2.1 Basic concepts

ISMF is an innovative process for manufacturing sheet metal products by numerical control machines (CNC) based on simple forming tools for plasticity deformation to form metal sheets according to the desired shape. The controllable motion of the forming tool allows deforming three-dimensional profiles. This forming method offers many advantages in rapid prototyping of sheet metal products, which were directly constructed from CAD 3-D models to a complete traditional product without middle stages for designing and manufacturing molds. There are two main deformations of ISMF according to concave surfaces (Figure 2a) and convex surfaces (Figure 2b). They show the workpiece surface where the tool is shaping motion. The actual experimental setup used in ISMF is shown in Figure 3. The forming limit curve (FLC) of ISMF process is much higher than the forming limits calculated from the theory of plasticity as well as obtained from traditional test [2]. The forming limit curve from conventional deformations is V-shaped. But, recently studies have shown that ISMF process achieved greater formability and FLC shape almost like a straight line with negative slope in the principal limit strains (major strain, ε1, and minor strain, ε2). In order to estimate the forming limit curve at fracture (FLCF) in ISMF for a cold rolled, Nguyen et al. [3] proposed the combination method for predicting FLC based on in-plane test (M-K model) and ductile fracture criterion of Clift et al. [4]. In the previous study [5], cold rolled steel sheet improved formability by ISMF process and is also used to manufacture automotive structure [6] as shown in Figure 4. In ISMF process, the effects of parameters such as size-step, tool-down step, tool radius, etc. on formability are very

Figure 2. Forming concave surface (a) and convex surface (b).

combination of forming movements in the local plastic deformation region. The deformation process is slow and time-consuming, so it is only suitable for rapid prototyping of products or in series production. However, this method allows for greater formability than conventional deformation methods of material sheet. Forming tools are simple and inexpensive and develop products in a short time. This method contains new and creative contributions in sheet metal forming

Rapid prototyping products from ISMF: (a) dental, (b) medical, (c) headlight, (d) sculpture, (e) automotive

• A new type of tool path generation in ISMF to create complex surfaces

• Improving the formability of sheet metal when comparing to the traditional

such as:

98

Figure 1.

cover.

forming process

Automotive cover panels

Mass Production Processes

Dental, medical

Table 1.

Small ship body panels

Sculpture, architecture, decoration Required shape by customers Cover for lighting equipment

Special parts for aerospace and aviation

Potential application areas for ISMF [1].

Other chassis sheet metal parts of an automobile

Figure 3. ISMF experimental setup: lower mold, clamping, metal sheet and forming tool.

be fixed on the clamping system. This is a suitable machining method for small series production, prototype production, and shaping of complex surfaces used in aerospace, automotive, shipbuilding, medical industries, and so on. This method is being applied to reduce costs related to specialized molds used for processing mass

Square cups formed by rotational incremental sheet forming: (a) 45° wall angle, (b) 60° wall angle, and

between the parameters that affect the formability of the sheet material.

Table 2 lists the basic parameters used for ISMF. The influence of these parameters on the formability of different materials has been studied by many researchers around the world. The conclusions about the influence of parameters on various sheet materials are different, and there is no general rule for each specific effect except the effect of the tool diameter. In general, the conclusions can be generalized as follows: when increasing the thickness of sheet metal, reducing the size of the tool and reducing the down step will tend to increase the formability of the sheet metal. It can be explained why the results are not uniform because the parameter areas used for each research are different. In addition, there is a reciprocal interplay

The history of ISMF was started in 1967 when Leszak [9] obtained a patent for the solution: "Equipment and process of ISMF." The idea was to be ahead of its time, but subsequent studies were not conducted until the 1990s when studies were conducted mainly on circular-shaped workpieces and products could be shaped on

In 2001, along with the development of three-axis CNC milling centers, the method of ISMF was continuously deployed. Previously only specialized CNC machines were used for this shaping process. This is the starting point for studies conducted outside Japan. Some of the most active researchers since then can be listed as Jeswiet et al. [10]. Although it has been more than three decades of research and development, the technology of ISMF applied in rapid prototyping is still a hot

production in traditional deformation machining.

(c) 70° wall angle at which point cracks occurred [7].

Rapid Prototyping for Sheet Metal Products DOI: http://dx.doi.org/10.5772/intechopen.88435

2.2 History of development

horizontal lathes.

101

Figure 5.

Figure 4. Incremental sheet forming for automobile shape [6].

important. The influences of the main material parameters of the sheet material on the formability of ISMF had been studied in several published papers. In order to demonstrate the formability improvement for sheet metal by ISMF process using rotational tool (RISF), both empirical and simulation studies [7, 8] have been carried out for a magnesium sheet alloy. They concluded that heat generation in the contact zone between forming tool and metal sheet would affect formability of light alloy sheet materials such as titanium and magnesium alloy. With light alloy structures, titanium alloy and magnesium alloy have many advantages when compared to steel, cast metal, and aluminum alloy. However, the structure of titanium and magnesium alloys is limited by the formability at room temperature due to the tightly packed hexagonal structure. In order to apply these light alloys widely in industry, many studies have investigated the ability of these alloys to form at elevated temperatures and concluded that magnesium and titanium alloys have the best formability in the temperature range of 200–800°C by experiments and corresponding simulations as shown in Figures 5 and 6, respectively.

When the mold has a convex surface shape, the forming device must be equipped with a hydraulic clamping system to hold the metal sheet firmly in the proper working position. In the case of concave molded surfaces, metal plates can

#### Figure 5.

Square cups formed by rotational incremental sheet forming: (a) 45° wall angle, (b) 60° wall angle, and (c) 70° wall angle at which point cracks occurred [7].

be fixed on the clamping system. This is a suitable machining method for small series production, prototype production, and shaping of complex surfaces used in aerospace, automotive, shipbuilding, medical industries, and so on. This method is being applied to reduce costs related to specialized molds used for processing mass production in traditional deformation machining.

Table 2 lists the basic parameters used for ISMF. The influence of these parameters on the formability of different materials has been studied by many researchers around the world. The conclusions about the influence of parameters on various sheet materials are different, and there is no general rule for each specific effect except the effect of the tool diameter. In general, the conclusions can be generalized as follows: when increasing the thickness of sheet metal, reducing the size of the tool and reducing the down step will tend to increase the formability of the sheet metal. It can be explained why the results are not uniform because the parameter areas used for each research are different. In addition, there is a reciprocal interplay between the parameters that affect the formability of the sheet material.

#### 2.2 History of development

The history of ISMF was started in 1967 when Leszak [9] obtained a patent for the solution: "Equipment and process of ISMF." The idea was to be ahead of its time, but subsequent studies were not conducted until the 1990s when studies were conducted mainly on circular-shaped workpieces and products could be shaped on horizontal lathes.

In 2001, along with the development of three-axis CNC milling centers, the method of ISMF was continuously deployed. Previously only specialized CNC machines were used for this shaping process. This is the starting point for studies conducted outside Japan. Some of the most active researchers since then can be listed as Jeswiet et al. [10]. Although it has been more than three decades of research and development, the technology of ISMF applied in rapid prototyping is still a hot

important. The influences of the main material parameters of the sheet material on the formability of ISMF had been studied in several published papers. In order to demonstrate the formability improvement for sheet metal by ISMF process using rotational tool (RISF), both empirical and simulation studies [7, 8] have been carried out for a magnesium sheet alloy. They concluded that heat generation in the contact zone between forming tool and metal sheet would affect formability of light alloy sheet materials such as titanium and magnesium alloy. With light alloy structures, titanium alloy and magnesium alloy have many advantages when compared to steel, cast metal, and aluminum alloy. However, the structure of titanium and magnesium alloys is limited by the formability at room temperature due to the tightly packed hexagonal structure. In order to apply these light alloys widely in industry, many studies have investigated the ability of these alloys to form at elevated temperatures and concluded that magnesium and titanium alloys have the best formability in the temperature range of 200–800°C by experiments and

ISMF experimental setup: lower mold, clamping, metal sheet and forming tool.

Figure 3.

Mass Production Processes

Figure 4.

100

Incremental sheet forming for automobile shape [6].

corresponding simulations as shown in Figures 5 and 6, respectively.

When the mold has a convex surface shape, the forming device must be equipped with a hydraulic clamping system to hold the metal sheet firmly in the proper working position. In the case of concave molded surfaces, metal plates can

2.3 Formability of ISMF

Rapid Prototyping for Sheet Metal Products DOI: http://dx.doi.org/10.5772/intechopen.88435

When comparing the deformations of ISMF with other traditional forming process such as stamping, clawing, pulling, bending, and so on, researchers have shown that the forming limit diagram (FLD) of ISMF is raised much higher than the traditional forming limit diagrams calculated from the theory of plastic deformation as well as obtained from experiments through traditional testing methods. The forming limit diagrams of traditional deformations are V-shaped. But studies have shown that the formability in ISMF is larger and shaped almost like a straight line in the minor-major strain space. In order to obtain the FLDs of ISMF, they could be based on the ductile fracture criterion of Clift et al. as shown in Eq. (1). The points on FLDs are calculated based on the initial point of the minor-major strain point convergence at the equilibrium strain region; the following points are calculated according to the relationship between the minor-major strain ratios (Eq. (2)) and

the equivalent strain function for the plane stress state (Eq. (3)):

<sup>ε</sup> <sup>¼</sup> Rm <sup>þ</sup> <sup>1</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>2</sup>Rm <sup>þ</sup> <sup>1</sup> <sup>p</sup>

ening equations as indicated in Swift's Eq. (4):

as a constant Eq. (5):

103

2.4 Applications of ISMF method

ðεf 0

> <sup>β</sup> <sup>¼</sup> <sup>ε</sup><sup>2</sup> ε1

> > 1 þ

where K is the plastic deformation coefficient of the curve and n is the hardening parameter of the curve. After substituting Eq. (4) into Eq. (1) and performing the integral calculation, we can solve the equivalent strain value at failure point of ISMF

To determine C<sup>1</sup> parameter, the values of the minor-major strain at the equilibrium biaxial strain position are used in combination with the fracture values on the traditional forming limit curve. After determining the value of C1, we can use this value to calculate the different points of the FLC during ISMF by giving the deformation ratio β changes in the permissible zone and replace in Eqs. (2), (3), and (5). Figure 7 depicts the forming limit curves in ISMF based on the forming limit curves of the traditional method (V-shaped) for various experimental forming tools [3].

ISMF method can be considered a new rapid prototyping method without cre-

ating expensive molds, and the time to create parts from the idea of the final product is less than 24 hours. ISMS method can also be distinguished as a layered

where ε<sup>f</sup> is the equivalent strain at the ductile fracture strain point, σ is the equivalent stress, ε is the equivalent strain, C is the constant of the material, β is the minor-major strain ratio, Rm is the anisotropic coefficient, and ε<sup>1</sup> and ε<sup>2</sup> are the minor and major strains, respectively. In addition, material tensile tests give a relationship between stress and strain, and they are often expressed through hard-

r

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

<sup>β</sup> <sup>þ</sup> <sup>β</sup><sup>2</sup>

<sup>σ</sup> <sup>¼</sup> <sup>K</sup>ð Þ <sup>ε</sup><sup>0</sup> <sup>þ</sup> <sup>ε</sup> <sup>n</sup> (4)

ε<sup>f</sup> ¼ C<sup>1</sup> (5)

2Rm Rm þ 1

σdε ¼ C (1)

(2)

ε<sup>1</sup> (3)

Figure 6.

Deformed shape in finite element simulation: (a) 45° wall angle, (b) 60° wall angle, and (c) 70° wall angle [8].


#### Table 2.

Basic parameters of ISMF.

topic to be further studied by the following reasons: Accuracy of deformed products are still limited; the heat generation by the contact and friction between the forming tool and the material sheet is significant; there are high surface roughness and low productivity. Some recent applications of ISMF process have been summarized by several researchers [11–16].
