light sensitive emulsion that has not been hardened by the ultra-violet light. This leaves you with an open stencil which corresponds exactly to the image that was supplied on the film. The screen is then secured onto the automatic printing machine and basic settings such as screen height and squeegee draw length are adjusted to suit.

The flat polypropylene box to be printed is placed in position under the screen and ink is placed on the top side of the screen -  the frame has an additional function in that it also acts as wall to contain the ink . A rubber blade gripped in a wooden handle (called a squeegee) is pulled across the top of the screen in the same way that a windscreen wiper would travel over glass. The ink is pushed through the mesh onto the surface of the flat polyprop box. The process is then repeated as the squeegee floods the screen again with a return stroke before printing the next impression.

At Polypropylene boxes, we use an automatic screen press which means that the printed product runs through a conveyor belt which carries the item into our drying oven or  UV curing system as the machine continues to print.

Many factors determine the quality of the impression made such as the composition of the squeegee, the size and form of substrate printed, the height of the screen and the angle, pressure, and speed of the squeegee drawing across the image.

With over 40 years experience in printing, our team can advise on all technical aspects of printing helping you to make the right choice for your desired polyprop box.

 

Digital Print

 

Digital print is now virtually as good as full four colour litho print and can produce very high resolution prints with precise dot placement accuracy resulting in vivid image quality.

This brilliant new service is ideal for proofing and small runs. It is easy and uncomplicated: no film, no plates, no make-ready, and fast turnaround times. You only order what you need, and when you need it. We archive your materials digitally, so re-ordering another batch is quick and easy.

Direct printing on opaque flexible PVC, opaque polypropylene with the option of a white underprint on the clear or frosted and addition of clear varnish if required, giving outstanding print quality.


 

Lithoprinting

 

Lithoprinting is the ideal printing process for the recreation of toned images and pictures. A characteristic of lithoprinting is that the result is a smooth contoured print which accurately reflects complicated photographic information in a precise manner. Set up times are more intense than with silk screen printing which means that the process is more viable for medium to large scale runs.

The Process

The lithoprinting process is based on the elementary principle that oil and water do not mix.

In lithography, flexible aluminium plates are mounted onto rollers which transfer the ink onto the polyprop substrate. The plates have a roughened texture and are coated with a photosensitive polymer emulsion.

The plates are prepared through exposure from a light source with the film on top of the plate. The emulsion is then chemically treated to remove the unexposed portions of the emulsion thereby leaving a positive image.

When the printing plate is made, the printing image is rendered grease receptive and hydrophobic, or water repelling. The non-printing areas are rendered hydrophilic, or water attracting, and ink repelling. The plate is mounted on the plate cylinder of the press which, as it rotates, comes into contact with rollers that have been treated with a dampening solution or water. This adheres to the rough, or negative portions of the image. The plate then comes into contact with the roller coated with ink, which adheres to the smooth, or positive portions of the image.

In order to cater for different thicknesses and surface textures of polyprop, the plate does not directly print onto the substrate material. Instead, a cylinder covered with a rubber surface, called a blanket, is rolled over the plate. The blanket squeezes away excess water, and picks up the ink. The cylinder is then rolled over the polyprop, transferring the ink. Because the image is first transferred to the blanket cylinder, we call this process “offset lithography” because the image is offset to the drum before being applied to the flat polyprop box material.

 

 

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