The following 3 articles discuss the uses and procedures of various type of cryogenic finishing.
1) By Robin A. Rhodes, Cryogenic Institute of New England, Inc. rrhodes@nitrofreeze.com
Cryogenic Deflashing is employed to remove undesired residual mold flash that remains on molded parts after they are removed or ejected from the mold cavity. Typically, this flash occurs in areas where different sections of the mold come together (and apart) and is known as “parting line flash”. The remaining mold flash typically traces around where the different mold sections “mate” and is created when the liquid mold material escapes out of the mold cavity into the tight area where the mold sections press against one another.
Mold flash can be caused from old or worn mold cavities that no longer fit tightly together. Other times, the complexity of the part requires so many mating pieces with such precise geometries that it is almost impossible to create a perfect fit on every impression. Most often, the type of material being molded, and its attendant viscosity in its liquid form, is the primary factor that leads to the creation of the unwanted mold flash.
To remove flash, manufacturers can turn to a cryogenic deflashing process. This is often performed by an outside specialty finishing contract shop. By processing the parts in a frozen state, the flash becomes stiff or brittle and breaks away cleanly during its processing.
The process typically works as follows: Parts are loaded into a basket that is placed into an insulated chamber. Using liquid nitrogen, the temperature of the chamber (and the parts) is lowered to a programmable point, typically between minus 50°F and minus 200°F. The parts are tumbled at a predetermined rate (5 to 50 RPM) and blasted with a cryogenic grade polycarbonate media that is sized at between 0.015″ and 0.060″. The temperature, tumble rate, size of the blast media and process time varies widely depending on the part size and geometry as well as the type of material being processed.
Cryogenic deflashing offers the advantage of not degrading or otherwise affecting the finish of the parts. In addition, only the undesired flash is removed and the integrity of the part shape is fully maintained. Sharp edges are not rounded from the process and the media can penetrate into recessed sections and clean blind and through holes with remarkable precision.
A wide range of molded materials can utilize cryogenic deflashing with proven results. These include:
- Silicones
- Plastics — (both thermoset & thermoplastic)
- Rubbers — (including Neoprene & Urethane)
- Liquid Crystal Polymers
- Glass Filled Nylons
- Aluminum Zinc Die Cast
Typical examples of applications that use cryogenic deflashing include:
- O-Rings & Gaskets
- Catheters and other in-vitro medical
- Insulators and other electric / electronic
- Valve stems, washers and fittings
- Tubes and flexible boots
- Face masks & goggles
In some instances, cryogenic deflashing does not utilize a blasting action, relying instead only on the tumbling of the parts to remove flash on the outer edges. This early technique, still in use today, was widely employed in the rubber molding industry for automotive components.
Once developed, cryogenic deflashing process recipes offer consistent results from batch to batch. Cost per part is generally well below any alternative technique.
Today, many molding operations are using cryogenic deflashing instead of rebuilding or repairing molds on products that are approaching their “end-of-life”. It is often more prudent and economical to add a few cents of production cost for a part than invest in a new molding tool that can cost hundreds of thousand of dollars and has a limited service life due to declining production forecasts.
In other cases, cryogenic deflashing has proven to be an enabling technology, permitting the economical manufacture of high quality, high precision parts fabricated with cutting edge materials and compounds.
2) From the Fall 1997 issue of Cold Facts, by Rolf Wieland, Business Development and ALTEC International Expert Food and Cryogenics, Air Liquide Industrial U.S. LP:
Since almost all materials embrittle when exposed to cold temperatures, cryogenic size reduction utilizes the cold energy available from liquid nitrogen to cool and inert materials prior to and/or during the grinding process. All materials which due to their specific properties at ambient temperatures are elastic, have low melting points, contain volatile or oily substances, have low combustion temperatures and are sensitive to oxygen, are ideal candidates for cryogenic size reduction. In addition, due to the embrittlement of the product through the cold temperature, cryogenic size reduction allows much higher production rates, by decreasing the energy required to shatter the product, than when these same materials are ground under ambient temperatures. Cryogenic size reduction also permits smaller particle sizes, and tighter control of particle size distribution and shape.
Physical Properties of LN2
Liquid nitrogen is produced by the separation of air into its components in an air separation plant and is distributed in vacuum insulated transport vessels to the end user where it is stored in a vacuum insulated storage vessel till it is used. At atmospheric pressure, liquid nitrogen is at a temperature of -320 degrees F and possesses a latent energy content of 85.6 BTU/LB and a sensible energy content of 94 BTU/LB resulting in a total cooling energy content of 179.6 BTU/LB. Nitrogen is a non-flammable, non-toxic and inert gas which makes up 78.09% of the air we breathe. It has the characteristics of an inert gas, except at highly elevated temperature, and does not form any compound under normal temperatures and pressures. Drawn from the liquid phase, nitrogen generally has a purity of 99.998% with a dew-point less than -100 degrees F and is very dry.
Primary materials which require or are suitable for cryogenic size reduction:
Thermoplastics: To which Nylon, PVC, polyethylene, and polypropylene belong are commonly used in powdered form for, but not limited to, a variety of applications, such as: adhesives, powdered coatings, fillers, resins and plastics sintering and molding. These powders generally can only be produced in high production rates and fine particles sizes utilizing cryogenic size reduction.
Thermosets: To which natural and synthetic rubbers belong are important recyclable materials. Under cryogenic size reduction, these materials can economically and at high production rates be ground into fine powders and, used as a filter, be recycled.
Adhesives and waxes: These materials at ambient temperatures are generally pliable and sticky and when ground would form excessive deposits in the mill building up heat, increasing energy requirement and eventually shutting down the size reduction process. Under cryogenic temperatures these products become brittle and can be pulverized with much less energy and without forming deposits.
Spices: Spices contain essential oils which give them their flavor and aroma but which in an ambient size reduction process, due to heat buildup, become volatile and tend to form deposits and gum up the mill. In addition, the volatility of these aromatic substances causes them to evaporate and thereby reduces the quality of the ground product. In the case of nutmeg and sesame seeds, which both have a very high oil content, grinding under ambient temperature quickly releases these oils forming a pasty mass which quickly gums up the mill, causing it to shut down.
In addition, the essential oils contained in spices quickly react with the oxygen in the air and would oxidize, thereby again reducing quality. Utilizing cryogenic size reduction, the volatility of the essential oils is vastly reduced, thereby maintaining aroma and flavor. In addition, mill deposits and gumming up are eliminated thereby allowing high production rates.
Explosives: Explosives explode when their ignition temperature, in the presence of oxygen, is achieved. Cryogenic size reduction performs two tasks when grinding explosives: 1) it reduces the temperature of the material well below its ignition temperature and removes the oxygen from the system, thereby eliminating the possibility of combustion.
Typical Cryogenic Size Reduction System Design:
The product to be ground is filled into the volumetric screw feeder where it is metered at a specific rate into the cryogenic pre-cooler. In the cryogenic pre-cooler, liquid nitrogen is injected and combines with the product, thereby cooling and embrittling the product. The product is then transported, along with the cold gas generated by the evaporation of the liquid nitrogen, to the grinding mill where it is pulverized. The pulverized product then goes through a classifier where it is separated into various particle sizes and packaged. Should oversize material exist, this can be fed back into the volumetric feeder and recycled into the system. The cold gas from the mill is recycled through the filter or bag-house and as makeup air back into the mill. Excessive cold gas is vented out. In addition, the cold dry nitrogen gas keeps both the classifier and bag-house free of moisture and inert, preventing the possibility of dust explosions and buildup of product.
The primary components of a cryogenic size reduction system are the volumetric feeder, cryogenic pre-cooler, grinding mill classifier and the cold gas recirculation. The right combination of these primary components determines the efficiency, productivity and operating costs of the system.
Plant design depends primarily on the type of product, desired production rates and the required particle size distribution. There are numerous grinding mills available, such as, but not limited to, pin, hammer, attrition and beater mills. These mills each have very specific characteristics which influence the production rate, grinding efficiency, particle size and quality of the product to be ground. Grinding mill construction needs to take cryogenic temperatures, and the effect these cold temperatures have on the materials of construction, into consideration.
The key to an efficient, cost-effective assembly of a cryogenic size reduction system is experience and a very good understanding of cryogenics and what effects this has on the product to be ground, size reduction physics, mill design characteristics as well as product flow characteristics.
Understanding Cryogenic Deflashing Systems:
Cryogenic deflashing has been in general use since the 1950s and is a highly efficient process for the removal of skins and flashes from molded rubber and plastic parts. Flashes on rubber and plastic parts are generated in the molding process where the two sections of the mold come together. The flash configuration is determined by the mold quality and the extent to which the mold sections fit together or have worn. In the cryogenic deflashing process, molded rubber parts are loaded into a deflashing chamber where they are tumbled, cooled by means of either liquid nitrogen or liquid carbon dioxide injection to approaching their glass transition temperature, and simultaneously interact with one another or are blasted with a blast media.
The interaction of the parts with one another removed external flashes only, whereas blasting the parts with media removes both internal and external flashes. Once the parts have been processed for a specific time cycle, the parts are unloaded and continue in the production cycle. The cryogenic deflashing processes can be effectively used for precision deflashing of complex parts, or for parts that do not require fine tolerances. Designed for a wide range of end user applications, it offers high operating efficiency for both large and small batches and consolidates coarse and fine deflashing in one operation. All types of rubber, plastic, as well as rubber to metal bonded parts, which become brittle at low temperatures, are suitable for cryogenic deflashing.
Deflashing Equipment
The selection of good cryogenic deflashing equipment is a crucial part of an efficient deflashing operation and determines the economic impact of said operation. The primary types of commonly used deflashing equipment are:
Cryogenic Tumblers: In these units, the parts are cooled in a rotating hexagonal drum with liquid nitrogen or liquid carbon dioxide to a specific temperature and are tumbled for a specific period of time only. The tumbling action allows the parts to interact with one another, removing external flashes. In some cases media such as ball bearings or various ceramic shapes are added to the parts for the purpose of internal flash removal. These units are generally low cost and operate efficiently where only external flashes are present and where medium-to-low-quality deflashing is required. This type of cryogenic deflashing unit is becoming less common due to its limited deflashing ability and will not be covered in the remainder of this article.
Mesh Belt Type Blast Deflashing Units: In this type of equipment the parts are loaded into a u-shaped rotating mesh belt chamber where they are cooled with LN2 or LCO2 to a specified temperature and then impacted or blasted with plastic media accelerated to a specific velocity by means of a variable speed impeller wheel, usually located directly above the parts, to remove internal and external flashes. The blast media is recycled and cleaned of flash externally.
Drum or Basket Type Deflashing Units: In this type of unit the parts are loaded into a perforated drum or basket where the parts are rotated and moved multi-directionally within the drum via mixing flights, colled with LN2 to a pre-specified temperature and then impacted or blasted with plastic media accelerated to a specific velocity by means of a variable speed impeller wheel located either in the back wall or in the front opening of the drum or basket.
Several factors differentiate these cryogenic blast deflashing units. The primary factors which have the most significant impact on efficiency are:
Part movement in the deflashing chamber. The part movement within the deflashing chamber is very important because it determines how the parts are exposed to both the cold and the blast media. The more intense the movement of the parts in the deflashing chamber, the more consistent the deflashing quality and the shorter cycle time required to deflash the parts.
In mesh belt type machines the parts are rotated in one circular direction only. The parts build a cylinder which results in overexposing the outside of the cylinder to the cold and blast media and underexposing the inside. This is turn results in inconsistent deflashing quality, reduced load capacity and extended cycle times.
In basket of drum type machines the parts are moved in the deflashing chamber by means of rotation and mixing flights which in turn moves the parts not only from the bottom to the top of the chamber but also from the rear to the front and back to the back of the chamber, generating a dual axis movement of the parts. This intense dual axis movement of the parts results in an even exposure of the parts to both the cold and blast mdeia which in turn achieves a high degree of deflashing quality in a minimal amount of cycle time.
Insulation: Due to the extreme cold temperature at which most cryogenic deflashing equipment operates, it is necessary to insulate all of the cold areas of the deflashing unit that are exposed to atmosphere. Any areas which are not insulated well transmit heat from the outside environment to the inside of the unit, requiring additional liquid nitrogen to maintain deflashing temperatures, thereby higher than necessary nitrogen usage. In addition, insufficiently insulated surfaces, exposed to the atmosphere, collect moisture in the form of ice, which when the unit warms up, thaws and results in water collecting in the unit, generating shot and parts flow problems, ultimately shutting down the unit. When selecting cryogenic deflashing equipment, evaluate how well the unit is insulated and sealed against moisture penetration. This will minimize problems with shot flow, moisture, excessive nitrogen consumption and down time.
Blast media handling: The blast media is the workhorse of a cryogenic deflashing unit and if blast media flow is impaired, deflashing of the parts will be inconsistent or cease totally. There are three main factors which impair shot flow: moisture, flash saturation of media and blast media containment within the unit; one may result in the other, and vice versa. Moisture enters the system through breaks in the media transfer mechanisms or when the unit is opened for loading or unloading. If the unit is not well insulated, this moisture, usually presented as ice, thaws and freezes, causing the media to clump together. Flash saturation occurs when the unit’s separator can no longer perform its function. This is usually caused by moisture or ice collecting on the separator. Once flash or dust saturates the media, it can no longer flow correctly and tends to cake.
Temperature monitoring and control: it is essential for efficient and consistent deflashing quality to maintain an accurate deflashing chamber temperature. Precise and accurate deflashing chamber temperatures are achieved through a) Precise placement of LN2 injection orifices in reference to part and blast media distribution within the deflashing chamber. b) Insulation of the deflashing chamber against LN2 leakage and heat infiltration. c) Precise placement of the temperature sensor to allow accurate monitoring of part and chamber temperature. It is essential that temperature sensing devices not be located where they are exposed to direct LN2 injection stream but monitor the actual chamber temperature.
Deflashing cycle parameters: Determination of the ideal deflashing cycle parameters is essential to the efficient utilization of the primary cost factor in cryogenic deflashing – liquid nitrogen consumption. Cycle time and temperature play the greatest role in the deflashing cycle and have the greatest impact on operating costs. The following table illustrates this.
3) From the Spring 2010 issue of Cold Facts. Submitted by the Cryogenic Institute of New England.
In every manufacturing operation, burrs are created during machining processes including grinding, drilling, engraving, milling and turning. Burrs are small pieces of material that remain after machining. In most cases, residual burrs will cause otherwise acceptable parts to fail quality standards.
As time has progressed, solutions to remove burrs have become more advanced and have further increased efficiency. Hand deburring, which requires steady hands, painstaking precision and significant amounts of time, has given way to new technologies. Many of these new processes are capable of producing high-quality results in a shorter amount of time, while eliminating the human/operator variable. One of these relatively new technologies is cryogenic deburring. Cryogenic deburring requires liquid nitrogen to assist in the removal of burrs from parts made of plastics, polymers, composites and metals.
The cryogenic deburring process can be summarized in the following steps. A batch of parts is loaded into a basket, which is then inserted into the machine. The door to the system is closed and the temperature is lowered using gaseous nitrogen via an onboard computer. Once the required temperature is achieved, media begins to blast at the parts while they tumble in the basket. This then continues for a programmed amount of time. Upon completion of the cycle, the parts tumble for a set amount of time to help dislodge any residual media. The basket is then removed from the system and the parts are placed into a work holder, where they are allowed to return to ambient temperature.
In the cryogenic deburring application, liquid nitrogen serves several functions to make the process feasible. First, liquid nitrogen allows the chamber of the cryogenic deburring system to be cooled to or near to the material freeze point of the parts being processed. Liquid nitrogen is pumped through a vaporizer before reaching the cryogenic deburring system. At this point, the liquid nitrogen is converted into a gas.
This allows for a gaseous nitrogen atmosphere within the cryogenic deburring chamber, which will safeguard the parts from thermal shock. Second, since liquid nitrogen cools the parts to be processed to near to or at their respective material freezing point, the parts will not be damaged. As the media is blasted at the parts, the burrs will be removed. The surface finish of the parts will look the same as before they had been processed.
Third, the conversion of liquid nitrogen to gaseous nitrogen allows for the flow of media throughout the machine. The media is pushed under pressure by gaseous nitrogen through the throw wheel of the cryogenic deburring system. The advantage of using nitrogen is that the media and parts to be processed will not become contaminated.
The fourth and final benefit is that liquid nitrogen makes the process environmentally friendly. The air that we breathe is made up of roughly 78% nitrogen. Therefore, cryogenic deburring utilizes a renewable resource to complete the process of burr removal without any pollution.
The cryogenic deburring process is a modern, efficient way to remove burrs from precision machined components, providing consistent and repeatable results from part to part while retaining surface finish and critical tolerances.
