Winter glue joint failure

 

Q. It is very cold outside. Whatconcerns do we have with this frigid weather in our plant?

 

Answer: Let’s consider four aspects ofcold weather on wood manufacturing: Machining, gluing, finishing and movement.

 

1. Cold wood machines the same,from a practical point of view, as does warmer wood…if the humidity and moisture content is the same.

 

2. Cold wood glues quitedifferently than warm wood. The real issue is that when the adhesive contactsthe cold wood, even if the adhesive is warm, it will quickly cool and becomemuch thicker than normal, so it does not flow well to spread out and fill themicroscopic nooks and crannies. Additionally, the drying and / or chemicalreactions that are necessary to cure the adhesive slow down or may not evenoccur. Obviously, the surfaces to be glued and the adhesive should be around 70degrees. (Note that in the summertime, we might want to alter the glueformulation a bit to avoid too much thinning from the warm temperatures.)

 

3. Temperature greatly affects thefinishing process, including poor curing, fish eyes, and a bunch of otherissues. The wood, the air and the finish all need to be at the correcttemperature. Finishing is also affected by low humidities in the plant that oftenaccompany cold weather.

 

4. Cold weather almost alwaysmeans low humidities in heated areas, including your lumber storage area, yourmanufacturing plant and customer’s home or office. This in turn means that the woodwill be drying out a bit and trying to shrink. If shrinkage is too much, thenwe can see cracking in the finish or wood and even sometimes a bit of warp. Inserteddoor panels can shrink and exposed unfinished areas. Door latches may not fitperfectly. Drawers may stick.

 

The reason all this happens isthat when air is heated, the humidity drops. For example, outside air at 100percent RH (28 percent MC in wood or 28 percent EMC) and 0 degrees F whenheated to 20 degrees F will drop down to 40 percent RH (8 percent MC in wood or8 percent EMC air condition). So, obviously, by the time we heat the air to 65degrees, the RH is really low.

 

What to do

 

Most customers have humidifiers intheir homes and also have moisture additions from bathroom showers, cooking andplants. Offices may not have such sources of moisture. Many wood manufacturingfacilities have humidification equipment to avoid the extremely low RHs that wesee in cold weather. Because a customer’s home is likely at a low of 30 percent RH (maybe abit lower on the coldest days, but these brief excursions are not an issue), weshould consider keeping our manufacturing plant at 30 to 35 percent RH when thetemperatures are frigid. Lower RHs are likely to cause manufacturing issues;higher RHs will mean more drying and shrinkage when the wood reaches thecustomer’s home.

 

How about when finished goods areshipped and warehoused? Shipping is seldom a problem, as there is no heat, sothe RH is higher—50percent to 65 percent MC (9 percent to 12 percent EMC) perhaps average outside.Similarly for a cold warehouse. The finish on the product will slow the moisturemovement, plus in cold weather, the moisture moves very slowly. The temperatureitself is not an issue, as wood does not shrink or swell due to temperaturechanges alone. However, for a heated warehouse, we have the same potentiallylow RH issues that were already mentioned.

 

One final comment: Wood can absorb a small moisture change without muchdifficulty when the change occurs slowly, such as we see from summer to winterto summer. The problem with defect development in finished goods will almostalways occur when there is a rapid change; moderate humidity to a low humiditycondition, or when higher MC wood is exposed to a low RH or EMC condition. Adjustingmanufacturing humidities and wood MC levels for cold wintertime weather isprudent to avoid warp, open glue joints, cracking and so on.

  

Gene Wengert, wooddoc@uwalumni.com

Gene Wengert, "The Wood Doctor," has been training people inefficient use of wood for 35 years. He is extension specialist emeritus at theUniversity of Wisconsin-Madison.

 

Ask the wood doctor

 

Q. We are gluing maple panels andhave been for years. This winter, the cold weather seems to be making jointsweaker than normal. Your earlier article had some good ideas which we areadding to our operation as appropriate, but is there anything further aboutglue joints from a clamp carrier?

 

Answer. I do hope you keep thatFebruary article available and review it again next November when the heatcomes on. You should never get an increase in glue joint failure in thewintertime if you follow those suggestions.

 

In your case, here is likely whatis happening. First, the cold weather means that the interior humidity in yourshop is quite dry. A $30 digital hygrometer from a store like Radio Shack willtell you the actual RH in your shop; do not use a dial RH sensor like is usedon your office desk.

 

In dry weather, the ends ofindividual staves will dry out just a bit, making the ends a bit narrower thanmost of the piece. This drying can happen within 30 minutes. (The lumber can be7.0 percent MC, but the shop might be drier at 25 to 30 percent RH.)

 

So, If you have clamps on yourclamp carrier at the ends of a piece, when you tighten the clamps, you willapply too much pressure to much of the joint. Why? When you squeeze the skinnyends to get the ends closed up enough to glue properly, the center sections aretoo tight with most of the adhesive squeezed out. So the ends are good but muchof the joint is weak due to the high pressure and squeeze out of too much glue.

 

Too little pressure, on the otherhand, will mean the ends will not be glued well and will split. The cure is tocut the staves to size and glue them within a few minutes aftercutting...before the ends can lose moisture and shrink in width. One client ofmine did this and found that joint failure went from over 10 percent in thewintertime to under 2 percent -- a tremendous benefit with no capital expense.

  

Gene Wengert, wooddoc@uwalumni.com

Gene Wengert, "The Wood Doctor," has been training people inefficient use of wood for 35 years. He is extension specialist emeritus at theUniversity of Wisconsin-Madison.

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