Pocono Mt. Screen Supply, Inc.

Squeegee Tips for Screen Printers

Introduction

The squeegee is certainly one of the most important tools used in the screen printing process. Unfortunately, it is also one of the most neglected. After investing countless sums of money on equipment, mesh, pre-press preparation, ink and substrate, the low-tech inexpensive squeegee is the primary factor that controls the ink deposit, affects the color and sharpness of the printed image, and ultimately controls the quality of the finished product. The squeegee alone forces the ink into the screen mesh, keeps the mesh in contact with the substrate, adapts the mesh to the shape of the substrate, and removes the excess ink from the mesh.

The purpose of this tech tip is to help you understand the choices available to you in the selection of the proper squeegee and how to properly and effectively use, care for, and maintain your squeegee once your selection is made.

Squeegee Selection

Pocono Mt. Screen Supply offers the following polyurethane squeegees:

  1. Single Durometer

    The single durometer squeegee blade is a single piece of polyurethane material designed to balance the squeegee's resistance to all the different inks available while still maintaining a high resistance to abrasion.

    Single Durometer squeegees are available as rubber-only or with a wood or aluminum handle. They are available in 60, 70, or 80 durometer in all standard and custom sizes.

  2. Triple Durometer

    In the “old days,” in order to prevent the squeegee from bending, printers would insert a strip of sheet metal between the back of the squeegee holder and the squeegee. This would allow the printer to utilize a soft blade that would conform to surface irregularities but still keep the upper portion of the blade from bending over. In order to solve this problem and avoid the use of sheet metal, the triple durometer squeegee blade was developed.

    The triple durometer squeegee is a “sandwich” type with two softer outside layers and a harder durometer in the middle. The inside hard layer prevents excessive bending while still allowing the edge to conform to surface irregularities.

    Pocono Mt. Screen Supply offers two types of triple durometer squeegee, 60/90/60 and 70/90/70, available rubber-only or with wood or aluminum handle in all sizes.

Squeegee Function

The squeegee has four primary functions:

  1. Forcing the ink into the mesh
  2. Keeping the mesh in contact with the substrate
  3. Adapting the mesh to the surface of the substrate
  4. Removing excess ink from the mesh

All of these functions are influence, to a greater or lesser degree, by the squeegee durometer (hardness), height (free height), profile, angle, length, speed, pressure, and edge condition.

Squeegee Durometer

The durometer of the squeegee is the measure of the squeegee's hardness on an arbitrary scale known as the A Scale (frequently referred to as the Shore A Scale). There is often confusion with terminology as the instrument used to measure the hardness is also called a “durometer.”

The durometer instrument or gauge has a small indenter pin that is pressed onto the surface of the material to be measured, and the resistance to that pin is shown on a dial gauge. The hardness of squeegee material usually falls between 50 and 90 with 50 being the softest and 90 being the hardest.

Generally, the substrate, mesh count, and screen tension will dictate your choice of durometer. The softer durometers will conform better to hard irregular surfaces, varying garment thickness, and various contours and will transfer more ink onto the print surface. Unfortunately they tend to bend due to high squeegee pressures and resolution will frequently suffer.

The harder durometers will generally wear better and resist solvents better. Finer detail and thinner deposits of ink result from use of the harder durometers. The increased pressure required to print at high speeds or with high opacity, high viscosity thick plastisol inks necessitates the use of the harder durometers.

The middle durometers are basically a compromise between the two extremes.

Polyurethane squeegees are frequently color-coded with a specific color representing a specific durometer, the obvious purpose being to facilitate the identification of a squeegee's hardness without the necessity of having a durometer gauge. Unfortunately, there is no standardization of colors throughout the industry and each squeegee manufacturer uses its own color code scheme.

Squeegee Angle, Pressure, and Speed

There is no hard and fast rule as to what is the best squeegee angle, squeegee pressure, or squeegee speed.

The optimum angle between the squeegee blade and the screen will depend upon the blade free height, printing pressure, mesh opening, durometer, blade profile, screen tension, off contact, type and viscosity of ink, and characteristics of the substrate. The setting of the proper angle will be determined by all of these different properties. If your square edge blade is held at exactly 90 degrees to the screen, you will most likely be unable to exert sufficient pressure to force any ink into the screen. If you reduce the angle too much, you will push too much ink through the screen.

The general rule to remember to obtain the optimum squeegee pressure is “less is better.” If all the variables described above are carefully considered and carefully set, the amount of printing pressure necessary to obtain the best results should be minimal. Aside from unwanted printing characteristics, excess squeegee pressure will cause excessive wear on your screens, rapid deterioration of your squeegee, and it will waste ink. The manual screen printer should also have a strong incentive to keep squeegee pressure at a minimum as studies have indicated that excess manual squeegee pressure is a factor in the development of carpal tunnel syndrome.

The rate of ink flow into the mesh is generally a function of the squeegee speed and most inks tend to print better with higher squeegee speeds. Too high a speed, however, may result in poor coverage. The optimum squeegee speed varies with the viscosity of the ink and the mesh being used and both factors must be considered to obtain the proper squeegee speed for the proper ink coverage. If the ink's viscosity is altered during production (due to manual thinning or heat generation) the squeegee speed may need to be adjusted.

Squeegee Storage

Most squeegee manufacturers rotationally cast their squeegee in a centrifuge so the squeegee already has a slight “coil” to it. Even if a manufacturer casts their squeegees in a flat mold, due to difficulties in shipping long straight lengths, virtually all manufacturers ship the squeegee to their dealers in a coiled form. By the time the squeegee reaches the printing press it will most likely have developed a slight coil. The cardinal rules of squeegee storage are:

  1. Do not store the squeegee in a roll but lay each length out on a flat surface.
  2. Do not allow the edge of the blade to come in contact with another edge or any other object.
  3. Store away from the sources of heat and in an area with fairly constant temperatures. While cold temperatures will not have as drastic of an effect as heat, extremes of hot and cold will affect the physical properties of the squeegee.
  4. Store away from sunlight and UV light producing operations. Do not store near welding operations.
  5. Do not store the squeegee in or around areas containing solvents. Solvents in the atmosphere will, over long periods of time, be absorbed into the squeegee.
  6. NEVER leave the squeegee soaking in ink or solvents.

Squeegee Maintenance

There are two causes of squeegee “failure,” both of which can be prevented by the proper selection of a squeegee and then the proper maintenance of that squeegee. The first is “excess wear” and the second is “chipping.”

  1. Excess wear is most frequently caused by:
    1. Too much squeegee pressure. Reduce the squeegee pressure.
    2. Incorrect free height. If your squeegee has been sharpened too many times it is too short to flex.
    3. Incorrect squeegee angle (too large or too small). If the angle is too large the squeegee may “chatter” which causes excessive wear. A too small angle results in a larger area of the squeegee edge coming in contact with the mesh.
    4. Old squeegee. All squeegee polymers lose some of their physical properties over time, even when they are not being used. If your squeegee is old, discard it.
    5. Improper rotation (see Squeegee Rotation section)
  2. Chipping (edge breakdown) is the breaking off of tiny pieces of the squeegee edge, leaving streaks in the image, and is most frequently caused by:
    1. Immersion in the ink for too long a period of time. All inks will, eventually, degrade the squeegee. Even water-based inks will affect polyurethane squeegees over time. In addition to squeegee rotation, squeegees must be thoroughly cleaned immediately after use. All ink must be completely removed from the blade and holder. Never soak squeegees in solvents to clean them. The squeegee will absorb the solvent and the possibility of chipping will be substantially increased.
    2. Old squeegee. See item 4 above.
    3. Improper hardness. Generally, the harder the squeegee the better resistance to solvents. If you can maintain your optimum printing characteristics with a harder squeegee, try 5 or 10 points higher durometer.
    4. Improper squeegee. Perhaps your squeegee polymer is not formulated to withstand the solvents or other chemicals in your ink. Try a tougher squeegee that is particularly formulated to resist very strong solvents and UV inks, such as Saati's Dura-Life Blades.

Squeegee Rotation should be in the forefront of any program designed to prolong and promote the useful life of a squeegee. What does “squeegee rotation” mean? If you leave your squeegee in production for too long a period it will suffer one or more of the failures listed above. To avoid this, you should keep one or more sets of squeegees for each printing station and replace each squeegee with a new one before the squeegee in production exhibits any of the possible characteristics of failure. The removed squeegee should be cleaned as described above and only put back in production after it has returned to its original state. The problems with this procedure are twofold:

  1. The cost involved in having more than one squeegee per print station. With proper maintenance and proper rotation, however, you will not have to replace your squeegee as frequently, ink costs will be reduced, production down time will be reduced, and print quality will be improved.
  2. How long is too long? There is no clear-cut formula to determine the time period to keep a squeegee in production nor the time period to allow a squeegee to rest. These periods will vary with each job. A general guideline to follow is to time the period the squeegee is in production until it begins to exhibit either of the two types of failure: excessive wear or chipping. Replace the squeegee and keep the new squeegee in production for at least one hour less than the first and determine if this solves the problem. Allow the removed squeegee (after proper cleaning) to rest for as long a time as is practical (24 to 48 hours at the ideal) and then time its next run to determine how it performs. Adjust the “rest” time in accordance with its performance.

Squeegee Sharpening

Squeegee manufacturers try to manufacture their blades with as sharp an edge as possible. However, while you may not be able to see it, the edge of the squeegee begins to wear from the very first impression. Eventually, this wear will cause the following conditions:

  1. The edge will become rounded, leaving a progressively heavier deposit of ink.
  2. The edge may become so highly polished it will tend to skip over the ink rather than spread it. This happens primarily with poster, water-based, or plastisol inks.
  3. The edge may become swollen and chip due to aggressive inks.
  4. The edge may become uneven due to wear at some spots and not others.

The simplest and most cost effective solution to these problems is to sharpen the edge.

There are two types of squeegee sharpeners available from a number of manufacturers. The newest type is a sharpener that slices the squeegee edge with a knife. The second utilizes a grinding wheel or belt.

The advantage of a knife type squeegee sharpener is that it provides a very sharp and smooth edge fast and without grinding marks. The smooth cut-surface may also enhance the squeegee's solvent resistance. Their disadvantage is that the units currently on the market are difficult to use, they remove too much material, and the uniformity and repeatability of the cut edges vary.

The advantages of a grinding type sharpener are their flexibility and ease of use. These sharpeners can provide profiles, remove small amounts of material, and can be highly accurate and repeatable. The disadvantage of this type of sharpener is that it does not produce as smooth an edge as the knife type without extensive polishing.

How often you can sharpen a squeegee depends on the free height of your new squeegee and the remaining usable height after multiple sharpenings. A general guideline (but not a hard and fast rule) is to sharpen the squeegee down to no less than 60% of its original free height.

How do you sharpen the squeegee?

  1. Never sharpen a squeegee that has just been removed from production. It may be solvent laden and swollen (even if not visible to the eye). Allow the squeegee to complete its “rest” period.
  2. With a “grinding type sharpener” remove large nicks or chips with a coarse grit and then fine polish with finer grit.
  3. How well the grinding sharpener performs is a function of the hardness of the squeegee, the coarseness of the grit, and the speed of the grinding wheel (or belt). Every squeegee can be sharpened but care must be taken to ensure that the temperature of the squeegee blade is not allowed to become so high that the blade melts.
  4. Remove as little material as possible. This is difficult with a knife type sharpener.

 

Pocono Mt. Screen Supply offers a variety of standard and custom size squeegees. They are available rubber-only, with wood or aluminum handles, or with our patented Power-Stroke handle, specially designed to reduce operator fatigue and prevent carpal tunnel syndrome. We also offer sharpening machines to keep your squeegees performing at their best.