Plastic Blast Cleaning Medias Finishing Equipment and Technology CRYOGENIC DEFLASHING: A REVIEW

Size: px

Start display at page:

Download "Plastic Blast Cleaning Medias Finishing Equipment and Technology CRYOGENIC DEFLASHING: A REVIEW"

Joseph York
6 years ago
Views:

1 Maxi-Blast, Inc. ISO 9001:2000 Registered Plastic Blast Cleaning Medias Finishing Equipment and Technology CRYOGENIC DEFLASHING: A REVIEW 3650 North Olive Road South Bend, Indiana (574) Fax No. (574)

2 A. Introduction 1. Rubber molders, like all manufacturers, are constantly striving for gains in productivity through technology advances. One area in which significant productivity gains can be realized is in the area of deflashing. 2. Prior to final inspection and packing, the vast majority of molded rubber parts must undergo a finishing process to remove excess rubber flash. This process is, therefore, termed deflashing. a. Manual Deflashing: Originally, deflashing was a manual operation. Dozens of workers, seated at small work stations, would take each part and trim the excess rubber off with scissors, knives or grinders. Hand deflashing has several drawbacks. The quality of work done by hand is often inconsistent. Customers, as evidenced by widespread shift to ISO standards, demand consistent, repeatable quality. 1. Another problem is scheduling in an environment of fluctuating production. If a company staffs the deflashing room for slow or moderate periods, they can expect a bottleneck in finishing during peak production periods. If they staff for peak periods, they must bear the financial burden of excess workers during the slower periods. Current customer buying practices i.e., just in time orders require the molder to be able to shift production levels smoothly and cost effectively. In addition, complex parts with complicated interior flash simply cannot be deflashed properly by hand. 2. Due to its labor intensive nature, hand deflashing has become more and more expensive. Increasingly complex part configurations and rapidly escalating labor costs are largely to blame. The answer, as is often the case, is to automate the process. b. Vibrators and Tumbling: Designing a machine to deflash rubber parts is difficult due to one basic fact about rubber flash. Rubber flash, unlike thermoset (thermodur/duroplast) plastic flash, is flexible at ambient temperatures and will bend and spring back without breaking. Traditional deflashing equipment used for thermoset parts will not work with rubber. The answer was found in the development of cryogenic (low temperature) technology. This technology uses cryogenic agents such as dry ice, carbon dioxide or liquid nitrogen to freeze flash. The embrittled flash can then be broken off through a variety of processes. 1. The first cryogenic deflashing machines were vibrators and tumblers. In these machines, the embrittled flash was broken off by the impact of the parts against each other, often in conjunction with some kind of media. Plastic or ceramic media are generally utilized; even pop (soda) bottle caps have been tried. In wide use today, vibrators and tumblers are effective on a broad range of parts, and they represent a significant productivity advance over manual deflashing. They are limited, in that they are effective primarily on outer dimensional (OD) flash. They are less effective on inner dimensional (ID) flash and on parts with complex configurations.

3 B. Cryogenic Shot Blast Deflashing: The First Generation 1. The most dramatic productivity advancement in rubber deflashing came with the development of the cryogenic shot blast deflashing machine. 2. In a shot blast machine, the parts slowly tumble on a continuous mill belt or in a barrel or basket and are simultaneously blasted with media propelled by a throwing wheel. The reusable media is then separated from the flash and recycled for blasting again. 3. At the time, shot blast machines were being used to deflash thermoset plastic parts and to deburr die castings. In order to be used on rubber parts, the machine was modified to inject liquid nitrogen into the blast chamber for flash embrittlement. The entire machine was then enclosed in a cold room designed to help insulate the machine and control liquid nitrogen usage. Typically, the media used was a fine mesh steel shot. 4. This machine yielded unprecedented levels of product through-put and eliminated a large number of manual deflashing personnel. Furthermore, it was extremely effective at removing ID flash and in deflashing parts with complex configurations. 5. Despite its productivity, the machine had several disadvantages. Relatively high cost (US $170,000 to $210,000) and a large space requirement put it out of reach for many molders. Wide fluctuations in temperature created moisture in the system and in conjunction with steel shot, caused a rust and contamination problem that usually necessitated a post-deflashing parts washing operation. Color-sensitive parts often could not be done at all. 6. The major problem was in the area of maintenance. The inherent abrasiveness of steel shot eroded the mill belt, wear plates and the elevator belts. Because the entire machine was enclosed in the cold room, major machine components such as motors and bearings were subjected to temperature variations and resulting moisture related problems. 7. In order to service the machine, it had to be completely shut down so a worker could safely enter the cold room area without the danger of suffocation in the nitrogen charged atmosphere. C. Cryogenic Shot Blast Deflashing: The Second Generation 1. Twenty years after the introduction of the original cryogenic shot blast deflasher, another major development in rubber deflashing technology occurred. 2. The new system featured a stainless steel cabinet insulated with dense polyurethane foam. Located inside the cabinet were the blast chamber with a continuous mill belt, the liquid nitrogen nozzle, and the opening for the throw wheel. Significantly, all major components of the machine (motors, bearings, etc.) were located outside of the cabinet and isolated from the temperature variations and moisture. In addition to reducing component failure, this new design allowed maintenance personnel to easily service the machine with no danger of suffocation.

4 3. The new system was designed to use plastic media (shot) instead of steel shot and featured a sophisticated vibrating screen media separation system, which allowed some flexibility in types and sizes of media. The use of plastic greatly reduced the maintenance problems associated with the abrasiveness of steel shot to the machine itself. Plastic media does not cause rust or part contamination, and part washing is often unnecessary. 4. The system provided high-quality, high-output deflashing with one sixth the space requirement, reduced nitrogen consumption. At a price of US $70,000 to $130,000, the new units were cheaper to purchase. They had wide appeal to molders of all different volume levels and product mixes and soon became accepted as the industry standard. D. Adjustable Operational Parameters 1. In addition to the type of shot blast system selected, engineers have four basic parameters to work with in striving for the most efficient, cost-effective deflashing operation possible. These parameters are: cycle time: wheel speed; media type and size; and temperature. These four parameters are so interrelated that analysis of any single one must take into account the effect of the others. a. Cycle Time: Ranges from 1.5 to 6 minutes. Factors that have a bearing on cycle time are: number of parts per batch (which is, dependent on the size of the parts); flash thickness; wheel speed; and type/size of media used. One way to select cycle time would be to set the temperature and media parameters, run at full wheel speed (assuming no part damage), and then begin with a short cycle time and work up until the cycle produces a 100% yield. 1. Consider a large molder with two of the first generation cryogenic shot blast deflashers. One machine is set up to use steel shot. The other machine has been modified to use plastic media. Since steel shot is significantly more aggressive than plastic, one would expect that the machine using plastic would have a longer cycle time. In fact, they operate with the same cycle time. The difference is that the steel shot machine runs with a 1,900 r.p.m. wheel speed, and that the plastic machine runs at 2,400 r.p.m. or more b. Wheel Speed: The speed of the throw wheel determines the impact force of the media particles against the parts. In the original, first generation machines, the wheel speed was often pre-set at 2,400 r.p.m., with some of the machines being altered in the field to provide variable control. Second generation machines had a variable speed up to 3,600 r.p.m. Although most parts are run in the 3,000 to 5,000 r.p.m. range, it is a general rule of thumb to use the maximum speed possible without damaging the parts. The reasons: to conserve nitrogen and to run as many parts per hour as possible. 1. In a classic case of user-generated technological advancement, several companies modified their second generation deflashers to operate up to 6,000 r.p.m. They reported that the extra r.p.m. made a startling difference in the performance of the machine. In one instance, cycle time was reduced from 6 minutes to 2 minutes. Subsequently, machine

5 manufacturers have begun offering units with up to 8,000 r.p.m. capability. c. Media Steel Shot: Most first generation deflashers still use fine mesh steel shot in the (0.127mm) range. Despite maintenance and contamination problems, steel shot is excellent in several applications. Steel shot is particularly effective on rubber-to-metal bonded parts, in which part of the rubber flash to be removed must be cleaned from a metal surface. It is also effective when the deflashing process is expected to remove any excess bonding agent from the metal. Steel shot is also used on parts such as multi-hole connectors where the media must be less than (0.356mm) to avoid lodging problems (sticking of media particles in part itself). d. Media Plastic: The most common type of plastic media used in cryogenic shot blast deflashers is cylindrical polycarbonate pellets. At the time the second generation deflashers were developed, polycarbonate media was being used in ambient shot blast deflashers on thermoset plastic parts. In order for this media to be used with higher wheel speeds and in temperatures down to -300 degrees F (-184 degrees C), it became necessary to make the polycarbonate more durable and impact resistant. This enhanced polycarbonate can effectively withstand 8,000 r.p.m. at low temperature with good life. 1. Polycarbonate media is available in 0.015, 0.020, 0.030, 0.038, 0.045, 0.060, and (0.38, 0.50, 0.76, 0.96, 1.14, 1.52, and 2.02 mm) cylinders: cylinder height is the same as cylinder diameter (0.76 mm) and (1.14 mm) are the most frequently used sizes. Factors which determine which size to use are: size of the parts, part wall thickness, flash thickness, and lodging considerations. For example, a part with a (0.76 mm) groove will have a lodging problem with (0.76 mm) media. A general rule is to use a larger size if no part damage occurs. The larger the pellet, the longer it stays in the system before removal by the separation system. Part damage is usually evidenced by chipped edges. Before reducing media size try adjusting wheel speed downward (assuming a corresponding increase in cycle time does not occur). 2. It has been determined by one large molder that the shape of the pellet has little effect on the deflashing action. When the pellet strikes the part, the force shatters the embrittled flash. This is a function of the mass and acceleration of the media particle. It is a misconception to think of the deflashing process as a cutting or scraping away of the flash. Deflashing is by impact, not by cutting action, and is fundamentally different than surface cleaning. 3. The second type of plastic media used is Styrene-Divinylbenzene Copolymer beads, which are spherical. This media is much less dense than polycarbonate and is used primarily on very delicate parts. Hearing aid parts are one example. This material ranges in size from.039 to.006 (0.99 to 0.15 mm) in diameter, and it can be used on parts that require very accurately sized, delicate media.

6 E. New Frontiers 4. To use media less the (0.76 mm) in most second generation deflashers, one must usually change the separator screen. e. Temperature: The temperature necessary to properly embrittled the flash will vary depending on the compound. SBR, butyl (IIR), nitrile (NBR), and polychloroprene (CR) compounds can usually be deflashed in the -70 degrees to -100 degrees F (-58 to -73 degrees C) range. Other compounds, such as silicones and EPDM, may require temperatures down to -300 degrees F (-184 degrees C). 1. The important task is, for each compound, to determine the embrittlement point as closely as possible; the embrittlement temperature may be somewhat higher than the Tg, or glass transition temperature. After a thorough study, one molder determined that it was going colder than necessary on almost 80% of its parts. Precision on this issue can mean large savings in nitrogen consumption. 1. The past ten years have been a highly productive period in the advancement of cryogenic shot blast deflashing technology. This period saw the development of refined second generation deflashers, cryogenic polycarbonate media, and the high r.p.m. throw wheels. 2. The pace of development is quickening. Innovative cryogenic deflashing equipment manufacturers are currently engaged in testing and innovation. Work is being done to make the equipment more efficient and productive. 3. Close study of the machinery s points-of-heat-transfer should yield advances in design and insulation. Work is being done to solve moisture and freeze-up problems. Better methods of controlling and measuring the flow of liquid nitrogen are being attained. Computer enhancements, such as programmable controllers with numerous deflashing recipes for automatic operation, are now widely used along with telephone modem hook ups and RS 232 ports that allow the use of printers for report generation. These innovations have enhanced function and facilitated the molder s transition into the world of ISO For companies molding parts with flash, cryogenic shot blast deflashing is an area with great promise for solving finishing problems and thereby gaining that competitive edge.

A deflashing media with high dimensional stability, which is also tough at low temperatures, is impact and abrasion resistant

A deflashing media with high dimensional stability, which is also tough at low temperatures, is impact and abrasion resistant Your Benefits at a Glance Emsodur, with its excellent properties such as high