THE CRITICAL SQUEEGEE IN SCREEN PRINTING:
Considering that every screen printer’s goal should be to produce high quality images, and at the same time, optimize printing performance, one needs to pay attention to the squeegee as an important component of the process.
Of critical importance is squeegee selection, maintenance, and press settings involving angle, pressure and stroke.
In the printing process the squeegee has four primary functions:
- The forcing of ink into the mesh, keyed by mesh size and viscosity of ink.
- The maintaining of mesh contact with the substrate, keyed by screen tension and off-contact.
- The fitting of the mesh to the printed substrate surface, overcoming the substrate unevenness, hardness and roughness.
- The removal of excess ink from the screen, which influences the next printed layer of ink and the image.
Selecting a Squeegee
Factors to be considered are: type of material, durometer, blade profile and cost.
Material
Currently, urethane is the preferred material due to its superior abrasion resistance, performance with a variety of ink chemistry, and durability, combined with low cost. A variety of polyurethanes are available including (MDI) Methylenebisdiphenyl, and (NDI) Napthalene Diisocyanate. MDI is in greater use with excellent cost/performance benefits. NDI offers greater abrasion and chemical resistance at a higher cost.
Durometer
Durometer is a measurement of the hardness of a polyurethane squeegee material as measured on the Shore A scale. Squeegee blades may be constructed of single, dual or triple durometer layers. Single durometer blades have difficulty controlling deflection as pressure is applied. The triple durometer blade features a sandwich of a harder durometer layer surrounded by two softer layers, which affords optimum ink shear at high stroke speeds. Dual blades offer similar advantages, but best results are most often obtainable from the triple blade.
Profile
This characteristic describes the shape of the squeegee blade. Most common is the straight rectangular edge, but single and double bevel, bull nose and other shapes find use.
Cost
While cost of materials is always a factor in printing, the small cost of using the best performing squeegee will be more than offset with the performance and quality gains obtained.
Maintenance
Good squeegee maintenance is an important factor in being able to consistently reach high quality and performance goals. Yet, it’s easy and effective to achieve.
Cleaning
An aqueous cleaning solution that is nonflammable, non-hazardous, non-abrasive and friendly to the environment, can be effectively used. A re-circulating tank or parts washer is capable of making the job easier and will also isolate waste ink in one place. Cleanliness, involving squeegees, will result in improving printing performance, minimizing waste, and improve profitability. On the other hand, get sloppy, and produce costly, wasted, production. This occurs easily when a squeegee blade dirty with a small amount of ink from a previous job, contaminates the next job, shifting color or tinting a white. It doesn’t take much!
Sharpening
Two types of squeegee sharpeners are available, grinding wheel and knife-cutting. Grinders, available in a variety of manual and automatic sizes are the most commonly used. In contrast the knife type uses a razor or knife blade to remove a thin strip of the squeegee edge to produce a renewed sharp edge. A dull squeegee requires more pressure, and a slower stroke speed, to push ink through a screen, to effectively shear ink. This results in poorer print quality, and lost productivity. How is it determined that a squeegee can benefit from sharpening? It’s simple; just move a finger along the blade edge.
If the ridges of your finger print cannot be felt, blade sharpening is required. Replacement should not be overlooked as a means of improving printing. Squeegee materials deteriorate after time, and repeated use and repeated sharpening eventually removes too much material for the squeegee to be optimally effective.
Squeegee Printing/coating settings
Proper squeegee settings are essential to effective printing/coating.
Angle
Typically, the more perpendicular that the squeegee is positioned to the screen, the more ink that is deposited, and the lower the angle, the less ink that is deposited. The printer needs to seek a setting that will produce target print objectives with a regard for the substrate, ink, mesh and squeegee durometer being selected for use.
Pressure
The ideal is to print/coat with the least squeegee pressure, because heavy pressure just wears the screen, produces more dot gain, registration issues, and poor images.
Stroke
A stroke is defined as one complete movement of the squeegee across a screen. Screen printing utilizes two different strokes: the flood stroke and the print stroke. The flood stroke initiates the print process. It spreads ink across the screen in preparation for the print stroke, pre-loading the mesh openings with ink.
Once the mesh openings are filled, it takes very little effort on the part of the print stroke to transfer that ink onto the surface of the substrate. There is no standard print stroke speed. Stroke speed is a variable, depending on factors such as the viscosity of ink, thread count of the mesh, and the substrate. The only rule is that the print stroke has to allow the ink time to flow onto the substrate.
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