Pad printing is the process of transferring 2D images onto 3D parts. When skilled artisans and industrial grade machinery are combined, the process of pad printing accurately transfers artwork images onto a substrate, en masse, providing a cost effective decorating solution. The process is particularly useful for the medical and industrial industries, where volume, precision, and consistency are paramount.
While pad printing will provide precise and repeatable image registration on irregular surfaces, ink adhesion is equally important. The ink should adhere and resist fading for the usable life of the product. But, adhesion also poses some of the most significant challenges of successful decorating. Selecting the right ink system for the substrate may not be enough. Often, pretreatment and/or post treatment processes are introduced to further improve ink adhesion.
Ink Adhesion and Pad Printing
The medical industry, in particular, has stringent standards in place to prevent ink from presenting a health risk to patients. The ink system, together with the material to be decorated, must be matched carefully to ensure durable ink adhesion and pass bio-compatibility testing.
Additionally, while ink manufacturers may claim their ink system is suitable for a given substrate, adhesion can be subjective. Similarly, resin manufacturers may claim their trade name substrate is suitable for decorating. But, who can be certain without testing? And testing can get expensive quickly. A third party resource with proven experience matching ink systems to substrates can be a valuable resource.
If the surface tension of a substrate isn’t within the correct range for a given ink system, pretreatment and/or post treatment may be required to improve adhesion results. Read on to learn more about the preparation and finishing of decorated surfaces for optimal adhesion results.
What Is Surface Tension?
Surface tension is the physical force that explains why ice cubes float in water. The density of the water is higher than the ice cube (hydrogen bonding).
So — how does this relate to ink adhesion? The surface tension of the substrate, measured in Dyne, requires a minimum value for ink adhesion. Most substrates require a Dyne level of at least 42 for durable adhesion with most pad printing ink systems.
Fortunately, there are affordable tests available to determine the Dyne level of a substrate (See below). Bottom line, the lower the Dyne level, the lower the ink adhesion qualities. Substrates with low surface tension, such as polypropylene and acetal, will require a pretreatment process to increase the Dyne level to achieve desired adhesion results.
Determining Substrate Dyne Level
A Dyne pen is the most economical and practical solution to determine the surface tension of a substrate. We have the capabilities to perform this test and advise if pretreatment and/or post treatment processes may be required.
Which Substrates Do Not Require Treatment?
Common substrates that often do not require pretreatment or post treatment processes include:
- Stainless Steel (Heat Post Treatment May Further Improve Adhesion)
- Thermoplastic Elastomer
- Polyetherimide (Meat Post Treatment May Further Improve Adhesion)
Which Substrates Require Treatment?
If durable ink adhesion is required, the following materials may require low frequency plasma (corona discharge pretreatment), high frequency plasma discharge, flame pretreatment, and/or heat post treatment processes:
- Polypropylene (Corona or Flame)
- Polyethylene (Corona)
- Polyarylamide (Corona)
- Polyurethane (Corona)
- Acetal (Corona and Heat Post Treatment)
- Nylon (Corona and/or Heat Post Treatment)
- Polycarbonate (Some Instances, High Frequency Plasma)
- Silicone (Heat Post Treatment)
- Fluorosilicone (Heat Post Treatment)
Corona Discharge Pretreatment
Corona discharge treatment “causes a change in the physicochemical properties of the polyamide fiber surface,” oxidizing the surface fiber, and improving adhesion. A high voltage, low-frequency corona discharge is generated near the part that is being treated.
The release is attracted to the material, which severs the molecular bonds on the surface of the material. As a result, the severed bonds are highly energized and attracted to coatings, inks, and other adhesives.
Corona discharge works well with a mold of any shape, but is more suitable for smaller parts. It’s a safe and clean process that will not blemish the component.
When flaming, a mixture of low pressure air and gas (coal gas, methane, propane, or butane) is directed at the surface to be decorated. The process requires extra skill since too much flame will damage the surface. But, not enough flame will prevent ink adhesion.
Flame pretreatment is generally used to treat materials like biaxially oriented polypropylene, polyethylene terephthalate, polyethylene terephthalate, polyethylene, coextruded films, metal foils, paperboard, and foams.
Flaming will also burn off rough surface fibers to create a smooth, flat surface that’s perfect for coating.
Obviously, flame treatment is a more delicate (and dangerous) process than corona discharge, but it’s still highly effective when deployed correctly.
Heat-Curing Post Treatment
Substrates like Acetal and Nylon may require further treatment after the printing process is complete. Certain Ink systems offer better adhesion results when exposed to forced heat compared with ambient air.
Does Ink Type Make a Difference?
Yes! There are numerous pad print ink systems formulated specifically for the substrate to be decorated. And, each manufacturer has a unique formula for each ink system.
In our experience, certain manufacturer ink systems offer better adhesion qualities for a specific substrate. Experience with multiple ink systems is key when it comes to adhesion.
Ink System Auxiliaries
Once the pad printing ink system has been chosen the decorator must decide which auxiliaries should be included.
For example, which thinner will be used? Most often, the appropriate thinner will be sourced from the same ink system manufacturer.
The decorator must also decide whether the ink system will be a one or two-component based formula. A two-component system includes a hardener agent.
There are other additives available to further improve adhesion qualities, but additives can become a slippery slope. A lot of time can be wasted with a trial and error approach.
Other Adhesion Considerations
Cure Time Requirements
Ink requires time to cure to achieve expected adhesion results. Generally, ink manufacturers will recommend 7-10 days ambient air cure time.
Heat baking, in some instances, may reduce the time significantly, but will add to process costs. In practice, 24 hours is often sufficient to layer pack or individual bag decorated components without the risk of ink smudging/smearing.
However, some substrates should allow for a full 7 days of cure time before packaging. Cure time often depends on the specific substrate trade name and adhesion expectations at the moment when the decorated parts are returned.
Trial shipments are recommended to determine the minimum required cure time to achieve the fastest turnaround.
“MOLD RELEASE” – Two words that strike fear in the hearts of decorators and should be strongly reviewed early in the product design stage for decorated components.
Mold release, quite simply, is an adhesion killer. While this additive can be very helpful to release injection molded components, mold release (external or internal) should be carefully reviewed at the product design stage and communicated to relevant parties if the component will be decorated.
External mold release agents often leave a slick, clear residue on the decorating surface which may not be visible to the print operator and will compromise ink adhesion qualities.
Fortunately, most external mold release agents may be cleaned with common cleaning agents like isopropyl alcohol. A pretreatment cleaning process will be required prior to decorating and must be communicated to the decorator beforehand.
Internal mold release agents present a much bigger adhesion problem. Internal mold release is resin additive that cannot be removed after the component has been molded. Internal mold release agents may create a surface tension well below 42 Dyne.
Corona and flame pretreatment and/or heat baking post treatment processes are generally ineffective at raising the surface tension for required adhesion.
However, in some instances, high frequency plasma pretreatment has been tested and proven effective to raise the surface tension suitable for pad printing.
In our experience, internal mold release agents are commonly found in polycarbonate substrates. Be certain to confirm the existence of internal mold release agents with the resin manufacturer and plan accordingly.
Clean Surface Area
It is vital that the location of the surface area to be decorated is clean and free of foreign material.
During the manufacturing process, oils and other contaminants often find their way onto a manufactured component which may not be clearly visible. Oil remnants create a barrier, similar to external mold release, which will prevent durable ink adhesion.
Isopropyl alcohol wiping, with a lint-free cloth, is recommended for most plastics. Harsher chemicals, such as acetone, may be required for cleaning metals.
Adhesion Testing Post Printing
Adhesion is a subjective term. What qualifies as durable adhesion? Many decorated products are single-use in nature, have a very short life, or may not be handled often.
Is it economical or practical to add pretreatment and post treatment processes for such a component? Perhaps. Products like medical syringes, even if single-use application, require very strong adhesion qualities for patient safety concerns.
Other decorated products found in agricultural heavy machinery like control displays, bezels, and housing components may have a life cycle over 20 years and will require years of wear resistance.
Ultimately, adhesion expectations may come down to end user requirements and economics.
Common Adhesion Tests
Common adhesion testing methods include;
- Tape Testing (more on this below)
- Alcohol Wiping
- Fingernail Scratch (yep)
Tape testing is often the most widely accepted form of adhesion testing. But, beware, tape testing is an analog testing approach that may be difficult to repeat precisely and could lead to false adhesion assumptions batch to batch.
In fact, an entire ISO standard has been created to provide a framework for tape testing. Do your adhesion requirements justify a tape test process that conforms to ISO standards? If so, budget for the added expense.
If you plan to tape test to evaluate ink adhesion, without following ISO standards, we recommend a documented process that addresses;
- Tape selection. Select the tape to be used and do not deviate. And, mind the expiration date.
- Cure time. Only perform the tape test after the full cure time recommended by the ink manufacturer.
- Acceptable ink transfer. Obviously, the goal is 0% ink transfer onto the tape. But, that may not be practical or possible. The ISO standard speaks to percentages of transferred ink onto the tape. Is 1-5% ink transfer, of the test area, acceptable? Or, 5-15%? Again, 0% may not be possible and the costs associated with attempting to achieve 0% may get out of control.
- Test batch size. If 1000 parts were decorated, how many parts would be tested?
- Pass/Fail Matrix. Created a spreadsheet matrix for your test which includes pass/fail. For example, randomly select 25pcs (per 1000pcs) test and record each result. If you decided previously that 5-15% is a “Pass,” then record each pass/fail test accordingly.
- Determine the random sample “Fail” percentage that requires further investigation. What’s the magic number? Is 10% the right number? If 3 of the 25 random samples fails the test should an investigation occur?
Successful product decorating via pad printing requires skill and experience. For 20 years, Liberty Clark has been providing contract decorating services to manufacturers. Our artisans average over six years of direct experience. And, our catalogue of ink systems and processes to achieve durable adhesion for a given substrate (and trade names) grows almost daily.
Your project is important. Your components are expensive. You deserve the best there is to offer. Contact us to learn more about our solutions for your pad printing project.