Elastoforming 101

Elastoforming, also known as rubber-pad forming, is a manufacturing process for sheet metal that utilizes an elastomer pad for one side of the forming tool and a solid die or punch on the opposite side.

What is Elastoforming?

Elastoforming, also known as rubber-pad forming, is a manufacturing process for sheet metal that utilizes an elastomer pad for one side of the forming tool and a solid die or punch on the opposite side. Rubber-pad forming was originally developed in the late 1930s by Henry Guerin to manufacture structural aircraft components and has since become a prerequisite for many aerospace applications. It is estimated that over 60% of all sheet metal aviation parts are shaped using flexible tools, typically employing polyurethane elastomers, to produce products of superior finish quality with fewer process related defects.

Elastoforming is an effective method for minimizing initial tooling costs and simplifying the forming process by replacing metal tools with flexible pads. A large variation of part sizes and multiple part configurations are easily achievable. High quality parts can be produced in a single pressing cycle that require no additional finishing or polishing. The simplicity in process design and application makes elastoforming a great option for both prototype development and low to mid volume production. Applications can be found in aerospace, automotive, and consumer goods sectors. Figure 1 below is an example of a hydraulic press designed for elastoforming.
Fig. 1: A 7700 Ton multi-frame elastoforming press.

Materials Considerations

Any ductile sheet metal can be formed by elastoforming, however, due to the limited strength of the elastomer there are restrictions on the sheet metal thicknesses that can be processed. Some common materials and typical maximum thicknesses are presented in table 1 below.
Sheet Material Grade- Condition Thickness, max typical
Austenitic Stainless Steel 304-Annealed 1.3 mm / 0.05”
Austenitic Stainless Steel 316 – 1/4 Hard 0.8 mm / 0.03”
Aluminum Alloy 2024-O 4.7 mm / 0.19”
Aluminum Alloy 7075-W 4.7 mm / 0.19”
Aluminum Alloy 2024-T4 1.6 mm / 0.64”
Titanium Alloy, heated Ti-6Al-4V 1.0 mm / 0.04”
Table 1: Example sheet metal materials for elastoforming.

Using elastomers in forming tools can be advantageous due to; excellent abrasion resistance, ability to recover quickly from deformations, and a high percentage of elongation before fracture. Natural Rubber (NR), Silicone Rubber (SR), and Styrene-Butadiene Rubber (SBR) are common elastomers that have been used traditionally in rubber-pad forming, however, modern day elastoforming processes most often use Polyurethane (PU) pads due to superior durability, scuff resistance, and the ability to withstand higher forming pressures compared to other elastomer alternatives.

The hardness, or durometer value, of the polyurethane, and the shape factor have a direct impact on the compressive load required to form a part. Shape factor [SF] is the ratio of the surface under load compared to the total area of unloaded surfaces.  Figure 2 below depicts an example of a polyurethane pad with different shape factors related to the stress-strain relationship of the elastomer material during compressive loading.

Fig. 2: Stress-Strain relationship of different shape factors of polyurethane with shore A hardness 70

Advantages of Elastoforming

Some of the advantages of elastoforming as compared to conventional forming processes are:

  1. Only a single rigid tool half is required to form a part.
  2. A flexible pad takes the place of any die shape.
  3. Tools can be made of low cost, easy-to-machine materials due primarily to uniformly applied forces.
  4. Forming radii decrease progressively during the stroke, unlike the fixed radii on conventional dies.
  5. Thinning of the work metal is reduced considerably.
  6. Different metals and material thicknesses can be formed in the same tool.
  7. Parts with excellent surface finish can be formed as no tool marks are created.
  8. Set-up time is considerably shorter as no lining-up of tools is necessary.

Some limitations are:

  1. The flexible pad has a limited lifetime that depends on the severity of the forming and the pressure level.
  2. Lack of sufficient forming pressure can result in parts with less sharpness or with wrinkles, which may require subsequent hand work.
  3. The production rate is comparatively slow, making the process more suitable for lower part volumes.
  4. Elevated forming temperatures are restricted to the temperature range of the elastomer.

When to Use Elastoforming

Hydraulic elastoforming presses provide the ability to rapidly and cost effectively manufacture high precision sheet metal components for aerospace, automotive, and appliance applications.  The process is ideally suited for low to medium production volumes.

Parts made from aluminum alloy, stainless steel, and titanium alloy are the most commonly formed with elastoforming.  Elastoforming is a manufacturing process that can be used where a complex part, that may have required multiple steps and processes of cutting, bending, and forming of joints, can be formed into a single sheet metal part within a single press.

The aerospace industry widely uses elastoforming to manufacture parts because technical regulations and safety standards do not allow the sheet metal parts to be formed with conventional metal tools.  Thinning, marring and stress concentrations can be alleviated with the use of elastomer tools while maintaining the quality and structural integrity required for airframe safety.
Process Considerations

Deciding if elastoforming is the preferred process for part manufacture is a complex challenge. Consideration must be given to production volumes, production rates, part cost, part function, and manufacturing tolerances. In addition to metal forming, both blanking and piercing can also be accomplished with elastoforming, as illustrated by Figure 3 in the next column.

Fig. 3: Different elastoforming operations
Alternative Processes
Processes capable of achieving similar results include;

Metal Stamping: Can be employed if the material being formed is ductile at room temperature and the required part quality can tolerate a closed die forming process. More traditional part and die design methods for managing spring back are typically required to produce accurate parts.

Warm Stamping: Typically used for non-ferrous alloys that are difficult to form at room temperature. A controlled cooling and ageing process is often required after forming to restore solution hardened microstructures present before the sheet is annealed by heating.

Hydroforming: An advanced sheet and tube forming process that uses hydraulic pressure instead of a fixed punch to produce geometries not suitable for stamping, including undercut or bulged shapes.  
Elastoforming Process

Overview

The basic setup for elastoforming typically includes a rubber pad, contained in a pad retainer, fixed to the upper ram of a hydraulic press, and a form block, contained on a platen located in the bed of the press. A blank, which can be held in place with the use of pins and/or nests, is placed on the form block. As the ram descends, the elastomer will move and take shape around the form block creating the workpiece. The pad retainer is a frame that fits closely around the platen to enclose the elastomer and prevent lateral flow when pressure is applied.
Fig. 4: Cutaway of tooling and setup for elastoforming.

The elastomer pad acts somewhat like a fluid and exerts nearly equal pressure on all workpiece surfaces as it is pressed and flows around the form block. Rubber-pad forming is designed to be used on moderately shallow, recessed parts having simple flanges and relatively simple configurations. The form block height is usually less than 100 mm, or 4 inches.

Success Factors

A comprehensive examination of all parameters that affect elastoforming is beyond the scope of this paper. Instead, we will look at a few key factors that translate well for acceptable process economics: complexity and part size, tooling considerations, production rates, and defect reduction.

Complexity and Part Size:
The elastoforming process is commonly used to perform low to medium sized runs of light metal parts. Due to the low compressible nature of elastomers, a high enough pressure cannot typically be achieved to prevent wrinkling in deep drawn parts. Elastoforming is ideally suited in forming moderately shallow, recessed parts where pressure uniformity across the part can be achieved. In most cases, elastoforming is suitable to produce parts with a sheet metal thickness of 4mm or less.
Tooling Considerations:
Low tooling cost is one of the major advantages of rubber-pad forming and is why it is often the choice in prototyping and development. Because the upper elastomer die can be used with separate and different lower metal form blocks, the process is rather inexpensive and much more flexible in comparison to conventional metal forming processes. The metal half of the tooling is not worn as quickly, however, the elastomer pads of the tool have the potential to wear quicker than metallic counterparts. Extending the life of the pads can be achieved by having lower profile form blocks and allowing for as generous corner radii as possible. These techniques enable the flow of the elastomer at lower stress levels during the forming process.

The elastomer pads may be constructed either from a solid casting or in extruded and laminated layers that can be bonded together. Laminated layers are advantageous as the working surface can be restored efficiently by merely replacing the top layer. Also, different hardness levels can be used in different layers allowing for increased production flexibility. In general, the harder the elastomer material, the greater its load bearing capacity.
Production Rates:
Production for elastoformed parts is often slower in comparison to other metal forming processes. Although initial set-ups can be very quick, flexible and inexpensive, they are not built to last for many runs over long periods of time. Therefore, quantity and output of parts needs to be weighed as determining factors when considering the use of elastoforming.
Defect Reduction:
The use of elastomer pads is an effective method to reduce defects in comparison to alternative processes.  Because the workpiece is sandwiched between an elastomer and a metallic form block, the surface finish is remarkably improved, therefore reducing both visible defects and subsequent polishing costs.  Defect free and high-quality parts are one of the major advantages of using an elastoforming process, as polished and/or sharpened surfaces remain unchanged.  This process will also allow pre-painted and pre-finished metals to be used.

Uniform thickness in metal formed parts is desirable, and often necessary to meet safety and regulatory standards. Pressure is distributed evenly across the blank resulting in significantly less thinning of the workpiece compared to other forming processes.

The use of accessory equipment such as draw clips, cover plates, wiping plates, forming rings and bars, dams and wedge blocks can be placed in specific locations during the forming process to adjust the pressure to aid in forming more complex shapes and features.  Accessories can also be used to assist in preventing wrinkling and distortion, however, effective part and tool design is often the first line of defense in limiting defects in parts.
Part and Tool Design to Prevent Defects
The main controlled process parameters for rubber-pad forming that can result in improved part and tool design are the hardness and properties of the elastomer pads, form block height and radii, and the workpiece thickness.

Elastoformed parts should be designed with suitable radiused corners as 90-degree corners cannot be fully achieved with the use of elastomer pads.  The actual size of a elastomers natural radius when deformed will be dependent on the pad material used, and should be considered in the design of the finished workpiece.  Allowing generous radius on parts will allow the elastomer to flow appropriately with good pressure distribution along the entire workpiece during forming.  Approaching uniform pressure distribution during the forming process results in parts with good die detail.
Fig. 5: Sample sheet metal part for elastoforming process

One of the challenges of elastoforming is wrinkling in deeper formed parts because it is difficult to achieve high enough pressure in the elastomer due to its low compressive strength.  It is important to consider the depth of the formed part as elastoforming is more suited to produce shallow parts. Typical elastoforming jobs include forming flanges around flat pieces, raised ridges, adding rigidity to flat sheet metal by forming beads, creating embosses and trimming.

Press Configuration

Basic Parameters

Configuring a press to produce elastoformed parts starts with the requirements for the part manufacturing process. The configuration requires information regarding material type, part specifications, production volume, production speed, and target pricing. These parameters have a direct influence on the handling and forming requirements, which in turn influence the specifications of the installation and tooling. Setup time is often lower with elastoforming in comparison with conventional metal forming. Only one half of the tool is metal, therefore lining up tools is far less critical and still results in high quality parts.

Bed Size

The bed of a press must be able to accommodate the footprint of the largest expected toolset. For rectangular or complex blank shapes, orientation of the part within the bed will determine overall bed dimensions. An estimation of bed size can be calculated based on blank size(s). Additionally, the bed size must be capable of containing the outer limits of the pad holder that fits around the form block on the bed of the press.  The pad retainer must be strong enough to contain the pressure of the elastomer and is typically 20-30 mm deeper than the pad itself.

Tonnages and Speeds

Modern elastoforming is executed using a programmable hydraulic press to repeatably control the forming tonnage and to optimize cycle times. Any press size up to 20,000 tons can be configured.

Tonnage calculations can be rather complex as the press, die, material, radii, and part size and geometry are all contributing factors.  With simple geometries, it is possible to calculate the forming tonnage required with the use of tables and formulas.  For a formed hemisphere, the following equation can be used as a simple guideline:

Where;
d = diameter of finished part
t = material thickness
TS = tensile strength of material

For example, an 8″ diameter hemisphere made of 0.040” thick 304 stainless steel (TS = 73,200 psi), would require;


73,588 pounds (36.8 tons) is required to form the part.  In this example, a 50-ton press could be recommended.

With more complex parts, sophisticated tonnage calculator tools and analysis software, combined with programmable hydraulic presses, has become more widely used to ensure a successful forming process.
Stroke and Daylight
Enough daylight must be available between the fully retracted elastomer pad and the form block to allow removal of the finished part from the press. In practice, this translates to daylight that is approximately equivalent to the press stroke. If a press will be used to produce more than one part, stroke and daylight should accommodate the largest expected tooling.

Maximum stamping depths can be increased by using thicker pads and more powerful presses, however the amount of wear on the elastomer and the impact of using more powerful presses are factors that require consideration.
Fig. 6: A Macrodyne 4000 Ton Elastoforming Press
Advanced Capabilities

Some advanced elastoforming capabilities include:

  1. Progressive tool design for continuous production of strip metal parts with complex geometries. Blanking, forming, and piercing can all be combined into a single pad process.
  2. The use of elastomers, in micro forming, a process of creating miniature parts with intricate details, can help in controlling the microscale of material deformation during the forming process due to the small size of parts. Factors such as minimizing defects, dimensional accuracy of the formed parts, and surface finish are even more significant in production of parts of this size.
  3. Shaping of circular tubes into triangular cross-sectioned columns of different angles can be achieved with the use of elastoforming processes.