Hydroforming  

a unique process for forming complex pressings 

 

Hydroforming, otherwise known as the SAAB fluid forming method or simply Fluidforming, was developed in the 1950’s as an extension of the rubber pad forming method (referred to under such names as Guerin, Verson,Wheelon, Marform methods). Both methods were developed for industries with low volumes and high varieties of relatively thin gauge sheet metal formed parts,such as in the production of aircraft components. And again both methods were similar in that they had the advantages of low tooling (and tooling proving) costs and, due to their inherently better control of metal movement, permitted more “severe” forming than conventional tools thereby also enabling more complex shapes to be produced whilst maintaining finished part integrity.

Both methods reduce the degree of marking which can occur with conventional forming methods and, as it is a “smooth” forming process they eliminate any cracking and wrinkling. Hydroforming has the advantage over rubber pad forming in  forming pressures (up to 20,000 psi) can be achieved allowing stronger materials to be formed e.g. high strength alloys of aluminium, nickel alloys and titaniums.

In basic terms the principle in both rubber pad forming and Hydroforming is that one half of the conventional solid, hard metal punch and die forming set is replaced by one half of the tool being made from a flexible “tool” and the other half, either the die cavity or the punch,remaining solid. With Hydroforming this flexible half is an oil filled fluid cell as opposed to a rubber block or pad. This fluid cell is made up of a steel cylinder, a fluid chamber and a rubber, or urethane, diaphragm. There are two methods employed to pressurise the fluid i.e. in the case of a self-contained unit the oil is pressurised by the action of the ram of the hydraulic press machine on the telescopic cylinder which, acting through the hydraulic oil, exerts pressure on the diaphragm; these self-contained units are commonly referred to as being SAAB units. Alternatively special purpose Fluid cell presses have an integral fluid cell which is mounted on a stationary ram whereby pressure is exerted on the diaphragm via the machine’s hydraulic pumps pumping high pressure hydraulic oil directly in to the fluid chamber.

Advantages of using SAAB self-contained units are that they can be moved around and attached to anyone of a number of different hydraulic presses, or can be removed completely to allow the press to be used in a conventional way. Whereas a purpose built dedicated machine is quicker to set up, can have an infinitely programmable pressure range (up to the maximum rating) which is easier to control, and it is often quicker to operate. In both cases the diaphragm enables the pressure to be exerted evenly around the whole of the component blank throughout the forming process and the metal is formed into the cavity or around the punch. This smooth forming process eliminates cracking and wrinkling whilst containing material thinning to a minimum if not none at all. It is typically suitable for sheet metal gauges up to 4swg/6mm.

The following diagrams show the SAAB method and help to illustrate the process. Note that the dedicated press’ integral unit follows the same principle but the ram is replaced by a hydraulic pump system which directly energises the pressure cell.

Stage1 – blank in position across tool

Stage2 – pressure cell in position, ram pushes down & forming begins                              

Stage3 – ram continues to push down until desired pressure is reached                               

 

Stage4 – forming cycle is complete and unit is withdrawn leaving finished part

     

Besides the obvious advantage of saving on the cost of the individual tooling sets, and of the proving of such tooling, as there is only one rigid half to manufacture and then develop if necessary, tooling costs as a whole are much less because the fluid unit can be used on a wide varietyof components/jobs and each job/component therefore only requires one solid half. Whilst slow in comparison to conventional forming/pressing methods the hydroforming technique is ideally suited to low volume, small production runs of relatively complex shape and from high strength sheet metals requiring to be formed to print with no subsequent hand working/correction.

In summary the advantages of hydroforming are:-

  • Low tooling costs
  • Non marking of the formed component
  • No wrinkling
  • No cracking
  • Minimal material thinning
  • Complex shapes can be formed, including C flanges and under-cutting
  • Controllable force of forming via control of the pressure
  • Components are formed right to print with no further expensive hand working being required
  • Formed parts with deep cavities can be produced
  • High strength materials can be formed e.g. alloys of aluminium, nickel & titanium alloys.
  • A cold forming process straight from sheet material

MSM aerospace fabricators has been hydroforming aircraft and aerospace parts for over 40 years. They have seven SAAB fluid cell units ranging from 8”/200mm to 19”/475mm diameter delivering pressures up to 10,000psi/689 bar/69 MPa, and each of these units are interchangeable on a number of their four hydraulic presses (ranging from 500 to 1500 ton). In addition MSM has a dedicated Fluid cell press with a 40”/1m diameter table capable of forming pressures up to 14,000 psi/ 965 bar/97 MPa. Furthermore MSM-aero has a bespoke software simulation system which can perform a virtual pressing operation enabling the hydroforming process to be optimised prior to any metal being cutor expense incurred. This also enables the solid half of the tooling to be designed right first time without the need for expensive press-measure-modifications which are typically required to determine spring-back allowances,etc. This in itself reduces the development time for the introduction of new designs and the associated costs of tooling trials.

If you have a component which may be suited to the above method then please contact MSM aero who’d be pleased to provide you with a technical proposal and a competitive quote. More information on MSM’s other specialist NADCAP approved processes is on our website.

Alternatively complete the contact us sheet and we will contact you.