MDF - Reducing its thickness and effects

[kayak23]

Can anyone help to settle some doubts I have here in our workshop regarding MDF and reducing its thickness?

As far as I know, when MDF is manufactured it is subjected to a huge amount of pressure which creates an outer 'skin' of very dense fibres and as you work your way into the material, the core is softer until you again reach the other, harder side, a symmetrical balance of fibre-structure and pressure.

Now, Person 1, lets call him...Me, believes that if you upset this balance by thickness sanding your MDF down in a panel-sander and subsequently veneering it to suit your rebate/groove or for a lack of thinner material to hand, then ultimately this may be a bad thing. It may cause the base material to become unstable and cup or bend, and also the surface could become way too porous creating problems when veneering by sucking up too much glue.

Person 2, lets call him...my colleague, has no such reservations, and will happily take small amounts right up to mm's off the thickness of MDF board and later veneer it. He sees no issues with this at all.

What do you all think about this? Be interesting to hear your views.
Thanks.

 

[Gary]

I thought that if you did something to one side of mdf, you had to do the same to the other. I read somewhere if you added formica to one side of mdf in a router table top you had to do the underside to stop it warping.

But then again I may be wrong.

 

[moz]

Person 1 is correct. I've done it. It goes like a banana. Just try ripping off the dense face from one side of a piece and you will see it will bend. It may be that there is enough tension in veneer on both sides to hold it straight again but I wouldn't risk it.

John

 

[woodbloke]

I've seen mdf sanded like this to bring it to an exact thickness for veneering in a hydraulic press. We used to find that it would only veneer well in certain places in the press so when we measured the mdf with a vernier we found that it wasn't a uniform thickness...there were variations across the board so my boss at the time used to pass it through a panel sander just to get it to an even thickness, but he wasn't taking off a huge amount - Rob

 

[Racers]

Hi,

I thinned down some 25mm to about 21mm on one side only and it stayed straight.

Pete

 

[kayak23]

Person 1 is correct. I've done it. It goes like a banana. Just try ripping off the dense face from one side of a piece and you will see it will bend. John

Lets assume that equal-ish material is removed from each side.

Another point may be that with a reduced harder skin, the mdf becomes weaker?

 

[RogerS]

Person 1.

Why else would the best-pracice be to put a balancing veneer on the other side when veneering ? Same principle applies, surely.

 

[9fingers]

I thought that if you did something to one side of mdf, you had to do the same to the other. I read somewhere if you added formica to one side of mdf in a router table top you had to do the underside to stop it warping.

But then again I may be wrong.

I agree. Whether natural timber or processed panels, all the advice I have seen suggests that both faces should be protected/treated similarly to manage moisture ingress and stress in the material.

However Mdf has very short fibres and is less likely to suffer from stress throughout the panel surface and provided the thickness were reduced parallel to the original surface I would not expect much distortion due to stress.
Moisture ingress is different. The core fibres have less resin it would appear and exposing those could give problems. You can usually measure the effects by taking an offcut of MDF that has been around the shop for a while and making a fresh cut through it. The thickness at the exposed edge is normally thicker than the fresh cut one as moisture travels into the open edges.

This effect is less noticeable with MR MDF.

As far as veneering and glue take up is concerned I would consider priming the sanded surface with dilute pva and maybe, just maybe running through the sander at the original setting to knock off any high spots before putting down the veneer.

All in all a lot of faffing about and I wonder if you could modify the design to use stock sizes?

Bob

 

[moz]

I wanted to taper in thickness some 100mm wide lengths of 30mm MDF from 18mm at one end to about 28mm at the other. Made a simple tapering jig. Ripped off one face and they curled immediately. Therefore, nothing to do with moisture. I ended up tapering pieces the other way, i.e. with sawn faces on both sides with no problem. Clearly, as person 1 says, it is inbuilt tension. It sounds like a fool's errand anyway since it's much easier and more controllable to modify the width of your groove.

John

 

[TEO]

Isn't there high density MDF (I know it's an oxymoron) would that do maybe? You'd probably still need to balance both sides.
hth,
T

 

[9fingers]

Isn't there high density MDF (I know it's an oxymoron) would that do maybe? You'd probably still need to balance both sides.
hth,
T

erm! that would be HDF of course!

 

[TEO]

Isn't there high density MDF (I know it's an oxymoron) would that do maybe? You'd probably still need to balance both sides.
hth,
T

erm! that would be HDF of course!

Of course, i was looking at this: http://www.pbplywood.co.uk/high-density ... ing-grade/

I'll go with your term Bob.

 

[alex8_en]

i,m making some doors that are supposed to resemble v sheeting(t&g) what would the effects be if i router them out in 18mm mr mdf?

 

[EssexChris]

We used to use mdf sheets as the base board on the cnc, once the surface was worn we would skim the entire sheet to obtain a flat surface again.
Never had any problems with reduced thickness sheets.