Bulletin 1 - Technology Alternatives for Screen Reclamation
The screen reclamation process can be one of the most hazardous operations in a screen printing facility. Typically, highly volatile solvents are used which may be hazardous to the health of employees if inhaled, ingested, or absorbed through the skin. Th ese products may also be hazardous to the environment if they are not disposed of properly. Traditionally, when reclaiming screens, employees vigorously scrub the screens in a wash-out booth, with their faces close to the reclamation chemicals. This incre ases the likelihood that they will inhale the chemical vapors.
To reduce the hazards of screen reclamation to workers and to the environment, screen printers can use alternative techniques for screen reclamation. These technologies help to reduce the employee exposure to hazardous chemical vapors either by speeding u p the reclamation process, or by enclosing the process, or by eliminating the use of volatile solvents.
The DfE Screen Printing Partnership identifies several potential substitute technologies that can be environmentally safer than traditional screen reclamation, including: high pressure water blasters, automatic screen washers, sodium bicarbonate spray, media blasting, pulse light energy technologies, stripping technologies, and emulsion chemistry.
This bulletin highlights three of these technologies:
High pressure screen washers and automatic screen washers are two commercially available technologies that can reduce a facility's usage of traditional solvent-based ink removers. Sodium bicarbonate spray is a technology now under development that could f urther reduce the costs and potential health risks of screen reclamation. This bulletin provides comparative cost, performance and risk information for these reclamation technologies, when available.
It should be noted that these technologies were evaluated using a case study approach; these were not rigorous, scientific investigations. Instead, much of the information presented here is based on printers' opinions of these technologies as they are use d in production. This bulletin compares the alternative screen reclamation techniques to manual application and scrubbing of traditional screen reclamation chemicals. The traditional system used in the comparison consists of: lacquer thinner as the ink re mover, 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.
|High Pressure Screen Washers|
High-pressure screen washers reclaim screens using pressurized water, usually in conjunction with some reclamation chemicals. Typically, excess ink is carded off the screen prior to cleaning. No ink remover is applied to the screen. An emulsion softener o r remover is applied and allowed to work, usually for from ten seconds to one minute. The ink and stencil are then removed with 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, in the systems evaluated, the emulsion and haze removal products were formulated to allow discharge to sewers. Where ink residues in the rinse water exceed wastewater permit concentration limits, su ch as for suspended solids, manufacturers also supply a variety of filters. The greatest environmental benefits are realized for systems using improved filtration systems which allow rinse water to be reused. Filter wastes are typically disposed of as haz ardous waste.
In general, the benefits of high pressure washers are that they reduce both chemical use (eliminating ink removers) and worker exposure (less scrubbing required). The DfE Screen Printing Partnership found that the occupational risks of this system were notabl y lower than the risks associated with the manual application of traditional solvent-based reclamation chemicals. For the traditional screen reclamation system, health risks associated with both daily inhalation and skin contact with the chemicals, partic ularly 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 .
Switching to this type of screen reclamation technology can reduce both your facility's releases of hazardous materials and your regulatory burden by reducing the amount of cleaning solvents you use. Contact your state and local regulatory authorities for information specific to your location.
Performance of a high pressure water blaster was evaluated by DfE staff at a volunteer printing facility where the technology was in place. Overall, the high-pressure screen washer reclaimed the screen efficiently and effectively. When demonstrated on scr eens with solvent-based, water-based inks, or UV-curable inks, the stencil dissolved easily, leaving no emulsion residue. Ink stains on these screens were completely removed by the haze remover even before the waiting period or pressure wash.
The DfE Screen Printing Partnership also estimated the cost of equipment, labor, and chemicals for the high pressure wash. Assuming that 6 screens are reclaimed daily and each screen is 15 ft2 in size, the cost estimate for the high pressure washer totaled $4 .53 per screen reclamation. 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 of the traditional syste m 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 from $1,300 to $12,000, or maintenance and operating costs which may also vary widely.
|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 Partnership 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 auto mation (such as conveyors), system capacity, and complexity of the equipment.
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 the health risks associated with skin contact of the chemicals remained high, these risks could virtually be eliminated if gloves are worn while handling the screens. Since the automatic screen washer evaluated was used for ink removal only, the risks associated with emulsion and haze removal remained the same as the traditional system's risks for these steps.
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 all the equipment that is commercially available, performance demon strations of automatic screen washers were not conducted in this project.
The DfE Screen Printing Partnership estimated costs for two automatic screen washers, assuming that the washers were used for ink removal only and that six screens (15 ft2 each) were reclaimed per day. Screen reclamation costs using an automatic screen washer ranged from $4.13 to $10.14 per screen compared to $6.27 for traditional reclamation. The largest cost component, and the cause of the variability in costs, is typically equipment cost. For many print shops, especially hi gher volume printers, the equipment pays for itself through savings in reduced chemical use. 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 onc e 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.
|Sodium Bicarbonate Spray|
| A sodium bicarbonate (baking soda) spray technology
was evaluated by the DfE Screen Printing Partnership to determine
if it is potentially adaptable as an alternative screen reclamation
technology. This technology is currently being 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 success of the sodium bicarbonate spray in
other applications, it appears to be a promis ing substitute for chemical
screen reclamation systems. Because the sodium bicarbonate spray technology
has never been tested for screen reclamation, DfE staff conducted
a one-day site visit to the equipment manufacturer's facility. Three
imaged screens w ere inked with three types of ink. Each inked screen
was individually placed inside an enclosed cleaning booth, and the
screen was passed, back and forth, under the sodium bicarbonate spray.
No other chemicals other than the sodium bicarbonate were used d uring
The DfE project did not undertake a risk assessment of this 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.
Several different methods for screen reclamation with the pressurized sodium bicarbonate spray were demonstrated. Performance was best when the sodium bicarbonate spray was delivered through a pressurized water spray. Typically, the emulsion came off in s tringy rolls, and ink flaked off rather than dissolved. A 100 in 2 area took approximately 15 minutes to clean. Following this cleaning, haze or ink residue spots remained. Cleaning of UV-curable inks was ineffective. No ev aluation of subsequent use of these screens was made.
Based on these limited demonstrations, initial results indicate that with further testing and research, this may develop into a promising new screen reclamation technology. Modifications are needed to reduced the cleaning time required for reclamation and to reduce the 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 th e 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.
Since the available equipment was not designed specifically for screen reclamation, it was 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 cost of 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 purchased, and approximately one pound is sprayed per minute. If this technology p roves to be a feasible alternative for screen reclamation after further developments, a more detailed cost analysis can be conducted.
For more information
For more information on the technologies discussed here, contact your equipment suppliers, For more detailed information on other technological and chemical alternatives, see the summary booklet, Designing Solutions for Screen Printers - An Evaluation of Screen Reclamation Systems. Additional bulletins are also available. For hard copies of bulletins, other DfE materials, or more information about the Screen Printing Partnership, please contact:
Pollution Prevention Information Clearinghouse
Screenprinting and Graphic Imaging