Using MDF for Composites Tooling

MDF is not the best material for building plugs and molds - not by a long shot!

There are many ways to build composite tooling with beautiful specialized products that cut like butter and finish like glass… and yet, so much MDF.  It’s popular because it is the cheapest material to build small plugs and direct molds - and it works well enough. I have cut literal tons of it and would not hesitate to recommend it for a great many applications where money and time are tight and you need something that’s just going to work.

This article is the distillation of years of failure and success with MDF tooling.

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WHAT IS MDF?

Medium density fiberboard is just wood fiber mixed with resin pressed into sheet form in a monster heated press.  There are lots of varieties - lots of resins and densities. It is available from ¼” up to several inches thick. It was traditionally made with a urea-formaldehyde glue but there are formaldehyde-free options available.  MDF is more dense than most plywood and is quite heavy and hard to deal with - especially in 1” sheets or giant glue-ups. One major factor is that being a wood product it has water in it - sometimes a lot. It is fundamentally renewable and in theory it will biodegrade - but not for a long time once we goop it up with resin.


WHEN TO USE IT

I have some rules of thumb for when to use MDF, and like most rules of thumb, there are exceptions and “rubbery” areas where you can get away with stuff if you are careful.  Generally if you can afford something better, go for it. For small molds like the ones shown above it can be a great cost-effective and quick tooling option. I will use the word “mold” to mean a shaped surface - if you substitute “plug” in its place that is fine.

MDF works well for molds that are small enough to be bonded together in single vacuum bagged “glue-ups.”  If you can bag it together and get it on the machine, it will probably be ok. For me this has meant a practical maximum of 5’x12’ (1.5m x 4m -ish) and rarely more than 12” (300mm) thick.  The bigger you go, the more problems you will have.

MDF is good for one-off prepreg parts that don’t have to be beautiful right out of the tool or extremely geometrically accurate.  It can be cooked as long as it is envelope bagged and not heated above about 200F/95C. It will shrink in all dimensions when you cook it, but most noticeably in the direction of the stack.  For tall stacks this will result in a measurable scaling of your part - and as moisture leaves it can crack too. Don’t count on a nice surface finish unless you use release film or adhesive teflon.  Also don’t count on your molds lasting more than a few cooks.

For plugs that do not have to be vacuum-tight, machined blocks can be assembled into larger volumes, but there is a risk of joints printing into parts if heat from curing resin gets too high.  This falls into the realm of “cheating” and you can get away with tons of stuff as long as you have a good feel for the materials and the process. You can also have big expensive mistakes!


CHOOSING MDF SHEET PRODUCTS

Not all MDF is created equal. Some of it creates very unhealthy dust from the formaldehyde used to glue it together.  Cheaper MDF products tend to have a variable density across the thickness of the sheet. The edges are hard and dense and the middle is like fluff and can split easily.  

I strongly recommend that you buy a formaldehyde-free moisture-resistant premium grade of MDF.  There are several brands producing similar products, but the one I am most familiar with is Roseburg Medex.  Higher quality MDF can be twice the price of lumberyard / big box standard stuff but if you are going to finish it to any sort of sealed, sanded painted finish, it will pay off dollar-for-dollar in labor savings.  The bonus will be safer dust, more stability and less tendency to fracture. If you want to compare mechanical properties, take a 4” wide strip of each and break it over a sawhorse or the front of your dumpster. Feel the difference in stiffness and look at the structure of the break.  

But don’t believe me, buy some of each and see what you like better.  Keep track of all the variables and don’t just buy more of the cheap stuff because it’s cheap - and don’t let your boss or the people in purchasing do it either.  You get what you pay for - and nobody should be breathing formaldehyde.

Thickness is another decision.  I have found that ¾” sheets are as big as I want to handle.  If you have a panel saw and two people, 1” is probably ok too.  Check the price per sheet vs. the actual volume of the material because it is often not at all equal per unit volume.  The one problem with thin stock is that it takes more glue joints to build a given thickness - and more labor to cut the stacks.  Calculate and decide.


GLUING BLANKS

I call the multi-layer blocks of MDF that you glue up “blanks.”  Not sure why but probably I worked somewhere and they called them blanks.  I also call them “glue-ups” but only if they are more involved and not rectangular solids.  

You’re going to want to glue blanks with clamps because it seems easier.  I have always wanted to and unless it was for something very small and low-priority it has always been a bad idea.  Vacuum bagging is the way to go. Once you have a system down it’ll be faster and cheaper when you take labor, material and the risk of screw-ups into account.  

The problem with clamps is that they do nothing to pull air out of the glue joints.  There are never enough clamps and they provide uneven pressure. The clamps also make the layers slide around and everything gets covered in glue.  You will end up with big flat bubbles in there and when you machine through the glue joint lines the bubbles will be unbonded areas. You really can’t fix this.  You can inject resin in there and it will be ok but it is almost always poor use of time and if you vacuum bag and/or cook the stack of MDF later it will make all those unbonded areas move around.


Vacuum bagging is relatively clean and it keeps the glue off people a little better.  It also allows you to pull the bag down slowly and make sure all the layers are aligned.  If your blank is simple, you can put cardboard packing angle over the corners and that will align things automatically.  You don’t need peel ply or breather fabric - just a few strips of infusion flow media in a grid over the part. You can machine it off as long as you remove the dry bits that aren’t saturated and glued to the part.  Dry stuff wraps around the cutter. Use the cheapest vacuum connect there is - a piece of infusion hose wrapped in flow media. If you are a wizard at not gooping up the expensive aluminum fittings, hoses and quick connects then go for it - I have ruined more than I care to admit!

There is the question of bagging the stack of MDF sheets down to a table or envelope bagging it on a flat surface.  I’m a fan of envelope bagging if things are small or thick. It saves release coating the table and the risk of gluing the MDF to the table by mistake.  The blank can also be glued up well before it will be machined and left in the bag (with or without vacuum) relatively safe from any moisture that could cause warping.  If you have a large or complex glue-up, or it absolutely needs to be flat, bag it to a table - but put some thin painter’s plastic underneath the first layer (and release coat the table!) to help when you do to “demold” it from the table surface.  

Some handy things to do when you have a complicated glue-up and you need to keep layers organized and aligned:

  • Assemble it dry and run a Sharpie marker around the edges of each layer so you know where to put the glue and a have a visual reference for placing each layer.

  • If you have layers that have to land accurately away from an edge, use tiny blocks of MDF (or tooling board) glued around the edge with 5-minute epoxy while the glue-up is stacked dry.  You only need three to index the next sheet. You’ll have to be careful handling things to not bust them off but it is very easy and intuitive.

  • Carefully lay out the layers in the order you will need them and label the pieces by layer.  Once the glue starts to fly things get less obvious.

GLUE

I recommend using epoxy for bonding MDF blanks.  Ideally you’d use a resin system of medium viscosity that can develop a full cure at room temperature.  You can mix in batches if you’re worried about resin curing too fast - once the resin is in a thin layer between the sheets you’ll have plenty of time.  Obviously there are limits - so test your resin and don’t get caught gluing a big job on a hot day and having it gel before the bag gets sucked down.

You can also save some money using yellow carpenter’s glue.  This isn’t as good as epoxy but it is ok for things that will get laminated over or that don’t need surfacing beyond a sheet of release film or adhesive teflon.  

How much glue do you need?  I have settled on 1.5oz/ sq ft. of joint area.  Less can work but it’s risky - you’ll get a feel for the resin and MDF you use and can set a standard.  Don’t leave this unspecified if you are delegating the labor - this is not the kind of variability you need!  

So if you’re gluing a 3’x2’ block with three layers (two joints) you’d have:

2 x (3 x 2) x 1.5 = 18oz of epoxy.  

Be sure to put it on relatively evenly and stay back an inch or so from the edge.  You need only apply resin to one surface. Leaving the other side (bottom of the sheet you’re adding to the stack) uncoated allows a way for the air to escape the joint - up into the dry sheet and out the edge.

Infusion resin seems like a good idea but I am concerned that it gets sucked up into the MDF and starves the joint.  I haven’t done any science to test this but I have seen instances that made me worry. Also, don’t add fillers or thixotropic modifiers to your resin.  If you have a good reason to use additives, please let me know what it is because I may be missing something - but I have never had good results.


DESIGNING THE STOCK VOLUME / DRAWING UP BLANKS

I will assume you are going to be machining your MDF blocks with a CNC machine, so you will be working from a 3D model.  You can use this model to design the geometry of the MDF glue-up you will create, and then you can use the 3D model of the MDF as a stock model for the CAM operations.  There is a lot of judgement involved in choosing how closely to approximate your final tooling surface, and how much effort you are willing to put into the glue-up vs. how much rough machining you want to do.  Roughing MDF is relatively slow, and sometimes it pays off to spend two more hours on the block glue-up and save four hours on the CNC.

You have to make the glue-up bigger than the finished surface model, so choose a minimum oversize amount and make the layers come no closer to the surface than that.  If you cut it too fine, small mistakes in alignment will leave you under size when you machine - and you’ll have to come back and glue chunks in and wait for the glue to cure before finish machining.  

Here is an example of a 3D surface (yellow) and a stock model:

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The stock is modeled to be both easy to assemble and close to the final surface.  On the model above, the bottom two layers are left long at the ends for clamping to the machine bed.  It is good to think about how you will fixture your material on the machine and this is a good time to plan for tabs or extra material for strap clamps or screws or whatever you use.

Sometimes I have found it worth actually cutting the layers out on the router - and using puzzle joints to align the multi-piece layers.  This is usually only worthwhile for strange shaped parts where there is a lot of volume that would have to be roughed out of pockets in the geometry.  Usually it makes sense to stick to assemblies of rectangles that can be cut on a tablesaw or panel saw. Here’s a bowsprit mold before glue-up. These layers were machined out on the router to keep the block light and save glue-up time:

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Once you have modeled the layers of the block, and have a set of sheet pieces and a 3D model of the block, the challenge is to present the glue-up in a way that will make sense to whoever is gluing it.  I have often gotten an earful for giving somebody a sketch for a glue-up that is not clear to anybody but me. This isn’t a good strategy and will end up causing mistakes as well as costing time and making people cranky.  Having a 3D model of the stock you are machining is no good if it doesn’t very closely resemble what is actually on the machine - ask me how I know! If you are going to get fancy, make sure there are enough measurements and it is clear what is intended - perspective views and color coding seem to help a lot.  It is often much easier to nest the parts from sheets on the computer even if the cutting will be manual - and it saves mistakes and confusion.

It’s not MDF, but here is an example of a glue-up and some color coded parts and guidance for nesting parts out of sheets:

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MACHINING

MDF machines pretty nicely.  I only ever use carbide cutters.  It is pretty abrasive as wood goes but a carbide cutter run with a reasonable chip load so it doesn’t heat up will last tens of hours in the cut.  Be careful of the dust. You want to use extraction and clear the area around the cutter pretty well. If you are cutting and there is a layer of chips several inches deep, you are creating an insulation blanket for the hot chips coming off the tool - and fire is a real risk.  Be careful!

You also don’t want to breathe the dust.  Even without formaldehyde in there, wood (and whatever other mystery stuff is in your MDF) dust is bad to breathe.  

For sheet cutting, compression spiral cutters leave clean edges and can be run in two passes, as long as the first pass is slightly deeper than the up part of the cutter.  My go-to is a ⅜” compression spiral though I have used 1/2 “ and ¼” too. For cutting out 2D shapes, you want a but that is stiff enough to take a heavy cut, but there’s no reason to cut a wider slot than necessary.  

For 3D machining you will have roughing tools and finishing tools.  For roughing, square end mills are typical, but a radius corner mill will be more resistant to chipping and will be quieter running.  They’re not as common though and more expensive. Indexable/insert mills are great as long as you are certain to cut only with the insert - rubbing a shank or trying to “cut” deeper than your insert can cause lots of problems - including fire.  My go-to for machining large volumes of MDF is a large (relative to machine Hp) two or three flute carbide upcut spiral end mill. More than three flutes and the chips don’t clear easily.

Try to rough within ⅛” /3mm if the final surface before running a surfacing pass.  The uneven load as the finishing cutter runs through the thick and thin of the roughed surface can flex the cutter and leave uneven-ness in the finished surface.

For finishing contoured surfaces a ball end mill is usually the way to go.  You can figure out a step-over for each pass by estimating the cusp height of the overlapped ball profiles, but usually it is ok to just estimate a standard step over for a given cutter diameter and desired finish.  Finishing is typically broken up into the “vertical part” and the “horizontal part” of the surface. You might use a contouring or “waterline” finishing strategy for the areas of the surface that are from 45 degrees to vertical, and then another parallel strategy for the 45 degree to flat part.  Do the vertical part first so you don’t risk bumping the bit into vertical walls of material at the edges where the roughing pass hasn’t cleared all the way to the finished surface!

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If you are cutting an extrusion-like profile (say a wing, tube or foil-shape) and finishing back and forth across the section - here a square end-mill can take a wider step-over and the flat spot is leaves at the bottom may be ok.  

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ASSEMBLING MULTI-PART TOOLING


There are going to be lots of times when you want to build a mold/plug from many machined components. This is great, and will save lots of machine time and make it possible to do complicated large composite parts for cheap money. Be careful to have a good gluing strategy for this. I try to model tooling like this with assembly in mind. If you can create a sheet-good “egg-crate” to support the backs of blocks of MDF, that can be a huge help and make it possible to handle large molds as a unit without breaking them. Often you can integrate machined MDF detail pieces into larger stick built tooling this way.

Here’s a hull plug for a boat showing the MDF parts as solid volumes. These were all machined, and the remaining smoother surface was strip planked. Note: some sheet-good structure is omitted in this image for clarity!

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One place you can get in trouble is with blocks shifting under heat or vacuum. If you are going to envelope bag a mold (which you’ll need to do with MDF!) be sure to bag the multiple blocks together when assembling. If you don’t bond the separate blocks very well and with a lot of pressure, they will shift or crack when you are making the part - especially if heat is involved. It is easy to end up with a crack or a step where two separately machined blocks meet.

FINISHING

Once you have your milled block of MDF off the machine, you have to choose how to turn the MDF surface into something you can use.  This is probably the most complicated part - and also the most tedious. I have done this hundreds of times and wished I had done it differently almost as many times.  You’ll have to experiment to see what you like best but here are a bunch of options and a few things you’ll want to be sure you don’t do.

The simplest way to use an MDF mold is to cover it with some type of release film and just go for it.  You can use packing tape even. I have used a non-perforated release film and glued to to the MDF with spray adhesive (Super 77, Airtac, etc.) The release film is so thin that wrinkles don’t matter much.  This really works best with prepreg, and only when the surface is going to be sanded and painted because the resulting finish is rough and - well it looks like MDF! One clever variation I saw online but haven’t tried, is to vacuum bag high elongation bag film (Stretchlon or similar) over the MDF and apply a release coating to the bag - may be worth a try for some applications...

One step up is to use a PTFE adhesive release film (like Airtech Tooltec or Taconic 6085-05 or any of the many similar options.) This is like a big roll of light fabric impregnated with PTFE (Teflon) on one side with an adhesive (acrylic or silicone) on the other.  You just peel the backer off and stick it to the mold surface and then you have a release system! It doesn’t do compound curves so well so it needs to be tailored (wallpaper cuts and little flaps) but if you are going to post finish your surface it can be a huge labor saver.  It costs between $2-$5 per square foot so it’s not cheap but it is awesome for one-off parts. Different adhesives are either one-use (acrylic) or can be peeled off and repaired (silicone) for low volume tooling. My preference is to epoxy-coat the MDF and sand it before using an adhesive release film, and you will find it needs a dust free and hard surface for good adhesion - but if you’re in a real hurry it can be stuck directly to wood, MDF and fillers- check it out!

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Most of the finishing steps require a sealer on the MDF.  My go-to for this is epoxy, but it does have some potential problems and you can mess stuff up pretty bad if things go wrong.  

First off, polyester sealers like Duratec clear sealer can work ok, but they aren’t strong and don’t provide the “hard candy shell” that a good coat of epoxy will.  Try it though - they are faster and don’t have cure-inhibition problems if you apply a polyester primer over. You might find it results in a quicker surface and is good enough - especially if heat is not involved in your processing.  You can use gelcoat and build a shiny surface that way and it will work fine - it just might not stay stuck to the MDF.

There are two things you can do to make epoxy sealing work really well and both are additional work - but totally worth it if possible.  The first is to heat the MDF block up until it is warm to the touch. Be careful not to heat it too much or for too long or it will crack. When you have this warm block, brush a medium viscosity epoxy resin onto the vertical parts of your mold first - it will soak into the “end grain” of the MDF faster and drip down into the flat areas anyway.  Because the block is warm things will need to move fast and you’ll use a lot of epoxy. Really load it up but try to avoid puddles - we’ll deal with them next. You will want to coat all the sides - even the bottom - or your MDF block or it can warp. Doing this in two steps is fine.

The second critical step: after it is fully coated, give it a few minutes to soak in (but not start to gel!) and get some cloth rags - not paper towels.  Work your way over the surface wiping up any excess resin. Ideally there will be no drips or puddles and the entire surface will have a dull fuzzy look and an even color. If you have light spots, add some more resin, let it soak in and wipe up the extra. Even if you didn’t heat the MDF first, this wiping step is critical to getting a consistent and easy to sand surface for your remaining finishing steps.

If you are going to use an epoxy primer to fair, you can go ahead and sand the surface as soon as it cures.  If you are using a polyester or vinyl-ester primer, you will need to post cure the epoxy sealing coat. If you don’t, it can keep the primer from curing and you’ll have to wipe it all off with acetone - ask me how I know!

From here on out it’s a finishing problem.  I am not a master painter by any stretch so my advice ends here.  The best finishing advice I ever got though is to “make sure you sand with something that is the shape you want the surface to become.”

The MDF will hold up ok, but don’t expect too much.  If you cook it it will eventually crack. Cook low and slow and try to keep it under 200F/95C.  If you have finely featured geometry, it will eventually break. Make sure you have a reliable release coat system that you test.  Release systems that work on production tooling will sometimes still take chunks off an MDF surface. But it was fast, and it was cheap and sometimes that’s a winning combination!

Note:

This article contains information that reflects my opinions - I make no promises about its usefulness!  It may contain mistakes (please let me know if you find some!) and will include prejudices based on my limited experience.  If you disagree with anything here, please get in touch. This is not just for me to share what I know, but to learn from others.  I will gladly insert additional information and differing opinions so readers are more aware of the diversity of “right” answers!