US3432968A - Liquid nitrogen deflashing apparatus - Google Patents

Description

March 18, 1969 C, P, CROFT ET Al. 3,432,968 n LIQUID NITROGEN DEFLASHING APPARATUS Filed March 22, 196e sheet of 2 March 18, 1969 c. P. CROFT ET A1. 3,432,968

LIQUID NITROGEN DEFLASHING APPARATUS Filed March 22, 196e sheet 2 of 2 WJ 52 A [0% NZH Y if? W w 24 20 lfi' I I g l -nil 'l u' l ,1,

iii tates LIQUID NTROGEN DEFLASHING APPARATUS Charles P. Croft and William W. Long Hl, Hagerstown,

Md., assignors to rThe Pangborn Corporation, Hagerstown, Md., a corporation of Delaware Filed Mar. 22, 1966, Ser. No. 536,349

U.S. Cl. Slt-8 9 Claims Int. Cl. B242 3/00; B24b 3]/00 ABSTRACT F THE DISCLOSURE This invention relates to an apparatus for removing trimmings, flashings, ns, etc., from molded resilient pieces, and particularly to such an appar-atus which applies liquid nitrogen to the pieces for freezing the excess material so that the pieces can be readily deflashed.

Apparatus for removing excess material from molded pieces by supporting and turning the pieces while they a-re subjected to the application of a freezing medium, have been devised heretofor. In these, the excess material is frozen and is easily removed by an abrasive blasting treatment. Some examples are described in U.S. Patent Nos. 3,110,983 and 2,996,846. Carbon dioxide or liquid nitrogen is used as the rfreezing medium.

The use of liquid nitrogen as the freezing medium has a number of distinct advantages over carbon dioxide. For example, a liquid nitrogen system is more economical when production volume is high enough to justify full time use of the system. Since liquid nitrogen boils off at the rate of 1 percent per day, intermittent use is not recommended. However, despite this boil-off, liquid nitrogen is still more economical.

Liquid nitrogen has a higher heat transfer coefficient than carbon dioxide with reference to the same part. This same relationship is true for example, when comparing nitrogen gas with carbon dioxide gas. Additionally, a larger percentage of the liquid nitrogen provides effective cooling than with carbon dioxide. The liquid nitrogen adheres to the workpieces and internal parts of the blast cornpartment as it boils off causing the refrigerant action; the carbon dioxide converts into 54 percent gas and 46 percent solid particles and it is the solid particles only that provide effective cooling. Since the dispersion of the solid particles of the carbon dioxide is a questionable point and since it is a known fact that some of these particles drop out of the blast compartment and never provide refrigerant action to the workpieces, equal weight of liquid nitrogen and carbon dioxide provide different cooling effects. Liquid nitrogen provides considerably greater temperature reduction. Moreover, lower temperatures are available from liquid nitrogen and therefore a wider selection of work can be deflashed; including silicone rubber compounds.

The net result of the above features is that less liquid nitrogen is required per lbatch and usually the batch cycle times are shorter. It is unrealistic to assign finite values to the generally expected savings of a liquid nitrogen system over a carbon dioxide system. The user, his operators, and the users plant environment, not to mention the configuration of the various parts to be deflashed, put enough variables into the question to make a clear-cut answer imatent O 3,432,968 Patented Mar. 18, 1969 possible. However, a reasonably broad test guide might be 20 to 50 percent less liquid nitrogen is required per batch for a 10 to l5 percent shorter batch cycle.

An ideal liquid nitrogen control system should take into account the following considerations: The shortest possible cool-down time should be maintained when rst commencing operation. The components of the system should be compactly arranged and sufliciently insulated to minimize, or efficiently utilize, the inherent Iboil-off of the liquid nitrogen. The arrangement of the components should 'be such that there is no entrapment of the liquid nitrogen between points Where there is no pressure relief available. The distribution of the liquid nitrogen in the blast compartment should be uniform. The liquid nitrogen should be in the liquid state when it reaches the workpieces to effect the maximum cooling, and yet the spray device should be located in a position within the blast compartment so as not to obstruct the tumbling action of the work or the blast stream. The flow of the liquid nitrogen should be controllable by varying the flow rate and time cycle in proportion to the rate of B.t.u. dissipation required for a particular workpiece (to assure that the body of the work undergoes a minimum temperature drop and therefore does not become brittle; while at the same time, the flash undergoes a temperature drop adequate to embrittle it so 'that the impact of the abrasive will fracture the flash for rapid removal).

When considering the advantages and disadvantages of the liquid nitrogen system of the above ideal type as cornpared with carbon dioxide, the importance of uniform refrigerant distribution should 4be taken into account. It is more important in a liquid nitrogen system to have uniform distribution of the refrigerant in the blast compartment than for a carbon dioxide system. When liquid carbon dioxide (at 300 p.s.i.g. pressure and 0 F.) is forced from the nozzle of the header and enters the atmosphere, it converts to 54 percent cold gas (-109" F.) and 46 percent solid particles. These particles drop down in among the workpieces, and it is this physical contact that allows the greatest heat transfer. The tumbling action of the workpieces aids in the equal distribution of the solid carbon dioxide particles. Since the heat transfer coeicient between the work and solid carbon dioxide particles is lower than that of liquid nitrogen and the work, then the carbon dioxide system must tumble the parts longer to effect the same degree of cooling. This longer tum-bling cycle allows more time for 'the carbon dioxide particles to distribute themselves. In the case of liquid nitrogen, however, there is a faster heat transfer from the work 'to the liquid nitrogen and therefore the work is cooled in a shorter period of time. Since there is a relatively less tendency for the liquid nitrogen to distribute among the work once it has contacted a workpiece and the time for distribution is shorter, the initial spray of liquid nitrogen in an ideal system must be evenly distributed from the header in the very beginning.

As can be appreciated, the lack of flow rate control in prior systems is a serious drawback. For example, lack of a precise ow rate control device in the system will make the accomplishment and repeatability of accurately controlled injections of liquid nitrogen into the blast cornpartment very diflicult. Additionally, lack of such a device will make maximum economy impossible. Moreover, a system without a flow rate control is dependent on the pressure of the users storage tank to determine the weight of liquid nitrogen that will flow through -a piping arrangement of fixed cross-sectional area.

An object of this invention is to provide a liquid nitrogen deflash-ing apparatus which fulfills the above criteria of an ideal system.

A further object is to provide such an apparatus in which the rate of flow of the liquid nitrogen is accurately and conveniently controlled.

Novel features and advantages of the present invention will become apparent to one skilled in the art from a reading of the following description in conjunction with the accompanying drawings wherein similar reference characters refer to similar parts and in which:

FIGURE 1 is a section of one embodiment of this invention;

FIGURE 2 is a top view of a portion of the apparatus shown in FIG. `l;

FIGURE 3 is a section of the portion of the apparatus shown in FIG. 2; and

FIGURE 4 is an enlarged detail partially sectioned of a header used in the apparatus of FIGS. 1-3.

lAs shown in FIGURE l, lapparatus 1t) is generally of the type described in Patent No. 3,110,983. Apparatus 410 includes a dehumidifying housing 12 and a tumbler `14 which supports and turns the workpieces. The freezing medium is applied to the Workpieces through spray nozzle or header 16, while the embrittled excess material or flashing is removed by blast machines 1S which propel abrasive material against the cooled workpieces.

The liquid nitrogen system is best illustrated rin FIG- URES 2 3. As indicated therein, the liquid nitrogen is supplied through pipe 20, past relief valve 25 and into entrainment separator 22 which maintains as nearly as possible 100% liquid nitrogen to the header 16. From the entrainment separator 22 the liquid nitrogen ows through conduit 24 past flow control valve 26 and is eventually discharged from header 16 inside the housing 12 against the workpieces 28. Advantageously a shut-off control valve 30 is provided in line 24. The system also includes a vent line 32 which communicates between entrainment separator 22 and tumbler v14. A solenoid valve 34 is provided in this vent line 32. The various valves may be conveniently controlled through control panel 36 provided on the outside of housing 12.

An important feature of this invention is the provision of flow control valve 26. When designing for a certain cool-down rate it ris best to design for an optimum time. It would Ibe easy to design a system having no flow rate control and having all components of a thin walled construction. In such a system, all components would cool very quickly. However, a iow rate control is necessary if the system is to have broad range capabilities for a wide range of workpieces. Additionally, as later described, there are distinct advantages to having some of the components of a thick walled design. Flow rate valve 26 is so constructed that the liquid nitrogen system operates substantially independent of storage pressure. Of course, any sizable change in storage pressure will affect the flow. The important point being, however, that in the system of this invent-ion the flow rate is not dependent on the storage pressure change for a change in ow rate. Advantageously the desired ow rate can be set by the operator through panel 36. Flow rate valve 26 operates 'by the pressure from carbon dioxide which ows through line 38 to chamber 40 to control the Imovement of valve element 42 disposed in liquid nitrogen line 24. The ow rate valve 26 may be for example, a parabolic needle type valve in which a pressure indicator is calibrated to establish a linear relationship between the pressure of the carbon dioxide and the extent of valve closing of valve element 42 in line 24. Valve element v42 may for example, be attached to a moveable diaphragm wherein the movement of the diaphragm 39 is determined by the amount of carbon dioxide and thus the pressure in the upper portion of chamber 40. The flow of carbon dioxide is controlled by manual control valve 41, adjacent pressure indicator 43 (FIG. 3).

As indicated above, control of ow rate of liquid nitrogen is maintained by the use of a diaphragm operated How control valve 26. The diaphragm, which controls the parabolic orifice in the ilow control valve is activated by the use of dry carbon dioxide gas where orifice opening and/or ow rate are in direct proportion to the carbon dioxide pressure. The carbon dioxide pressure is controlled manually by the operator at control panel 36.

By the provision of ow rate contr-ol valve 26, the system of this invention is more economical than if the system were dependent on storage pressure to change the ow. For example if the system were dependent on storage pressure any time a different ilow was required, the storage tank pressure would have to 'be changed. When an increase in pressure were required, the pressure gauge setting on the storage tank would have to be adjusted and the operator would have to wait for the pressure to build up. When a decrease in pressure were required, then the storage tank would have to be vented and nitrogen gas expelled. The waste that occurs during this adjustment varies with many factors; a few of which are the liquid level in the tank, the change in pressure desired, and the level of saturation of the nitrogen gas in the tank.

Another important feature of this invention is the provision of header 16. As best illustrated in FIGURE 4, line 24 terminates in T mounting 44. A nozzle '116 is for example threadably secured in each leg of mounting 44. The threaded engagement permits the nozzles to be rotated to change their orientation as required for different workpieces. As later described, nozzles 46 are thick Walled having for example, a one inch outside diameter and a 214,4 inch inner diameter. Thus, the spray aperture 48 are for example, approximately 11/,2 inch long. Additionally, the diameter of each spray aperture l48 decreases from T mounting 44 toward the outer end of each nozzle 46. The distance between each aperture -48 may be for example, two inches.

The above described header 16 is speciiically designed to provide even distribution of liquid nitrogen. Normally liquids undergo a pressure drop as they move from the center T of the header toward the extreme ends. Liquid nitrogen, however, actually increases its pressure by its expansion from a liquid to a gas. The size and arrangement of apertures 48 are designed to compensate for this characteristic and thus assure an even flow from the header along its entire length.

Not only is the design of the header so very important for the reason described above, but its ability to maintain the greatest percentage of nitrogen in the liquid form right down to the time when it contacts the work, is another key function of header 16. In this sense, the thick walled construction of nozzles 46 is not in accordance with the generally accepted practice of manufacturing the header of a thin walled tube. This departure to thick Walled tubing, however, has two great advantages. First, the spray apertures 48 provide an orice effect that propels the liquid nitrogen greater distances more effectively with less vaporization than a hole in a thin walled header. Secondly, the thick walls tend to lessen the heat transfer from the atmosphere to the liquid nitrogen within the header. Combined, these two features permit the nitrogen to be delivered in a liquid state to the work.

With header 16 it is possible for example, to provide a system wrich requires 9 pounds of liquid nitrogen and only about 20 minutes to cool down to an operating temperature of -320 F. Thus, the deflashing can take place with greater eciency than in prior apparatus.

The effectiveness of the header 16 would be lessened if 1GO percent liquid -tlow were not provided to the header. As described hereinafter, entrainment separator 22 maintains a liquid level at all times and therefore there is 10() percent liquid fiow through the flow rate valve 26 to the header 16 every time shut-olf valve 30 is opened.

An important advantage of this invention is the utilization of any boil-oit to act as supplementary coolant. Since liquid nitrogen will boil-orf despite any reasonably economic preventive methods, this system not only minimizes the boil-off but actually uses the nitrogen gas to aid in the cooling action. In this respect, the system is physically compact and well-insulated. For example, as shown in FIG. 2 entrainment separator 22 has foamed-in-place polyurethane foam 50` bet-Ween the inner and outer stainless steel walls 52, and the entrainment separator 22 is designed for minimum external surface for minimizing heat transfer from the atmosphere to the liquid nitrogen therein. The boil-off gas in entrainment separator 22 is advantageously conveyed through line 32 to the tumbler 14 (FIG. 3). This helps maintain a lower temperature within the blast compartment and also limits the vaporization of the liquid nitrogen as it exits from header 16. The gas flow due to boil-olf (-320 F.) is controlled through shut-off solenoid valve 34 to avoid over-cooling certain workpieces.

The components of the system are located for maximum safety and avoid the danger of entrapping liquid nitrogen in an area where the expanding gases could cause an explosive condition. The arrangement is also compatible with the users main storage tank and the associated pressure and shut-olf controls. Moreover, strategically placed pressure relief valves give positive protection.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In an apparatus for dellashing excess material from workpieces including a dehumidifying housing, means for supporting the workpieces within the housing for simultaneously supporting and turning the workpieces, freezing medium applying means for applying freezing medium to the workpieces to lower their temperature thereof, and abrasive blasting means supported within the housing adjacent the supporting and turning means for propelling abrasive particles against the cooled workpieces to remove excess material therefrom, characterized in that said freezing medium applying means includes means for connection to a source of supply of liquid nitrogen, an applicator head adjacent the supporting and turning means, conveying means for conveying liquid nitrogen from the source of supply to the applicator head, means -for varying the rate of ow of the liquid nitrogen in said conveying means substantially independently of the storage pressure of liquid nitrogen in the source of supply, and means for conveying boiled-off nitrogen gas to the supporting and turning means.

2. In an apparatus for deashing excess material from workpieces including a dehumidifying housing for simultaneously supporting and turning the workpieces, freezing medium applying means for applying freezing medium to the workpieces to lower their temperature thereof, and abrasive blasting means supported within the housing adjacent the supporting and turning means for propelling abrasive particles against the cooled workpieces to remove excess material therefrom, characterized in that said freezing medium applying means includes means for connection to a source of supply of liquid nitrogen, an applicator head adjacent the supporting and turning means, conveying means for conveying liquid nitrogen from the source of supply to the applicator head, means for varying the rate of flow of the liquid nitrogen in said conveying means substantially independently of the storage pressure of liquid nitrogen in the source of supply, and said means for varying the rate of flow includes a tlow rate valve operable by a control gas other than nitrogen.

3. In an apparatus as set forth in claim 2 wherein said means for varying the rate of flow of liquid nitrogen includes a manual shut-olf valve for the control gas.

4. In an apparatus as set forth in claim 3 wherein said manual shut-ott` valve is disposed upon a control panel mounted externally of the housing.

5. In an apparatus as set forth in claim 1 wherein said applicator head includes a dual header comprising a T mounting, a pair of nozzles secured in said T mounting, a plurality of spray holes in each of said nozzles, and the diameter of each spray hole decreasing from said T mounting to the tip of each nozzle.

6. In an apparatus as set forth in claim 5 wherein adjustable mounting means secure said nozzles to said mounting for varying the orientation of said spray holes.

7. In an apparatus as set forth in claim 1 wherein said conveying means includes an entrainment separator between the source of supply and the supporting and turning means, and said means for conveying boiled-off nitrogen gas including a conduit between said entrainment separator and the supporting and turning means.

8. In an apparatus as set forth in claim 1 wherein said conveying means includes a shut-otf valve for controlling the on and olf ow of the liquid nitrogen.

9. In an apparatus as set forth in claim 1 wherein said conveying means includes an entrainment separator between the source of supply and the supporting and turning means, the walls of said entrainment separator being made of foam material disposed between layers of stainless steel, said means for conveying boiled-olf nitrogen including a vent line leading from said entrainment separator to said supporting and turning means, means for varying the rate of flow of liquid nitrogen including a parabolic needle valve, a moving member connected to one end of said needle valve, the other end of said needle valve being disposed for seating in said conveying means, means for conveying carbon dioxide control gas on the side of said moving member remote from said conveying means to move said needle valve in accordance with the pressure of the control gas, a manually operable carbon dioxide pressure regulator connected in said means for conveying the control gas, said conveying means including a safety valve between said entrainment separator and said header, a shut-off control valve in said conveying means, said shut-off control valve being disposed on a control panel mounted externally of the housing, said header including a T mounting, a pair of outwardly extending nozzles threadably secured to said T mounting, a plurality of spray apertures in each of said nozzles, each of said apertures being approximately 11/32 inch long, and said apertures being equally spaced from each other and decreasing in diameter away from said T mounting.

References Cited UNITED STATES PATENTS 2,859,064 11/ 1958 Nelson. 3,110,983 11/1963 Moore 51-13 X 3,137,101 6/1964 Leliaert 51-13 3,255,597 6/ 1966 Carter 62-64 X 3,298,138 1/ 1967 McCormick 51-13 JAMES L. JONES, JR., Primary Examiner.

U.S. Cl. X.R. 51-163

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