Archive for : July, 2016

The Perception of Color

I believe we can all agree that color is the single most misunderstood property of the products that we decorate. If you don’t believe me, ask me about the customer who faxed me a color chip.

For someone to perceive any color you have to have three things: a light source, an observer, and an object. These three components of color perception are commonly referred to as the ‘color triplet’. Each component plays an important role in how color is perceived. This article will identify the variables inherent to each component and explain how minimizing their influence on the perception of color is the key to successful color representation.

Light Source

Varying lighting conditions will change a color’s appearance. For example, the color of an object may be perceived differently under fluorescent lighting than under natural sunlight. Therefore, when evaluating the appearance of color on a pad printed part, it is imperative that you do so under the same lighting conditions as your customer does.

Short of having a customer representative on hand to evaluate color with you each day, the best way to ensure that you’re both on the same page with regard to lighting conditions is to have your customer specify the lighting conditions to be used for evaluating color. The most obvious method for accomplishing this is to use standardized light booths.

Observer

Unless you’re using a color measurement device such as a spectrophotometer for color evaluation, the process will always be subjective. No two human observers will evaluate a given color the same, regardless.

In all cases, the proportion of long-wavelength- sensitive cones to medium-wavelength- sensitive cones in the retina as well as the profile of light sensitivity in each type of cone and the amount of yellowing in the lens and macular pigment of the eye differs from one person to the next. Second, a given person’s perception of color can vary depending upon his mood, what he recently ate, or even his body temperature. Finally, physical variables exist with which to contend such as the viewing time, angle, and/or distance.

Once again, it is important to standardize things like the color of the background, viewing distance, angle, and time used by all evaluators. The same applies even if using a color measurement device. The device must be properly calibrated and configured using the same measurement parameters (i.e., light source, angle, spectral inclusion or exclusion, etc.) as the device used for final approval.

Object

The majority of problems arise from misunderstanding the physical characteristics of the object. (For this discussion, ‘object’ and ‘sample’ are synonymous.)

When the manufacturing methods and/or materials used to create the visual standard vary from those used to create the object (sample) that is being evaluated v. that standard, numerous problems arise. Therefore it is important that a visual standard is obtained that most accurately represents the ‘target’ (i.e., compare apples v. apples, not apples v. oranges).

For example, problems arise when the following conditions are present:

  • The standard has a matte finish and the sample is glossy.
  • The standard is made of natural materials and the sample is synthetic.
  • The standard is painted and the sample is printed.

In each case, it is important to realize that the range of colors that can be reproduced by a given process is limited by what is commonly referred to as the ‘color gamut’ of that process.

Injection molding, painting, powder coating, hot stamping, screen printing, offset printing, and pad printing all have their own, unique color gamut. Sometimes colors that are created using one process cannot be recreated using another process. When that happens, such colors are said to be ‘out of gamut’. When color matching, if the previously mentioned variables are not, or cannot be, controlled, the best that can be achieved is a metameric match.

Metamerism

A metamerism is the situation in which two color samples with different spectral power distributions (reflected wavelengths) appear to be the same color when viewed side by side under one set of conditions, but not necessarily under another. Such colors are referred to a metamers.

Metameric matches are extremely common. In fact, the basis for nearly all commercially available color image reproduction processes such as photography, television, printing, and digital imaging is the ability to make metameric color matches. Four types of metameric failure exist, including illuminant, observer, geometric, and field size.

Sound familiar? That is because the terms correspond directly to the three members of the color triplet.An illuminant metameric failure is a result of varying lighting conditions (i.e., colors that match in cool white fluorescent light do not match in natural sunlight).

An observer metameric failure is the result of having two different observers (i.e., the decorator and the customer, two different color measurement devices, etc.).

A geometric metameric failure results when material attributes vary between the standard and the sample/object. Normally, these attributes (translucency, gloss, or surface texture) are not considered in color matching. However, geometric metameric failure can occur when two samples match when viewed from one angle, but then fail to match when viewed from a different angle. A common example is the color variation that appears in pearlescent auto finishes.

A field size metameric failure results when the size of the two color samples varies significantly. For example, two 1” square color samples may appear to match when viewed side by side, but when the sample colors are viewed as 1″ square and 4″  square samples, they don’t appear to match. This is a result of human physiology: the perception of the sample color is affected by colors that are reflected to the eye from the periphery (area around the sample).

Color Measurement Devices

Assuming you have a measurable standard and sample, color measurement devices are useful tools, even when metameric failures exist.

The benefit of color measurement devices is that they can be configured to eliminate variables involving light source, viewing angle, viewing time, field size, and gloss level. It is when they are relied upon as the sole means of qualifying a color that problems arise.

A spectrophotometer works in guiding you to a successful color match the same way a global positioning system works to guide you to a destination. Sometimes the computer will say that you have an acceptable match when, in reality, you don’t – just like a GPS can say you’ve arrived when you’re actually in the wrong neighborhood. If a human, and not a computer, is the one using the product, shouldn’t the humans have the final say in whether a color is acceptable or not?

As decorators, we must educate our customers as to how color is perceived, and assist them in eliminating as many variables from the process of obtaining an acceptable color match as possible. Work with them to identify and document specifications for light source, standard and sample field size, viewing distance, viewing angle, viewing time, and (where applicable) color measurement device configurations. Failing to do so may be a costly mistake.

Sources:

Wyszecki, Gunter and Stiles, W.S. (2000). Color Science – Concepts and Methods, Quantitative Data and Formulae 2nd edition, New York: Wiley-Interscience. / R.W.G. Hunt. The Reproduction of Color 2nd edition, Chichester: John Wiley and Sons, 2004. / Mark D. Fairchild. Color Appearance Models, Addison Wesley Longman, 1998.

John Kaverman is the technology coordinator for Innovative Marking Systems of Lowell, Mass. He holds a degree in printing from Ferris State University and has over 20 years of combined screen and pad printing process experience. He may be reached at john@padprinters.com.

Troubleshooting the Polymer Cliche Making Process

The manufacturing of polymer cliches has become more and more popular for pad printers of all types. Shorter lead times and costs are two big reasons the market has seen this increase. As with everything else, polymer cliche making has its challenges. However, if the right preparation and equipment are used, it can be a very successful endeavor.

Most challenges people experience are the results of one or more of the following:

  1. Developer
  2. Cliche material
  3. film density and orientation
  4. line screen
  5. exposure equipment

Manufacturing polymer cliches is must like baking a cake. Think of the cliche material, developer, and the film & line screens as the ingredients and the exposure equipment as the oven. If you have bad ingredients to start or your oven is not the right temperature, you’ll end up with either a hockey puck or a cake pan full of lumpy pudding. The same goes for making polymer cliches. If you have a bad developer, expired or “stressed” cliche materials, poor film density, the wrong line screen, or a deficient exposure unit, your cliche wont turn out any better than your cake.

Fortunately, a series of simple tests exists that you can perform to help you identify potential causes and remedies in your cliche making process.

Developer

Depending on the specific type of cliche material you use, you will probably be developing with either water, alcohol, or a mixture of both. More specifically, you will develop with either distilled water, or denatured ethyl alcohol that is diluted with distilled water. Tap water is not the same as distilled water. If a recipe called for baking powder, would you add powdered sugar just because it looks the same? You never know what is in tap water. Municipalities add chlorine, fluoride, and other chemicals that may not be compatible with the cliche material. Distilled water will be much more consistent for developing the cliche.

Alcohol developing solution is best purchased from your cliche material supplier. You can buy a gallon of “Denatured Alcohol Solvent” from the local hardware or building supply superstore, but how do you know what’s in the can? the chemicals or amount of chemicals have been known to be inconsistent when you purchase from a retail store. Your cliche supplier will have the right consistency for you application. I can’t tell you how many times I’ve had someone call me after they have been making ciches for two years to complain about what has to be a “bad batch of cliches,” only to find that they ran out of developer and had to use “stuff from the hardware store” instead. One final word regarding developer- it works better warm than it does cold. Use water or alcohol that is at or above room temperature.

Testing Developers

To test your developers, simply develop an unexposed piece of cliche material for the recommended time. If the polymer comes off all the way down to the adhesive layer that holds it to the steel backing material then your developer works fine. If it doesn’t develop, either your developer is weak or your cliche material is expired.

Testing Cliche Material

Cliche material typically has a shelf life. Most manufacturers will recommend storing it in a cool dry place. Prolonged exposure to varying temperatures and periods of extremely high or low humidity can damage cliches. Cliches are light sensitive and can easily be exposed by accident, so be sure to work with them under appropriate lighting conditions: lighting that is free of ultraviolet wavelengths. Fluorescent office lights give off UV light, so you should always shield them or use an alternative light source.

Cliche materials vary significantly from brand to brand, and between grades from the same manufacturer. Be advised that different materials may have different exposure times. If you can tell your supplier a little information about your exposure unit’s light source (number of bulbs, wattage, distance from cliche) they should be able to give you a well-educated guess of where to start exposure times.

The test for determining if your material is expired or has been accidentally exposed is the same test you used above: if emulsion comes off in the recommended time, your cliche is good.

Testing Film Density

Film density is a killer. Everyone wants to use laser-printed overhead transparencies, vellum or some other alternative that costs less than ‘real film.’ A laser printed film media can work for a high percentage of applications but a piece of ‘real film’ will normally yield the best result. The first important thing is that the image is density sufficient. To test density, expose the cliche with the film only for the recommended time, and then develop it. Using a magnifying glass (10x), look in the image area. If you can see areas where the emulsion is still in the image area, chances are these areas are not dense enough on your film.

Testing Film Orientation

Film orientation is the other film related culprit. The image has to be on the side of the film that comes in contact with the cliche during exposure. The correct terminology would be “emulsion down film positive,” which means that when you can read or see the image as you wish it to be etched onto the plate, the emulsion is “down” or on the backside.

the emulsion side of the film is almost always dull, while the base side is usually glossy. Some film may be slightly matte on the emulsions side. Matte film is recommended for better evacuation of the air when the vacuum draws down in the exposure unit.

If you aren’t sure, and there is some emulsion on a non-image area of the film, scratch it with a film knife or scissors. The emulsion will scratch off. Once you determine what orientation the film needs to be in for exposure, it is recommended to cut a little piece of the upper right hand corner of the film off for future reference.

Having your film orientation wrong will usually result in a loss of image resolution during exposure. This is especially noticed in detailed areas of the image.

Line Screen Film

If you’re using a cliche material that requires a line screen exposure, the manufacturer or supplier should recommend a couple of line screens. For most applications, 120 line/cm 90% dot is a good choice. If you want to transfer a little more ink, you may want to consider using a 100 line/cm 90% dot. If you are wanting to be on the edge of transferring too much ink, such as when printing white on black, 80 line/cm 90% dot is recommended.

Regardless of the film you use, the important thing is to get the line screen film on the cliche in the right orientation, which is emulsion down.Like the film that holds your image for the first exposure, line screen film is glossy on the base side and dull on the emulsion side.

Keep your line screen clean and free from scratches and kinks. If you damage a line screen, replace it.

Testing Line Screen Film

If you are using line screen film, the best way to know if your exposure time is correct is to perform a step test, using 15 or 30 sec intervals. After development and post-exposure/drying, gently draw a fingernail over the image area. If the line screen exposure is correct, the peaks of the dot pattern should be roughly at the same level as the top surface of the cliche. You should be able to hear you fingernail scraping across the rough surface, but you should not be able to catch your fingernail on the edge of the etched image. If you can catch your nail, it means your cliche is too deep and that your line screen exposure time should be increased.

Using 10x magnification, observe the non-image areas of the cliche. If you can observe a very shallow dot pattern over the entire surface it indicates that your exposure with the film was not sufficient.

Testing Exposure Equipment

People tend to overlook several equipment related variables, including the power output of the light source, vacuum pressure. UV light bulbs have a pretty specific lifetime.

Depending on the manufacturer, the power output of the bulbs drops off significantly somewhere between 400 and 700 hours. If your exposure unit has an integrator that senses the output and adjusts exposure times accordingly, you’re fine. If not, then you should know the life expectancy of the bulbs you have and change them on a consistent basis. Generally, the lower the wattage, the better control you have. It is recommended to use 15 or 20 watt bulbs. If you use 40 watt bulbs the power output is not double that of 20 watts, but rather the square of the output of the 20 watts. This is why people trying to make cliches with a 1000 watt metal halide (screen printing exposure unit) light source have so many higher quality (more sensitive) cliche materials.

Vacuum pressure should be sufficient to eliminate air from being trapped in between the film or line screen film and the cliche surface during exposures. Normally, when people have sufficient density yet lose detail, or have varying etch depths, poor vacuum is the cause of the problem.

You can periodically test your exposure equipment by exposing a cliche with a test film (having sufficient opacity), using the same exposure time each time. That exposure time should be sufficient to result in a developed cliche having a depth all the way to the adhesive layer of the cliche material. If you use the same film and exposure each time you perform the test, it should always wash out to the adhesive. If it doesn’t, it means your light source output is deteriorating.

In sum, the tests that we have discussed will help you pinpoint the root cause of polymer cliche making problems. Remember, polymer cliche making is a process, not a craft or an art. Using a systematic approach, you can troubleshoot and successfully correct most problems.

John Kaverman is the technology coordinator for Innovative Marking Systems of Lowell, MA. He holds a degree in printing from Ferris State University and has over 20 years of combined screen and pad printing experience. He may be reached at john@padprinters.com

Choosing the Right Decorating Process

Manufacturers decorate their products to differentiate them from those of their competitors. Often product designs are approved, marketing campaigns are launched, and expensive tooling is ordered without anyone ever confirming the design’s feasibility with regard to the decorating process that has been specified.

This paper will pair up some key design characteristics with the appropriate decorating process while identifying the benefits and limitations of one process versus another. Processes that will be discussed include pad printing, screen printing, hot stamping/heat transfer, in-mold decorating and piezoelectric DOD inkjet.

In researching this paper one of the first things I did was create a “Plastics Decorating Matrix” which I sent out to dozens of people within our industry; I chose both people that specialize in one process and people with experience in several processes. They were asked for their professional opinions in rating the processes mentioned for their suitability over a range of substrate materials, part and imaging characteristics, and sensitivity to conditions within the production environment.

While the responses were disappointingly limited, they confirmed what I know from 24 years of experience; very few plastics decorating applications have only one “right” solution. Each application is unique in some way, just as each process has benefits and limitations that play an integral role in determining whether or not it provides the “right” solution

Processes were rated on a scale of 1 to 4 (with the highest highlighted green in the table)

  • 1 = Poor – process not suitable
  • 2 =  Fair – process is suitable with several limitations
  • 3 = Good – process is suitable with minimal limitation
  • 4 = Excellent – process is suitable without limitation

Compound Angles, Concave/Convex, and Spherical Parts

Pad printing has long been recognized as the process of choice for direct printing onto compound angles, concave and convex surfaces, and spherical objects. That is largely due to the transfer pad’s ability to pick up an image from a flat plane and conform to transfer it onto irregularly shaped surfaces with little or no distortion.

Alternatively, IMLs can provide a solution where run size requirements are larger and/or indirect decorating is preferred. IMLs, or pre=printed and/or pre-formed film inserts, are generally more expensive to produce, and may require that the artwork be pre-distorted. While IML enjoys (with exceptions) and advantage over pad printing for large image areas, it is generally not cost effective versus pad printing when the image only covers a small portion of the decorated surface.

Flat Parts, Raised, and Recessed Graphics

Screen printing and hot stamping/heat transfer are generally the “go-to” processes for use in decorating flat parts. Screen printing being the obvious choice for areas of large coverage, and hot stamping/heat transfer decorating is not affected by extremes or variations in temperature and relative humidity within the production environment.

Raised and recessed graphics are generally decorated via the vertical hot stamping process; unless there are surface irregularities in raised graphics, in which case a roll-on press is generally used.

IML applications work best on flat surfaces or on large radii. Long narrow images with pointed corners do not work very well because the force of the material flow in the mold tends to move the label out of position during molding.

Cylindrical Parts

Cylindrical items, such as 5 gallon paint pails, are typically screen printed. Rotary pad printing is also useful for smaller format parts such as syringe barrels and closures, and can reach extremely high speeds (400-600 parts per minute), even with U.V. curable inks.

Heat transfers may also be applied to cylindrical parts. For coverage of less than 85 degrees, a curved silicone rubber die can be made to fit the curve of the plastic part. For coverage up to 360 degrees, transfers can be applied by rolling the parts under and flat silicone die, with pre-registered transfers sandwiched in between on a web.

Hollow Parts

Hollow parts such as blow molded containers, are usually screen printed or labeled with pre-printed PSA (pressure sensitive adhesive) labels. For more rigid parts with smaller diameters and/or thicker sidewall constructions, hot stamping/heat transfer can be an alternative.

Pad printing is becoming more widely used when hollow parts have complex geometries that are too difficult or impossible to screen print, label, or dimensionally stable it is essential to inflate the parts equally, at each print station, to ensure registration of the image.

Textured Parts

Textures are almost always a gamble. In an attempt to find a correlation between characteristics of different textures and printability, I pad printed a series of sixty-odd black “visual texture standard” plaques, provided by Mold-Tech. I used an automotive-approved, two-component, white ink-thinned (15 percent by weight), a steel cliche (etch depth of 23 microns) and a 9-shore (scaled c) transfer pad with a high angle.

Each plaque was single-printed at one end and double printed on the other, then allowed to dry per the ink manufacturer’s recommendation. The plaques were then visually inspected for coverage under a uniform, non-directional (unfocused) light source at a distance of 18″ for approximately ten seconds (per Ford Motor Company visual inspection procedure for automotive interior parts). Acceptance or non-acceptance was determined by the presence of any visible defect or void resulting from insufficient coverage of the texture.

The results indicate that the frequency of the texture plays a significantly larger role in achieving an acceptable print than the depth of the texture or the angle of the sidewalls. One texture having a depth of .0055″ and 8 degrees draft couldn’t be printed successfully at all. The difference was the number of peaks and valleys in the texture within the image area. The obvious conclusion: the higher the frequency, the lower the likelihood of acceptance.

Textures can be compensated for, however, by depositing more ink. That makes screen printing a great solution for flat or cylindrical parts. For large image areas and/or complex geometries, your only choice may be to apply IMl technology

Image characteristics are a little harder to quantify than part characteristics, mostly due to the fact that many applications are comprised of a wide variety of different kinds of graphics; bold and finely detailed, metallic and matte, etc.

Wet-on-wet/Personalization/Serializing

Piezoelectric DOD inkjet is the only process in the survey that received an “excellent”, and it was for wet-on-wet. For short run CMYK and personalizing or serializing, you would have a hard time finding a better solution. with today’s U.V. inkjet formulations, more and more specialty advertising, small format signage, and promotional marketing applications are going with DOD inkjet.

Multiple Color and Fine Detail

New generation YAG lasers, laser cliche materials, and digitally controlled, multiple-axis pad printing systems in printers are producing lines only microns in width, and CMYK images with 1200 d.p.i. resolution, even on complex geometries. Screen printing provides nearly equivalent quality, as does IML.

Heat transfers are a viable alternative for those that don’t wish to become printers. Transfers are generally manufactured using one of three processes: screen printing, rotogravure, or flexographic. Screen is very similar in that it allow for brilliant colors. rotogravure provides superior resolution but is more attune to larger quantities due to the increased setup time. Flexography is least expensive but does not provide the brilliance of screen printing or the resolution of rotogravure.

Metallic Finish

Hot stamping continues to be the process of choice for high gloss, metallic finishes like those found on cosmetics. In fact, hot stamping is the only process where the bright gold and silver metallic finishes found on cosmetic packaging can be produced. IML is generally not recommended for applications requiring foils or metallic inks because they will not adequately maintain a static charge (required to stay in location within the mold) and because they can arc to the mold surface, causing pitting.

John Kaverman is the technology coordinator for Innovative Marking Systems of Lowell, MA. He holds a degree in printing from Ferris State University and has over 20 years of combined screen and pad printing experience. He may be reached at john@padprinters.com