# something that has been bugging me..



## BCLuxor (Aug 24, 2010)

Referring to the Yew laminate I sent Quarterinmynose I have a question for the community to be answered as detailed as possible. How does penatrive and surface finishing affect the strength of glue bonds?.. the situation specifically was two identical frames one further finished than the other sat in the same hot location the un finished but sanded slingshot crept with the heat and exposed a hollow weak point along the join .. the finished slingshot with thin satin CA finish and wax buff had no sign on weakness.. all joins were over level and held under pressure for 24 hours... additionally what would the expected outcome be of just a wax finish for example or just tru oil/linseed ? Would the joints move ? Thanks guys ..


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## AlmightyOx (Mar 4, 2013)

I don't claim to be a scientist but I can only assume that the more shiny or finished a slingshot is, the more reflective it will be. Unless they were in a dark area that wouldn't be the case. I'm just guessing though.


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## Crac (Mar 3, 2013)

1. How does penetrate and surface finishing affect the strength of glue bonds?

-I think most epoxies should be ok, standard gluing practice applies (keep it clean and a close fit) and the technical data sheets can (DO) contain testing with key chemicals, temperature and joint gap.

2. What would the expected outcome be of just a wax finish for example or just tru oil/linseed?

- Hydrocarbon based fuels, lubricants and solvents are commonly tested, thus the glues can be very resistant. If you're happy the glue was given enough time at a suitable temperature, there's not much else we can do.

3. Would the joints move?

...? No, as in q2.

If you pimped the pieces out then tried to glue them together, that surely breaks the rules.


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## ash (Apr 23, 2013)

What kind of glue?


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## Jaximus (Jun 1, 2013)

Was there high humidity as well?


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## BCLuxor (Aug 24, 2010)

In this instance polyurethane .... but I am interested in the p.v.a type also.


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## flipgun (Nov 14, 2012)

Well, The CA is basically a plastic finish and not as subject to the vagaries of temperature as most glues. Tru-Oil is a polymerized linseed oil, which give it the characteristics of a plastic coating also. To my mind such coatings are acting as binders on the covered material. The adhesive on the more finished one probably was affected as was on the unfinished, it just had no freedom to move. :twocents:

(Were your gluing faces sanded or roughish? )


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## BCLuxor (Aug 24, 2010)

wood faces were roughed up and alcohol wiped the plaatic spacer was very smooth and completely non pourus I guess this could explain poor bonding ...


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## flipgun (Nov 14, 2012)

I'm sure that is exactly the problem. Roughing the plastic should fix that. Check out Nathans tut on gluing laminates, wood to metal.


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## PorkChopSling (Jan 17, 2013)

BCluxor said:


> wood faces were roughed up and alcohol wiped the plaatic spacer was very smooth and completely non pourus I guess this could explain poor bonding ...


Yes, and what Flipgun said, I had the same problem and I realized I just needed to rough it up more then I had, in fact I cut little hash lines in certian points so that the glue had more to grip on to. Hope this helps, good luck!


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## DawnEvil (Apr 28, 2013)

When bonding different materials together you create a complex set of problems. First is humidity, wood expands and contracts a different amount whether with the grain or across it. Plastic and metal on the other hand are not impacted by changes in humidity. They use to make flutes with metal sleeves on them but found out that the wood would crack because of expansion and contraction due to changes in humidity. It may be possible to so seal the wood that it is unaffected by humidity. However, there is another problem. Thermo expansion. Most materials are affected differently by heat. Metals will expand more that plastic or wood. In fact, different metals expand and contract at different rates with temperature change. They use this characteristic to make thermostats. The only solution for this problem is to keep your slingshots out of extreme temperature changes. I hope this helps.


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## ash (Apr 23, 2013)

I think DawnEvil is on the right track here. Gluing to plastic is a tricky proposition at the best of times, but gluing wood to plastic is worse still.

The likely situation is that one piece of wood was sufficiently stable and did not de-laminate, while the other wanted to move and separated from the stable plastic side.

The movement could have been because it was unfinished, allowing moisture in the wood to escape too quickly, or it could just have been that the piece of wood in question had a kink or imbalance in the grain that was going to move no matter what.


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## Sst der Kleine Steinschlag (Dec 5, 2011)

I guess Heat is not the problem, it´s the humidity in the air. As the more finished piece is saturated with oils and sealed with wax the wood can not take up moisture at all or at least not so fast as the unfinished one . At the same time rougher wood has a much bigger surface to expose to air humidity so the effect of expanding/contracting of the cells will be far more apparent. in addition you used a plastic Spacer that won´t be affected by the influence of moisture at all. It is no wonder that they reacted differently. i once had the root wood burl veneer come off the dashboard of a rental limousine in summer due to this effect, so i consider a small crack in an unfinished slingshot the lesser pain.

Take care, Steinschlag


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## Adirondack Kyle (Aug 20, 2012)

Moisture is the enemy, therefore any finish that will protect the glue joint from exposure to the elements would be best, poly or ca should protect your glue bond.
Yes , plastic will always be more difficult to glue up, 
Wood is somewhat pourus and will bond better, 
Creating divits like a golf ball surface will help to bond the plastic.


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