Glazing With Robots
While robots and automated machinery can be used throughout the manufacturing facility in a variety of operations, one of the most beneficial areas for automation in a whiteware plant is often in the glazing process. Manual glaze application is typically a hand spray operation using a simple air spray or high-velocity low-pressure (HVLP) gun. Field data puts the efficiency of this process at about 35%. Automating this process can mean anything from installing a fixed spray gun to implementing a comprehensive robotic system (see Table 1). For the purpose of this discussion, let's consider a typical six-axis robot as robotic glazing and simpler types of motion, such as telescoping guns and reciprocators, as "fixed automation." For plants that are producing high volumes but need the flexibility to change sizes, shapes and colors to respond to market demand, robotic glazing is typically the best choice.
Robots vs. Humans
Modern six-axis robots can precisely mimic the delicate, experienced movements of the most skilled glaze applicator. One program development technique is to actually videotape an experienced hand sprayer and "teach" the robot to use similar movements. In fact, the program can be fine-tuned to maximize the spray time and glaze transfer efficiency so well that no human operator can match the appearance and cost-efficiency of the robotic spray process-especially not with the repeatability that a robot performs on every piece of ware.The basic economic arguments for robots vs. humans have been made many times over and are probably familiar. The most relevant observations might include:
- Robots do not take vacations, get sick, or require lunch or bathroom breaks. They do not need health benefits or show up with hangovers, and they do not quit or ask for raises. They will work seven days a week and every holiday if required. Additionally, robots do not have language barriers-robots in Mexico, China or the U.S. all understand the same program.
- Robots, once programmed correctly, never make a mistake. Provided the equipment is maintained, they will coat each piece of ware the same as every other, every time. A new robot also requires no training or learning period. Install the correct program, and the robot will immediately make perfect parts.
- Robots can work in difficult environments-even those that would be unpleasant or unhealthy for people. For instance, a robot can apply glaze in an environment with far more air contaminants than a human could tolerate. This may have advantages in booth air recirculation design.
Improved Transfer Efficiency
Another significant benefit of using robots in glazing operations is their transfer efficiency. Not long ago, the whiteware industry turned a deaf ear to this topic. Glaze was cheap, so why worry about improving the efficiency of the application process? Today, however, there is an increasing awareness of the costs of neglecting system efficiency. All aspects of the operation that contribute to healthy bottom-line profitability are important.The cost of oversprayed glaze is not just a material cost consideration-oversprayed material must also be treated, collected and disposed of properly. There is a direct link between the amount of overspray and the labor time required to clean the booth and equipment, as well as the time required for a color change or spray equipment setup from one product style to the next.
Additionally, the lifetime and replacement costs of other spray components (e.g., guns, hoses and spray tips) are directly related to the amount of glaze sprayed through them. Increasing the efficiency of that same volume of glaze has a direct bearing on the components' cost and the plant's profitability.
Another increasingly important aspect of controlling transfer efficiency is controlling the plant emissions of glaze that is not captured by the reclaim system.
In 1999, a series of transfer efficiency studies were conducted at a plant that had both manual and robotic spray operations. The data revealed that the repeatability of a human operator, as measured by the weight of glaze applied to consecutive pieces by the same sprayer, varied by ±9.0%. The same measurements made on ware coated by a robot showed a variation of ±1.1% (see Figure 1). Clearly, robots present an opportunity for significant savings in wasted glaze on a large portion of the production.
These same transfer efficiency studies also have another implication. While some ware may go out the door with too much glaze, roughly the same amount of ware receives too little glaze. This may mean rework of a defective piece after the ware is fired or a bad part shipped to the customer. In either case, the situation results in added expense (for wasted materials and/or rework) or lost future revenue.
The Real Costs of Robotic Glazing
It would be naive to think that switching to a robot is as simple as buying a robot and having it installed. This approach would seriously underestimate the true costs involved in integrating a robot into an overall system that works properly. Too often, the "bare bones robot-in-a-box" number is used in budgeting, and the day of reckoning is an unpleasant one.It is not uncommon for the cost of the robot itself to be a fraction (perhaps one-third) of the final integrated cost of the robot. Companies that are evaluating robotic glazing systems must also consider the cost of part fixtures, presentation and product identification; spray booth and conveyor design; worker safeguards; and the necessary training, tools and related parts.
Part Fixtures, Presentation and Product Identification
The inherent "hand-eye coordination" of a human operator is difficult (and expensive) to replicate in a machine. A great deal of equipment and programming would be required to allow a robot to "see" parts in the same way a human does. Instead, most robotic systems rely on the assumption that a known part is placed in front of the robot in a pre-defined spot and orientation. The same program can then be executed time after time.If the part is placed somewhere else, or if it is a different part altogether, the original program will produce an unsatisfactory result. Small variations (fractions of an inch) are usually tolerable, since the spray pattern of the gun is wide enough to have some margin for error, but proper part presentation is a key design element for any robotic glazing system. This is often a key factor in determining how easily an existing glazing line can be converted to robotic spray.
Ware holders can be designed to allow loading and unloading in a predictable and repeatable manner. However, existing ware boards may need to be replaced with those designed with such a purpose in mind. Similarly, many glaze systems are outfitted with rotating spindles for spraying ware, but these often have no means of locating the absolute position of the spindle in any specific orientation.
Many times a robot indexing "turntable" is used, which has two, four or more "arms" to hold ware. The arms swing into the spray position using either a precise mechanical indexer or servo motor technology. The end of each arm might be outfitted with a spindle rotator that also has a mechanical or electronic means of movement. The spindle can be rotated in either direction in tiny increments, if needed, to optimize the spray cycle time as the robot executes its pattern. Normally, the indexer and robot communicate electronically to ensure that the part is in its "home" position (often by locating a mechanical detent or using an absolute encoder electronically) prior to the robot receiving a "start" signal.
While the more sophisticated equipment and controls often cost more, they can provide the optimum flexibility for a manufacturer that produces a wide range of products.
Robotic Spray Booth Design
In some cases, it might be desirable to retrofit an existing spray booth for a robotic applicator. While this type of retrofit is possible, robots are generally larger than people are, so it is imperative to plan accordingly. Robots often require more clearance around them than a manual booth may provide. However, systems that carry a single spray gun do not require as high a booth air velocity as many automated or manual booths might.In some robotic glazing system booth designs, the physical location of the robot can be a problem. The robot is usually mounted on a large pedestal that is cumbersome and difficult to work around. On robotic indexing tables, it is common to try to locate the robot in the center of the table, since oversprayed glaze must be collected without a robot between the ware and the spray booth. However, the physical size of the robot pedestal requires a large booth with little room to move around for cleanup and maintenance.
A clever solution to this problem is the inverted robot design. The robot is hung from the ceiling so that the arm holding the spray gun hovers above the part to be sprayed. This configuration keeps delicate components out of the spray zone and frees up the area around the parts and material handling system for easier accessibility.
Conveyor Design
A reliable conveyor system is imperative for reliable, consistent part presentation. The technology also exists for the conveyor to supply speed data to the robot using an electronic encoder installed on the conveyor drive. This device signals the robot precisely when the next part is in the correct position for spraying.
Worker Safeguards
Proper system design for a robot always places worker safety high on the list of requirements. A human can be hurt or even killed by the rapid, unexpected movement of a powerful robot.Safety begins in the programming stages, since the likelihood of an accident is highest during this phase. Most robots have both software and built-in hardware to limit movement during programming, but workers involved in this operation should also be trained to ensure the highest possible level of safety.
Robotic spray booths also commonly use a worker detection system-such as pressure sensitive floor mats, door interlocks, electron beam arrays and perimeter safety pull-cords-to safeguard people working near the robot. If these devices detect the presence of a worker too close to the robot when it is signaled to move, the robot's program will be halted and require manual resetting.
Training, Tools and Parts
A robotic glazing operation will also require some special preparation that wasn't needed previously in the plant. For instance, engineers, operators and maintenance personnel must be properly trained to program, operate, maintain and even make some repairs on the robot. Nearly all robot suppliers have a wide range of both off- and on-site training available.Some specialized tools and spare parts must also be kept on-hand. Usually this also includes some equipment and software to make path programming easier (such as a teach pendant).
The Bottom Line
Other U.S. industries, such as automotive manufacturing, were forced several decades ago to either modernize or die. These manufacturers embraced the use of robots for assembly, painting and other manually intensive operations to compete with low-labor-cost suppliers like Japan.The whiteware industry can learn much from the successful adoption of robots by these companies. Today's robotic glazing systems are higher quality, more user-friendly, and require a much lower investment than in the past. As with all new equipment, manufacturers must weigh the options and carefully analyze the return on their investment to determine whether a robotic glazing system is right for their operations. However, in many cases, such an investment will pay dividends in higher productivity, reduced waste-and the ability to compete in the global marketplace.
For more information about robotic glazing, contact Rodgers Finishing Tools at 1120 S. Patterson St., P.O. Box 625, Lebanon, IN 46052; (765) 482-7266; fax (765) 482-7456; e-mail sales@rfinc.com; or visit http://www.rftinc.com.