METHOD OF PRODUCING LARGE THIN-WALLED SAND CASTINGS OF HIGH INTERNAL INTEGRITY

Abstract

A process for high integrity castings of metals and their alloys includes the steps of providing at least a sand mold at desired elevated temperatures, delivering a molten metal into the mold, and supplying a predetermined amount of coolant to contact the surfaces of the casting at desired rates, times, and durations to achieve an acceptable level of progressive solidification from the distal end of the casting towards the riser until the casting has reached desired temperatures.

Claims

1. A process for the casting of metals and their alloys, comprising of the steps of: preparing sand molds containing at least an aggregate and a binder to form a cavity to make castings; bringing at least one mold to predetermined elevated temperatures with a certain temperature distribution; introducing a molten material into the mold cavity to form castings; delivering a predetermined amount of a selected coolant at predetermined rates, times, and durations to contact the surfaces of the solidifying casting and to maintain an acceptable level of progressive solidification from the distal end of a casting to the riser or downsprue; and controlling the cooling of the casting to maintain the distance between the dendrite front and the solidus front within a predetermined range during the solidification of the casting by controlling the mold temperatures and the coolant cooling until the metal is totally solidified.

2. The process of claim 1, where the heating of the mold is achieved by any method that is conventionally used in the casting industry, including, but not limited to, torch heating, oven heating, and infrared heating.

3. The process of claim 1, where the local temperatures of the mold can be managed by using insulation materials, exothermic materials, and embedded heating devices in the mold, or by other means that are conventionally used in the metal casting industry.

4. The process of claim 1, where the mold temperature is heated to in a range between 100° C. to the solidus temperature of the molten material.

5. The process of claim 1 wherein the mold temperature is heated to in a range between 200° C. to the solidus temperature of the molten material.

6. The process of claim 1 wherein the mold temperature is heated to in a range between 300° C. to the solidus temperature of the molten material.

7. The process of claim 1, where the molten material is introduced into the mold cavity by gravity or by pressure.

8. The process of claim 1 wherein the coolant is a liquid, a gas, a mixture of gases, or a mixture of liquids and gases that contact the surface of the introduced metal to achieve high cooling rates at the region of contact until the metal is cooled to predetermined temperatures.

9. The process of claim 1, where the contact region of the coolant to the casting moves from the distal end to the feeder of the casting at controllable speeds.

10. The process of claim 9 wherein the controllable speed is variable and is between a range of 0 mm/s to 100 mm/s.

11. The process of claim 9 wherein the controlled speed is variable and is between a range of 2-40 mm/s.

12. The process of claim 1, where the molten material is a molten aluminum alloy.

13. The process of claim 1, where the molten material is a molten magnesium alloy.

14. The process of claim 1, where the distance between the dendrite front and the solidus front along the centerline of the wall of a solidifying casting is between a range of 1 to 10 times the wall-thickness of the casting.

15. The process of claim 1, where the distance between the dendrite front and the solidus front along the centerline of the wall of a solidifying casting is between a range of 4 to 10 times the wall-thickness of the casting.

16. A process for the casting of metals and their alloys, comprising of the steps of: preparing sand molds containing at least an aggregate and a binder to form a cavity to make castings; bringing at least one mold to predetermined elevated temperatures with a certain temperature distribution; introducing a molten material into the mold cavity to form castings; delivering a predetermined amount of a selected coolant at predetermined rates, times, and durations to contact the surfaces of the solidifying casting progressively from the distal end of a casting to the riser or downsprue; and controlling the cooling of the casting to maintain the openness of the feeding channel for the liquid from the feeder to feed the solidification shrinkage of the casting by controlling the mold temperatures and the coolant cooling until the metal is completely solidified.

17. The process of claim 16 wherein the coolant is a liquid, a gas, a mixture of gases, or a mixture of liquids and gases that contact the surface of the introduced metal to achieve high cooling rates at the region of contact until the metal is cooled to predetermined temperatures.

18. The process of claim 16, where the mold temperature is heated to in a range between 100° C. to the solidus temperature of the molten material.

19. The process of claim 16 wherein the coolant is delivered to the surfaces of the casting progressively towards the feeder with speeds, constant or variable, between the range of 0 mm/s to 100 mm/s.

20. The process of claim 16 wherein the coolant is delivered to the surfaces of the casting progressively towards the feeder with speeds, constant or variable, between the range of 2 mm/s to 40 mm/s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0023] FIG. 1 is a schematic view of a layout of one embodiment of the present invention.

[0024] FIG. 2 is a schematic view of a layout of one embodiment of the present invention.

[0025] FIG. 3 is a schematic view of a casting, the molds and the locations where tensile specimens are taken from the plate-shaped casting.

[0026] FIG. 4 depicts the relationship between the solid fraction and the temperature of the A356.2 alloy.

[0027] FIGS. 5A, 5B, and 5C show the cooling curves, the evolution of fraction solid in a sand casting on cooling, and an SEM image of the fractured surface of a tensile specimen, respectively.

[0028] FIGS. 6A, 6B, and 6C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.

[0029] FIGS. 7A, 7B, and 7C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.

[0030] FIGS. 8A, 8B, and 8C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.

[0031] FIG. 9 is a photograph showing the HMPC process in operation.

[0032] FIG. 10 is a photograph showing two castings made by the HMPC process.

[0033] FIG. 11 shows the tensile strength of selected castings and forgings.

[0034] FIG. 12 shows the elongation of selected castings and forgings.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0036] The present invention is also explained in detail in the article recently published in an international journal [22].

[0037] In a preferred embodiment, the present invention relates to a method of manufacturing large thin-walled sand castings with high internal integrity using hot mold with progressive cooling (HMPC) process. The term “large thin-wall” defines the shape of a casting when its length or width, whichever is the greatest, is 4 times to orders of magnitude greater than its average wall thickness. Such a casting is unable to be fed using a single riser by prior art in the metal casting industry. The conventional wisdom in the art of metal casting is that the feeding distance of a riser is 2.5 times the thickness of steel castings and is less than 10 times the thickness of aluminum castings [8]. The term “high internal integrity” refers to the internal quality of a casting that is free from shrinkage porosity. By such a definition, large thin-walled castings of high internal integrity are unable to be manufactured using a single riser by prior art because the solidification shrinkage of the casting cannot be fed, resulting in the formation of shrinkage porosity in the casting. Hot molds in the present invention include those that are heated up to elevated temperatures using known conventional technologies in the metal casting industry. Hot molds also include molds that contain exothermic materials or insulating materials at their localities. For example, molds can be heated up using furnaces, ovens, or infrared lamps. Exothermic materials can be placed in certain locations in a mold to heat up the mold locally. Insulation materials can be placed in certain locations to maintain the local temperatures in the molds. The term “progressive cooling” refers to cooling methods that maintain the freezing front movement with selected speeds, which are under control, from the distal end of the casting to the riser, feeder, or downsprue. The uniqueness of this present invention is in the use of combined technologies in mold heating and casting cooling to ensure the ease at which molten metal can fill a large thin-walled mold cavity and feed the solidification shrinkage during the solidification of the molten metal.

[0038] Cavities for a large thin-walled casting are usually difficult to fill by molten metal because the molten metal tends to solidify when it flows into the cold cavity. The length of a thin-walled cavity that a molten metal can flow before being frozen depends on the size of the flow channel, the temperature of the molten metal and the molds, the pressure driving the flow, and other factors. Molds with higher temperatures allow a molten metal to flow a greater length than molds with lower temperatures. Given sufficient pressure, a molten metal can flow to fill a cavity of any length provided that the mold temperatures are higher than the freezing temperature of the molten metal and the channel thickness is not extremely thin. Still, the metal casting industry prefers not using molds of high temperatures partly because the hot molds slow down the solidification rates in the casting, promote one-dimensional heat transfer from surfaces to the centerline of the thin-wall, and lead to increased formation of porosity in the final product. Molds of high temperatures are usually used for making large thin-walled castings that have no strict requirements on their internal quality.

[0039] The new idea of this present invention is to use the hot molds to facilitate the mold filling of a molten metal and to maintain the solidifying metal at temperatures where the colder freezing front can be fed by hotter metal ahead of the front by either a liquid feeding or mass feeding mechanism. To keep the freezing front sufficiently fed, the freezing front has to travel from the distal end of the casting to the feeder/downsprue while the solidifying metal ahead of the front is still at adequate temperatures. Such a condition has to be satisfied by forcing the freezing front to travel through the casting within the time frame where shrinkage feeding can still be maintained. The present invention deals with the utilization of technologies for mold heating and progressive cooling to produce large thin-walled shape castings of high internal integrity. The use of hot molds is 1) to ensure that extremely large thin-walled cavities can be filled by a molten metal or material, 2) to ensure that the solidifying metal in predetermined regions of the mold can be maintained at predetermined temperatures before the coolant is applied locally, and 3) to allow sand mold with certain conventional binders to be used for controlled penetration of the coolant to cool the solidifying casting. The use of hot mold technology combined with the progressive cooling technology is to ensure that shrinkage porosity can be totally avoided in the solidifying casting. Furthermore, a controlled mold temperature distribution combined with a controlled variable progressive cooling can be used for controlling porosity distribution in a casting so that critical areas in the casting are porosity free but non-critical areas contain a controlled amount of porosity, leading to a mass reduction of the resultant casting.

[0040] FIG. 1 illustrates a method of one embodiment of the present invention on the production of large thin-walled sand castings with high internal integrity using HMPC technology. Sand molds 12 and 16 are made with or without flasks. The mold, 12 or 16, is composed of an aggregate and a binder that are conventionally used for making green sand molds, and can be made using conventional mold making methods including, but not limited to, mold making machine, sand blower, 3D printing, etc. Molds thus made are heated to predesigned elevated temperatures and become hot molds. Cavity 10 in the molds 12 and 16 is used to make a casting. Cavity 14 in the mold 12 is used as a feeder or a downsprue where a molten metal is introduced to form the casting. A coolant is provided through devices 18 and 20 and applied on the solidifying casting. The coolant may first be applied at the distal end of the casting through device 18 which is fixed between mold 12 and mold 16. The coolant delivered from device 18 to the casting promotes the formation of a freezing front near the distal end of the casting. As soon as the freezing front is formed at the distal end of the casting, the coolant can then be applied through device 18 which travels from the distal end of the casting towards the feeder in cavity 14 to completely solidify the metal. In some cases, device 20 is not needed. Coolant can be directly applied through device 18 as it progressively travels from the distal end of the mold 12 to the feeder of the casting in cavity 14. The speed of the device 18 is defined as a relative translational speed between the device 18 and the molds 12 and 16. Such a relative motion can be caused either by moving the device 18 while the molds are stationary, by moving the molds 12 and 16 while the device 18 is stationary, or by moving both device 18 and molds towards each other. The speed of the relative motion can be either constant or time dependent for either removing porosity completely or for controlling porosity distribution in the solidified casting. The temperature distribution in molds 12 and 16 can also be controlled for controlling porosity distribution in the final solidified casting.

[0041] FIG. 2 depicts the operation of producing a large thin-walled sand casting with high internal integrity using the present invention of HMPC technology. When coolant is applied through the hot molds 12 and 16 on the solidifying casting 24 as described in the paragraph

[0042] , freezing fronts are formed and move towards the feeder in the casting 24. Two important freezing fronts are illustrated in FIG. 2. The solidus front 20 is closer to the coolant delivery device 18 and the rigid dendrite front 22 is further away from the coolant deliver device 18. A dendrite network is considered as rigid when the fraction solid is about 0.25, which is also the dendrite coherency point. Therefore, the rigid dendrite front is also termed as the front of the dendrite coherency point.

[0043] There are two features shown in FIG. 2 that are closely related to porosity formation. One is the angle between the normal direction of the front to the central line 26 along the casting wall thickness. The other is the distance between these two fronts, 20 and 22.

[0044] For each front, there is an average angle of the front to the central line 26 along the casting thickness. The temperatures of the molds 12 and 16 and the coolant conditions including the coolant amount and the speed at which the coolant delivery device 18 travels should be controlled such that the average angle of the front 20 has to be greater than a critical value. When the average angle is greater than the critical value, solidification shrinkage of the front 20 can be fed by the liquid from the feeder so that shrinkage porosity can be avoided if the distance between these two fronts is small. For a controlled distribution of porosity in the casting, the mold temperatures and the cooling conditions have to be controlled such that the average angles in the regions where shrinkage porosity has to be maintained small or the distance between two fronts has to be large.

[0045] The distance between the fronts shown in FIG. 2 affects the ease of liquid feeding to the solidification shrinkage at front 20. Dendrites existing between these two fronts tend to restrict the liquid ahead of front 22 to flow towards front 20. The use of hot molds in this present invention is to reduce the distance between these two fronts so that the liquid ahead of front 22 has less difficulties in feeding the solidification shrinkage at front 20.

[0046] FIG. 3 illustrates a plate-shaped casting, the mold assembly, and the locations and orientation of the tensile samples taken from the plate-shaped casting for mechanical property measurement. The thickness, width, and the length of the casting were 13 mm, 70 mm, and 400 mm, respectively. The length to thickness ratio of the plate-shaped casting was about 31, much greater than 4-10, indicating that shrinkage porosity should form in the casting under conventional sand casting conditions. A356.2 aluminum-silicon hypoeutectic alloy was used to make the plate-shaped casting to demonstrate the benefits of the present invention of HMPC process over sand casting and ablation casting processes.

[0047] FIG. 4 shows the relationship between the fraction solid and temperature for A356.2 alloy. The primary dendrite phase, which is a fcc phase, forms at about 615° C. where the fraction solid is zero. The eutectic silicon phase forms at the eutectic temperature, T.sub.EU, about 572° C. where the fraction solid is about 0.5. Below the eutectic temperature, a number of intermetallic phases form. The solidus temperature of the alloy is about 550 to 560° C. where the fraction of solid is 1.0 [23].

[0048] A validated model [15, 22] was used to calculate the cooling curves of the plate-shaped casting under conventional sand casting conditions and HMPC conditions by applied ablation cooling at a constant translational velocity of 10 mm/s from the distal end of the casting to the gate/feeder.

[0049] FIG. 5A illustrates the cooling curves of a sand casting. The molten alloy at 680° C. is poured into the mold cavity. After mold filling as shown in FIG. 5A, the local temperature at the distal end of the casting drops to the liquidus temperature, T.sub.L, in about 18 s, to T.sub.EU in about 80 s, then to the solidus temperature, T.sub.S, in about 160 s as local solidification completes. The temperature at the gate/feeder drops slower than that at the distal end of the casting. There is a temperature plateau at T.sub.EU on both cooling curves. The evolution of fraction solid in the sand casting during 55 to 80 s after mold filling is shown in FIG. 5B. There are a number of fronts illustrated in FIG. 5B. The solidus front is shown as the gray front where the fraction of solid is 1. The eutectic front is outlined by the boundary between yellow and orange where the fraction solid is 0.5 and the corresponding temperature is T.sub.EU. The dendrite coherency point is outlined by the boundary between red and brown where the fraction solid is 0.25 according to Chai et al. [24]. For this alloy casting, the eutectic front almost overlaps with the solidus front. One can use either the solidus front or the eutectic front to represent front 20 and the front of dendrite coherency point to represent front 22 shown in FIG. 2. It can be clearly seen that when the solidus front just reaches the distal end of the plate-shaped casting, the front of dendrite coherency point has already entered the feeder/downsprue of the casting. The distance between these two fronts is the entire length of the casting, and the fraction solid in the entire casting is greater than 0.25. It is extremely difficult for the liquid in the feeder to flow through dendrites over such a long distance to feed the solidification shrinkage of the solidus front. As a result, shrinkage porosity occurs in the casting. FIG. 5C is a scanning electron microscopy (SEM) image of the fracture surface of a tensile specimen taken from the center section of the casting as shown in FIG. 3. The SEM image reveals a few large shrinkage pores on the fracture surface. The fracture surface is widely accepted as an ideal surface to show larger pores that are more detrimental to the mechanical properties, especially ductility, than smaller pores in a casting [25-26].

[0050] FIG. 6A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 100° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. Such conditions are similar to ablation casting [10-11] where the mold temperatures are at room temperatures, slightly lower than 100° C. The cooling curves of the sand casting are represented by the black or red dashed lines. The dashed cooling curves are similar to that shown in FIG. 5A except that the solidification times, the times from the moment of pouring to the moments when the cooling curves reach the solidus temperature, are longer because the mold temperatures are slightly higher than those shown in FIG. 5A. The application of a progressive ablation cooling reduces the solidification times significantly as shown in the solid curves in FIG. 6A. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 6B. The solidus front appears at the distal end of the casting at 55 s while the front of dendrite coherency point has already reached the middle of the casting. The distance between these two fronts is increased at 60 s and then is decreased afterwards. It appears that the feeder/downsprue is capable of feeding the solidification of the solidus front when the front moves close to the feeder towards the end of the casting solidification. However, the majority portion of the casting, especially the middle portion of the casting, cannot be sufficiently fed. Shrinkage porosity does exist in the tensile specimens as shown in FIG. 6C.

[0051] FIG. 7A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 200° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. The cooling curves of the sand casting, represented by the black or red dashed lines, are similar to those shown in FIGS. 5A and 6A except that the solidification times are longer because the mold temperatures are higher. The application of a progressive ablation cooling reduces the solidification times significantly as shown in the solid curves in FIG. 7A, which is very similar to the curves associated with ablation cooling in FIG. 6A. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 7B. The solidus front reaches the distal end of the casting at 55 s while the front of dendrite coherency point is within 1-2 thickness distance away from the solidus front, indicating that the liquid ahead of the front of dendrite coherency point is capable of feeding the solidification shrinkage of the solidus front. The distance between these two fronts is increased to more than 4 times the wall thickness of the casting when the cooling time is about 70 s, but the distance then is reduced afterwards. Feeding problems could occur for steel casting when the distance between these two fronts are greater than 4 times, but this is not a major concern for aluminum casting. The SEM image, FIG. 7C, of the fracture surface shows only two small pores, indicating the internal quality of the aluminum HMPC casting is reasonably good.

[0052] FIG. 8A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 350° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. The cooling curves of the sand casting indicate that the solidification times in the sand casting in molds of higher temperatures are longer than those in molds with lower temperatures. However, there is no significant difference in solidification times of the HMPC castings solidified in molds held at different temperatures. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 8B. The solidus front reaches the distal end of the casting at 55 s while the front of dendrite coherency point is within 1-2 thickness distance away from the eutectic front. In fact, the distance between these two fronts is within 1-2 times the thickness of the wall throughout the entire solidification process of the plate-shaped casting, indicating that the liquid ahead of the front of dendrite coherency point is capable of feeding the solidification shrinkage of the solidus front. Indeed, the SEM image shown in FIG. 8C depicts no shrinkage porosity on the tensile sample taken from the center portion of the casting, indicating that the internal quality of the HMPC casting is excellent, similar to that of forgings.

[0053] Results shown in FIGS. 5-8 suggest that the present invention of HMPC technology is capable of producing thin-walled castings free from shrinkage porosity if the mold temperature is sufficiently high. The invention can also be used for controlling porosity distribution in a casting by varying the local temperatures since pores form in regions where the mold temperatures are lower and are eliminated in regions where the mold temperatures are higher. Porosity distribution in a casting can also be controlled by fixing the mold temperature and varying the speeds of progressive cooling. Pores should form when the speed is too low or too high.

[0054] The key idea of this present invention of HMPC technology is to control the mold temperature and the delivery of a coolant to the solidifying casting in such a manner that the distance between important solidification fronts is within a certain limit to eliminate porosity formation and outside the limit for allowing pores to form in the casting. This limit seems to be in the range of 4 to 10 times the wall-thickness of the thin-walled casting. The use of a hot mold also ensures that the large thin-walled casting can be filled by a molten metal so that extremely large sized castings can be made.

[0055] The invention further provides examples of the present invention of HMPC technology. The examples provided below are merely meant to exemplify several embodiments and should not be interpreted as limiting the scope of the claims, which are delimited only by the specification.

EXAMPLE

[0056] FIG. 3 illustrates a plate-shaped casting, the mold assembly, and the locations and orientation of the tensile samples taken from the plate-shaped casting. The thickness, width, and the length of the casting were 13 mm, 70 mm, and 400 mm, respectively.

[0057] 2.5 kg of A356.2 alloy was melted in a graphite crucible using electric resistance heating, heated to 720° C. in 20 min, modified with 0.05 wt.% Sr, fully degassed while the melt cooled down from 720° C. to 680° C. before poured into the cavity in steel metal molds for making permanent mold castings, sand molds with sodium silicate as binder for making sand castings, or preheated sand molds at various temperatures (100, 200, or 350° C.) for making HMPC castings using the HMCPC technology described in one embodiment of this present invention. Silica sand with conventional sodium silicate binder was mixed in a sand mixer for making sand molds.

[0058] For comparison, forgings of the same dimensions of the plate-shaped casting were obtained. These forgings were plastically deformed, at high forging temperatures, by 70% along its wall thickness to close out any cavities that might exist and to breakup silicon particles into small fragments.

[0059] Molds for the HMPC process were preheated in a muffle furnace to desired temperatures. The preheated molds were then filled with the A356.2 alloy and transferred to an ablation cooling setup shown in FIG. 9 for making plate-shaped castings. The ablation cooling setup consisted of an array of water nozzles and a translation conveyer traveling at rate of 10 mm/s over the filled molds, ablating away the sand molds to cool the solidifying casting. Castings made are shown in FIG. 10. Ablation cooling technology [10-11] failed to ablate the sand molds held at room temperatures under controlled manner. As a result, no castings with satisfactory surface quality were successfully made by ablation cooling when the molds containing silica sand with a conventional binder were at room temperatures.

[0060] FIG. 11 shows the tensile strength of the forgings and castings made using various processes where TM is the mold temperature. Castings with satisfactory surface quality were made when TM was 100° C. or higher. Generally, the tensile strength obtained on this alloy was lower than those reported in the literature partly because the mass of metal melted was only 2.5 kg in this example. It is very difficult to refine molten aluminum of small quantity.

[0061] FIG. 12 illustrates the ductility of the forgings and castings made using various processes where TM is the mold temperature. Castings with satisfactory surface quality were made when TM was 100° C. or higher. The elongation of the sand casting and metal mold casting were around 3% or less. The HMPC process with mold temperatures higher than 200° C. is capable of increasing the elongation of the thin-walled plate-shaped casting to 16-19%. Such an elongation level is very close to that of the forgings and is 8 times greater than that of the sand casting.

[0062] While the invention has been described in connection with specific embodiments thereof, it will be understood that the inventive methodology is capable of further modifications. This patent application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth and as follows in scope of the appended claims.

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