Permanently Bonding Metal Inserts

[gerrym]

If I want to permanently bond in load bearing metal inserts into the fibreglass, do readers have any tips?.

From my reading, the original bobbins mainly rely on mechanical retention due to their shape and the layup around the bobbin.

For stainless steel, it appears crucial to grit blast and then bond with epoxy (I'm using West System for this) within 4 hours. Aluminium is very tricky and really requires a conversion coating. Otherwise the resin will slowly disbond overtime due to corrosion at the surface. Again the West System manual has some details on all this.

I've got in my support strips on each wing for the boot boards in the Plus 2 and also through thickness bobbins for the underdash handbrake bolts.

Regards

[john.p.clegg]

Gerry

"For stainless steel, it appears crucial to grit blast and then bond with epoxy (I'm using West System for this) within 4 hours."

I can understand the grit blast to roughen/clean the surface,but why the "within 4 hours"?

John

[gerrym]

John, I'm certainly no expert here. In any case that would require test coupons that had been aged over several years. The 4 hours stipulation is from the West Manual. I guess it is because an oxide will grow over time, plus a general risk of surface contamination. Just a guess. Do we have any boat builders here amongst us? They may have some good practical experience and observations.

Regards

Gerry

[GrUmPyBoDgEr]

It's a long time since I was in School but from my Materials Technology Classes I remember that stainless Steel very rapidly builds up a thin, impervious, invisible Oxide layer, immediately after being cut, machined, blasted etc.
This the property that makes it stainless Steel.
By the way it's that property that also demands welding to be done with inert Gas which is not necessary for other Steels.
It's interesting that, that Bonding System is so critical & needs to be applied prior to heavy Oxide formation, I'd have never given it a thought.
It would be interesting to know West's theory.

Cheers
John

[alaric]

Hi. John's spot on with the stainless - it has a passive Chromium oxide layer over the surface that 'flows' when the metal is scratched.

Bonding anything to stainless is difficult, and will fail in time. You can bond polyurethane or epoxy with appropriate primers on the stainless, and you may need two primers layered depending on what you're trying to bond.

So, the best way to secure the inserts is mechanically i.e. build up layers of fibreglass around them as per the original bobbin design.

I'm moving away from any form of bond between plastic and stainless or other metal in my profession.

An alternative to using metal, if you do want to rely on a bond, however, is a ceramic material such as PEEK (http://en.wikipedia.org/wiki/PEEK). This is a machineable ceramic that has good mechanical properties and doesn't oxidise at all, and is easy to bond to with epoxy / pu. It can be bought in standard tube sizes etc. I made a couple of sonar arrays using PEEK as the 'strongback' to which the Pu front or the arrays is bonded, in place of stainless, to prove that they wouldn't fall apart due to cathodic delamination in sea water. Worked very nicely. I had helical inserts into the PEEK to bolt down an o ring sealed rear plate. I did look into buying small quantities at one time and think you should be able to get some.

All the best.

Sean.


wow, it's now possible to buy PEEK screws: http://www.theengineer.co.uk/news/rino% ... 06.article

Ouch, it's not cheap.

[gerrym]

Sean, thanks for your replies. It seems the best thing to do is to avoid at all costs bonds between the glass and metal. Good thing we aren't talking about Europa's and Elise's !.

For the boot supports, I think I can manage with core and mat. Any trapped fastener could be mechanically trapped and thus not really reliant on a bond. Around the scuttle/handbrake mounting I'll have a rethink.

Separate thought. I'm pretty sure that many of the bobbins in our 30-40 year old bodies with be fairly loose, as the chemical bond has probably disappeared and it's only the mechanical grip (ie layers of glass) within the bobbin shape that hold them securely in place.

Thanks for the tip re the PEEK.

Regards

[alaric]

Oh yes I forgot about the elise, their Aluminium chassis is bonded isn't it. My comments about bonding are a bit negative really aren't they - trouble is I'm designing sonar arrays and they fall apart if you have a metal to plastic bond whatever primer you use, due to the harsh environment - they generally look very shiny until you put them into an electrolyte with other dissimilar metals. So my negative mist is due to salt water cathodic delamination.

I found this on another lotus forum;

Lotus Bonds with Aluminum

By Kermit Whitfield, Senior Associate Editor Kermit's Bio Write Kermit

If aluminum-intensive cars are ever to become more than an occasional curiosity, automakers may have to give up their weld shops. At least that's the conclusion you could draw after talking with the people at Lotus Engineering (Hethel, England). Lotus has been building cars with aluminum chassis for many years, but none of them are welded: they are held together with screws and adhesives. When Lotus first introduced the method on the low-volume Elise in 1996, company leaders were worried about market acceptance for what is essentially a glued-together car, but the technique proved so successful (over 23,000 cars produced with no reported failures) that it has become the basis of a new higher volume venture that may help to bring aluminum-intensive vehicles more into the mainstream. The new project, which uses Lotus' Versatile Vehicle Architecture (see AD&P February 2004, "Lotus' Versatile Venture"), tweaks the lessons learned on the Elise for volume production, but remains true to the fundamental concept: to get the most out of aluminum structures you must design for the material, not treat it like a steel substitute.

Bond, Adhesive Bond. First and foremost, that means not welding it. Why? "The yield strength of aluminum goes down by half once its welded," explains Richard Rackham, vehicle architect at Lotus. So, getting the same strength in a welded aluminum chassis as in a bonded unit requires doubling the amount of material used; since aluminum is usually chosen for its light weight, that dilutes its key benefit. Another big disadvantage of welding aluminum is that stresses are localized along a point or a line, which can lead to material fatigue. Stresses are distributed over a wide part-mating when bonding is used. To gain the full merits of using adhesive, Lotus had to come up with ways to optimize its properties. For example, after determining that the optimum bond gap between parts is 0.2 mm, the question became how to maintain that gap uniformly over the bonded surface. The answer: Lotus designed tiny protrusions, or "pips" on the parts that held them exactly 0.2 mm apart.

Aluminum bonding methods

The shape of things to come? Lotus is bringing the aluminum bonding methods it pioneered with the Elise chassis to higher volume production. The first of the new vehicles are scheduled to be produced at the end of 2005.

To fabricate the main chassis components Lotus chose a process well-suited to aluminum: extrusion. Chassis supplier Norsk Hydro ASA (Oslo, Norway) extrudes the closed-box parts out of 6000 series aluminum and bonds them to folded sheets of recycled 3000 series. Rackham says that one reason extrusions were chosen is because they can be inexpensively produced (he estimates the cost of a die at about $5,000), which helps to offset the higher material costs of aluminum. Another is that they can be formed into complex shapes that serve multiple purposes and help keep parts count down. The proof: the entire Elise chassis consists of only 27 different extrusions.

Higher Volume. The challenge Lotus now faces is translating the aluminum production methods for an essentially hand-made $40,000 sports car to affordable vehicles that can be mass-produced. It's current initiative, which is being conducted for an unnamed automaker, looks to build sub-$30,000 vehicles in the 40,000 to 50,000 annual unit range. To do that, Lotus is making some changes. According to Kerry Osborne, principal engineer, the hand-applied flow drill screws that are currently used to knit the Elise chassis together are being replaced by self-piercing rivets which can be shot more quickly, though they require application tools that generate at least five tons of pressure. But perhaps the biggest change is in the bonding. Realizing that no mass-production operation can afford the Elise's 50-minute curing time (nor would it wish to incur the expense of multiple ovens), Lotus is replacing the heat-cured single part epoxy adhesive used on the sports car, which required temperatures of 180?C, with one that will cure in the lower heat of the paint oven. (Both adhesives are sourced from Dow Automotive [Auburn Hills, MI].) The savings in process time garnered by these changes could be enough to peak the interest of volume automakers and get them to consider bonded aluminum chassis as a viable alternative for niche vehicle production.

[bill308]

gerrym,

Last summer I replaced a lower door pivot bobbin on my S2 and basically followed the factory recommendation. It came out great. The important thing is to hold the bobbin in place while slipping the slit glass/resin matrix into place and forming it around the bobbin with surgical gloved fingers. If I recall correctly, I laid up and saturated all the layers on a flat surface, placed the matrix on my fingers and lifted it into place, allowing the slit to slide over the bobbin. After the cure, I filled the top surface crack between the bobbin and supporting wall with a few drops of resin to finish the cosmetics.

Looking around at all the other bobbins on the car, all still appear to be solidly anchored, a testament to the basic idea. A chemical bond is simply not necessary for the bobbins.

I also campaign an old fiberglass Lightning sailboat on which I've done a lot of fiberglass work over the years. While West System Epoxy is great stuff, I would only use it where it was absolutely necessary. I find generic polyester resin more forgiving, less expensive, and suitable for almost all work. I was once told one can always apply epoxy over polyester but one shouldn't apply polyester over epoxy. So given the choice, I perfer to work with the polyester.

alaric,

Thanks for sharing the Whitfield article, a very interesting read.

As a student in engineering school, my senior design project was to design, construct, and race a human powered amphibius vehicle. I was responsible for the chassis which was built up of folded and bonded sheet aluminum with internal Styrofoam filling and external Styrofoam hull features to obtain a reasonable hull shape. It was basically a floating tricycle with 2-wheels in the front for steering and a single drive wheel in the rear. A kayak paddle was used to propel it in the water. Ready to go, it weighed about 55 lbs. During testing we damaged the end of one seam on the hull with careless handling. In general, all joints were extremely strong as they were loaded in shear. The failure was in a peel mode where one panel was pushed down relative to the other. In retrospect, I should have used a mechanical fastener at the end of each seam such that the fastener would take the peel load. Fortunately the damage was in a relatively lightly loaded area and was not sufficient to prevent us from racing the next day

All joints held for the 2-runs we made which included non stop dives from the banks into 2-ponds. Our driver had to pull the vehicle out of the water with a rope to continue the road and field course that was about 2-miles long.

Bill