Indirect Manufacturing Sheet Metal Forming with MJP Parts

 

Indirect Manufacturing Sheet Metal Forming with MJP Parts

Best Practice

p/n 33-D279 Rev A

 

It is often advantageous to use rapid prototyping in the fabrication process of another part. The 3D printed part is typically destroyed or discarded after being used and can make one or more of the final desired part. This type of process is termed "indirect manufacturing" or "indirect prototyping" and the 3D printed parts are commonly referred to as "indirect tooling". Many traditional manufacturing process can leverage rapid prototyping in this way, the most common uses being plastic injection molding, thermoforming, sheet metal forming, sand/investment/flack casting, indirect and direct printed silicone/urethane molding and wax molding for investment casting.

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Injection Molding Thermoforming Sheet Metal Forming

 

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Investment Casting Direct Printed Eggshell Mold 2-Half Direct Printed Mold

 

Direct Printed 3D MJP Molding Capability
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Indirect Manufactured MJP Mold

(Using MJP as a Master Pattern)

Wax Molding

(for casting)

Rapid Tooling / Indirect Manufacturing Examples using MultiJet Printing Technology

 

Metal forming is the process of fashioning metal parts and objects through mechanical deformation. In this process, the work piece is reshaped without adding or removing material, and its mass remains unchanged. The metal is plastically deformed such that the physical shape of the material is permanently deformed into a final desired shape. Sheet metal forming is a subset of metal forming and is typically defined for metal sheet thicknesses les than 0.25in (6.35mm). Thicker metal sheets are referred to as a "plate". In most of the world, sheet metal thickness is specified in millimeters. In the US, the thickness of sheet metal is commonly specified by traditional, non-linear measure known as its gauge. The larger the gauge number, thinner the metal. Commonly used sheet metal range from 30 gauge to about 7 gauge. Gauge differs between ferrous metals and nonferrous metals such as aluminum or copper. Copper thickness, for example is measured in ounces, which represents the weight of copper contained in an area of one square foot.

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Some of the standard metal gauges (Wikipedia)

 

Sheet metal parts deliver a lot of value to product design and are extremely common in nearly all product categories. Sheet metal is strong compared to injection-molded plastic and can carry current for electrical grounding purposes and/or electromagnetic shielding. Parts can be made with good dimensional accuracy and surface finish and there are a wide range of materials available (strengths, conductivity, weight, and corrosion resistance, etc.) including aluminum, steel, brass, copper, tin, nickel and titanium. There are numerous custom finishes available (anodizing, plating, power coating, and painting). For these reasons, there are vendors worldwide with the capability to create, prototype, and mass-produce sheet metal parts and the parts can be fabricated relatively quickly and inexpensively. Sheet metal parts are formed from a single sheet of metal with constant thickness. This creates a number of strict design rules; however, a good designer can create a wide variety of functional geometries for different needs. A sheet metal part is created using Computer Aided Design (CAD) and a flat pattern of the final formed part in generated from the bent design.

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Left: flat pattern, Right: bent part (both from SolidWorks CAD

 

Typically, a laser or waterjet is used to cut out the flat pattern and the flat pattern is bent and formed into the final desired shape. Traditional forming tools are typically used including brakes, punches and dies which are able to form angled bend and features. 

One of the main uses for rapid prototyping is in the creation of embossed or formed features in sheet metal parts. Sheet metal is purchased in sheet or roll stock and is typically not very flat and has very little bending stiffness compared to its tensile strength. For this reason, complex embossed, stamped, or formed features are often used in the design of a sheet metal part as they dramatically increase the stiffness of the part and greatly improve flatness. Also, weight, cost, and/or functionality can often be improved by using the thinnest material possible which benefit the most from embossing/forming operations. Such features are typically complex and unique and therefore not general machine exists for such needs.

 

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Sheet metal cover with numerous formed features for stiffness,flatness, screw head clearance, aesthetics and wire routing

 

Besides flatness and stiffness, forming can also be used to create a clearance in a specific design like for the case of a screw head or for wire routing.

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(A)

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(B)

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(C)

(A) Formed features for screw head clearance, (B) & (C) Formed features for wire routing

 

Formed vents are often cut into sheet metal to allow for airflow and cooling.

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Formed sheet metal cooling vents

 

Finally, embossing can also be used for purely aesthetic purposes or for physical numbering or marking.

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Embossed letter L to identify the left side of the device. (Also bent length-wise for stiffness)

 

Sheet metal forming utilizing 3D printed rapid tooling is very powerful and commonly used in prototyping; however, it has numerous advantages and be the cost effective even for the manufacture a final part in low to medium volume. The alternative to a 3D printed part is a complex CNC machining operation, likely with numerous part and machine setups. The materials and printer capability of the ProJet MJP 2500 are able to replace these laborious and time-consuming traditional processes. Functional updates are simple and geometric complexity of the tool is easily created. 3D Systems MJP plastic dies are faster to generate and cheaper than traditionally manufactured dies made out of metals and yet they can be used to form tens to hundreds or even thousands of parts depending on the specific design. With the design freedom you gain from rapid tooling, one can design a more complex shape for functional purposes or aesthetics needs. More complex designs allow for reduced weight or improved functionality often leading to higher durability and lower cost. 3D printed tooling using MJP should be considered for all applications including aerospace, automotive, consumer products, electronics, energy, industrial and robotics among others. Military uses are also possible as well as they will often produce one-off replacements for the repair of damaged vehicles and aircraft using metal forming processes.

Designing Sheet Metal Parts with 3D Printed MJP Technology

There are numerous design principles that must be followed to create highly functional and error-free products using metal forming processes. The design in CAD software is often quite different compared to how they are actually fabricated on the shop floor. For example, holes expand at higher temperatures causing spacings to misalign, fasteners to loosen and bends to have spring back (spring-back is the unwanted tendency of sheet metal to retain or go back to its original flat form after the forming process). Also, bends and formed/embossed features stretch the metal and this can interact with other features and lead to unwanted hole deformation. Because of this, manufacturers can spend up to 50% of their time fixing errors, particularly related to manufacturability. There are numerous design rules that are well published (for example DFM Guidelines for Working with Sheet Metal, KashyapVyas, HiTech CADD, Machine Design Sep. 21 2016). However, the use of MJP rapid tooling does not substantially change these basic design rules. Therefore, any literature, experience or testing a person may have related to their design most likely transfers.
There are a number of important printer and material requirements that enable a safe and functional use of rapid tooling in sheet metal forming. Of course, the printer needs to reproduce geometry sufficiently well to create the needed features and the build size is important to allow the most ease-of-use and flexibility. However, the main requirement is in the materials ability to handle a very high compressive load and in its surface and bulk durability under high and repeated compressive loads. One of the great strengths of 3D Systems MultiJet Printing (MJP) technology is in its broad and individually optimized material set focused on different customer applications. For example, all the Rigid and Engineering materials can be used for drilling/taping/machining/pressing and achieve the good surface finish, sharp corners, fine features, and create high fidelity true-to-CAD parts that is well known for MJP technology. The Rigid white, clear, and gray materials (M2R-CL/WT/GRY) are optimized for general-purpose properties. They are fairly rigid with good heat deflection temperatures, and yet maintain adequate elongation. They are good for concept modeling and light end-use prototype part needs. The Engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) were designed for the most aggressive Engineering and Rapid Tooling applications. Proflex is not as stiff as Armor, but has the highest toughness and elongation capability. ProFlex behaves similar to polypropylene and can be substantially twisted, flexed and deformed without cracking or breaking. Armor also has very high toughness and elongation, along with a moderate stiffness making it the typical of choice for sheet metal forming applications. Armor should be tried first, but for molds without small features, with sufficient thickness and/or that requiring extreme pressure, ProFlex may be the better material. ProFlex features will deform more for a given load and so the features in parts will have a larger radius. A few examples will be presented next to facilitate such knowledge; however, ultimately each design is unique and one must
iteratively test to determine the appropriate load/pressure requirements for their specific needs (material, thickness, and geometry).

 

Example 1 : Simple Embossed Part

A simple mold was made that forms a square and triangular emboss in a sheet metal part. The molds were 1” (2.54cm) thick and measured 3”x5” (7.6x12.7cm) in dimension. Both features were 1” (2.54 cm) in size and extruded about to 0.2” (0.5 cm) depth.

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CAD view of embossing/forming tool

 

Two dowel pins were used to alignment the two mold halves and the sheet metal to the mold. In one side, a compressive fit was achieved to hold the pins by simply printing the same size hole as the pin diameter. The other side used a clearance of 0.2mm on the diameter allowing the molds halves to slide together. No post machining or other special processes were needed for any of the molds.

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Embossing/forming tool with alignment dowel pins

 

Both aluminum and steel were tested according to the following thicknesses.
- Aluminum: 0.032”, 0.050”, and 0.063” (22ga, 18ga, and 14ga) - 0.76mm, 1.27mm, and 1.60mm
- Steel: 0.036”, 0.048”, 0.060” (20ga, 18ga, and 16ga) - 0.91mm, 1.21mm, and 1.52mm

A manual, 50-ton hydraulic press was used to form the metal. A very thick plate of aluminum was used on the top of the mold to distribute the load across the mold. A steel plate was used on the bottom.

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50 ton capacity hydraulic press

 

For each sheet metal part the load was increased until the two halves of the mold came together and then the load was recorded and removed.

All of the parts worked with this mold design in this setup and none of the molds were damaged. There were some expected part defects given the range of materials and thicknesses tested. This was due to normal sheet metal design rules and had nothing to do with rapid prototyping. There were also some noticeable differences
in the formed feature based on the materials used. For example, the .030” aluminum plate showed tearing on the top edges of the triangular part on the corners of the square part. Some wrinkling occurred at the edges of the sheet metal due to the feature of the tool being so close to the side of the tool that deformation occurred.

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0.030 aluminum sample sheet metal tear and wrinkling at the edge

 

The steel parts were harder to remove from the mold after forming compared to aluminum plate. A light coating of silicone release agent allowed for easier removal. We also observed same design-related defect in the steel parts, i.e., wrinkled surface on the square side of the tool.

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Separation of the mold after forming operation

 

Since steel strength is higher than aluminum the applied force was higher than the aluminum testing application. The following loads were measured for each sample.

Thickness Aluminum Steel
0.032'' 500 lbs (0.25 ton, 2.2 kN)  
0.036''   12,000 lbs (6 ton, 53 kN)
0.050'' 1,500 lbs (0.75 ton 6.6 kN) 16,000 lbs (9 ton, 71 kN)
0.060''   20,000 lbs (10 ton, 89 kN)
0.063"

2,000 lbs (1 ton, 8.9 kN)

 

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Aluminum, left ot right: M2R-CL, M2G-CL, M2G-DUR

Top to bottom: 0.032'', 0.05'', 0.63''

 

 

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Steel, left to right: M2R-CL, M2G-CL, M2G-DUR

Top to bottom: 0.036'', 0.05'', 0.60''

 

The stiffer/stronger materials created the sharpest corners. This makes sense because the less stiff ProFlex material deforms more under a similar load.

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M2G-CL (Armor) vs. M2G-Dur (Proflex) edge radius comparison, 16 gauge steel parts

 

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M2G-CL (Armor) vs. M2G-Dur (Proflex) edge radius comparison

 

Example 2: Complex Formed Feature Testing
In conjunction with the testing above, numerous other different diagnostic parts were created and tested with different thicknesses of both Aluminum and Steel. The idea was to stress the material and look for useful trends and failures. The following is a list of key findings.

  1. The M2G-CL Armor parts were found to have limit of approximately 15 tons of pressure before experiencing compressive fracture. This was much higher force than that needed to form the parts. The Armor parts also had a high endurance. For example, a 1” thick flat part of Armor material could be subject to 10 ton (20,000lbs/89kN) force numerous times and would bounce back.

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M2G-CL Amor can sustain massive mold 15 tons of force and experience massive mold deformation without failure. The mold would return to shape and could continue to be reused.

 

  1. Higher pressures and/or more rigid mold material typically resulted in an increase in part detail for both aluminum and steel.
  2. Draft angle and release agent had a significant effect on part release from mold.
  3. The transparency of the Armor and Proflex material was sometimes advantageous as it could assist in alignment.
  4. The features were photographed after numerous cycles and showed little to no signs of wear.
  5. Small shallow features of molds had more difficulty appearing on thicker metal parts.
  6. Dowel pins need to be significantly long in order to ensure pre-pressing uniformity and symmetrical fit as dowel pins that are too short will not reach into the female mold with metal sheets between both halves of the mold
  7. The more the material was stretched by the mold the more warping occurred on the sheet metal by the dowel pins. Alignment dowel pins need to be placed sufficiently away from the formed features.

Example 3: SolidWorks Workflow and Multiple Part Fixture

  1. Create the desired part in SolidWorks. This example was done feature-by-feature using the sheet metal package. The side edges and back of this part were created with a slight radius for aesthetic purposes. This would be very difficult to form using traditional methods. Also, the top surface required some formed embossed features for added strength and flatness. For this this example, the entire flat pattern will be bent and formed in a single step to create the final part using a single MJP 3D printed tool. This includes bending all four side flanges and forming four top embossed features.

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Sheet metal part to be formed with 3D printed MJP rapid tool.

 

  1. Create a forming die that will emboss/form/stamp the desired shape. This example was done feature by feature using SolidWorks solid modeling. Test this forming die with your sheet metal part to achieve the desired result. That functionality is built into SolidWorks. Take note that in order to form this pattern into your sheet metal part, you will need to save your part into the SolidWorks Design Library as a forming tool. There are many tutorials available for this type of function.
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Forming tool created in SolidWorks

 

  1. The forming tool above is able to form your sheet metal part in SolidWorks and it show you visually what your final part will look like. However, to form the actual part in the shop will require a 3D printed MJP top and bottom half. One way to build both these two parts is to reuse the forming tool above. To do this, start by creating a large flat piece of sheet metal with the appropriate thickness in SolidWorks. Make this part oversized so that there will be extra material left on the edges after the forming process in SolidWorks. Use your forming tool to form this oversized sheet metal part.

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Oversize part made in SolidWorks using a blank piece of sheet metal and the forming tool

 

You can see in this assembly picture, that the oversize sheet metal part is identical in shape and thickness to the desired final sheet metal part.

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Oversized formed sheet metal part identically matches sheet metal geometry

 

  1. Create another solid part file that will be used to make the two halves of the 3D printed mold. Keep it simple at first with just a single large-solid feature that has the approximate size of the total A and B halves. You can make final modifications later. Make an assembly of this new solid part and the oversize sheet metal part made above. Finally, use the cavity tool or a Boolean subtraction of the two parts (make sure you unclick the “merge part” checkbox so that you end up with two separate bodies in your part file). Now you have the exact geometry needed to make both the top and bottom halves of the 3D printed molds that will both bend and form your flat pattern into the final geometry. There are other methods possible including offsetting the surface or other CAD package specific capabilities that could be considered as well.

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Use the cavity tool or a Boolean subtraction to form the functional
features in the top and bottom mold halves

  1. Finish up the molds halves by cutting away extra material to create the desired mold thickness and final shape. Consider adding special cut-away features to allow easier removal of the sheet metal part after forming. In this example, numerous threaded features were added to the top and bottom of the molds that allowed it to be attached with screws to the supporting top and bottom metal backup plates in the hydraulic press. These functional threads were directly printed with the MJP printer.
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Left: Top of mold, Right: Bottom of mold

 

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Top and bottom mold assembly with threaded holes for fixture assembly needs and front cutout to assist in removing the sheet metal part after forming operation

 

  1. For this example, 300 of these sheet metal parts were needed. Therefore, we attached 3x of these assemblies to machined aluminum top and bottom backup plates. A single set of 3x MJP tools were used to make the 300 parts. So, each tool easily made 100 parts each without any noticeable degradation of part quality.

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Complete fixture with 3x of the MJP 3D printed tool attached to aluminum parts that fit into the jaws of the hydraulic press used in the shop.

 

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300 parts were generated without noticeable degradation, 100 from each 3D Printed MJP Tool.

 

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MJP Sales Kit Part showing engraved logo and entrapped support marking
that identifies the tool front