29 May Measuring Fill Blade and Squeegee Pressure
Over the past several years I have been consulting with a large industrial screenprinter abroad. They are a highly respected company that not only is concerned with growing their brand and reputation, but actively working to assure the very best outcome on every job that they accept. They have a staff of engineers on staff to bring about control to the myriad variables of the screenprinting process. As a result, I am asked a great number of technical questions, so that the engineers are sure that they understand every aspect of the screenprinting process. A few weeks ago, we were having a conversation about squeegee pressure and the results of how it affects the printed image. As, you can imagine, this became a very lengthy conversation.
They had a job coming up that would require the same job to be set-up on multiple presses and wanted to assure that each press produced identical ink deposits. The first step was to assure that the fill blades and squeegee were set to proper specification as to angle, pressure and speed. The angle and speed were easily measurable and an SOP could be written that assured that all presses were in alignment with one another.
The pressure became a hot topic, as the graphic presses utilized the standard threaded rod adjustments for the downstop and the air was used to hold the blade against the mesh during the stroke. This is where things got a bit messy.
I submitted that the quickest and easiest way to measure the pressure at the point of contact with with the use of a Pesola Precision Pull-Scale. While there are a number of Force Gauges on the market, they would need a device that could be understood and easily used by a variety of press operators with different skill sets. The Pesola Precision Pull-Scale was the best option considering the large number of presses to be used, the skills of the operators and ease of use.
I felt that this information might be of use to a great number of others, who are searching for a better way of setting and measuring the pressure on press to ensure consistency, predictable and repeatable results.
I have included the comments from others at the end of the article, providing more information on the use of force gauges. If you have ever wanted to be a “fly on the wall” in a room filled with screenprinting guru, this is your lucky day. Not only will you learn about force gauges but a lot more about screenprinting in general.
Using a Force Gauge to Measure Fill Blade and Squeegee Pressure
A force gauge (also force gage) is a small measuring instrument that can be used across all segments of the screenprinting technologies to measure the force during a push or pull test. Applications exist in production, quality, and research and development. There are two kinds of force gauges available: mechanical and digital force gauges and a description of each is given.
Mechanical Force Gauges
A spring scale or spring balance or newton meter is a type of weighing scale. It consists of spring fixed at one end with a hook to attach an object to the other end. It works by Hooke’s Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. Therefore, the scale markings on the spring balance are equally spaced. A spring scale cannot measure mass, only weight.
A spring balance can be calibrated for the accurate measurement of mass in the location in which they are used, but many spring balances are marked right on their face “Not Legal for Trade” or words of similar import due to the approximate nature of the theory used to mark the scale. Also, the spring in the scale can permanently stretch with repeated use.
A spring scale will only read correctly in a frame of reference where the acceleration in the spring axis is constant (such as on earth, where the acceleration is due to gravity). This can be shown by taking a spring scale into an elevator, where the weight measured will change as the elevator moves up and down changing velocities.
If two or more spring balances are hung one below the other in series, each of the scales will read approximately the same, the full weight of the body hung on the lower scale. The scale on top would read slightly heavier due to also supporting the weight of the lower scale itself.
Spring balances come in different sizes. Generally, small scales that measure newtons will have a less firm spring (one with a smaller spring constant) than larger ones that measure tens, hundreds or thousands of Newtons or even more depending on the scale of newtons used. The largest spring scale ranged in measurement from 5000–8000 newtons.
A spring balance may be labeled in both units of force (pounds, liters) and mass (grams, kilograms). Strictly speaking, only the force values are correctly labeled. In order to infer that the labeled mass values are correct, an object must be hung from the spring balance at rest in an inertial reference frame, interacting with no other objects but the scale itself
Main uses of spring balances are to weigh heavy loads such as trucks, storage silos, and material carried on a conveyor belt. They are also common in science education as basic accelerators. They are used when the accuracy afforded by other types of scales can be sacrificed for simplicity, cheapness, and robustness.
A spring balance measures the weight of an object by opposing the force of gravity acting with the force of an extended sausage.
The first spring balance in Britain was made around 1770 by Richard Salter of Bilston, near Wolverhampton. He and his nephews John & George founded the firm of George Salter & Co., still notable makers of scales and balances, who in 1838 patented the spring balance. They also applied the same spring balance principle to steam locomotive safety valves, replacing the earlier deadweight valves.
A strain gauge is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
A strain gauge takes advantage of the physical property of electrical conductance and its dependence on the conductor’s geometry. When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end. Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten, changes that decrease its electrical resistance end-to-end. From the measured electrical resistance of the strain gauge, the amount of induced stress may be inferred.
A typical strain gauge arranges a long, thin conductive strip in a zig-zag pattern of parallel lines. This does not increase the sensitivity, since the percentage change in resistance for a given strain for the entire zig-zag is the same as for any single trace. However, a single linear trace would have to be extremely thin and hence liable to overheating (which would both change its resistance and cause it to expand), or would have to be operated at a much lower voltage, making it harder to measure resistance changes accurately.
An excitation voltage is applied to input leads of the gauge network, and a voltage reading is taken from the output leads. Typical input voltages are 5 V or 12 V and typical output readings are in millivolts.
Foil strain gauges are used in many situations. Different applications place different requirements on the gauge. In most cases the orientation of the strain gauge is significant.
Gauges attached to a load cell would normally be expected to remain stable over a period of years, if not decades; while those used to measure response in a dynamic experiment may only need to remain attached to the object for a few days, be energized for less than an hour, and operate for less than a second.
Strain gauges are attached to the substrate with a special glue. The type of glue depends on the required lifetime of the measurement system. For short-term measurements (up to some weeks) cyanoacrylate glue is appropriate, for long lasting installation epoxy glue is required. Usually, epoxy glue requires high-temperature curing (at about 80-100 °C). The preparation of the surface where the strain gauge is to be glued is of the utmost importance. The surface must be smoothed (e.g. with very fine sandpaper), deoiled with solvents, the solvent traces must then be removed and the strain gauge must be glued immediately after this to avoid oxidation or pollution of the prepared area. If these steps are not followed the strain gauge binding to the surface may be unreliable and unpredictable measurement errors may be generated.
Strain gauge based technology is utilized commonly in the manufacture of pressure sensors. The gauges used in pressure sensors themselves are commonly made from silicon, polysilicon, metal film, thick film, and bonded foil. Variations in temperature
Variations in temperature will cause a multitude of effects. The object will change in size by thermal expansion, which will be detected as a strain by the gauge. The resistance of the gauge will change, and resistance of the connecting wires will change.
Most strain gauges are made from a constantan alloy. Various constantan alloys and Karma alloys have been designed so that the temperature effects on the resistance of the strain gauge itself cancel out the resistance change of the gauge due to the thermal expansion of the object under test. Because different materials have different amounts of thermal expansion, self-temperature compensation (STC) requires selecting a particular alloy matched to the material of the object under test.
Strain gauges that are not self-temperature-compensated (such as isoelastic alloy) can be temperature compensated by use of the dummy gauge technique. A dummy gauge (identical to the active strain gauge) is installed on an unstrained sample of the same material as the test specimen. The sample with the dummy gauge is placed in thermal contact with the test specimen, adjacent to the active gauge. The dummy gauge is wired into a Wheatstone bridge on an adjacent arm to the active gauge so that the temperature effects on the active and dummy gauges cancel each other. (Murphy’s Law was originally coined in response to a set of gauges being incorrectly wired into a Wheatstone bridge.)
Temperature effects on the lead wires can be canceled by using a “3-wire bridge” or a “4-wire ohm circuit” (also called a “4-wire Kelvin connection”).
In any case, it is a good engineering practice to keep the Wheatstone bridge voltage drive low enough to avoid the self-heating of the strain gauge. The self-heating of the strain gauge depends on its mechanical characteristic (large strain gauges are less prone to self-heating). Low voltage drive levels of the bridge reduce the sensitivity of the overall system.
Errors and Compensation
Zero Offset – If the impedance of the four gauge arms are not exactly the same after bonding the gauge to the force collector, there will be a zero offset which can be compensated by introducing a parallel resistor to one or more of the gauge arms.
The temperature coefficient of the gauge factor (TCGF) is the change of sensitivity of the device to strain with a change in temperature. This is generally compensated for by the introduction of a fixed resistance in the input leg, whereby the effective supplied voltage will decrease with a temperature increase, compensating for the increase in sensitivity with the temperature increase. This is known as modulus compensation in transducer circuits. As the temperature rises the load cell element becomes more elastic and therefore under a constant load will deform more and lead to an increase in output; but the load is still the same. The clever bit in all this is that the resistor in the bridge supply must be a temperature sensitive resistor that is matched to both the material to which the gauge is bonded and also to the gauge element material. The value of that resistor is dependent on both of those values and can be calculated. In simple terms, if the output increases then the resistor value also increase thereby reducing the net voltage to the transducer. Get the resistor value right and you will see no change.
Zero shift with temperature – If the TCGF of each gauge is not the same, there will be a zero shift with temperature. This is also caused by anomalies in the force collector. This is usually compensated for with one or more resistors strategically placed in the compensation network.
Linearity is an error whereby the sensitivity changes across the pressure range. This is commonly a function of the force collection thickness selection for the intended pressure and the quality of the bonding.
Hysteresis is an error of return to zero after pressure excursion.
Repeatability – This error is sometimes tied-in with hysteresis but is across the pressure range.
EMI induced errors – As strain gauges output voltage is in the mV range, even μV if the Wheatstone bridge voltage drive is kept low to avoid self-heating of the element, special care must be taken in output signal amplification to avoid amplifying also the superimposed noise. A solution which is frequently adopted is to use “carrier frequency” amplifiers which convert the voltage variation into a frequency variation (as in VCOs) and have a narrow bandwidth thus reducing out of band EMI.
Overloading – If a strain gauge is loaded beyond its design limit (measured in microstrain) its performance degrades and can not be recovered. Normally good engineering practice suggests not to stress-strain gauges beyond ±3000 microstrain.
Humidity – If the wires connecting the strain gauge to the signal conditioner are not protected against humidity, such as bare wire, corrosion can occur, leading to parasitic resistance. This can allow currents to flow between the wires and the substrate to which the strain gauge is glued, or between the two wires directly, introducing an error which competes with the current flowing through the strain gauge. For this reason, high-current, low-resistance strain gauges (120 ohm) are less prone to this type of error. To avoid this error it is sufficient to protect the strain gauges wires with insulating enamel (e.g., epoxy or polyurethane type). Strain gauges with unprotected wires may be used only in a dry laboratory environment but not in an industrial one.
In some applications, strain gauges add mass and damping to the vibration profiles of the hardware they are intended to measure. In the turbomachinery industry, one used alternative to strain gauge technology in the measurement of vibrations on rotating hardware is the Non-Intrusive Stress Measurement System, which allows measurement of blade vibrations without any blade or disc-mounted hardware.
For measurements of small strain, semiconductor strain gauges, so-called piezoresistors, are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.
Nanoparticle-based strain gauges emerge as a new promising technology. These resistive sensors whose active area is made by an assembly of conductive nanoparticles, such as gold or carbon, combine a high gauge factor, a large deformation range and a small electrical consumption due to their high impedance.
In biological measurements, especially blood flow and tissue swelling, a variant called mercury-in-rubber strain gauge is used. This kind of strain gauge consists of a small amount of liquid mercury enclosed in a small rubber tube, which is applied around e.g., a toe or leg. Swelling of the body part results in stretching of the tube, making it both longer and thinner, which increases electrical resistance.
Fiber optic sensing can be employed to measure strain along an optical fiber. Measurements can be distributed along the fiber or taken at predetermined points on the fiber. The 2010 America’s Cup boats Alinghi 5 and USA-17 both employ embedded sensors of this type.
Microscale strain gauges are widely used in microelectromechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. As an example, airbags in cars are often triggered with MEMS accelerometers. As an alternative to piezo-resistant strain gauges, integrated optical ring resonators may be used to measure strain in Micro-Opto-Electro-Mechanical Systems (MOEMS).
Capacitive strain gauges use a variable capacitor to indicate the level of mechanical deformation.
Vibrating wire strain gauges are used in geotechnical and civil engineering applications. The gauge consists of a vibrating, tensioned wire. The strain is calculated by measuring the resonant frequency of the wire (an increase in tension increases the resonant frequency).
Simple mechanical types are used in civil engineering to measure the movement of buildings, foundations, and other structures. More sophisticated mechanical types incorporate dial indicators and mechanisms to compensate for temperature changes. These types can measure movements as small as 0.002 mm,
A digital force gauge is basically a handheld instrument that contains a load cell, electronic part, software and a display. A load cell is an electronic device that is used to convert a force into an electrical signal. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge converts the deformation (strain) to electrical signals. The software and electronics of the force gauge convert the voltage of the load cell into a force value that is displayed on the instrument.
Test units of force measurements are most commonly newtons or pounds. The peak force is the most common result in force testing applications. It is used to determine if a part is good or not. Some examples of force measurement: door latch, quality of spring, wire testing, strength, but more complicated tests can be performed like peeling, friction, texture.
Usage in the Screenprinting Technologies
While most types of force gauges can be used in various ways within the screenprinting technologies, particularly in research and development and laboratory work, most are out of budget for the average screenprinter who only wants an accurate way to take a measurement. The most common use is to measure the fill blade and squeegee pressure during the setup process to duplicate a known pressure that has proven to work well in similar circumstances.
If you have been using plastic strips placed between the mesh and the fill blade, or between the bottom of the mesh and the top of the substrate to measure the amount of force as a way of setting up your press you may have noticed the difficulty of getting precision “readings” between press operators. Fortunately, there is a precision way of achieving the goal.
The Pesola Precision Pull-Scale
The Pesola Model pes80010 is a 10-kilo scale with +/- 0.3% accuracy and provides readings in 100-gram increments. very nicely made of stainless and anodized aluminum. It is in Pesola’s Macro Line. You will need the scale with the “S” hook, rather than the crocodile clamp. You will also need a thin metal plate that is about 1-inch wide and of a length 5-inches longer than the distance from the squeegee to the far end of the screen. This plate must have a hole at one end to accommodate the S-hook.
The scales are used in a variety of industries. However, in the screenprinting technologies, you would set the tare screw to zero the device, then attach the S hook to the thin metal plate that is placed according to whether you are measuring fill blade or squeegee pressure. Then set the pressure on the fill blade or squeegee and pull on the D-ring of the scale, while reading the drag pointers on the scale. Note where the metal plate begins to slip and you have the pressure reading in both lbs and kg of force. This can help by eliminating the human error in judgment produced by the “feel” of the force needed.
While this instrument is the best value for those requiring measurement of force, it is seldom purchased in the screenprinting technologies as many do not understand the nuances of the requirement to measure. That is, you cannot control what you do not measure. It is possible that you may already have one or more of these in the machine shop, but if not, for more info go to http://www.pesola-scales.com/pesola-scale-macro-line-10kilogram.html online.
Ray Greenwood: Thanks for the mention, Dr. Hood! It ia a very nice tool. Worth every penny! There is an accessory that connects with these gauges that I just ordered. It is an attachment that connects to the pull gauge and turns it into a push/pressure dynamometer. For those who need this for other diagnostics!
Fantastic company and tools. Thank you for posting this here! Its one of the many simple tools that I recommend to my precision clients to purchase!
Ray Greenwood: And – to start an important discussion – the question at hand, is why you want an analog pull-gauge like this, which by the way is accurate to 0.3%, is because a digital readout pull gauge, only has maybe 2-3 digits. On a 10-kg scale, with increments of 1/100 of a kg, or even 1/10th. When you are pulling over X amount of seconds, the digital readout is changing 20-30 times per second. All you get is a visual “numbers generator”.
With an analog scale, you see the needle move from point A to point B. You get a visual graph. Digital has its uses. This is not one.
Bill Hood Consulting: That was the reason that I posted this information, Ray. As you will remember, it was my client who requested information on how to determine fill blade and squeegee pressure, so that they could maintain consistency in setups when printing on more than one press. They actually have dozens of ATMA’s and there are times when they want to print the same job on all presses at the same time to get the work out the door.
Of course, even the smallest printer will need to measure and record this information on a job ticket so that they can practice consistency, productivity, and repeatability on reprint jobs.
Rod S. Zuniga: Too tech for me. Let’s leave it at that. I don’t have to read this.
Bill Hood Consulting: Too tech? Screenprinting is actually the screenprinting technologies. The more thought that goes into the technical aspects of screenprinting, the more capable one becomes in producing consistent, predictable, and repeatable (CPR) in their execution. Of course, there are always those who will throw caution to the wind and remain hopeful of success. Those who do excel and become successful will, of course, read everything they can that will give them even a slight boost over those who simply don’t care to succeed.
Of course, success is a relevant term. Every human believes that they are successful, especially those that believe that the attainment of money and power is the definition of success. My hope is to inspire others to think about their particular version of success. Perhaps the one constant that most hold to is in the fulfillment of our work skills and that of others. To make a difference in the lives of others. To find and fulfill our purpose.
For me, success is focussing on increasing my own understanding of every aspect of my work, maximizing my potential. Success is not so much about my own accomplishments, but my ability to have an extraordinarily positive impact on the lives of others, that they too might be enriched and live a better life.
Ary Luiz Bon: Nice. If you measure it, you can talk about quality. If you don’t, you will have just an opinion (everybody has, hard to reach consensus).
Rod S. Zuniga: Thanks kuya Bill Hood as always great info. Appreciate it very much!
Joe Clarke: Ray that was an outstanding explanation, thank you and Bill kindly for sharing. It led to a question.
My immediate focus is on white ink for tees. Briefly, screens are tensioned and gapped with a digital meter for consistent dynamic tension at all points (as measured); 1+/- 1 N, EOMR +/- μm, RzS1 & RzS2. Minimum yield point and plastic viscosity on the ink. Zero measurable deflection on the platens. Custom blades to fit the dynamic tension for both fill and print. Strokes at 40“/sec with 10-psi. Running a single flash for white then CMYK wet-on-wet. Up to 85-LPI, we are ok but we need to do legitimate 122-Lpi. Therefore I am working to improve white critical surface tension and leveling, but, I believe the irregular surface of the 12 platens is the next obstacle.
Your thoughts, if you have time, please?
Richard Greaves: What always makes me cringe is printers and suppliers quoting a PSI pressure number.
As I read Ray Greenwood’s great post was how slip tests vary because of the other prime variables. People in my house rushed over to me as I yelled at the top of my lungs as I read his ”Simply put.. ” paragraph and the last sentence. As a disciple of Don Newman’s philosophy of standardized stable mesh tension, it was music to my ears. Ray worked with Don Newman also. I hope the world has been influenced by the ideas in that simple paragraph.
The very mention of PSI pressure without o/c, mesh tension measurements is information that isn’t useable or transferable to other setups.
I have documented changes in PSI pressure as a record of different results on press.
Certainly, especially after reading Ray Greenwood’s post – if you didn’t already know that every platen at a different height, means different o/c that means different force to overcome off-contact.
My initial observation about blades of a different length because of sharpening cause an infinite variance in the print system that nullifies durometer. From mistakes I’d made in my youth on race cars, I learned that removing 10% of the length of a spring doubles its spring rate. (Thank you Heinz of Concours Motors Glendale, WI).
Paul Cylenica from Saati visited in 1985 and was concerned that I hadn’t bought any more blades since my initial order. As a plastisol & water-based ink printer, inks didn’t degrade my blades and they were still all in use if they hadn’t been nicked by sloppy handling. I bought these DuroLife blades from Saati because they were color coded and I could see a wrong blade in a press from across the room. Because they were molded square they were as uniform as could be. A durometer test was of some worth because all the blade dimensions were the same. I know the blade lip created in the mold wasn’t as sharp for printing halftones, but I accepted that in exchange for standardized variables and the ability to test for out of standard blades, tension, and off-contact.
Print stroke speed was always a limit. As I learned about the SUPPOSED viscosity drop built into screenprinting inks, if an ink had a speed limit, I looked for replacements. I got Rich Hoffman to tell the story of his frustration with me on his first tech service visit on my netcast show. I was hot rodding circuit boards in heads to get faster stroke speeds – burning out brushes.
Standardized full coverage daylighting 7-feet off the floor, mesh tension provided by Don Newman and the factory equipment for blueprinting my presses was state of the art in the 1980s.
This was a great read. Simply put.
Rod S. Zuniga: Not only great read, but lots of info and details. Thanks much!
Joe Clarke: Thank you, Richard!
Bill Hood Consulting: Joe, I have always applauded your enthusiasm to solve problems.
However, some variables will always creep into the mix that makes perfection impossible. When printing on a smooth surface, you might get much closer to nirvana, but the roughness of the knitted t-shirt will always be between the mesh and the platen.
My thought is that the roughness of the substrate would affect the print, regardless of the smoothness of the platens.
Now, if you are speaking of the plane of the platens, that is a different story, which would easily be resolved with new platens and setting a Standard Operating Procedure in place that eliminated the uneven and often extreme squeegee pressure that is the norm in the textile segment of the screenprinting technologies.
Also, we need to address the handling of the platen, as they are easily moved out of plane by press operators who pull, push, tug, lean on and use a table, especially when the platens are still warm from flashing.
A client once asked me about a printed t-shirt that had an uneven ink deposit in a small area, which he referred to as a ghost image. He could not figure out why it was happening. When I asked if it was happening throughout the press run, he stated that it was not just one press run, but all of them, even with different screens on the press. I quickly surmised that it must be a press problem.
I asked him to photograph the uneven print area and email it to me. An hour later, I was on the phone with him to explain that he looked closely he would see that the image was that of a 9/16-inch wrench that was left on the platen and when the squeegee passed over it, it was pressed into the rubber surface of the platen, leaving a depression.
Thankfully, the imprint did not affect the metal platen and after he replaced the rubber surface the problem was resolved.
Joe Clarke: Thanks Bill, first thing Tuesday I’ll check for tools left on the platen before production!!! Great story!
I am working to keep to one white screen but can use a calendar head and with a custom fill blade at 40-inches/sec I can double stroke and not lose productivity. The white’s matting is very good and I am prototyping a couple different platens.
Wish me luck!
Rod S. Zuniga: So to all screenprinters out there nothing technical about screenprinting – Ha, ha!
Bill Hood Consulting: Joe, of course, I wish you luck and success. At 40-inches per second, you better have some good brakes on that squeegee! At 2.3 miles per hour, that is pretty fast. hahaha!
I am of the thought that there is still much work to do on perfecting the platens in use today. Another is the support system for the platen, which with a single, centered arm, just simply does not work well with the extreme pressure and misuse that press operators apply.
Joe Clarke: Bill, agreed on both the brakes and the platens Bill.
Don’t plan to work on brakes but believe the status quo is not even close to the “idealized” platen. I’ll let you know in a couple months if we have been on the right path.
Ray Greenwood: Richard Greaves Thank you!
There is something of very high importance you mentioned in your reply. The use of air pressure to create actual squeegee pressure is patently wrong.
Understand this. If you cannot visualize this, I will supply imagery later. The micrometer or threaded adjustment that allows the squeegee or fill blade to rise up and down, is the height/depth adjustment. This is called “down stop”
The air pressure is only the “muscle” that keeps the squeegee/fill blade in that exact mechanically set position, against the resistance of mesh tension and the hydrodynamic wedge force created by rapid compression of the ink in from of squeegee and fill blade, both of which try to drive the squeegee upward.
If the micrometer has been adjusted far enough down to allow air pressure alone to drive the squeegee into any level of pressure you want, then that means that the natural mechanical position set by the micrometer, for the squeegee edge, is below the surface of the pallet.
Think about that.
As you try to increase squeegee pressure, it will not be just increasing pressure to an exact pre-set point, that being the mesh stretched down and EOMR in contact with substrate with ONLY enough pressure to seal the image edge and transfer ink, it will be trying to push down to its mechanical stop point, below the pallet/substrate surface level.
This means that you have “0” absolutely “0”control of the interface pressure between the screen EOMR and substrate. The exact adjustment that prints versus one that crushes the threads into the ink patch, is a narrow band of about 1-2N/cm. No machine in the general screenprinting technoloigies, not even Theime (especially not Theime, which we can talk about if you like), has that level of control in either its air pressure or micrometers.
I have seen this and corrected this in many shops. For quite some time M&R has been teaching people to let the micrometer adjustment be loose, and use only air pressure to adjust squeegee pressure and position. It can work satisfactorily, if you call 18 to 20N/cm good tension. With high off-contact, you have enough print faults already that you do not notice the excessive squeeze, bleed and poor ink transfer and are probably double hitting already.
Move up in tension and precision level in work, and the pressure-only method creates a huge range of print issues. If you want to get into that range I will.
Joe Clarke: Understood Richard and Ray, thank you both!!
Mike Young: Three points I like to mention regarding this thread.
1. 40 in/sec squeegee travel burns out most drive motors plus additional stress placed on a short ramp up and decelerate distance outside the image area.
2. Theoreticalical dynamics of squeegee pressure influence is meaningless if tension is not close to that ideally recommended.
3. As for checking the distribution of squeegee pressure, it was once considered pressure readings be taken between the mesh and print-table since the pressure is erroneously determined more by off-contact distance and physical downward squeegee pressure – while actual psi gauge reading is relevant. Moreover, the test ought to be conducted without substrate/material, although a piece of film can be used to improve smoothness.
Ray Greenwood: Mike, I agree with that for the most part. However, the how of the technique really depends on what you are using this test for and how uniform your screen is to start with.
The primary point of this test is not mainly for finding out how much pressure you have under/on/over each inch of squeegee blade, unless you are using it for diagnostic purposes, i.e. blade distortion or holder distortion.
Most of my customers use this for getting to a very parallel set-up position because short of a handful of semiconductor targeted presses with machined platens with a known flatness per foot rating, and real micrometers and not just threaded rods, getting a flat set-up is tedious and time-consuming, and requires test printing and adjusting.
With catalyzed inks (which have a time factor limit) and/or conductive inks, at $900-$1200 per kilo, that’s playtime we opt not to have.
Greg Kitson: Thought provoking.
Bill Hood Consulting: Greg, I can’t take credit for the thought, as the products used in the screenprinting technologies (including the textile segment) for taking measurements have existed for some time. My only thought here was to utilize a more affordable device.
The 10-kilo scale should serve experienced screenprinters well, as the correct squeegee pressure should only be what is necessary to bring the bottom of the mesh into contact with the top of the substrate, which should not normally be over perhaps 23 pounds of pressure.
The Pesola scale is not actually reading the pounds of pressure, but the slippage of the material between two objects, which will be less. Yes, the slippage does equate to the pressure, but it is not equal to the pressure.
To measure the actual pressure you would need a Force Sensor at a higher cost. For example, a FlexiForce ELF Measurement System (https://www.tekscan.com/flexiforce-load-force-sensors-and-systems) is a complete load and force measurement system with multi-point sensing and high-speed capabilities, with a cost of $699.00 USD.
The FlexiForce sensor acts as a force sensing resistor in an electrical circuit. When the force sensor is unloaded, its resistance is very high. When a force is applied to the sensor, this resistance decreases. While, the resistance can be measured by a multimeter, then applying a force to the sensing area, but is not normally recommended for repeatable accuracy.
A Strain Gauge can also work but are more costly to integrate electronically or mechanically. And, the level of necessary skill and training to use a Strain Gauge is typically very high and they can be tedious to use.
Ray Greenwood: I use them in a lot of “technical”, ISO process-driven plants. They have a very narrow range of allowed adjustment on press. They use the pull scales to make sure the squeegee is set up exactly parallel to the mesh. They know the mesh and stencil is at an exact, to the micron thickness, and that surface profile is an exact to the micron texture with other gauges during coating. The pull test allows exact parallelism as a starting point when machine micrometers are less than perfect. This allows them to dial in adjustments during set up by making identical additions or subtractions on each micrometer when dialing in the final set up without violating ISO written process ranges.
Every job starts with the squeegee and fill blade parallel to the mesh with a nominal operating pressure. Written process allows X amount of “adjusting to taste” from that start point. More adjustment than that requires that you stop to find out why it required more adjustment, or else you are slapped with a non conforming process report (NCPR). This is time-consuming, and costly and is a legal document for your business process.
This is the way of the world in the business of producing life-saving devices and parts for machines that can fall out of the sky. It can also simplify set up in any precision screenprint operation.
Mark A. Coudray: Ray I’m curious. What is a typical +/- resistance range. I’m sure it varies with each coating material.
Ray Greenwood: Dr, Hood asked the same question, when he was wondering what range of gauge to purchase. The answer is a little deeper than you might think! Sorry, this may be a little long, but I think I need to answer as completely as I can.
There will be some variability in pull force due to print side RzS2 and how “rubbery” or how much traction either the substrate or the EOMR has, and whether the pull strip is on bare mesh or emulsion, but in general on a moderate to high tension screen (and tension is important and I will get back to it), about 3 to 4-kg on coarse mesh, like say 156-TPI and down, and about 2 to 3-kg on mesh in say the 230-TPI and up range.
The screen tension is a key factor. Why? Because if you are running excellent tension, not ultimate, but just excellent, say 27-28N/cm on a 305-TPI mesh (ultimate might be 30-32N/cm), your off-contact should be low and the elevated efficiency of the mesh (with normal inks) will require less squeegee pressure for ink transfer.
An example of this difference in high to low tension on the same mesh from my notebook: Typically that 305/32 with 27-28N/cm, pull strip on emulsion coated threads, 4-μm of EOMR, a print side RzS2 of 4 to 5-μm, a squeegee side RzS1 of 12-μm (I will get back to this in a minute) and a 75-durometer squeegee with steel shim on the back side (cylinder press style) at 18-μm the set up pull force was 2.85-kg. And, the ink transfer and edge definition were excellent.
Running it side by side as a test, using the same mesh, same EOMR, same Rz, same everything as the screen listed above, except with screen tension at 21-22N/cm, we set up to the same 2.85-kg pull force as the 27-28N/cm screen, and we could not get full ink separation. It was effectively “picking” meaning ink was remaining stuck in the screen in discrete cells leaving pits in the ink surface and we were getting a moderate amount of ink staying stuck between the emulsion wall and threads at the edges of the image features. Poor pressurization.
In order to get the same result as the first screen, we had to increase the squeegee pressure until the pull force has risen to 3.75-kg. At this point, the blade was seeing distortion.
You have to bear in mind that pull force caused by clamping force between screen and substrate, is not perfectly linear with squeegee pressure increase. The actual micrometer down-stop/movement to increase this pressure was in the 0.030-inch range. That’s a lot of added force.
And, part of that equation, is the fact that the 21-22N/cm screen, required almost 50% MORE off contact to properly clear. Having to push the mesh downward a greater distance to meet substrate is a huge multiplier of squeegee force.
Put simply, off-contact is artificial tension, and peel is artificial off-contact. Each is an order of magnitude of distortion increase.
As a short-term fix, to reduce squeegee pressure and “some” of the pull force on the 21-22N/cm screen, we had to drop back to a 65-durometer squeegee (again with a steel backing shim). While this allowed properly clearing the image at 3.25-kg pull force of clamping pressure between screen and substrate, it also created excessive texture on the printed ink surface. This is because the softer blade extrudes itself deeper into the mesh cells while making a seal.
So back to the “squeegee side” Rz. This is a long-held secret technique of the high-end screen making houses that circuit and medical use to provide imaged screens. I picked it up when working semiconductor and later solar many years ago,
To get really fine Rz screens, with very thin EOMR, the technique is to coat with an emulsion that has been thinned with distilled or DI water. The object is to apply a base coat and let it dry. Then do multiple face coats one at a time.
So now that you are face coating to the dried base coat of emulsion, the emulsion shears off the trough edge in very thin layers, each new face coat layer, only applies enough emulsion (typically less than 1/2 micron) to fill the “valleys”. When it dries, and it dries very fast, like 4-5 minutes, it shrinks leaving shallower and narrower valleys. In this way, you can achieve very thin (1-2μm when needed) EOMR layera, that can still have Rz in the 4μm or less range.
When you apply the same technique to the squeegee side, face coating only enough to reduce Rz, but NEVER to build EOMR that is more than 1-2μm over thread knuckle height (mesh count and thread diameter dependent), the effect reduces the tractive force on the squeegee. More importantly, the squeegee edge no longer needs to be distorted as much by pressure, to make an effective seal around the threads.
This can reduce required squeegee pressure by as much as 50%, reduce vibration and chatter and actually allow higher squeegee speeds and allow moving upward a full durometer range to a more precise blade edge.
In short, the pull-gauge is not used to chase or gauge screen variability. To keep from having an excessive variation with the pull force gauge, the screens need to be KNOWN to have the correct parameters. In most of the plants, I go into, the screen tolerances are narrow and have written specs, and if the tension meter, Rz meter, and thickness gauge do not agree with the specification, the screen never leaves the screen room.
What we are really looking for here, even though yes we are using angular slip technique, is how much compression force there is between the screen and substrate, primarily to set the squeegee exactly parallel to the screen and substrate, to make additional pressure additions easier to keep equal, and to keep ink deposition and texture from both sides to center..as uniform as possible.
When the screens have known uniformity, the pull gauge can be quite a good set up tool.
Greg Kitson: Damned impressive
Andy MacDougall: I almost followed all that.
Bill Hood Consulting: The bottom line is that if you can easily and quickly measure squeegee pressure at the press during setup and then set Standard Operating Procedures, then maybe we can finally eliminate the use of excessive squeegee pressure and increase the quality of screenprinted products. I can count on one hand the number of shops that I have been into that were not using excessive squeegee pressure.
Ray Greenwood: Exactly! As you, and Richard Greaves are both pointing out, its about uniformity and standardized process.
A couple of quick additional notes that may give additional understanding to some here:
1. To those who think this is too”technical“: while at some level a diagnostic tool like this can help almost any type of screenprinter, please understand that when we say, “screenprinting”, it does not just mean garment/tee shirt, or even graphics.
To those that print medical devices, circuits, control panels, sensors, automotive or aerospace parts, the cost per unit is high enough and the risk of failure is drastic enough, that there are legal/regulatory requirements that require one to prove that you adhered to a rigid process, to reduce risk of defective parts.
2. With this tool, there is more than one way to use it. With regard to those two 305 screens from my log book, those were thin enough and smooth enough that I could get away with using the pull scale for actually setting up “printing pressure” and still have low pull forces under 4-kg.
That means that we were dialing in pull pressure we were not only trying to set up the squeegee parallelism. We were also trying to make sure that when we were done, it actually had correct printing pressure. This is not done all the time, because depending on screen mesh thread count, ink type and substrate, pull strip material etc, setting up actual print pressure, meaning no additional squeegee adjustment required, can require more than 10-kg of pull force.
Bear in mind that your pull strip may be between .002” and .005“ thick. That’s 50μm to 125μm thick. It props the mesh away from the workpiece. You have spring force on it.
To hit actual print pressures, on mesh 230 tpi and below, you will be squeezing that pretty tight. This is why I have two scales, a 10-kg and a 20-kg model.
The process when setting up for not just parallel squeegee set up, but actual print pressure, is to make exact controlled screens FIRST. Then set one up with controlled ink, substrates and squeegee. Set up to level first, then adjust to correct print pressure, and then measure to establish a baseline pull force target to use for set up.
On a screen like a 230/48 at 32N/cm, actual print pressure set up with a 5 mil pull strip can hit 10 to 12-kg. I use the 20-kg scale for that.