System and Methods for Producing a Deep Drawn Cup

The present disclosure generally relates to systems and methods for forming metal cups through a deep draw operation and, in particular, to implementing a double die designed to form a metal blank into the desired cup shape while ironing the sidewalls in a single drawing operation.

The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

Drawing refers to a specific technique of metal forming which implements a punch to force a flat precut metal blank into a die cavity of a press-mounted die. As the metal blank is pushed into the die cavity, it begins to take the shape of the cavity. Drawing may not be an ideal technique for thick and/or rigid metals as the end result may not achieve a desired uniform thickness or may have micro-cracks throughout.

Deep drawn cups are utilized in forming a variety of products, such as pressure vessels. When forming pressure vessels, material quality and uniformity in the thickness of the material after drawing are important factors for avoiding failure of the pressure vessels during use. Defects in the materials or thinner regions of a pressure vessel wall could result in a point of failure after pressurization. Conventional drawing methods have been limited in the types and grades, typically high grades, of metals that can be utilized as the drawing process can induce microcracking or other potential defects into lower grades of metals.

Conventionally, deep drawing is used to form two components of the pressure vessel that are joined by welding. To prepare the deep drawn cups for welding, the tops are typically machined down to provide a flat smooth welding surface. Machining processes, however, can introduce some defects into the cups.

As described further herein, the disclosure generally relates to apparatus, systems, and methods. This summary is not comprehensive and is necessarily limited to certain aspects of the invention described herein. Additional or alternative components, aspects, functions, or actions may be included in various embodiments, as described further below.

An apparatus for deep drawing a metal blank into a deep drawn cup having a cup sidewall thickness T, a cup inner diameter D, and a cup height H, the apparatus can include a top die comprising a bore opening having a tractrix profile region and a first draw line downstream of the tractrix profile region as viewed from a first end of the opening to a downstream second end of the opening, wherein the tractrix profile region has a tractrix curvature configured based on the outer diameter of the metal blank and outer diameter of the to be formed cup; a bottom die positioned immediately downstream of the top die and spaced a predetermined distance from the top die. The bottom die can include a draw die having a bore opening having a second draw line, wherein a second draw line diameter Dis smaller than a first draw line diameter D, and a die holder into which the draw die is press fit, the draw die holder having a bottom surface having a profile substantially the same as a profile of a bottom of the cup to be formed. The apparatus can further include a punch for applying a pressure to the metal blank to draw the metal blank into the top and bottom dies, wherein when the punch applies a pressure to the metal blank, the metal blank is drawn into the bore opening of the top die along the tractrix profile region and the first draw line to produce an intermediate cup, and then the intermediate cup is further drawn into the bore opening of the draw die and deformed along the second draw line thereby forming the deep drawn cup, wherein a first ironing of a sidewall thickness occurs as the metal blank is drawn through the first draw line and a second ironing of a sidewall thickness occurs as the intermediate cup is drawn through the second draw line. A ratio of a diameter of the punch to a diameter of the metal blank (P:C) is about 0.28 to about 0.64. The predetermined distance is measured as a distance between an upstream-most edge of a length of the first draw line and an upstream-most edge of a length of the second draw line. The predetermined distance is about 59% to about 78% of a theoretical height of the intermediate cup H.

A method of deep drawing a metal blank into a deep drawn cup having a cup wall thickness T, a cup inner diameter Dand a cup height Hcup, can include applying pressure to a metal blank using a punch to deform the metal blank drawing it into a bore opening of a top die, the bore opening having a tractrix profile region and a first draw line downstream of the tractrix profile region as viewed from a first end of the opening to a downstream second end of the opening, wherein the tractrix profile is configured based on the outer diameter of the metal blank and outer diameter of the to be formed cup, wherein upon drawing the metal blank through the bore opening of the top die the metal blank is drawn into a desired cup shape and a sidewall thickness is ironed to a first reduced thickness, and applying further pressure to the metal blank with the punch to further deform the metal blank drawing it into a bore opening of a bottom die positioned immediately downstream of the top die and spaced a predetermined distance from the top die, wherein the bore opening of the bottom die has a second draw line, a second draw line diameter Dbeing smaller than a first draw line diameter D, thereby further ironing the sidewall thickness to a second side wall thickness. A ratio of a diameter of the punch to a diameter of the metal blank (P:C) is about 0.28 to about 0.64. The predetermined distance is measured as a distance between an upstream-most edge of a length of the first draw line and an upstream-most edge of a length of the second draw line. The predetermined distance is about 59% to about 78% of a theoretical height of the intermediate cup Hof the deep drawn cup.

In any of the apparatus or methods herein, a diameter of the punch can be substantially same as the cup inner diameter D.

In any of the apparatus or methods herein, the first draw line diameter Dcan be determined by D=P+2(0.85T), where Pis a diameter of the punch and Tis a theoretical maximum thickness of a sidewall of a forming cup exiting the tractrix profile region.

In any of the apparatus or methods herein, the first draw line length Lcan be determined by L=1.2*T, where Tis a theoretical maximum thickness of a sidewall of a forming cup exiting the tractrix profile region.

In any of the apparatus or methods herein, Tcan be 1.22 times a thickness of the metal blank.

In any of the apparatus or methods herein, the second draw line diameter Dcan be determined by D=P+(2T), where Pis a diameter of the punch and Tis the sidewall thickness of the cup.

In any of the apparatus or methods herein, the second draw line length Lcan be determined by L=1.4*T, where Tis a thickness of the intermediate cup.

In any of the apparatus or methods herein, Tcan be 85% of a theoretical maximum thickness of a sidewall of a forming cup exiting the tractrix profile region T.

In any of the apparatus or methods herein, the first draw line irons a thickness of the forming cup about 15% to about 33%.

A method of forming an apparatus for deep drawing a cup, the apparatus comprising a top die spaced a predetermined distance from a bottom die, the method can include preparing a top die having a bore opening having a tractrix profile region and a first draw line. The tractrix profile is determined by determining points along a tractrix curve by articulating a segment having the target cup height in about 10° increments from a vertical segment representing the target cup height to a horizontal segment perpendicular to the vertical segment, and projecting the points along the curve to a space below the curve at equivalent distances to develop an inverted tractrix curve defining the tractrix profile. The first draw line is determined by defining a diameter of the first draw line Dsuch that D=P+2(0.85T), where Pis a diameter of the punch and Tis a theoretical maximum thickness of a sidewall of a forming cup exiting the tractrix profile region, and defining a length of the first draw line Lsuch that L=1.2*T, where Tis a theoretical maximum thickness of a sidewall of a forming cup exiting the tractrix profile region. The method can further include preparing the bottom die having a draw die having a second draw line. The second draw line being determined by defining a diameter of the second draw line Dsuch that D=P+(2T), where Pis a diameter of the punch and Tis the sidewall thickness of the cup, and defining a length of the second draw line Lsuch that L=1.4*T, where Tis a thickness of an intermediate cup exiting the top die. The method can further include selecting the predetermined distance between the top and bottom die as measured between an upstream most edge of the first draw line length to an upstream most edge of the second draw line length to be 0.59% to about 78% of a theoretical height of the intermediate cup.

An apparatus for preparing an opening of a deep drawn cup for welding can include a base for receiving the deep drawn cup, the base having a receiving area having a shape substantially matching the shape of the deep drawn cup, wherein the base has one or more sidewalls surrounding the deep drawn cup when the deep drawn cup is placed in the receiving area, the one or more side walls having a height between 0 to 24% of the thickness of the sidewalls of the deep drawn cup remains extended above the sidewalls; and a smash die for applying pressure to a top edge of the deep drawn cup sufficient to flatten the top edge, wherein the smash die has a substantially planar die surface for engaging with the top edge of the cup and a width wider than a diameter of the cup.

The apparatus and methods of the current disclosure implement techniques for producing a deep drawn cup using a double die configuration including a top die configured based on a tractrix curve. The die configuration can allow for an effective deep draw of high strength/low alloy materials and does not require custom materials to be able to obtain acceptable deep drawing results. The double die arrangement, including for example, the profiles of the bore openings of each die and spacing of the dies, has been found to improve the deep drawing operation and can product improved uniformity and reduced or eliminate defects in the drawn cups, even when using for example, high strength, low alloy materials. In particular, the double die allows for the shaping of the metal blank into the desired cup shape and ironing of the sidewall thickness in a single operation. This can be advantageous in avoiding overworking of the metal and with materials that strain harden when being worked and become brittle with increased working.

The draw operation of the current disclosure implements a double die to simultaneously thin the walls and shape the metal blank into a cup. By thinning and shaping the metal simultaneously, the current disclose avoids issues that can arise in the forming processes. During the drawing process, the sidewalls of the forming cup will thicken during the shaping processes. An ironing of these sidewalls is needed to achieve a uniform cup wall thickness. For conventional operations, this thinning of the walls in regions where thickening occurred is done after the drawing process, for example, by a secondary machining operation. However, this can induce defects or cause other material weakens, particularly in lower grade metals. For example, certain metals harden during the drawing process, making subsequent operations, such as thinning the walls more difficult and/or susceptible to inducing defects, such as microcracking, in the material. This is why conventional processes often require custom, high alloy metals for deep drawing operations that are specifically engineered to better handle the secondary machining process.

The apparatus and methods of the disclosure can be used with various high strength low alloy materials. For example, the metal blank can be a carbon steel with a carbon content of about 0.8 to about 0.15. For example, A1008 to A1015 steel can be used as the metal blank. The material of the metal blank can have a tensile strength up to 125,000 psi and/or an elongation of greater than 18%.

In the drawing operation of the current disclosure, cup shape formation and the ironing process are done in a single continuous drawing operation. As the punch begins to push the metal blank through the bore opening of the top die (also referred to herein as the tractrix die), the top die contours the metal blank while simultaneously thinning the material. Once the metal blank reaches a bottom die at the bottom of the top die, the material undergoes another thinning operation while also being forced into a cavity of the bottom die to form the cup shape. By thinning and shaping simultaneously throughout a single continuous drawing operation, the transformation will occur before the material hardens, which can reduce or eliminate potential defects in the cup material, while providing a more uniform cup thickness.

Once the cup is formed, a separate smash operation can be performed using an apparatus that has a base for receiving the cup and smash die that engages with a top lip of the cup to prepare the top lip of the cup for welding. The smashing operation and apparatus is described herein in greater detail below with respect to. The smash operation can be used to prepare the cup for subsequent welding such as for joining two cups for forming a pressure vessel. To obtain a strong weld, it can be advantageous to have smooth flat welding surfaces. The deep drawing operation can result in a cup with a tapered edge at the open-end of the cup, which may not be suitable or ideal for welding. The smash operation of the current disclosure can advantageously provide a flat top surface of the cup without otherwise deforming the sidewalls, inducing defects into the cup material, and/or fracturing the cup. To perform the smash operation, the cup may be placed in the base, where the cup protrudes from the base by a predefined amount, the smash die may then be pressed onto the base to compress the open-end of the cup. It has been found that the selection of the base dimensions and particularly the amount of cup exposed the base need to be controlled to achieve a flat top surface without imparting defects in the cup. In particular, it has been found that if too much of the cup sidewall is exposed above the base, the smashing operation can resulting in microcracking of the sidewalls when the flat top lip is formed. It has been further found that if not enough of the cup sidewall is exposed above the base, inadequate flattening may be achieved and/or the cup can be forced too far into the base and cannot be ejected from the base after the smashing operation.

Referring now to the figures,illustrates a schematic view of the deep draw apparatusincluding a punch, a double die (the top (tractrix) die, and the bottom die), and a blank locator ring, where each of the components of the deep draw apparatusmay be configured based on the desired specifications of the formed cup. In operation, the deep draw apparatusmay implement the punchto deform a metal blankthrough a bore opening of the top dieand subsequently through a bore opening of the bottom dieto form the cup. A blank locator ringmay be included to hold the metal blankin place in the top dieas the drawing operation begins. As explained above, the drawing operation performed by the deep draw apparatussimultaneously forms the metal blankinto the cup shape and thins the sidewalls of the cup as the punchforces the metal blankthrough both the top dieand then the bottom die.

The top diehas a bore opening through which the metal blank is pressed by the punch. The inner profile of the bore opening shapes the metal blank as it is pushed through. The bore opening of the top diecan include a tractrix profile region and a draw line downstream of the tractrix profile region. Downstream is viewed from a top, first end of the top diewhere the metal blank is initial positioned to a bottom, second end of the top die, in the direction in which the punch pushes the metal blankthrough the apparatus. The tractrix profile region can have a curvature based on a tractrix curve configured using the desired dimensions of the final formed cup.

The top diemay include at the top, first end, a metal blank receiving area. This receiving area can be horizontally flat region surrounding the bore opening, on which the metal blankcan reside. The profile of the bore opening of top diemay then slope down from this receiving area based on the tractrix curve to provide the tractrix profile region. Alternatively, the tractrix profile regioncan have the tractrix curvature initiate from a region spaced from the receiving area with an initial curvature from the receiving area defining an initiation region of the top die.

Downstream of the tractrix profile region, the bore opening of the top die has a first draw line. The first draw line region represent the narrowest diameter portion of the bore opening of top die. As the metal blank is forced by the punch through tractrix profile region, the curvature of the cup shape is formed. As the metal blank is further pushed through the first draw line, a first ironing or thinning of the sidewall of the forming cup is achieved. A diameter of the first draw line Dis determined using the following equation:

wherein Pis the diameter of the punch and Tis the theoretical maximum sidewall thickness of the forming cup as it exits the tractrix profile region. The theoretical maximum sidewall thickness Tis calculated based on an anticipated maximum thickening of the material by about 22% at the open end of the cup. The drawing process through the tractrix profile will results in a gradient of thickening of the sidewall where at or near a bottom of the cup the sidewall may experience little to no thickening and remain at the thickness of the original metal black T. The sidewall can thicken in a direction opposite the drawing direction, from a bottom of the cup to the open-end of the forming cup, with a maximum thickening occurring at or near the open-end of the forming cup. The thickening can occur as an increasing gradient in the direction opposite the drawing direction. Trepresents this theoretical maximum sidewall thickness which is at or near the open-end of the forming cup. That is:

where Tis the thickness of the initial metal blank.

The first draw linefurther has a first draw line length L. The first draw line length is the length along the inner profile of the bore opening having the narrowed first draw line diameter. This length defines, in part, the duration the metal blank is subjected to the ironing operation in the top die. The first draw line length can be about 1.2 times the theoretical thickness of the material of the forming cup as it exits the tractrix region. That is,

where Tis the theoretical maximum sidewall thickness of the forming cup as it exits the tractrix profile region of the top die.

The combination of the first draw line diameter and the first draw line length corresponds to the amount of thinning or ironing of the sidewalls that is achieved in the top die. It has been advantageously found that a reduction of about 15% to about 33% of the thickening using the first draw lineallowed for good processing operation and ultimately in combination with further ironing in the bottom die a final cup having the desired sidewall thickness with a uniform thickness throughout the sidewall.

provides a schematic illustration of bore opening profiles of a top dieand bottom dieof an apparatusof the disclosure. As illustrated in, the profile of the bore opening of the top diemay further include a tapered regiondownstream of the first draw line. The tapered regioncan taper outwardly, away from the opening. For example, the tapered regionmay have a taper of about 1.5 degrees outward. This taper can facilitate the exiting of the forming cup as it is transitioned from the top dieto the bottom die. The cup as formed through the top die is also referred to herein as the intermediate cup. The configuration of top dieis discussed in detail below with respect to.

The bottom dieis positioned downstream of the top dieand spaced a predetermined distance from the top die. The bottom diehas a bore openingthat is arranged in line with the bore opening of the top die such that as the punch pushes the intermediate cup out of the top die, the intermediate cup enters the bore opening of the bottom die. As illustrated in, the bottom diemay include a draw dieand a draw die holder. The draw diehas the bore openingand is configured to be press fit in the draw die holder. The draw die holderis configured to hold the draw dieand also includes a bottom surface(i.e., cavity) to receive the formed cup.

The bore openingof the draw diehas an inner profile. The inner profile includes a second draw line. Referring to, the inner profile can further include one or more tapered regionsand. For example, the inner profile can include a first tapered regiondisposed above or upstream of the second draw lineand a second tapered regiondisposed below or downstream of the second draw line. Alternatively, only a single tapered region could be included. Either or both of the tapersandcan be outward tapers about 1° to about 3° The second tapered region, like the tapered regionof the top diecan facilitate exiting of the formed cup from the bottom die. The first tapered regioncan facilitate entrance of the intermediate cup into the bottom dieand can be configured, for example to properly position the intermediate cup as it is received from the top dieand prepare it for the second ironing operation in the second draw line. The first tapered regioncan be, for example, an outward taper (away from the opening) of about 3°. The second tapered regioncan be, for example, an outward taper of about 1.5°.

As with the first draw line, the second draw linerepresents the narrowest diameter portion of the bore opening of the draw die. The second draw linecan have a second draw line diameter Dcalculated as follows:

where Pis the punch diameter and Tis the desired thickness of the final formed cup.

The second draw linefurther has a second draw line length L. The second draw line length is the length along the inner profile of the bore opening of the draw die having the narrowed first draw line diameter. This length defines, in part, the duration the intermediate cup is subjected to the ironing operation in the draw die. The second draw line length can be about 1.4 times the theoretical thickness of the material of the forming cup as it exits the tractrix region. That is,

where Tis the sidewall thickness of the intermediate cup as it exits the top die.

The Tis calculated based on an expected rate of ironing occurring at the first draw line. For example, a first draw linehaving an ironing rate of about 15% to about 33% will result in an intermediate cup thickness. The ironing rate can depend at least in part on the material of the blank. Tcan be determined as follows:

where I=(1−the ironing rate), and Tis the theoretical thickness of the sidewall after the tractrix profile region. Ican range for example from about 0.67 to about 0.85 depending on the material of the metal blank being drawn.

The cavityof the bottom dieis configured to receive the cup and has a shape substantially matching the shape of the bottom of the cup. The diameter of the cavitycan correspond to the desired outer diameter of the cup, and the depth of the cavitycan correspond to the desired height of the cup. By correspond, it is intended that the diameter and/or the depth of the cavity can have a substantially similar size to the respective dimension of the cup. The diameter and/or depth of the cavity, for example can be slightly larger than the respective dimension of the cup to allow the cup to be received into the cavity securely, but without further pressure or deformation imparted into the cup from the cavity. The cavityof the bottom diemay have a cross section that is wider than it is tall, to provide dimensional stability during the ironing operation A vertical wall of the cavitymay also have a 0.5-2 degree taper for proper press fit for dimensional stability during the drawing operation. The components of the bottom diemay be made out of any combination of D-2, 4140 or 4340 pre-hardened material, or other similar material.

During the drawing operation both the top die andand the draw diewill iron off the sidewall of the metal blankand intermediate cup, respectively as it is being formed into the cup. Typically, the top diewill iron approximately 15%-33% of the thickening that occurs as the metal blanks is formed through the tractrix profile region of the top die and the draw diewill iron the remaining thickness to achieve the desired final cup thickness. The amount of ironing achieved in the top diewill depend, at least in part, on the material of the metal blank being drawn. Each dieandis configured to iron a specific amount of the material throughout the drawing process. It has been found that if the top diedoes not result in sufficient ironing, then there is a high likelihood the resulting cupwill crack in the sidewall after forming. Further, it has been found that if the top dieexcessively irons, then there will be similar defects in the sidewall of the cup. The draw die diameters and lengths as calculated herein are based on a balancing of the amount of ironing to be done in each die to achieve effective ironing during the drawing process without resulting defects.

It has been further defined that the success of the ironing operation depends, in part, upon a spacing between the top and bottom dies. The top and bottom dies are spaced a predetermined distance. This predetermined distance His measured from a top (upstream most) edge of the first draw die length to the top edge of the second draw die length. It has been found that the predetermined distance Hshould be about 59% to about 78% of the height of the intermediate cup Hic. Other suitable values include about 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, or 78% of the theoretical height of the intermediate cup H. The theoretical height of the intermediate cup His determined as follows: