Sheet Fluid Forming and Sheet Dieless Forming Part 1.
Posted by: Trent Maki on Thursday, February 07, 2002 - 08:57 PM
The automotive and other industries are striving to overcome current forming limitations to make use of newer materials and form more complicated shapes. Moreover, cost effective production methods are desired and alternative manufacturing processes are being investigated. This paper introduces two possible solutions; sheet fluid forming (sheet hydroforming) and dieless NC forming. They clear most forming restrictions and can form intricate shapes at a lower tooling cost and in a shorter development time.
Author: Hiroyuki Amino (President, Amino Corp.)
Assisted by: Keiji Makita (Amino Eng. Dept.)
Trent Maki, P. Eng. (Amino Development Dept.)
Hydraulic counter-pressure forming was initially developed to overcome the restrictions of deep drawing. It was first applied by Amino to production thirty years ago. Further development led to forming of tapered and other complicated shapes. It has been used in the automotive industry over the last ten years to reduce tooling cost. Now it is being pursued to overcome formability restrictions of new materials: Aluminum, High Tensile Steels, Magnesium, Titanium and others. Hydraulic Counter-pressure forming is also known as sheet hydroforming, hydro-mech, aquadraw and for the purposes of this report, sheet fluid forming.
In addition, a new numerically controlled incremental forming process was recently developed to further reduce tooling cost and prototyping time. The dieless NC forming machine has many applications for rapid prototyping, manufacture of service parts and small lot production.
This paper looks at both methods and applications to the automotive and other industries.
2.1 Fluid Forming Process
2.1.1 Basic Principles
A sheet metal blank is formed by hydraulic counter pressure generated by the punch drawing the sheet into a pressurized water chamber, which is acting as the female die. The water pressure effectively pushes the sheet firmly against the punch to form the desired shape. The major advantage of fluid forming is an increased drawing ratio. It allows forming to be carried out in one process as opposed to two or three by the conventional methods. This comparison is detailed in Figure 1 and Table 1.
Fig. 1: Comparison of Conventional and Fluid Forming
Table 1; Comparison of Conventional and Hydraulic Counter Pressure Forming
In conventional deep drawing, low friction leads to free movement at the punch shoulder resulting in localized thinning and breakage. By adding hydraulic counter pressure, refer to Figure 2, the high frictional force, when the sheet is pressed firmly against the punch shoulder, causes an increase in the tensile resistance of the sheet. Breakage will usually occur in the overhang part near the die shoulder. Therefore, the friction effect on the punch increases the overall breakage resistance
Fig 2: Principles Diagram (1)
The sheet clamped in the binder rides on the edge of the die resulting in material thinning and breakage in the normal deep drawing process. With hydraulic counter pressure the material between the punch and the die is initially expanded reducing contact with the die shoulder and possible tearing during the drawing process.
When the water pressure increases past the set point of the relief valve, water flows out between the blank and the die. The die face is effectively lubricated by the escaping water and friction at the flange decreases facilitating the flow of material into the die.
The forming process is the same as the conventional deep drawing process, except that the die cavity is filled with liquid and hydraulic pressure is applied during the forming process.
126.96.36.199 Blank Set
Fig. 3: Blank Set
Figure 3 shows the typical tooling consisting of a punch that holds the shape of the part, blankholder to clamp the blank, the die and water chamber to provide hydraulic pressure. The blankholder, punch and the fluid pressure are computer controlled.
188.8.131.52 Blank Clamping
Fig. 4: Blank clamping
Figure 4 shows the slide descending and the blank firmly clamped by the blankholder. The chamber completely fills up and the water relief valve is set. Sometimes a seal is used when high forming pressure or a three-dimensional die face is required.
Fig. 5: Forming
Figure 5 shows the punch just entering the die. The fluid pressure builds rapidly to the relief valve setting. Tapered shapes require an increase in blankholder force. Figures 6 and 7 show the typical change in hydraulic counter-pressure and blankholding force per punch stroke.
Fluid pressure causes the material to expand between the die and punch, decreasing contact stresses on the die shoulder.
Fig. 6: Typical Blankholding Force and
Fig. 7: Typical Blankholding Force and Counter-Pressure curves for straight Punch Counter-Pressure curves for tapered punch.
184.108.40.206 Forming End
Fig. 8: Final Forming
Figure 8 shows the punch at full stroke. During this time the punch force has had to increase rapidly for forming as well as to counter the fluid pressure.
The fluid pressure will have reached the relief valve setpoint The pressure will continue to increase and the fluid will spray out between the die and blankholder providing lubrication to the blank. As the punch reaches the end, fluid will continue to flow out and hydraulic pressure will drop.
220.127.116.11 Features of Fluid Forming
Increased accuracy of the part.
Eliminates scratches and marring on the surface of the part.
Greater uniform wall thickness and less localized thinning.
Complicated shapes can be formed. Reduction in forming processes.
Tooling cost saving.
Lower grade of blank material can be utilized.
Suitable for small-lot production
2.1.3 Fluid Forming Techniques
The fluid forming is a very flexible process Shown below in figures 9~16 are different methods or techniques employed to reduce forming steps. Examples and descriptions of these methods will be shown in section 2.2 Fluid Forming Applications.
Fig. 9: Expansion Forming
Fig. 10: Taper Holder Driven
Fig. 11: Draw/Bend Synchro Method Forming Method Forming Method
Fig. 12: Plastic Sheet Forming
Fig. 13: Partial Lower Die
Fig. 14: Draw/Stretch Forming Method Synchro Forming Method
Fig. 15: Draw/Bend Synchro Forming Method
Fig. 16: Hydro Re-Strike Forming Method
following next: Fluid Forming Applications to Automotive and Other Industries