Please email any questions that are unanswered on our website.  The aim is for the information we have provided to help educate those who read it.  We may include it if it fits.
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Tube hydroforming is a conceptually simple technology, but can get quite complicated when attempting to determine how to apply it most effectively to provide the most value added features for a reasonable cost.  Past missteps have been numerous and some very costly.

We have included questions that address a number of issues, conceptions & misconceptions

(?)Does using higher pressure allow you to tube hydroform more complex parts?

(Ans.) While this seems conceptually sensible, it is more complex than this simple statement.  It is true that high pressure fluid has the greater impetus to force material to go where you want it to (ie more complex geometries) compared to low pressure.  The catch is that the material must not rupture or tear and theoretical shapes that could be formed, become more conservative and rounded when they actually are.

Though counterintuitive for many, experience shows that the relationship between part shape complexity and the pressure being used can also be characterized in another way.  More complex cross section shapes have been made using lower pressures, perhaps since the material has more opportunity to move relative to the die cavity surface.  High pressure inherently forces the material against the wall forcefully enough that sliding is prevented.

In summary it is best to work with a hydroformer that is adept with both approaches to allow use of the most suitable one, combination or innovation to achieve the most advantageous part design.

(?)Is high pressure needed to expand steel tubing? 

(Ans.) Not necessarily. It is common to combine straining the material (expanding) and changing the shape of the tube while forming it, which is what happens during the HPH process. The reshaping immediately before expansion substantially reduces the radius or Ri in equation 1. This greatly increases the pressure required to expand the material. Simultaneously, material starts to work harden, further increasing the pressure needed, which is the reason for the high in HPH.

Instead by expanding in the round relatively low pressure is required, usually 2-3,000 psi for normal tube used for automotive structural applications. This pressure is much lower than normal maximum LPH pressure. So the real difference is when during the process expansion is being done, rather than if it can be done at all. The relationship between pressure P, yield stress S, material thickness t and smallest corner radius Ri being formed is shown in equation 1.

(?)Why is tube hydroforming so slow?

(Ans.) This is dictated by the sequence of steps that are required to form the part and how the equipment is designed.  It has been common to have cycle times ranging from 20-45 seconds, with some even longer.  Cycle time also is somewhat proportional to the part size.  HydroDieForming™ offers the opportunity to substantially reduce cycle time with several strategies  suited to different part sizes.

(?)Why is tube hydroforming expensive?

(Ans.) There are a number of factors that contribute to this.  In summary capital or investment cost, as well as part cost are relatively high.  This is driven by the process design and the steps that are seen as necessary.  These needs are changed dramatically by using a different process approach.  The lowest cost, nigh capability approach that suits most needs is  HDF™. 

(?)Tube hydroforming is only suitable for high volume applications?

(Ans.) This is a function of the high investment cost usually associated with hydroforming production equipment.  The cost is distributed over a larger number of pieces making it more acceptable.  This cost is substantially reduced when using HDF™.

(?)Why does tube hydroforming always need special materials and ‘hydroformed’?

(Ans.) This is mostly a direct function of the formability required to enable the concentrated forming of corners when expanding in the hydroforming die. This is one factor that can be altered to facilitate forming the required or desired part shape. Others include the amount of expansion, end feeding, lubrication, and corner sharpness.

(?)Are galvanneal, aluminum and high strength steel incompatible with tube hydroforming?

(Ans.) Materials such as these are less formable that others more commonly hydroformed. Often design practices, especially to expand the tube in the hydroforming die, as is common with HPH, requires high formability material. However, these materials are in increasing demand for use to reduce the structural weight of the vehicle. It is best to use the process that offers the most design flexibility. HDF™ often exceed the capabilities of other hydroforming processes. Forming does not stretch the material, more suited to these types.

(?)Why is end feeding always required for tube hydroforming?

(Ans.) This is used to reduce the wall thinning effects of expanding the tube. It is only effective to the point where friction between the tube exceeds the column strength of the formed tube, beyond which it will crumple.

(?)How much of a problem is die wear for tube hydroforming?

(Ans.) This varies depending on the process used. For high pressure hydroforming with end feeding surface abrasion is high, usually requiring hardened cavity surfaces and inserted sections to allow replacement when wear becomes too great. Low pressure hydroforming (ie HDF™) does not bring about this intense contact and therefore wear is substantially less problematic. It is common that dies run millions of parts with no substantial wear.

(?)Why is lubrication always required for tube hydroforming?

(Ans.) It is needed to reduce friction, which reduces wear as discussed briefly above, to make forming & strain more even and to increase the amount of possible end feeding. HDF™ normally does not use lubricants.

(?)Do all tube hydroforming parts need to be expanded?

(Ans.) There is a perception that this is true, but there are more then 50 million production parts produced with low pressure to show that this is not necessary.  Expansion can add cost if not included in the design properly.  When expansion is larger there is a structural benefit, which can provide great opportunity to reduce cost and weight.  This is a key advantage of hydroforming that should be used to its fullest.  We can help in this and many other respects to design and make the part that will deliver the functions you require with quality parts that are light and cost effective.