Gap when joining plastic mouldings. What would you do ?

[ComeOnYouReds!]

I have to make a lot of simple square frames using Polcore or Artcore and I've always had the problem of some of the flat, 2cm wide ones, the basic type of flat topped mouldings, joining with 12mm wedges, leaving the inside of the joint open.
No matter what I do I cannot get the corners to meet tightly and the resultant frame looks cheap and amateurish.
The morso blades are newly sharp.
I think some of it is to do with the bottom of the moulding not being flat and that when pinning it rolls slightly and opens even with the downward pressure of the underpinner head.
I'm not going to start gluing them as I have hundreds to make.
The price I charge is ok for a plastic frame with glass and back but for my own sake I need to get the frames looking better.
Have any of you had experience with this and how did you overcome it, if at all.
And I'm not going to start with the outside corners being open ... that's another matter !

[Not your average framer]

Polymer mouldings do not cut exactly be same as wood mouldings and although your morso may cut perfectly with wood mouldings does not mean that it always cut as perfectly with polymer mouldings. It's just one of those things. In cold weather molymer mouldings which have just been delivered after a long journey in the back of a cold delivery vehicle are best left to warm up and aclimatise for 24 hours, before attempting to cut them. Also I have found that a better make of underpinner wedges can pull corner joints together better on polymer mouldings. Generally hardwood wegdes are supposed to be best for polymer mouldings, but that's not always true either, some mouldings work slightly better with soft wood wedges. I don't like polymer mouldings and have not used them for years.

Sorry to say this, but you still usually get what you paid for. If you wedges are trying to grip properly on the core of the moulding, which is mostly tiny air bubbles in fairly weak polymer material, expecting perfect results is not always likely to produce the best results without gluing the join first. Pushing the two pieces tightly together with the cyanoacrylate glue is intended to prevent gaps opening up while underpinning the joint. Cheap plastic ready made frames are not glued at the corners either and they have much more expensive equipment that you will have and with all that equipment and investment, the joints still look rubbish a lot of the time. Their answer is to shrink wrap the frames and pull the corners together with the shrink wrap. If you expect to avoid using glue like the ready made frame manufacturers, why do you think that the result is going to be so different?

Another thing is that the mass produced ready made frame manufacturers use electric circular saws to cut the mitres and get extremely precisely cut mitres, so that this reduces the effect of not using glue slightly and makes the joints look slightly tighter. Ever thought of getting a shrink wrapper to squeeze the corners a little, while the frames are on display. You'd be surprised how many bigger framing companies do this. It cost extra to do this, but it helps sell frames, so it's money well spent!

[Steve N]

The underpins are internal clamps that hols the corner together until the glue dries, so you are on to a loser if you are not using glue
How are you cutting the moulding saw or morso, I have found even with glue there are plastic moulding that are crap when cutting with a morso and leave gaps, with a saw there are better

[Not your average framer]

The practice of not gluing joints is known in the industry as dry pinning and was started long before polymer mouldings were available. Lots of volume framers do this as normal and does not usually cause any real problems in wood mouldings where the wedges have something solid to grip against, but these days polymer mouldings are produced with reduced density polymer core filling to save cost. This is why polymer moulding suppliers are often keen to sell cyanoacrylate glue to their customers,

The practice of not gluing polymer mouldings is not so sound in certain conditions. There are very good reasons why a particular width of polymer moulding will only get used upto a certain size of frame size if a large volume framer is not using glue, but a larger profile will be used instead. Many large volume framers still use woodern mouldings as well as polymer mouldings and no doubt there may be various reasons for this and price is probably one of these, but solving difficulties with not using glue could also be one of the reasons as well.

Getting the balance right and finding what work and what does not work is a major part of making the no glue technique work. Please don't get me wrong about this. I am not saying that not using glue does not work, but you may need to experiment a bit to get this right and investigating which polymer mouldings work best may be part of this. Also if you are not using the deepest wedges for these particular mouldings, then the gaps of the front face may not be able to pull fully together as they a joined by underpinning.

As wedges are designed and produced for use in wood mouldings, it is not surprising that no glue joining usually does not produce as many problems with wood mouldings. Perhaps considering adding a little bit of extra friction to the top clamp pad, of two a chevron spacer against the under pinner fences will help to resist any tendancy of the wedges to push the joint apart. I have several ready joined supplementary wooden spacer that I can insert again the under pinner fences which have been faced with fine sandpaper to prevent the two pieces of moulding pushing apart as the wedges are inserted.

I hold the two pieces of moulding tightly against the sandpaper using hand pressure and this stops mouldings being pushed apart as the wedges are inserted. I don't use polymer mouldings, but mouldings pushing apart with joining on an under pinner can sometimes happen with wood mouldings too.

[prospero]

Plastic being what it is, is slightly flexible so will flex slightly as the blades make contact. The sight edge will
get deformed before the blades start to bite. As it is a flat moulding, have you tried cutting it upside-down.
Exactly the same problem on the ally-wrapped moulding of yesteryear.

[Keith Hewitt]

Think its called plagiarism
Copied from the the USA framers forum
Posted by Jim Miller one of the top educators in USA
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Oh, wait. Are you saying that someone teaching a framing course taught you to underpin without glue? That would be wrong, and in that case, you probably should question everything else that you were taught.

There are several procedures to achieve good, tight miters with glue and v-nails, which is the correct way to join. Some framers glue & clamp until dry, then v-nail. Some are proficient enough to glue and then v-nail immediately. If you have a Cassese underpinner that enables you to lock the clamped corner in the machine for a couple of minutes, that would allow the glue to set up before driving the v-nails.
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What jumped out to me was "in that case, you probably should question everything else that you were taught."

[Not your average framer]

Well, I could easily agree with what has been said, when you are making hundreds of frames and the customer's proirity was a primary one of price, this is what happens and what other option is there? It's not bad to volume framers who produce low priced frames without glue. It's not what many would describe as best practice, but it gets the job done at rock bottom prices!

Whatever you may think, this is normal practice in much of the high volume framing market. The customer gets the frames that they wanted at the price that they wanted. What's the trade off? Gaps in the mitred corner joints, if using some types of polymer mouldings. Many of us may not like this way of doing this, but there's bespoke framers and then there's volume framers and for volume framers not gluing the joints is just a normal fact of life.

Lots of volume frames do the same with contract quality, wood mouldings and still make these frames without gluing the corners and nobody notices, because gaps in the corners are not that obvious. If you are a normal volume framer, who stocks a small range of wood mouldings and buys them in a quantity of 10,000 feet for each individual moulding, the prices can be so cheap that's almost crazy. Welcome to the world of volume framing!