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I'm starting to see more and more frames using investment casting for head tubes, drop outs, seat tubes, etc. Such as the new WTP c.r.e.a.m. frame . It looks nice, but I'm suspect on how much more strength this gives you over traditional welds. Does anyone have more info on this technology?

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Your question is pretty subjective. Maybe you could rephrase it to make easier to answer. My suggestion would be: "What are the advantages to investment casting". –  Jack M. Aug 25 '10 at 21:46
    
@Jack M. good point –  dotjoe Aug 26 '10 at 13:45
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3 Answers

up vote 6 down vote accepted

I think the investment casting of the entire headtube is something particular to BMX bikes. It makes sense, for reasons outlined below.

Typical parts of a bicycle which can be investment-cast are:

  • Head lugs
  • Dropouts
  • Seat clusters
  • Bottom bracket shells
  • Seat-stay and chain-stay bridges
  • Fork crowns

The easiest way to see why investment casting is preferable is to compare it to the alternative ways the same items can be made: (modified from here)

  • Investment Casting: This is the most common method of making high quality lugs. This is the oldest form of metal forming on the planet.
  • Stamped & Rolled: This is the lease expensive way to make lugs in large quantities, as they can be stamped from a flat sheet, and formed into the required shape.
  • Bulge Forming: Relatively rare, this involves placing a steel tube into a shaped mold and using hydrostatic pressure to expand and shape the tube.
  • Welding/Brazing Tubes Together & Filing: This is also a common method, but intricate and time consuming.
  • Machined/CNC'd: Possible but not really used. Wastes a lot of material.

The two biggest problems with all the alternative methods are:

  1. They are formed into a different shape from a cold, pre-formed piece of metal (Stamped, Bulge, Machined/CNC'd), thereby reducing strength, or
  2. They have some weak spot (Rolled, Welding/Brazing), thereby reducing strength.

In investment casting, the steel is allowed to settle into its final shape while it's hot and allowed to slowly cool. This means that in it's final form, the metal is under absolutely zero stress trying to maintain it's shape. Furthermore, it ensures very even distribution (for example, rolled steel may be thin on the convex side and thick on the concave side).

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nice answer...one thing I was concerned with is how it moves the TT and DT connecting welds to lower points. I assume that is why they leave a large surface area with those sleeves (if that's what you call them)? Do you lose strength connecting 2 tubes like that? I'd almost want to see a gusset go under the DT/HT connection. –  dotjoe Sep 2 '10 at 14:04
    
Not sure what they're called, but I'd share your concern--I think it reinforces the head-tube, but perhaps at a cost. You'll see such gussets on a lot of cheaper chromoly frames for that exact reason. –  Dustin Sep 2 '10 at 14:19
    
Yes, pretty much all bmx frames have a DT gusset somewhere. Internally, externally, sandwiched between the TT and DT like classic Standard STA frames, etc. –  dotjoe Sep 2 '10 at 14:50
    
You're slightly mistaken about the disadvantages of other techniques: forged parts are typically stronger than cast parts, not weaker. I imagine forged frame parts are rare because of the much greater expense of the tooling, particularly for complex shapes, relative to casting, rather than because of any mechanical inferiority. –  Tom Anderson Sep 9 '10 at 20:21
    
For dropouts, laser-cutting is also becoming much more common. It has the advantage of having minimal tooling requirements. –  lantius Mar 2 '11 at 7:09
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The advantages of investment casting are:

Excellent surface finish

High dimensional accuracy

Extremely intricate parts are castable

Almost any metal can be cast

No flash or parting lines

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Investment casting has always been used for lugs and dropouts. What happened is that bike manufacturers got away from using lugs about 20 years ago, when NC welders became available that could weld a frame quickly enough to not destroy the temper of high-quality chromoly or aluminum. What you're seeing is really just a return to the old techniques, in a way.

Probably also the development of computerized rapid prototyping systems to produce the patterns has made designing with investment castings more attractive.

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