Heat Shrinking Metal - Auto Body Repair Shop

Heat Shrinking Metal

Lots of controversy surrounds heat shrinking, yet I've successfully used this process for 40+ years. It not only makes a damaged panel repairable (and can save a job from totaling out), but it's also a less invasive repair.

Probably no other subject causes a more diverse reaction when a group of “experts” are gathered round a table than heat shrinking of metal. But as a collision repair technician — a “metalman” — I can’t understand why this process is so controversial. It has been a basic procedure that I’ve used almost weekly since my career started in 1963.

And as far as I’m concerned, the process is just as valid now as it was then, provided you take all the necessary precautions.

Like I said, though, not everyone would agree with me on that. For example …

I work with the Institute of Automotive Service Excellence (ASE) occasionally to develop new certification tests for the collision repair series of tests the Institute offers. A test-writing workshop is held in a single location and experts from associated fields related to automotive collision repair are brought in for a 2 1/2- to 3-day intensive test-writing workshop. These people are from large shops, small shops, consolidators, equipment suppliers, I-CAR, material manufacturers, auto manufacturers, technicians who have scored high on ASE tests, paint companies, and the list goes on. Generally speaking, the attendees are experts in their field, and I always enjoy meeting with people who truly enjoy what they do. That’s why they’re good at what they do.

When I wrote a question several years ago regarding heat shrinking, I was told by ASE staff — after about 30 minutes of non-conclusive discussion with the experts — that it was unlikely that a question regarding heat shrinking could be written for an ASE test.

I asked why.

I was told that no workshop was ever able to agree on what the process was enough to be able to write a question to it.

Well, telling an Irishman that something can’t be done is like trying to extinguish a fire by pouring gasoline on it. To me, that was a green light to go ahead and try.

Some of the comments that came up when discussing heat shrinking went as follows:

  • “I always get nervous when I see a technician approach a car with a torch.” — a representative of a large
  • “We don’t recommend heat shrinking.” — an auto manufacturer.
  • “That falls into the category of a lost art.” — a material manufacturer.
  • “Flipping amazing.” — my comment.

I did eventually achieve my goal, in a way. At a subsequent workshop, I was able to write a heat shrinking test question that was included in the body of an actual ASE Non-Structural Collision Repair test. However, the question was later withdrawn because it tested so poorly — which tells me that few technicians understand the process or how and when to use it.

Why Heat Shrink?

Are you noticing more vehicles “totaling out” nowadays? We had a 5-year-old Jeep in our shop recently, and we were glad to have it in our shop. Work was scarce!

The Jeep needed a front bumper, fender, two doors, a possible quarter and a rear bumper. Our estimator said, “That quarter will send the job to the salvage pool.” I said, “Hold on! We can straighten that and save the job.”

That’s one reason to heat shrink; it makes a damaged panel repairable and it saves the job.
Now please don’t misinterpret what I’m saying here. I’m in no way saying to do a “schlock” job. I’m saying, “do your job,” which is to repair automobiles, not replace parts.

We repaired that Jeep quarter panel (it looked great) and put our lifetime warranty on it.
To the OE guy who said, “We don’t recommend it,” I ask, “Do you recommend against it” or do you just like to sell parts?

Consider this. Somebody takes a ball bat and whacks a softball-sized dent in the side of a quarter panel. The metal is stretched. The overall damage is a small part of the entire quarter panel.

Let’s see which would be the less invasive repair? Properly heat shrinking and repairing the stretched damaged area, or replacing the entire quarter panel with the subsequent corrosion protection and adjacent panel blending issues.

It’s obvious that the better job would be the repair, rather than the replace. Of course, this will vary on a case-by-case basis, depending on severity of the damage and the skill level of the technician. And this does not require a quarter-inch of plastic filler after you’ve heat shrinked. I do it all the time with no filler, of course, depending on access to the backside.

Some Considerations

Be aware of what you’re
straightening and shrinking. This is a cosmetic process only and should not be used on unibody frame rails or high-strength steels. Some manufacturers will use high-strength steel in their outer body skins in selected locations — not many and not universally by one manufacturer, so verify what you’re working on before you start.

It’s called a repair plan. Is it steel or aluminum? No worries either way. Aluminum can be successfully shrunk with heat, too. But my point here is, know in advance what you’re working on because the result depends on it. You can destroy aluminum if you’re using a steel shrinking process on it. Be aware!

What’s behind that panel you need to shrink? Get access to it, if only to verify that nothing flammable is going to start a fire that you can’t readily and quickly extinguish. If it’s predictable, it’s preventable. Keep in mind the Physician’s Oath: First, Do No Harm.

I’m not trying to scare you here, just get you thinking. There are some people out there who think an oxy-acetylene torch shouldn’t be in a modern collision repair facility. That’s crazy! It’s an extremely useful tool, and not just for heat shrinking, but … it requires careful use and special considerations.

The Tools

The tools you need for the shrinking operation include:

  • Oxy-acetylene torch. Use a small tip. This could be a 000 or 00, depending on your set, or a 1 or 2. You want a small neutral flame that you can
  • Air blower with an on/off button or a lever for fast on/off action.
  • Smooth-faced body hammer. The hammer should be a quality, smooth, well-maintained body hammer with no nicks or gouges. You’ll be working on cherry red metal, so you don’t want to damage the metal with a poor quality hammer. Do not use a “so called” shrinking hammer or rough-faced hammer for this procedure. You’ll damage the surface and defeat your purpose.
  • Heel, toe or other dolly that has a good flat surface to it.
  • Device to hold your torch safely while you’re doing the hammering operation. I use a jack stand with a trim panel pry tool clamped to it to hold my torch.

The Process

I’ve always felt that if you understand the physics or chemistry of a process, it helps you achieve successful results — probably because you actually understand what you’re doing.

Metal that’s stretched is thinner than it was originally. You want to make it thicker again, thereby lowering the high or stretched area to its original contour. To do this, you’ll heat a high spot of the stretched steel about the size of a dime or a quarter to a cherry red color and then hammer immediately and rapidly around the hot spot, while “backing up” the area with a flat-sided dolly, lightly held on the back side.

Do not hit directly on the hot spot, since this will only stretch it. What you’re actually doing is driving the molecules into the hot, more fluid or liquid spot, thereby thickening it.
That said, let me walk you through the procedure, step by step:

Step by Step

Aluminum Shrinking: Yes, It Can Be Done

Yes, you can heat shrink aluminum too, so don’t worry yourself that it can’t be done. It’s basically the same basic process with this one major difference: Your shrink temperature is much, much lower (aluminum — 550 degrees F; steel — 2,000 degrees F).

Just be careful to monitor your heat because aluminum does not change color when it gets hot. Because of this, you need to check the surface temperature with heat crayons.

At 550 degrees F, you can shrink the aluminum without affecting the temper or hardness. (Welding suppliers carry a wide range of temperature crayons that melt off the surface when your desired temperature is achieved.)

Back your panel with a flat-sided wooden dolly and carefully tap around the hot spot. Work quickly and then cool.

Expand Your Skills

For years I’ve been saying that heat shrinking is a necessary and useable process. And I keep saying it because it’s true.

If you decide to adopt and master this technique, you’ll not only increase your value as a skilled collision repair technician, but you’ll be doing your job as intended — which is to repair automobiles, rather than simply replace their parts.

Writer Mike West is a contributing editor to BodyShop Business and the 2003 winner of the magazine’s “Executive of the Year” award. West has been a shop owner for more than 30 years and has been a technician for more than 40 years. His shop in Seattle, Wash., has attained the I-CAR Gold Class distinction and the ASE Blue Seal of Excellence. You can e-mail Mike at [email protected].

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