Hot Pressing, Cold Pressing, and Veneer Joint Methods

A discussion of different veneering techniques and their advantages. October 14, 2008

I need someone to manufacture doors for a job. Maple veneer on MDF with edgebanding. Client wants all straight grain, no figure whatsoever. No flame, birds eye, tiger stripe, mineral, etc. What should I look for?

Forum Responses
(Veneer Forum)
From contributor C:
No matter what shop does this for you, make sure you have the splice lines Diehl spliced or you will have hairline gaps from shrinkage in maple veneer.

From contributor E:
What is Diehl sliced? Is it like splicing trim, cutting joint at angle overlapping each other?

From contributor C:
After cutting the edges you spray a resin glue lightly on the veneer edges. They are then run through a tapeless splicer that heats the glue and pushes the edges together making a very tight edge joint. It is a hard glue line (not a hot melt). This will sand well and when properly done, leave an invisible joint. Other methods (taping and zig zag stitching) pull the veneer together but have no glue between the veneers so when they are heated in pressing or when they dry out after cold or vac pressing, some woods shrink and leave an open seam that may only show up after finishing. Maple is a problem in this regard. It's not a technique that most small shops use as it is a production machine and they are tricky to keep in top running condition.

Some smaller shops will hand tape and then fold the veneer back and put a light amount of white glue between the seams and fold back flat to dry. That works too, it's just not good for any volume work. Anyway, for the best work, you should specify this. If you use an oil finish, this may not show, but we do cat lac finishes and open joints will show up after the seal coat.

From contributor J:
I have to respectfully disagree with some (not all) of what contributor C has to say. A good hand-taped joint with no adhesive in the joint is the gold standard for many if not all veneers in a cold-pressing situation. The reason is pretty obvious: good joint with no glue in it = tighter, more invisible joint. A good tight stitched ("Kuper-style") joint is a close second for the same reason.

In a hot-pressing situation there is more potential for shrinkage and opening seams with some veneers - not all. But it is important to remember that hot pressing doesn't offer any advantage to the finished product; it only helps the presser speed up production. In fact, there are some veneers that should *never* be hot pressed, such as crotches. The bottom line is that any method that produces tight, invisible, durable joints is acceptable and any that does not is unacceptable. A bad glue line splice is as ugly or worse than any other bad veneer joint.

Many end users are under the impression that hot-pressing produces a superior product, which it does not. And some people, for whatever reason, have floated the idea that stitched joints are inferior, which they are not - forty years of veneered panels back this up.

Bottom line: get your panels from a layup house that produces quality work by whichever method.

From contributor C:
We produce many maple panels including sketch faces. There is no visible glue joint on our Diehl spliced joints. On occasion, we have to hand tape faces. You can tell that they were hand taped in finishing as the joints have hair line separations which have to be filled. Not all the time, but they can be seen.

Unlike most veneer plants, much of our production goes into product that we finish for high end office furniture. We have the advantage of seeing the results of our veneer technique daily which helps us maintain quality.

As far as hot pressing, the difference in hot and cold pressing is that we are able to use a better quality adhesive creating a catalyzed glue bond which is superior. There are times when we use a PVA glue (which is also thermal set), but we prefer a urea resin as do most panel people (and I assume you do). Also, resin glue is not water based, so the panels are more stable (but panel stability is another subject!). The thermal set products are better. Better grip, no creep, faster set.

Finally, on never hot pressing crotch veneer. We never press them with PVA, but we hot press these materials using urea glue all the time, including parts and lids for 8 ft grand pianos! You must back fill and lock in the grain in unstable woods which is what we do via hot pressing. The most respected processor of crotch and burl panels in the USA, Ed Zeller, not only uses hot pressing but they use a Tago film that requires 300 deg (F) to activate the glue!

As far as Kuper joints, we have a Kuper machine and 3 tapeless machines. The Kuper is for sale! For many things it works well, and for small shops it is a great tool. Like all things in woodworking, there are many paths to a desired result, and I'm sure you have your ways, as do we. Thanks for the input.

From contributor J:
This is turning into an interesting thread! I think that my main points were:
1. Glue-line splicing isn't a superior veneer joint; rather, it solves a problem that sometimes occurs with hot pressing.

2. Hot-pressing isn't a superior pressing method; rather, it solves a production issue in high-volume environments.

Regarding urea resin adhesives: I totally agree that they are superior to PVAs (we use them exclusively except when customers request no-formaldehyde panels or when we need to use PVA on tight radius faces) though I am very curious about the adhesive Columbia is using on its PureBond. I don't know if that will become available as a stand-alone.

But you don't need to hot-press urea; you just need more time. We "warm-press" a wide variety of species including all types of maples using both zig-zag and paper tape and have never had an open seam issue.