The Three Things You Need To Know About Doming

Doming (also known as lensing or capping) is the process of applying a clear resin onto a two-dimensional decal, nameplate or label to create a 3-D artwork that adds protection, value and “pop” to the finished work. This dome can be scratch and weather-resistant while enhancing a bold image that stands out from any angle. Three-dimensional parts always look more natural to the human eye because we live in a three-dimensional world. For example, think how a sculpture looks more realistic than a picture.

Basic knowledge of doming starts with: • Domingenvironment• Compositionoftheitemyouwishto

dome• Domingsystem

The success of these factors is determined by the overall commitment to doming. If there are any concessions or shortcuts taken, the shortcuts will be evident in the results.

Doming EnvironmentThe environmental needs for doming should not shock anyone who runs a high-quality printing operation. The three concerns that need to be controlled in a doming environment are temperature, humidity and cleanliness.

TemperatureJust like “The Three Bears” porridge, doming environments can be too cold, too hot or just right. The simple rule is to keep it comfortable for the operators and resin being used. I typically suggest operating temperatures of 21oC to 26oC (70oF to 77oF) for both material and operating environment.

Some resins require heating, which presents other issues that will not be discussed here. Cooler temperatures will cause the resins’ viscosities to build, which is detrimental to the flow and mix quality. It will also noticeably slow the resin’s cure time. If the temperature is much higher than this, the operators in all of their safety gear — gloves, lab coats and other gear — will be uncomfortably warm, causing sweat and increased ambient humidity, while the resin will spill more readily and cure faster in the mixer.

Humidity Most print shops have control of humidity, but a doming operation works best at a range of 30 to 50 percent. This creates a comfortable doming environment for the operators and will produce parts with the best gloss. Excessive humidity not only creates a dank operation but also runs the risk of creating surface bubbles or blisters, which are caused by a reaction with one of its components.

Excessive humidity also may be absorbed by the liners, substrates and ink that are in the room, waiting to be domed. If there is less than 30 percent humidity, the operators’ skin will be dry, making them more susceptible to a reaction with the component vapors and creating more dust.

CleanlinessApproximately 95 percent of dust is dead human skin cells and it’s a killer

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P r o d u c T I o n

by Lance Ewert, National Sales Manager – Industrial Division, Chemque Inc.

You can buy a $1 million “doming system” that will generate nothing but expensive scrap if you don’t treat all facets of the process with equal attention.

DOMING

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for wet or tacky domes. It can be picked from freshly domed work but once the resin has gelled, that dome is lost. Eliminating as many dust rejects as possible can be addressed by managing three areas.

Attire or proper clothing will not only help keep dust and lint to a minimum, it also will protect the operator from the resin, which is a sensitizer and tremendous adhesive. Gloves and long-sleeved lab coats are minimum requirements while head coverings, long pants, steel-toed shoes and any other industrial safety gear are recommended.

I dabble in woodworking, so I know a thing or two about dust. However, there are doming operations I have seen that rival a wood shop in the amount of dust generated! Nothing fibrous — cardboard, chipboard, boxes, paper towels, toilet paper or wood — should be located where doming or curing is taking place.

People are the main dust generators, so the more unnecessary people who are in your doming room wearing cotton clothes laden with lint that stirs up dust, the more of a problem you will have. I suggest keeping visitors to a minimum or behind a viewing window.

Air circulation, like attire, is an issue that addresses cleanliness as well as safety. We suggest the air be turned over at least six times per hour. This air should be brought into the room through HEPA-quality filters located near ceiling-height and opposite the area of the room where doming and curing is taking place. The air should be exhausted outside at floor-height where the curing and dispensing is taking place. Any vapors created during the curing of the resin are heavier than the air and will be immediately drawn away using this design.

Doming CompositionThe three areas of composition that need to be addressed within the part that will be domed are the cut, ink and substrate.

The fluid resin requires a clean, sharp cut at the edge of the work to stop the flow as the resin fills the part. The cut is frustrating because it’s easy to fix but not usually recognized until domes are spilling and it’s too late. The three cutting issues that cause doming problems are cuts that are too shallow, too deep or dull.

The shallow cut makes “weeding” (removal of the extraneous substrate from around the desired object) difficult and leads to decals that become stretched or misplaced within the sheet. If the operator needs to tear or stretch the decal during weeding, it will most likely cause a spill

when the wet resin is deposited.While the deep cut weeds well, it also

may draw the liner up to the level of the domed surface, making a convenient ramp for the resin to escape and ruin the rest of the sheet. Even a tiny filament brought up from the liner can act as a wick and cause a spill.

A dull cut rounds off the substrate edge creating another means for the resin to spill. The surface tension of the resin performs admirably in keeping the resin on the surface provided the edge is clean.

Inks offer a different set of concerns but can be tested before a production run begins. The three problems inappropriate ink can cause are spilling, undesirable flow and poor adhesion. All are caused by excessive or insufficient surface energy. An ink surface energy of 36 to 50 dynes per square centimeter will provide optimal flow and adhesion for polyurethane doming applications. Some larger printers will clear coat all their work to be domed to insure it falls in the middle of this range.

There are limitless substrates than can be domed, but the exceptions are governed by three rules:• Noporousorfibrousmaterials

(paper, fabric, etc). These materials absorb liquid and will make a terrible mess. There are ways to dome onto these materials, but a carrier must be domed first and then transferred.

• InertpolymerssuchasPE,PP,Teflon and other plastics will not allow the resin to bond unless surface treatments are employed. You also must beware of heavily plasticized plastisol and vinyl because the additives used will often discourage adhesion.

• Youmustbeabletocutthesubstratecleanly. The heavily plasticized materials previously mentioned tend to be cut-resistant and the edges will round-off or tear rather than leave a nice cut. If a thick, soft adhesive is used on the back of the decal, beware of it being stringy and causing the resin to wick off the label, much like that of a deep cut.

Doming SystemNow we get to the glamorous portion of the article — the Doming System. Unfortunately, some folks become distracted by shiny machines and moving parts, not taking everything into account. Youcanbuya$1million“domingsystem”that will generate nothing but expensive scrap if you don’t treat all facets of the process with equal attention: the resin, dispenser and material handling.

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ResinThere are two main polymer families and curing mechanisms used in doming. For the sake of our discussion, we will rule out epoxy (poor weathering, f lexibility and scratch resistance) and conventional UV-cured resins (poor weathering, size limitations and two to three times more expensive). The most popular doming resin in use today is two-component polyurethane.

Some novel resins are about to be released, but at this time, two-component polyurethane offers the widest variety of choices, the most versatility and highest performance of all the options. There are many variables to consider when choosing a doming resin, but the main one that affects your processing is cure time. Your customer will be interested in hardness and weathering.

Cure time should be as fast as possible. Two-component resins cure progressively in three stages: Gel time, tack-free and final cure. Gel time is when it has taken its final shape but is still sticky. Tack-free is when it no longer feels sticky but, despite its feel, is not completely cured. Final cure is when it’s shippable without sticking to or taking impressions of the mating liners.

While a fast, final cure is desirable to meet the demands of “I need it today” customers, there must be ample time prior to gel to QC the parts as well as ensure the resin flows together and completely throughout the part. Many factors are involved in determining a good compromise between speed and processing, but the solution without compromise is snap-cure technology. It allows generous time prior to the gel stage, yet achieves final cure before the end of the shift without heat.

Again, I will reference The Three Bears when discussing hardness of the cured dome. Hardness in a customer’s eyes may be too soft, too hard or just right. About 90 percent of the applications fall into the Shore D hardness range of 45–50. This provides the best combination of flexibility and surface feel.

There are rare applications for softer resins, in which increased flexibility or impact resistance is required, but the tradeoff is a gummy or tacky surface. For years, there was an assumption that the harder the dome, the higher the performance and better the durability. Not true.

Hard, inf lexible resins have limited applications and are restricted for use on flat surfaces and where temperature fluctuations are minimal. They also scratch and chip more readily than their resilient

counterparts, as evidenced by my car’s shifter, which has a hard domed decal that inexplicably has a chip out of it while the rest of the surrounding trim is coated with a flexible resin that looks brand new despite suffering abuse and being 6 years old.

Weathering is a consideration that usually means nothing or everything to a doming customer. Doming resins are categorized in three grades of indoor, general purpose and automotive OEM. Indoor grade is pretty self-explanatory. It is used for cheaper tools, electronics, promotional items and giveaways that will be lost or broken long before the dome’s deterioration is a concern.

Most of the world uses general purpose resin. It offers good performance at a good price. They offer the most variety of choices in ratios, properties and compositions. Europe and Japan are driving the initiatives to remove mercury from these resins and every toy recall from heavy metals expedites legislation. Mercury-free resins are becoming more popular and recent advances in this technology have eliminated their historical drawbacks. Mercury-free resins can now be tack-free in 10 minutes at 50oC (122oF), don’t need to be vacuumed and work well in your existing doming environment.

The automotive OEM industry long ago prohibited the use of mercury-based resins. US manufacturers also have the most demanding weathering specifications. The doming resin on a domestic economy car must pass a weathering standard three times that of some of the world’s most expensive luxury vehicles. These OEM resins also are used in demanding and ecologically conscious applications such as marine, RV and other outdoor industries.

DispenserNow that we have determined what resin we are going to use, we get to shop for the dispenser with which to apply it. There are a wide range of considerations ranging from $10syringekitsto$1millioncontraptionsthat resemble a carnival ride. I will assume you want to run production and make some money. With this in mind, we will pass over syringe kits and tabletop units, which are good for prototyping or hobbies but lack the quality or productivity of professional dispensers. Professional dispensing units all have three main components to consider when evaluating different units: metering pump, mixer and axis motion.

The metering pump is important because it is responsible for maintaining the ratio of the resin components and consistent shot sizes. The three most

popular metering pumps used for doming dispensers are positive displacement piston, bi-directional piston and progressive cavity and gear pumps.

Think of the positive displacement pump as a syringe that pushes material out in a positive direction and then has to be reloaded. Because the system has to reload, you are limited to defined shot sizes, and several shots may have to be taken to complete your work.

Another drawback to reloading is that it creates a vacuum in the cylinder that will cause the resin to degas just prior to mixing, which will create considerable bubbles in your parts unless costly, messy and time-consuming degassing is employed in day tanks prior to processing.

Bi-directional piston pumps solve the issues of degassing and limited shot size because they operate in both directions. This also means you can continuously dispense for the occasions in which you have large or multiple parts. Both piston pumps are sized on your initial needs for a shot size, small volumes for accurate applications and large volumes for greater productivity.

Progressive cavity and gear pumps offer all the advantages of bi-directional pumps. The same pumps that dispense accurately for precise applications also will flood large work as fast as any piston pump, because the output can be varied dramatically. The output of these pumps is tied to the speed at which they rotate, so the greater the speed of the pump, the greater the output.

Proprietary pump technology for one-component resins has been developed and should soon be common in doming shops by the end of the year. Currently, mixing is still required and an important part of the process.

The best pumps in the world will not help you at all if the two components are not thoroughly mixed. The job of the mixer is to blend the two components until they are homogeneous without introducing air that will create bubbles in the dome.

If the components are not thoroughly mixed, the result will be domes that are too hard or soft, striated or have wet spots that never cure. The dispenser manufacturer will have a specific diameter and number of mixing elements (helical obstructions that halve the mix with each pass) suitable for your application.

The disposable-static mixer is common, cheap and the best choice for short runs in which there are no breaks. They are stationary plastic elements in a plastic tube that last several hours before they lose

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effectiveness and, when they do, are simply thrown away.

A drawback to this mixer is that it is designed to do one decal at a time. I have seen mixers that are bonded to a system of small lines, but they are expensive and only last an hour or so before the resin builds up in the lines along with backpressure. At that point, they turn into projectiles that spew adhesive resin across the room.

There is a dynamic version of this mixer that uses a motor to spin the elements, creating much more mixing in a smaller volume. These mixers cost more and suffer the same drawbacks as disposable-static mixers. However, they are good for large flow rates that static mixers cannot handle.

The permanent-static mixer is usually made of stainless steel tubing with inexpensive and replaceable elements. These mixers are designed to be used with a cleaning solution or solvent so they can be rinsed as often as necessary, making them cheaper to use in the long run. Because they can be rinsed indefinitely, this also allows cleaning of manifolds and lines, reducing the high-productivity cost.

Productivity can be improved with the use of automated motion. Automated motion not only assumes the tedious task of moving the head around the work to be domed — so the operator can perform other essential tasks (e.g., preparation, QC, material handling, etc.) — but also it drastically improves the accuracy, precision and speed at which the work is domed. The three automation levels include single-axis, multi-axis and system integration.

Single-axis motion will move your mixer or manifold left and right, which works for most simple shapes — circle, oval, square and rectangle aligned in columns and rows. Programming the system is usually performed through simple buttons or dials located on the dispenser’s control panel.

However, some dispensers are capable only of depositing drops on designated areas, while more advanced controllers allow a “line” of resin to be applied, which is favorable for rectangles and ovals. If there is no need to dome complicated shapes or scripting, these dispensers are just as productive as multi-axis machines that cost twice as much.

Multi-axis machines quickly pay for themselves when your customers’ needs are more demanding than simple shapes. Commonly called XY or XYZ motion, multi-axis dispensers allow consistent, precise and fast doming of complex shapes, lettering and scripting as well as special-case simple shapes that require the perimeter to be dispensed in order to achieve proper flow and fill.

There are some older technologies that use pick-and-place motion (think of an Etch-A-Sketch™), which is fine for moving widgets from point A to point B, but does not provide the smooth motion necessary for scripting. Programming these units used to be dreaded when it called for manually inserting code and coordinates for every move, action and dispense point.

This is now achieved through PC-based programs that provide a target visual to be domed on a computer screen. The dispense path is quickly determined with a few clicks of a mouse. Flow and speed are input, and you are ready to dome. What used to take an engineer several days to program now takes a novice only minutes.

Material HandlingIntegrated doming systems combine

automated dispensing with varying degrees of material handling and accelerated curing capabilities to increase productivity while keeping labor to a minimum. Many of these systems are customized to suit the users’ need for specific productivity within a defined area or volume, and incorporate the use of palletizers, depalletizers, conveyors, ovens, racks, stackers and UV-gelling systems to accomplish the goals of the customer. These systems are typically employed when the work is of consistent size and very high volume.

As with most post printing finishing processes, doming is simple, right? I hope you realize that all three facets of a doming operation must be satisfied before a professional operation can be realized.

Lance Ewert is the National Sales Manager of Industrial Division at Chemque Inc. With more than 35 years of expertise, Chemque is the worldwide leading provider of superior doming polyurethane resins, advanced dispensing and curing equipment and manufacturing lines. Chemque also offers processes to create chrome and non-metallic emblems along with a full line of one-component, mercury-free, RoHS automotive grade doming resins.

lance.ewert@chemque.com

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While a fast, final cure is desirable to meet the demands of “I need it today” customers, there must be ample time prior to gel to QC the parts as well as ensure the resin flows together and completely throughout the part.

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