|Technology alternatives for Screen Reclamation
PUBLISHING, PRINTING AND REPRODUCTION OF RECORDED MEDIA # 10
This case study was developed for screen printers by the U.S. Environmental Protection Agencys (EPAs) Design for the Environment (DfE). Program with assistance from the Screenprinting and Graphic Imaging Association International (SCIA).
This case study focuses on different technologies that can be utilized in screen reclamation. The DfE Screen Printing Project identified seven potential substitute technologies that are environmentally safer than traditional screen reclamation, including: high pressure water blasters, automatic screen washers, sodium bicarbonate spray, media blasting, pulse light energy technologies, striping technologies, and emulsion chemistry. This study describes three screen reclamation technologies that may be used to change both the types and amounts of chemicals used: high pressure screen washers; automatic screen washers; sodium bicarbonate (baking soda) spray.
Of the seven alternative technologies listed above, these three were selected because screen printers expressed an interest in learning more about these technologies. This case study presents:
|Descriptions of two commercially available technologies that can reduce a facilities usage of traditional solvent-based ink removers.
|Description of a technology now under development that could further reduce the costs and potential health risks of screen reclamation.
|Comparative cost, performance and risk information for three reclamation technologies.
The costs and risks for each of the three substitute technologies are compared to the costs and risks of a traditional screen reclamation system. The traditional system used in the comparison consists of lacquer thinner as the ink remover, a sodium periodate solution as the emulsion remover, and a xylene/ acetone/ mineral spirits/ cyclohexanone blend as the haze remover. These chemicals were selected because screen printers indicated they were commonly used in screen reclamation. In both the cost and risk comparisons, it was assumed that these chemicals were applied manually to 6 screens per day, each 2,127 in2 (approximately 15 feet2) in size.
Cleaner Production Principle
Process/ product modification; Material substitution
Cleaner Production Application
The DfE Screen Printing Project identified seven potential substitute technologies that may be environmentally safer than traditional screen reclamation, including: high pressure water blaster, automatic screen washers, sodium bicarbonate spray, media blasting, pulse light energy technologies, stripping technologies, and emulsion chemistry. This study describes three screen reclamation technologies that enable to change both the types and amounts of chemicals screen printers use:
|High pressure screen washers
|Automatic screen washers
|Sodium bicarbonate (baking soda) spray
Of the seven alternative technologies listed above, these three were selected because screen printers expressed an interest in learning more about these technologies.
The costs and risks for each of the three substitute technologies are compared to the costs and risks of a traditional screen reclamation system. The traditional system used in the comparison consists of lacquer thinner as the ink remover, a sodium periodate solution as the emulsion remover, and a xylene/ acetone/ mineral spirits/ cyclohexanone blend as the haze remover. These chemicals were selected because screen printers indicated they were commonly used in screen reclamation. In both the cost and risk comparisons, it was assumed that these chemicals were applied manually to 6 screens per day, each 2,127 in2 (approximately 15 ft2) in size.
High-Pressure Screen Washers
Two high-pressure screen reclamation systems were reviewed. In addition, the performance of one system was evaluated in a print shop as part the DfE Screen Printing Project. High-pressure washers typically work as follows. Excess ink is carded off the screen prior to cleaning. No ink remover is applied to the screen. An emulsion softener or remover is applied and allowed to work, typically for anywhere from ten seconds to less than one minute. The ink and stencil are then removed by a high-pressure water blaster sprayed on both sides of the screen at pressures of up to 3,000 pounds per square inch (psi). If necessary, a haze remover is then applied and allowed to work. Again, the high-pressure water blaster is used to rinse off the haze and the haze remover. Cleaning usually takes place in a washout booth where the rinse water can be collected.
While this technology may require significant water use, most emulsion and haze removal products are formulated to allow discharge to sewers. Where ink residues in the rinse water exceed wastewater permit concentration limits, such as for suspended solids, manufacturers also supply a variety of filters. Some improved filtration systems allow rinse water to be reused. Filter wastes are typically disposed of as hazardous waste.
Performance of a high-pressure water blaster was evaluated by DfE staff at a volunteer printing facility where the technology was in place. During the demonstration, the technologys performance was very good. On screens with solvent- and water-based inks, the stencil dissolved easily, leaving no emulsion residue on the screen. Ink stains on these screens were completely removed by the haze remover even before the waiting period or pressure wash. Reclamation results were fairly similar for UV curable ink as well
Automatic Screen Washers
There are several different types of automatic screen washers, and although most are used for ink removal only, automatic systems for emulsion and haze removal are also available. The major benefits of automatic screen washers are reduced solvent losses, reduced labor costs, and reduced worker exposures. The DfE Screen Printing Project identified a wide variety of automatic screen washers on the market and found significant differences in the chemicals used and costs. Costs vary based on the level of automation (such as conveyors), system capacity, and complexity of the equipment.
The basic component of the automatic screen washers is the wash unit, an enclosed box that can house a variety of screen sizes (up to 60 in. by 70 in.). After a screen is clamped inside the wash unit and the top closed, the cleaning process begins. A mobile mechanical arm sprays solvent onto the screen through pressurized nozzles (30 to 150 psi) for any preset number of cleaning cycles. Since the systems are enclosed to reduce solvent losses, volatile solvents, such as mineral spirits, are often recommended because of their efficacy. There are, however, a number of alternative formulations offered by equipment manufacturers. Used solvent drains off the screen and is directed to a filtration system to remove particulates (inks and emulsion). Following the filtration step(s), reclaimed solvent is typically reused. Some systems have separate wash, rinse, and air dry cycles or separate tanks for washing and rinsing. Solvent reservoirs must be replenished intermittently and changed once or twice a year. Filter wastes are typically disposed of as hazardous waste.
As described above, there are several types of automatic screen washers, and for each type there are several manufacturers. Because of the resources required to do a full demonstration of the equipment commercially available, performance demonstrations of automatic screen washers were not conducted as part of this project.
Sodium Bicarbonate Spray
A sodium bicarbonate (baking soda) spray technology was evaluated by the DfE Screen Printing Project to determine if it is potentially adaptable as an alternative screen reclamation technology. This technology is currently used for removing coatings, such as paint, grease, or teflon, from metal parts. In these applications, the technology has been successful in replacing hazardous cleaning chemicals. Based on the technologys success in other applications, it appears to be a promising substitute for chemical screen reclamation systems. Because the sodium bicarbonate spray technology had never been tested for screen reclamation, DfE staff conducted a one-day site visit to the equipment manufacturers facility. Three imaged screens were inked with three types of ink. An inked screen was placed inside an enclosed cleaning booth, and the screen was passed, back and forth, under the sodium bicarbonate spray. No chemicals other than the sodium bicarbonate were used during the reclamation.
Environmental and Economic Benefits
In general, the benefits of high-pressure washes are that they reduce both chemical use (eliminating ink removers) and worker exposure (less scrubbing required)/ The DfE Screen Printing Project found that the occupational risks of this system were notably lower than the risks associated with the traditional solvent-based reclamation chemicals. For the traditional screen reclamation system, health risks associated with both daily inhalation and skin contact with the chemicals, particularly with organic solvents, were significant. For the high-pressure screen reclamation system, health concerns were related to unprotected skin contact with the reclamation chemicals. Dermal exposures could be reduced dramatically, however, by wearing gloves. Ecological risks from discharges to the water were not a concern for either the traditional system or the high-pressure blaster system. General population risks from air releases also were not a concern for either system.
Compared to manual application of the traditional screen reclamation chemicals, the DfE risk evaluation of automatic screen washers found that worker inhalation exposures to the volatile organics used in solvents (mineral spirits and lacquer thinner) were reduced by as much as 70 percent. Although health risks associated with dermal exposures to the chemicals remained high, these risks could be reduced dramatically if gloves are worn while handling the screens. Since the automatic screen washer is used for ink removal only, the risks associated with emulsion and haze removal remained the same as the traditional systems risks for these steps.
Because there are several types of automatic screen washers, and for each type there are several manufacturers, and because of the resources required to do a full demonstration of the equipment commercially available, performance demonstrations of automatic screen washers were not conducted as part of this project.
The DfE project did not undertake a risk assessment of sodium bicarbonate spray technology for a number of reasons. Sodium bicarbonate has been shown to be a fairly innocuous chemical and it is not a skin irritant. In addition, it is a common ingredient in baked goods, toothpaste, and detergents. If this technology proves to be a viable alternative for screen reclamation in the future, a detailed assessment of the human health and environmental risk should be conducted.
The DfE Screen Printing Project estimated the cost of equipment, labor, and chemicals for the high-pressure wash. Assuming that 6 screens are reclaimed daily and each screen is 2.127 in2 in size, the cost estimate for the high-pressure washer totaled $4.53 per screen. This estimate was compared to that of the traditional screen reclamation system (using lacquer thinner, sodium periodate, and a solvent blend). Using the same assumptions, the estimated reclamation cost using the traditional system is $6.27 per screen: 30 percent more than the high pressure wash, with the greatest savings coming from the reduced labor costs for the high-pressure washer. Equipment costs, estimated at $5,300 (installed) account for just 12 percent of the per screen costs. This estimate does not include filtration units, which range in price form $1,300 to $12,000, although maintenance and operating costs vary widely.
The DfE Screen Printing Project estimated costs for two automatic screen washers, assuming that the washers were used for ink removal only and that six screens (2,127 in2 each) were reclaimed per day. Screen reclamation costs using an automatic screen washer ranged from a low of $4.13 to $10.14, compared to $6.27 for traditional reclamation. The largest cost component is typically equipment cost. Additionally, the savings of switching to this technology would be greater if this costing accounted for the labor savings of workers moving on to other tasks once the screen is loaded in the washer. It is important to note that the cost per screen of the more automated, higher-cost washer would be much lower if it operated nearer to its capacity of over 100 screens per day.
In the sodium bicarbonate spray project, the available equipment was not designed specifically for screen reclamation, so we have assumed that the cost of equipment modified for screen reclamation would be similar to the cost of the equipment used in the performance demonstration. The available equipment ranges from $32,000 to $52,000, including a filtration system. The sodium bicarbonate itself costs between $0.65 to $0.75 per pound, based on amount of purchase, and approximately one pound is sprayed per minute. If this technology proves to be a feasible alternative for screen reclamation after further development, a more detailed cost analysis can be conducted.
The performance demonstration showed that cleaning the screen with a pressurized sodium bicarbonate spray alone, without water, resulted in excessive damage to the screen. Performance clearly improved when the sodium bicarbonate spray was combined with a pressurized water spray for screens with solvent-based ink and water-based ink. Typically, the emulsion came off in stringy rolls, and ink flaked off rather than dissolved. A 100 in2 area took approximately 15 minutes to clean. Following this cleaning, there were either significant haze or ink residue spots. Slightly greater spray pressures or slightly longer times resulted in visual screen damage or a ripped screen. Cleaning of UV-curable inks was ineffective. No evaluation of subsequent use of these screens was made.
Based on these limited demonstrations, initial results indicate that with further testing and research, this may be a promising new screen reclamation technology. Some modifications are needed to clean the screens faster and with less possibility of screen damage. For example, the physical support behind the screen greatly reduced the stress on the mesh. Use of hot water was suggested as a means of improving emulsion removal. Other modifications may include decreasing the sodium bicarbonate particle size, or modifying the delivery rate and pressure of the sodium bicarbonate and water sprays. Further testing is needed before a definitive evaluation of performance can be given.
This case study was submitted by the US EPA's Design for the Environment Program to UNEP IE. It was edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.