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checks in maple paper backed veneer doors12/31
I have some small checks showing up(along grain and slightly raised) on maple paper back veneer doors 3 months after finishing doors.
Procedure:mdf is edgebanded then glue is hand rolled on to mdf and the veneer and is immediately pressed in vacuum press at 18hg and heated to 80C for 2.5 minutes. The RH at press time was 45 to 50% and is now 30 to 35%. doors sat in shop for about 1 week before being finished with spay stain and converted varnish.
I am looking for input as to what is causing the checking. Is paper back veneer a good choice or is fleece or raw veneer a better choice??? Want to used veneer type with least chance of problems.
ps I also made some sample doors at the same time as checked doors and are still unfinished with no signs of checking.
Guess #1- Poor bond between paper & veneer. If you have a vac press why not use unbacked veneer? Rolling glue is OK but make sure you use the min amount, Check with a wet mil gauge (get one from your glue supplier).
Guess #2- Too much finish. Measure dry mil thickness and check with your finish maker.
I am new to the whole veneering world but not to wood working. Press has been used for thermal wrap up til now. In our testing we used raw and paperback , the raw tested was deeply pocked similar to birds eye chipped on a planer and took for ever to palm sand out .( is this the norm for maple veneer or any type of veneer??)
Is there any good literature or links on veneering using heated vac press pva glue humidity??
Sounds like you got a bad batch of raw veneer for your testing.
Raw veneer has what I call plus and minus grain. The tight/good side has some high grain and the back side has some low grain. With good quality veneer, the sides are really hard to tell apart. Worst case is lots of high grain on the front and "deeply pocked" grain on the back. The only maple I've gotten that had significant +/- grain was some highly figured western.
What was the moisture content of the veneer and substrate before you started, had they both had time to acclimate to the 45 to 50% environment? Since the RH went down and you have puckering, instead of splitting, the core had to have shrunk more than the veneer. Either the core had a higher moisture content before pressing, or it expanded more/absorbed more moisure during pressing.
I have no idea how heat affects the process. Is a heated platten in direct contact w/ the veneer, could it drive moisture out of the veneer and into the substrate?... Is the temperature enough to affect the bond between paper and veneer?
In my experience with PVA cold pressing at >= 20hg for 1/2hr with 1/4" MDF cover sheets, splitting would be more likely. My single side layups always dry flat or cupped on the veneer side, i.e. the glue/veneer contracts more than the substrate.
I wouldn't think a 15% change in RH would be enough to cause that puckering unless there was, at some point, a significant difference in moisture content between the materials and/or there was a pretty weak/non existant bond between two of the materials.
The core and veneer would have been in the shop for about a week before processing and week to process .
I do not have a moisture reading for veneer or the core.
The rep from the mill that layed up the veneers and applied the paper told me the temp that i am using would not affect the paper bond. The only way i tested the bond was to rip the veneer off with a pair of pliers and in that test the paper separated leaving paper bonded to veneer as well as the core. The veneer came of in small pieces .
The press heats the silicone membrane that i place a piece of 70lb craft paper between veneer and membrane.
PICTURE BELOW :
I included this picture thinking it may tell you experts something more about the checking as it is on a different core that had been aclimatized for several months.
What was the finishing schedule.
Re finishing schedule : our shop did not finish the doors or the 1/4" sample above but i will tell what i know about it. Doors were finished about a week or so mfg. they were spray stained with solvent base stain (no wipe) chem craft hawaiian tan. Then strayed with conversion varnish (chem life 24) dont know how many wet coats (i think he said 2 but not sure) or time between coats.
I took a sharp knife and cut some veneer and varnish off at a sharp angle then measured the cut piece with digital caliper at the point where stain and conversion varnish met it measured 4.5 mil . Did this 3 times and got same results each time.Not sure this is the right way to measure dry finish thickness but makes sense to me it would be accurate .
I'd do two samples, one with a waterbased finish and one with paperbacked from a local box store, i.e. change one potential culprit for each sample.
The only time I've had veneer pucker like that was when I glued paperbacked with contact cement. It was my first veneer job and I haven't used either since.
1- Ask the finish supplier what they think, now that you know the dry mil thickness. No one here can answer unless they know the actual composition of the finish
2-Cross cut a sample 90 degrees to the checking and use a strong magnifying glass to inspect. You should be able to see what in fact is checking or loose.
I have had the same issue but the checking shows within 24 hrs of the finish drying. It's not normal checking it's the reaction between the finish and the glue under the paper and veneer. The only sure resolution I have found is to use to wood backer.
As the finish dries it creates tension or pullback which lifts the veneer away from the paper. The paper is thin and the finish was able dissolve the bond between that and the veneer which weekend the bond enough allowing this to happen.
I have had seen this happen with water and solvent based products but always with a paper backed veneer.