Finger, Erik J., Tiyoda - Serec Corporation, East Providence, Rhode Island, April 1999, Revised February 2004
Traditionally, Powder Injection Molding (PIM) companies have performed their solvent debinding processes in Open Top Vapor Degreaser (OTVD’s). Since the Montreal Protocol most chlorofluorocarbons have been banned for use while most chlorinated solvents have been heavily regulated. Due to these regulations many PIM companies have found it increasingly difficult to utilize their current debinding equipment, namely OTVD’s.
For most large scale production needs, OTVD’s are not able to meet the emission or solvent usage guidelines documented by the government. This has caused PIM manufactures to seek alternative solvent technology for their debinding needs. This paper discusses the advantages of replacing OTVD’s with Airless™ degreasing systems for solvent debinding applications. It outlines the technical advantages, highlights the environmental and health benefits and relates how the control mechanisms can be utilized to increase throughput and reduce costs.
Over the past decade several original equipment manufactures (OEM’s) have proposed, airless and airtight systems. As defined in California’s South Coast Air Quality Management District’s (SCAQMD) Rule 1122, covering degreasing / debinding systems, airtight systems are basically OTVD’s with an over the top casing, capturing any open emissions. Airtight enclosures are also classified by their ability to withstand an internal pressure of 0.5 pound per square inch over atmosphere without leaking to the environment. Airless™ Vacuum Systems are classified as closed - loop systems and differ from the OTVD’s and airtight systems. These system s are defined as removing all atmospheric air from the degreasing / debinding chamber through a vacuum level of less than 25 torr (29.5”Hg) before solvent is introduced into the chamber effectively, eliminating all solvent / air mixtures. Traditionally in the OTVD’s, parts were transferred to the cleaning media by baskets, conveyors and / or overhead hoists. In a closed - loop system, the cleaning media is transferred to the parts to be cleaned in a sealed cleaning / debinding vessel.
Of the closed - loop systems available today, only one system is considered airless. Protected by U.S. patent Nos. 5,240,507 and 5,469,876 the Serec Corporation, based in Providence, RI, is the only OEM of airless vapor degreasing / debinding systems. A closed - loop system ot her than an airless system consists of an air - solvent interface throughout the system. In an air - solvent mixture you have a Vapor Vapor Liquid Equilibrium (VVLE) consisting of air, solvent vapor and solvent liquid. With the introduction of air, the thermodynamic properties become more complex to estimate, the 2 machine design becomes more indecisive, machine emissions increase and the process becomes more difficult to control.
Air, especially in humid environments contains moisture. Moisture at an air - sol vent interface has the ability to form an imiscible water layer on the chlorinated solvent’s surface. Water contamination as little as 50 - 100 ppm in such chlorinated solvents as Tetrachloroethylene, can form hydrochloric acid (HCl). HCl is highly corrosive to various types of metals which may damage expensive machine components. Additionally, acid formation deep in the pores of the PIM pieces could affect structural integrity.
In a Serec Airless™ closed loop system, as previously mentioned, air does not come in contact with the solvent. Air is removed from the system with the use of a vacuum package. Vacuum values of one torr or less are achieved for a removal of 99.97%. Without the presence of air in the system, full control of the temperatures and pressures are attained since only a pure substance (solvent) is present during processing. Temperatures and pressures are monitored and controlled by a programmable logic controller (PLC), so no monitoring or adjustments are required by the operator. Vapor concentrations and emission estimates are easily calculated. Troubleshooting and performance testing are simpler as well. Also, if no moisture is present, HCl cannot form. Based on our experience with water free solvent, Serec has successfully fabricate d the majority of its units using carbon steel as the material of construction. This air free, corrosion free, full control closed loop system is what makes a Serec system unique.
Airless™ Vapor Degreasing / Debinding Process Control Options
With a Sere c system the purchaser can equip its unit with what is necessary for its debinding operations. A standard dual debinding unit is shown in Figure 1.
Figure 1, A standard dual debinding unit.
Debinding machines that have been fabricated by Serec are e quipped with the following process control options:
- Vapor degreasing / debinding
- Warm stagnant soak
- Warm recirculating soak at variable circulation speeds
- Ambient stagnant clean soak
- Ambient recirculating clean soak at variable circulation speeds
Vapor degreasing / debinding Vapor degreasing / debinding is a necessary step for the operation of the Serec system. In a Serec system air is first removed from the cleaning chamber where the parts to be cleaned have been placed. Once the air is removed, pure clean vapors are introduced into the cleaning chamber.
The high temperature vapors immediately attack any cold surfaces within the environment due to the temperature differential. A detailed diagram is provided in Figure 2.
Figure 2, A conceptual diagram of a vapor cleaning application.
Once the higher temperature vapor comes in contact with the cold surface the vapor gives its heat to the lower temperature part, condenses and mixes with the solvent soluble wax 4 based contaminant. Remember, in a Serec system air is fully removed from the degreasing / debinding chamber, allowing full distribution of the solvent vapor over all lower temperature surfaces without the diffusional hindrance of air, especially for parts with complex geometries. With the aid of gravity, the wax solvent mixture that consists of a specific gravity larger than one, drains off the surface of the parts and is returned to the vapor producing sump called a Vapor Supply Tank (VST). Vapor degreasing / debinding continues to occur until the part reaches equilibrium temperature with the vapor environment.
Since PIM parts are porous, diffusion is not limited to the surface. As long as a heat differential is maintained within the part, diffusion of the solvent into the pores will continue. The additional process options cited are suited to aid in enhancing this diffusion process. Serec has a technological advantage in the understanding of flow through porous media and diffusional behavior since Serec originated from a 50 year old Impregnation company.
Warm stagnate soak Warm stagnate soak can be used to fully submerge the parts to be debound. For parts that are fragile and/or cannot be moved from its stacked position, soaking in a stagnate warm bath of solvent can enhance the diffusion rate without damaging the parts by movement. By submerging the parts, an infinite amount of high temperature low wax concentrated solvent is available for diffusion into the porous cavities as shown in Figure 3.
Figure 3, A conceptual diagram of a stagnate soaking application.
The thicker the part, the longer the diffusion rate may take to fully displace all of the binder contaminant for a series of deep non - uniform pores. For most PIM parts, soaking 5 is required due to the nature of the porous material. The solvent is supplied from and returned to the VST, and the temperature is controlled by the debinding temperature set point. For the vapor degreasing / debinding and the warm stagnate soak steps of the process, the removed contaminant collects in the VST. For high production units, the VST supplies a secondary distillation unit with the contaminated solvent for processing. The removed solvent is replaced with non - contaminated clean solvent from the storage tank to maintain a low concentration of solvent - contaminant mixture for cleaning. In the secondary still, the solvent is separated from the wax contaminate and the distillate is returned to the storage tank for reuse. The secondary still acts as the collection area for the waste. A full distillation will remove 95% or more of the remaining solvent from the collection vessel and the waste is manually drained into a waste receptacle. A common time allocated for a full distillation is 2 - 3 hours and is performed on an average, once a week.
For smaller debind units waste collection in the VST is permitted. Wax collection is at a minimum so the contamination level is below the threshold to allow continuous debinding. These units as well as the high production units will require a full distillation cycle once every one or two weeks.
Warm recirculating soak at variable circulation speeds The soak that is provided from the VST for the stagnate soaking process can also be circulated at variable speeds. By circulating the soak, a collection area of solublized wax in solvent particles at a pore’s entrance / exit location is prevented as shown in Figure 4.
Figure 4, A conceptual diagram of a re - circulating soak application.
Since diffusion is driven from a high concentration to a low concentration area, the removal of the collection area at the entrance / exit location helps to maintain a constant diffusion rate in and out of the pore. In the stagnate soak process the diffusion decreases over time since a collection area increases at the entrance / exit location (reference Figure 3) retarding the diffusion process. Maintaining a constant diffusion rate at the entrance / exit location decreases the diffusion time necessary for contaminant displacement compared to the stagnate soak option previously discussed. A re - circulating soak should be used when applicable. Variable speeds are available when different part configurations exist.
Ambient stagnate clean soak A stagnate ambient clean soak is sometimes required for specific PI M parts. PIM part manufacturing was originated to produce complex geometry parts that were previously too difficult to machine out of raw materials. Many of these complex geometries have the capability to cup solvent and contaminants. Traditionally, part s would be rotated creating a constant draining action to prevent puddles from gathering. PIM parts are fragile and in most cases must remain in a pre - stacked position for the following manufacturing step of sintering. By introducing a clean soak after a warm soak process we can aid in the flushing of any contaminant collection in a cupped area leaving only a near - pure puddle of solvent. Solvent collection is not an issue since the heat absorbed by the part is sufficient to flash any solvent puddle during the drying process. For parts that do not have the thermal properties to provide enough heat for drying puddled areas, Superheat is an additional available option that is discussed later in the article.
Soaking at ambient temperature will also give flexibility for a secondary degreasing / debinding process to occur within the same debinding process. For other degreasing systems this is commonly performed by spray. Here, as mentioned time and again, the force of spraying is not desired due to the fragility of the parts and their positioning therefore, stagnate soak is the best choice. The ambient temperature solvent cools down the part below equilibrium temperature initiating a final precision step of degreasing / debinding on the surface for removal of any remaining extracted binder contaminant residue from the pores prior to vacuum drying.
Ambient re-circulating clean soak at variable circulation speeds The ambient recirculating clean soak is available for the same reasons as the stagnate clean soak previously discussed. The additional benefit is a more thorough flush occurs with agitation.
Superheat Superheat is utilized to provide heat energy to the parts during drying through the use of superheated vapors. Vapors are superheated and distributed evenly over the surface of the parts with the use of the vacuum package. After all of the wax contaminant has been displaced from the part during the debinding portion of the process, solvent may still exist puddled in cupped geometries and/or within the parts pores. By the application of Superheat, the parts are capable of maintaining a high temperature to flash any remaining liquid solvent. Once the liquid is vaporized, it is removed by the vacuum 7 package. This process helps reduce or prevent solvent dragout after debinding which is a common, irremovable problem in OTVD and airtight solvent debinding equipment. Superheat is often used for larger high mass PIM parts.
Warm stagnate soak, warm recirculating soak at variable circulation speeds, ambient stagnate soak, ambient recirculating clean soak and superheat are time variable operations chosen by the operator.
Vapor removal follows the degreasing / debinding step. A vacuum is applied to the vapor rich cleaning chamber, removing the vapors, condensing them and transferring the liquid to a storage tank. As the pressure decreases in the degreasing / debinding chamber, the wet parts whose temperature is equal to the vapor temperature, gives its heat off to the thin film of solvent remaining at the p art surface. The liquid solvent reaches its vapor pressure as the pressure in the degreasing / debinding chamber decreases, and flashes off the part surface. This part of the process is known as drying. A vacuum level of one torr is repeated leaving les s than 25 ppm vapor concentration in the degreasing / debinding parts chamber. The parts are then removed clean, debound and dry. Weight removal recorded by PIM companies using our equipment is in the range of 5%.
Airless™ Vapor Degreasing / Debinding Performance on Porous Ceramics
Serec has performed extensive airless vapor debinding tests on ceramic porous parts. We have found that the ceramic parts are extremely temperature sensitive. Tests were performed at the 160 º F range. Frequently, test pa rts were cracking and / or breaking into several pieces at this higher temperature range. The higher heat application was vaporizing the solvent in the pores rapidly causing an overabundance of expansion, resulting in damage to the ceramic test part. By lowering the temperature, the heat application to the parts were reduced and the temperature differentials of the solvent and the debinder were minimized. The slower rate allowed the debinder to solublize with the liquid solvent at a controlled rate preventing any damage to the parts. Recorded weight removal for the ceramic parts were in the range of 9%.
The results of these tests are a principal example why full process control exhibited by the Serec system is important in sensitive cleaning applications. Serec systems have the ability to operate with any solvent with an atmospheric boiling point above 100 º F. Serec systems have been manufactured to operate solvents above their atmospheric boiling points at pressures above atmospheric pressure.
Environmental, Health and Safety
The Serec system is the industry leader for emission and solvent usage reduction. Serec is recognized by the State of California and the United States Environmental Protection Agency as both, Best Available Control Technology (BACT) and Lowest Achievable Emissions Rate (LAER). Our customers have reduced their solvent usage from tens of thousands of pounds to a single drum or less per year. Operator exposure is reduced to 8 approximately 1ppm or less per operating cycle which falls far short of the 200ppm one time exposure limits (for Tetrachloroethylene and Trichloroethylene) set by Occupational Safety and Health Administration (OSHA).
Because of the excellent solvent management capabilities of a Serec system, solvent change - out is unnecessary. In a traditional system, solvent degrades and as discussed earlier has the ability to turn acidic with the presence of air. In OTVD’s the air carries the moisture through the cold air blanket that is used to help control emissions. Part manufacturers would have to manually drain their existing system and perform physical cleaning. The solvent is handled and disposed of by the operators on a frequent basis, keeping their exposure time high. Since the Serec system is an airless syste m, solvent does not degrade and / or turn acid. For example, one of the first Serec systems was installed in July of 1995 in an aircraft manufacturing facility and is still operating with its original solvent. Although no equipment is maintenance - free, the Serec system is heated by steam in jacketed vessels. Electric heating elements are not used, eliminating this high maintenance item and maintenance staff exposure to the solvent. Typically, changing the vacuum pump oil is the only monthly maintenance item.
Economically, the Serec system has many benefits. Economic models are available at the customers’ request.
Although Serec systems are higher in capital cost than OTVD’s and airtight systems, operational economics generally erase this initial cost penalty in one year and the system generally returns all capital in 3 - 5 years through operational savings.
Labor to operate the Serec system is as easy as loading the parts, pushing the start button and unloading the parts at the end of a cycle. Once a cycle is initiated, the operator can be used for preparing the next load or any other task that may be required in the factory. Typical cost savings for labor is 50%.
Serec has provided debinding units with loading racks as shown in Figure 5.
Figure 5, Loading racks for PIM parts.
These racks are designed to accept parts that have already been preplaced on ceramic production trays. These racks eliminate costly time and labor that is used to transfer parts from process step to process step. Once the debinding process is complete, the ceramic trays with parts can then be removed from the rack and placed in the furnace for sintering.
For the above process and process options, existing debinding equipment is available in single and dual units. Current machines in operation using trichloroethylene have cycle times of 20 minutes plus soak time for an 800 pound production load. Larger and smaller unit designs are also available. These units require electric, compressed air, steam and cooling water. Boiler and chiller packages can be purchased as an option with the Serec system.
In conclusion, the Serec system is the front runner for debinding applications. The system is environmentally friendly, cost - effective and technological ly advanced. The system is the only airless unit available that will give you complete full process control, which will continue to allow further development of PIM part applications for larger, more complex and higher profitable parts.
About the author:
Erik J. Finger is the Vice President of the Tiyoda - Serec Corporation, N. Kingstown, Rhode Island. He received his BSME from Northeastern University, MS and PhD in Chemical Engineering from the University of Rhode Island. He has designed and developed Airless™ process equipment for over eight years. He can be reached at firstname.lastname@example.org.