Advantages of the screen printing process

Advantages: Environmentally friendly, as there is no material compound with packaging. Cost-effective form of decoration High-quality look and feel, high opacity of the printing colours. Almost photorealistic print motifs can be realised. Printing costs fall with increasing production batch size.

Disadvantages of the screen printing process

Disadvantages: Print templates must be created before the screen is created. Screen adjustment per printing colour required before first print. New artwork and screens are required for print image changes.

Screen printing on plastic packaging

Sustainable and high-quality decoration

Screen printing is a widely used printing process for decorating and finishing plastic, glass and metal packaging. This process enables high-quality and cost-effective multi-coloured decoration of round objects (body printing) and flat surfaces (flat printing). Screen printing can be used to create almost photorealistic motifs, complex colour gradients and shading by screening and colour separation of print motifs.

Compared to other decoration and marking technologies, such as labelling, inmould labelling or sleeving, decoration using screen printing is environmentally and recycling-friendly. By applying the screen printing ink to the packaging, no material composites are created that are difficult, if not impossible, to separate during the recycling process. The screen printing ink dissolves during mechanical recycling and prevents the reprocessing of the recycled raw materials.

How does screen printing on plastic packaging work?

Flat and round body printing briefly explained.

Screen printing is a stencil printing process. The stencil is a fine-meshed screen mesh that is stretched over a frame. The screen mesh is coated and has permeable areas depending on the print image. Using a squeegee, screen printing ink, which is fed into the printing screen, is pressed through the open areas in the screen mesh onto the surface to be printed. The screen is then lifted off the printed surface and the ink is dried using heat or UV exposure. For multi-coloured print images, the required colours are applied one after the other to the surface to be printed. A separate printing screen is required for each colour.

Screen printing unit Round body printing Plastic packaging — Screen printing unit for round body screen printing for plastic packaging

Handling CNC screen printing machine — CNC screen printing machine for plastic packaging

Sequence of the screen printing process

The screen printing process essentially consists of five processes, whereby steps two to five must be carried out separately for each printing colour in the case of multi-colour prints:

1. preparation of the print image: The print image is created digitally, divided into individual colour layers and these are then printed onto a transparent film using high-density printer ink with an inkjet printer. The so-called screen printing film is created.

2. creating the printing screens: The screen frame, which is covered with a fine-meshed fabric, is coated with a light-sensitive emulsion. Once this has dried, the screen frame is placed on the screen printing film in an exposure unit. The coating hardens, with the exception of the areas covered by the screen printing film. After exposure, the coating is washed out of the unexposed areas. This creates the finished printing screen.

3. preparation of the screen printing ink: The required screen printing ink is mixed according to the required colour shades (Pantone, HKS or RAL).

4. screen printing process: The prepared printing screen is clamped into the screen printing machine. The appropriate mixed ink is poured into the screen and then pressed through the open areas of the screen onto the surface to be printed using the screen printing squeegee.

5. drying and curing: After each ink application process, the applied screen printing ink is dried or cured. Depending on the ink used, this is done by heat treatment (solvent-based ink) or by irradiation with UV light (UV-curing ink).

Screen printing machine for circular printing at H&K Müller — H&K Müller has modern CNC-controlled screen printing machines for round body printing

The structure and components of a screen printing machine

How does a screen printing machine work?

H&K Müller has a total of five screen printing machines for up to four-colour screen printing, which have been specially designed and built to meet the requirements for printing on plastic packaging. Essentially, screen printing machines consist of a feed and conveyor belt, pre-flaming, screen printing units, exposure stations (for UV-drying printing inks), a drying belt (for printing inks containing solvents) and an optional print image control system using a high-speed camera.

The injection-moulded parts and packaging to be printed are placed on a feed belt, which then transports the items to the screen printing machine. In the machine, the injection-moulded parts to be printed are transferred to a transport carousel or conveyor belt, depending on the article geometry. The transport unit then guides the items past the individual stations (pre-flaming, screen printing unit, exposure station, camera check) one after the other before ejecting the finished decorated parts for packaging.

During pre-flame treatment, the molecular bonds in the surface of the plastic to be printed are broken by means of flame pre-treatment in order to increase the adhesion of the printing inks to be applied afterwards.

The printing screen with the desired print motif is clamped in the screen printing unit . Using a squeegee, the screen printing ink in the screen is pressed through meshes in the printing screen onto the item to be printed.

The applied screen printing ink is then exposed to UV light in the exposure station and cured. In the case of solvent-based inks, curing takes place after printing in a drying belt tunnel.

Finally, the printed injection moulded parts are scanned in a camera inspection station using a high-speed camera and the images are compared with reference samples. Defective items are ejected through a compressed air nozzle.

It should be noted that a screen printing station and exposure unit are required for each printed colour.