Here the coating is wiped through a pattern on a screen. Originally the screens were made of silk but now it is made of nylon fabric, polyester fabric or stainless steel mesh to get longer life and better reproducibility. The process is well suited for application of two or more colours and/or patterns depending on the number of heads added on the machine. It can be used with curved and flat surfaces.
Silk screen coating is now termed as screen printing because it is used to print bottles, moulded items, flat films and extruded shape. The basic components of the screen printing system are: 1. Screen, 2. Frame to hold the screen 3. Squeegee.
1. Screen: Nylon screens are preferable for plastic but with hot inks stainless steel must be used. Split screens are used for simultaneous printing of two colours.
2. Frames: Although most frames are made of wood, metal frames are desirable because the metal frames have longer life and they do not warp. Specially operated frames, stretching all the four sides of the screen simultaneously are available to increase the mesh life.
3. Squeegee: Its blades are best if they are made of resilient rubber or plastic. The shape of the printing edge of the squeegee determines the sharpness and deposit of ink. A double sided squeegee with a bevelled (slant) edge is used for direct printing on uneven surfaces, bottles or containers. Softer blades are recommended for rough surfaces and for depositing more colours. The blade should at least be 5 cm longer than the print. In high speed machine, the squeegee should print from either direction to prevent build up of ink on one side of the screen. Sometimes, a one way sweep is also necessary.
The squeegee is pulled across the screen so that the ink is forced through the mesh to make the print. Either screen frame or the squeegee can be moved.
Types of Ink:
1. Flame Dry Ink: It dries in seconds and do not require long dryers. They are under development and are used most widely in the dairy industry and are limited to PE.
2. Standard Inks or cold Inks: They are not to be heated before use. They are composed of a variety of material based on the type of material, gloss, product resistance and drying time. eg. PVC products require a vinyl base ink.
The ink fuses to the product in many cases; hence the adhesion in the screen printing is excellent.
Both PE and PP must be treated before being printed. Three methods are used:
1. Flame treatment: The most practical and widely used method.
2. Corona discharge and chemical methods are not widely used, but it has an advantage of not destroying the gloss of PP.
1. Nest carriers: to hold objects to be printed in a special manner like printing up to the edge of the part. A vacuum system may also be used to hold the part throughout the printing and feeding.
2. Static eliminators: are placed after the feeding mechanism and ahead of the printing head. Tinsel and tinsel bars, electric static eliminator and remotely controlled ionized air units which are expensive and efficient mechanical equipments are used. Permanent and temporary anti static compounds may also be used either in the resin itself or as a coating on the product.
3. Dryers: for semi automatic unit, a flat bed oven with circulating air, heated by gas or electricity is usually used. In automatic unit oven with special nests carrying the products through an up and down configuration are desirable.
4. Un scramblers are attached to orient the bottles before placing them in the detector.
5. Automatic registration in the printing station requires a notch in the part to match the registration mark in the printing station.
6. Expandable mandrels to hold pieces with irregular inner dimensions and soft plastic or open tubes are also incorporated.
7. Automatic feed and take off and step less variable speed drive is also available.