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ABC’S OF VARI-FORM
“The ABC’s of Hydroforming” provides useful information about the hydroforming process and its applications. VARI-FORM VS. STAMPED
Compare PSH assemblies with stamped assemblies – by number of components, total mass, and dollar investment. |
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ABC’S OF VARI-FORM
“The ABC’s of Hydroforming” provides useful information about the hydroforming process and its applications. VARI-FORM VS. STAMPED
Compare PSH assemblies with stamped assemblies – by number of components, total mass, and dollar investment. |
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A- Pressure Sequence Hydroforming (PSH) reshapes the tube while the die closes.
Once the die is completely closed the tube has been forced to take the shape of the die cavity without requiring the material to expand. High Pressure Hydroforming first closes the die on an undersized tube and then utilizes high internal fluid pressure to expand the tube to fill the die cavity.
A- Pressure Sequence Hydroforming (PSH) is a patented tube hydroforming process that utilizes low internal fluid pressure to support the tube while the die closes. Once closed the majority of the part profile has been formed. At this point the internal pressure is increased to lock in the form and provide backup for punching holes.
A- Pressure Sequence Hydroforming is the world leading technology for piercing holes in the hydroform die. Vari-Form currently forms as many as 83 holes in the hydroform tool per part. Hole size can range from as small as 2 times material thickness to as large as 50 mm X 200 mm. Holes can be extruded or clean pierced, and practically any shape including round, slot, square, hexagon, or rectangular. The resulting material slug is typically pushed back out of the way and left attached inside the tube, though there are techniques available to remove them when required.
A- The part to part or floor to floor cycle time for Pressure Sequence Hydroforming is in the range of 17 seconds for a small part such as an Instrument Panel Beam to 24 seconds for a large part such as a roof rail or structural member.
A- Pressure Sequence Hydroforming (PSH) is compatible with most metals, if it can be made into a tube PSH can form it. The process that normally establishes the required material elongation is the prebending operation. Vari-Form has formed HSLA materials in production for over 12 years and in 2008 will be entering production with DP 780. PSH has proven process compatibility with High Strength steel up to 960 Mpa UTS, Dual Phase, and TRIP steels. In addition to carbon steel the PSH process has been used to form both 5000 and 6000 series aluminum, and numerous grades of stainless steel.
A- The amount of Prepressure and Final Pressure is optimized for each application depending on the material specified and the profile being formed. Typically prepressure values are under 1,000 psi (65 bar) and final pressure is under 10,000 psi (660 bar).
A- While in theory any diameter of tube can be hydroformed, the majority of automotive structural products in production today fall in the range of 1 inch (25 mm) to 6 inch (150 mm) diameter.
A- Wall thickness limitations are generally set by the capability of the tube manufacturing process and not the hydroforming process. Vari-form has formed tubes with thicknesses between 0.5 mm and 6 mm, while production products generally fall into the 1 mm to 3 mm wall thickness range.
A- No, Pressure Sequence Hydroforming (PSH) reshapes the tube cross section into the required profile without stretching the material. The tube material thickness distribution found after hydroforming is the same as that present in the bent tube.
A- No lubricants are used in the Pressure Sequence Hydroforming process. Upon exiting the hydroform process the tube is ready for any post operations including welding without any cleaning or washing.
A- No, because the PSH process forms the finished part profile by reshaping the tube rather than stretching it, wall thinning does not occur in the hydroform tool thus eliminating the need to compensate for it. It should be noted that for thin wall (greater than 40 D:T) tube axial end feeding is only effective for approximately 100 mm from the end of the tube. Beyond that distance friction between the tube and die cavity will effectively negate the benefit of axial end feeding.
A- While PSH can be used to expand the tube, Vari-Form recommends using a constant periphery tube design to ensure a robust and cost efficient production hydroform process. Other benefits of using a constant periphery include the ability to change material grade, strength, and thickness without having to modify the hydroform tool or process.
A- Yes, the PSH design guide recommends a target value of 4 times material thickness for the outside corner radius of a cross section. However local cross sections have been formed with corners as small as 2 times the material thickness without using high pressure and no material thinning.
A- There is much concern, and rightfully so over the issue of material thinning in the cross section corners during hydroforming. Material thinning during hydroforming stems from Traditional High Pressure Hydroform (HPH) processes that utilize an undersized tube to prevent pinching when the die is closed. Once the die is closed high internal pressure is used to blow or stretch the tube to form the cross section corners.
Unfortunately the high internal pressure creates high friction between the tube and cavity. As the tube expands to fill the cavity, wherever it makes contact the material tends to freeze in place due to the high friction. As the material expands into the corners the stretching is distributed over a smaller and smaller area resulting in local thinning of the tube. Typically the thinnest area will be found at the tangent point of the corner, this is where the tube is most likely to fail.
To counteract the effect of friction most High Pressure Hydroform processes coat the tube with oil or dry lubricants prior to hydroforming. After hydroforming the lubricant residue needs to be cleaned from the tube prior to any post hydroform operations.
The Pressure Sequence Hydroform (PSH) process uses a completely different mechanism than HPH to form the corners. In the PSH process, the tool stops before it is completely closed on the tube, this is referred to as the prefill height. The tool dwells at this point as the tube is then filled with fluid and lightly pressurized. The die is then fully closed while the tube is supported by the prepressure. Using this support PSH forms the cross section corners while the die is closing under prepressure.
A- The presence of a weld seam does not create any particular issues during hydroforming. However it is good practice to position the weld seam near the neutral axis for bending purposes, and away from pierced holes and weld joints. With today’s complex hydroform products it often isn’t possible to satisfy all of the above conditions so the final position for the weld seam usually ends up as the best compromise between the factors.
A- In many structural applications hydroformed tubes can offer improved structure, reduced weight, lower cost, and tighter tolerances than a stamped and welded assembly.
i. Improved structure due to a continuously welded closed cross section in place of a spot welded two piece assembly
ii. Reduced weight due to the elimination of weld flanges
iii. Lower cost as a single tube and tool can replace multiple stamping tools and weld equipment
A- Vari-Form has produced volumes as low as 30,000 pcs per year and as high as 700,000 pcs per year. Building a successful business case for using a hydroform tube will depend on the cost of employing alternative technologies that could provide a similar part. Typical production programs fall into the 100,000 to 500,000 pcs per year range, but each potential product must be reviewed on it’s own merits.
A- Pressure Sequence Hydroforming is a dimensionally stable and robust process. Product features that are produced in the hydroform tool are typically very stable as the entire part profile and all piercing is completed in a single cavity. Typical tolerances at 1.67 Cpk are;
i. Surface profile tolerances generally fall between 0.5 mm to 1.00 mm depending on where the surface is in relation to part datums and major bends.
ii. Hole position tolerances generally fall between 0.5 mm and 1.25 mm depending on where the hole is placed in relation to part datums and major bends.
iii. Datum hole sizes (formed in the hydroform tool) are held to within +/- 0.05mm.
iv. Pierced conical holes for self tapping screws are held to + 0.0 / -0.13 mm.
A- No, Pressure Sequence Hydroforming is conducted at room temperature.
A- Yes, after the part is hydroformed the fluid is drained into a catch basin, the solids are separated, and the fluid is filtered then circulated back into the hydroform delivery system.
A- Engineering changes to production products are a fact of life, Vari-Form works closely with it’s tool suppliers to complete most changes such as hole additions, hole relocations, and surface profile changes over a 4 day window. The change is complete once all quality checks are signed off and the revised part is approved for production. The time from removing the tool from production to when it is OK to run is typically less than 7 days.
A- There are a number of engineering options available for reinforcing local areas along a hydroformed tube. To achieve the optimal result each situation needs to be studied prior to selecting an alternative;
i. The tube thickness can be increased in high stress areas by positioning a short sleeve over the tube. The sleeve is positioned over the tube prior to bending and is locked into position during hydroforming.
ii. In high stress areas structural foam can be placed inside the hydroform tube to improve structural and NVH performance.
iii. A local patch can be applied to the high stress area to reinforce the tube against the failure. The patch can take the form of a welded metal stamping or an adhesive bonded material.
A- Yes, because the Pressure Sequence Hydroform process harnesses the closing action of the press combined with light internal fluid pressure to form the cross section corners, variations in the material wall thickness and strength do not influence the hydroform process.
A- Most any fastener currently used in stamped and welded assemblies can be used in a hydroformed tube. The most cost effective fastening solution is a self tapping screw threaded into a conical extrusion that is pierced in the tube during the hydroform process. Other more costly threaded fastener options include but are not limited to weld nuts, pierce nuts, riv nuts, and snap nuts. Hydroformed tube can also provide holes compatible with pop rivets, christmas tree style clips, nut plates, and a wide variety of push pins.
A- Stampings require a weld flange to allow two or more parts to be assembled together thus forming the required closed cross section. Hydroformed tubes typically do not include weld flanges because they are not needed for assembly of the closed cross section. A local flange can be formed during the hydroform operation in areas where one is required for joining the tube to the vehicle body structure or for seal attachment.
A- Currently the most popular weld process used with hydroformed tubes is Mig welding. Both Gas Metal Arc Welding (GMAW) and Flux Core Arc Welding (FCAW) are in wide use by most hydroform manufactures. The chief advantages Mig welding has over other alternatives is it only requires single sided access which in turn allows high density of welds, and low capital and operating costs.
Spot welding shares Mig welding’s low costs, but limits design flexibility as it requires access to both sides of the joint. This drives the need to pierce holes in one side of the tube to provide access for the weld tip to the joining surface.
A- The bending and PSH processes can be computer simulated with very good results correlation. It is possible to take the strain map output from the tube bending simulation and input to the hydroform simulation. The resulting output from the hydroform simulation will provide a detailed map of the expected material thickness and strain in the final part.