5. Laminating the bottom High Density (HD) foam

There is still a way to go before we start to shape the deck of the board.  The next step is to laminate high-density foam to the bottom of the board.  This step also gives the first experience of the vacuum pump, and laying up fibreglass.  Too much to take in one step?  Dude, wait till we get to laminating the HD foam onto the deck!  It’s three times harder!  Look at this step as practice!

The key to successfully laminating the bottom HD foam is preparation.  The problem is that the epoxy only has a relatively short working time, so you need to be reasonably quick once the epoxy is mixed.  It’s all so much easier if you adequately prepare, as you have time to deal with the small problems that inevitably crop up through the process.

Here is your preparation list;

1. Cover the bottom mould with a sheet of polythene, and attach it using cheap drafting tape.  This stops any stray epoxy from bonding the board to the mould.
   
2. Make a vacuum bag.   The bag has to be big enough to enclose the board, bottom mould and vacuum table.  To give you an idea of size, I use a 1.5mx3m bag I use for a vacuum table that is 2.4m x 8’x2’.  That may seem excessive, but there needs to be enough slack in the bag to get into all the corners.  It looks huge when you first put it on, but the vacuum pump soon collapses the excess.   The edges of the vacuum bag are sealed with vacuum tape, which is double sided sticking tape.  You can only seal down three sides, and leave one end of the bag open (otherwise you will never get anything in it).  On the side that you leave open, put the tape down one side of the opening, leaving it ready to be sealed once you have your board in the bag.
   
3. Prepare your epoxy mixing table, which needs the following;
   
   1. Vapour mask. Disposable rubber gloves.
   2. Epoxy in easy to access containers (two jam jars in this case).
   3. A pair of syringes to measure the epoxy resin and hardener.
   4. A mixing pot and stirring stick.
   5. A squeegee.
   6. Acetone and kitchen roll (Mrs ECB loves it when her kitchen roll goes missing) to clean up.
4. Now, we need to prepare the HD foam.  This comes in 8’x4’ sheets, so for some boards you could consider cutting a single piece of HD foam.  In practice, it’s more efficient to use multiple pieces of foam taped together using your highest quality drafting tape.  Use the plan shape template to mark out the outline of the hull, then add the additional width for the rails in the same way as you did on the EPS foam.  Also mark a very clear centreline and your 15cm station lines, using a large set square.  It’s worth marking out both sides of the foam for the tail and nose sections, as you want to align the EPS and HD foam in the vacuum bagging process.  The centreline of the EPS foam board needs to match exactly to the centreline on the HD foam, otherwise there is all sorts of measurement problems later on.  By the way, I’m using 2mm Airex R63.80 here.  2mm is fine for this lightweight speed board, but you might consider using 3mm foam for a board that needs strength, like a wave board or likely-to-be-jmuped-and-landed-flat freeride board.
   
5. Put the HD foam on your vacuum table, and fetch your roll of 4oz S-type glass.  Roll out the cloth on top of the HD foam, and cut out a piece of cloth to the same size, using some very sharp kitchen scissors (or fibreglass shears if money is no object).  Mark the nose and tail ends of the cloth.  Also cut two pieces of 6oz carbon (or S-type glass if you have no carbon) about 12”x6” to reinforce the area around the fin box.
   
6. Clear the vacuum table, and then place the bottom mould on top of it.  Put the HD foam on the bottom mould (drafting tape side down), and align the centreline of the mould to the centreline of the HD foam.  Use a couple of pieces of high quality drafting tape to hold it in place, but make sure the tape doesn’t stick to anything other than the 5mm at the outside of the high density foam.
   
7. Place the 4oz glass on the high density foam. 
   
8. Prepare the vacuum apparatus. You need the pump with its hose connected, two through-connects, and the vacuum gauge.

   Preparation is complete.  Now comes the epoxy.  Have a cup of tea first (but not in the workshop, we don't eat or drink in the workshop through fear of ingestion of noxious chemicals), and read the rest of this section before setting off!

5.1 Epoxy and Fibreglass

Before we get stuck into the lay up process, a few words on the materials.  Epoxy and fibreglass are marvellous materials, and are fundamental to board construction.  Unimpregnated Fibreglass is cloth like and has no mechanical properties.  Cured epoxy is very brittle and again useless for our needs.  But fibreglass impregnated with epoxy has remarkable mechanical properties, and is light enough to be useful for windsurfer construction.

Carbon fibre is a variant in the world of fibreglass.  In terms of tensile strength, carbon has better properties than other types of glass.  This is the reason it is used so widely in boards, because using carbon means that you can get away with using less cloth (and therefore weight) for the same strength of board.  The problem with carbon, however, is that it is not strong in terms of receiving an impact compared to fibreglass.  So, slam your mast into your full carbon, and you could be into expensive repair territory.  It is for this reason that manufacturers make hybrid cloths with a carbon and (for example) Kevlar mix – they are trying to get the best properties of both cloths.  My solution is to use both carbon and S-type glass on the final lay up, using the impact resistance of the S-type glass in the final layer.

Unimpregnated fibreglass is relatively innocuous.  Epoxy needs a little more care – always follow the manufacturers safety guidance.  For the board builder, it’s worth having an appreciation of the dangers of fibreglass and epoxy dust.  Once the fibreglass has been impregnated with epoxy, when sanded it produces dust that is a major skin and respiratory irritant.   If you are sanding fibreglass and/or epoxy, you must wear a top quality dust mask (I use a 3M mask from B&Q), a long sleeve shirt (from Oxfam) and disposable gloves (from Anne Summers, or Machine Mart if you want them cheaply).  If I’ve finished a big sanding job, I come in and take a cold shower (thanks to Ian Fraser for this tip - a cold shower closes the pores in your skin, and hot shower opens the pores a can compound an already unpleasant problem).  I have had fibreglass dust on my skin and it really is unpleasant.  Imagine what it could do to your lungs. Cured epoxy dust also needs to be avoided. It won't irritate the skin like fibreglass, but is a hazardous chemical nevertheless. Always wear your dustmask and avoid contact with the skin.

5.1.1 Mixing the Epoxy

Epoxy comes in two packs – resin and hardener.  They have to be mixed in prescribed quantities by weight or volume.  It’s much easier to mix small quantities by volume using syringes.  For our board, we want to mix enough epoxy to wet out the 4oz cloth, and then an additional 20% contingency, because we don’t want to be mixing more epoxy half way through.

For the speed board I estimated I’d need 150ml of epoxy (from the ‘Volume Calculation’ sheet on the SP Systems website), and my SP115 states I need a resin to hardener mix of 10:4.  So I measured out 110ml of resin into my mixing pot, closely followed by 44ml of hardener.  I should point out, by the way, that I had my workshop temperature at 18C.  Any warmer than 25C, and the epoxy gels too quickly.  Any cooler than 15C and the epoxy doesn’t gel quickly enough.  The properties sheet with the epoxy will tell you the optimal workshop temperature.

Here is a great tip from Bill from BC Boards. The problem with wetting out the glass straight onto the high density foam is that the grain of the open cell foam will be essentially filled with epoxy. The resin taken in doing this is going to add significant weight to the board. Bill's tip is to fill the grain of the foam with thickened epoxy, and whilst this is still gelling wet out the 4oz glass on top. This leads to a really strong bond between the glass and the foam making a strong structure that is less likely to delaminate. A further tip from ECB. If weight is your primary concern, wet out the glass cloth on a sheet of polystyrene and transfer to the high density foam. This minimises weight but compromises on strength. If in doubt, fill the grain of the foam with thickened epoxy (I've done this on all boards with the exception of the speed board).

Spend at least 1 minute mixing the resin and hardener.  It has to be thoroughly mixed.

Now, pour 2/3 of the epoxy down the centreline of the 4oz glass and use the squeegee to wet out the cloth by pulling excess epoxy to dry parts of the cloth (apologies for the lack of photos here, but it’s difficult to wet out and take photos at the same time).  You have about 15 minutes (with SP115) to wet out all parts of the cloth.  Once the 4oz glass is wet out, place the carbon patches on top of the 4oz glass, centering on the middle of the fin box.

By the time you have finished, the cloth should be uniformly wet out, with no dry patches and no pools of excess epoxy.  Pools of epoxy give the board no additional strength, but do add weight.  Dry patches of 4oz cloth will impair the strength of the board and could lead to delamination.

 Now, place the EPS board on top of the fibreglass, and ensure that the centreline of the board is aligned of the centreline of the HD foam.  Also make sure that the nose and tail marks are aligned.  Hold the whole lot in place with a couple of pieces of high quality drafting tape.

So, we have a board sat on top of wet out fibreglass and HD foam, sat on a bottom mould, sat on a vacuum table.  All of that lot has to make it into your vacuum bag.  It’s good to have someone around to help with this, especially if you are doing it for the first time.  Lift the back end of the table and work the whole bag onto the back of the table (rather like, and I’m loathed to use the analogy, putting on a stocking).  Then, work the bag up the table until the whole lot is in the bag.   Before you seal the end of the bag, be sure to put the base parts of the through-connects into the bag.  The top parts screw through the bag, making a good seal.  In the picture, you can see the bag in place with the vacuum pump attached and ready to go.

Now, turn on the vacuum pump, and stand by as the pump takes the air from the bag.  The bag seems to collapse very slowly at first, but don’t be tempted to nip off for a cup of tea!  Watch the vacuum bag continuously, to make sure that it is not being stretched too much around the board and vacuum table.  The bag material is remarkably resilient, but if it is put under pressure around a sharp corner it could rupture.

As the last air is removed from the bag, it will start to crackle as it begins applying pressure to the board.  At this point, watch the vacuum gauge like a hawk.  Too much pressure and the board could be written off at the first hurdle.  Use the valve on the vacuum pump to moderate the vacuum pressure, until you have a reading of 4-5” of mercury on the gauge.  Watch the gauge for 10 minutes to ensure that the pressure remains constant.  Readjust the valve as necessary.

By the way, you can see here that I have mounted the bottom of my through-connect valve on 6 squares of breather fabric.  There are two reasons for this.  Firstly, mounting on breather fabric means that the vacuum is applied over a larger area than the footprint of the through-connect.  This in turn means that the foam under the through-connect is less likely to be damaged.

Now, make sure that the workshop is going to stay at a good temperature, and leave the vacuum pump running for 12-16 hours.  Or, if you built yourself an oven, throw the lot in and set the temperature at 21C or so.

Do not be tempted to stop the pump and have a look inside the bag after 6 hours.  I made this mistake once and wrote off a project, a very expensive and time-consuming lesson to learn (Mrs ECB had to buy a PlayStation 2 to console me).

After 12-16 hours, disconnect the pump and switch it off.  Remove the bag and take the board off the bottom mould.  Place the board on the stand and admire your work.  You’ve stuck two pieces of foam together with a bit of fibreglass in between!  A small achievement perhaps, but a composite structure nevertheless.

Carefully remove all drafting tape.  Now you see the benefit of using a quality tape – it actually comes off.  Low quality tape would get soaked with resin and bond to the board.

The next step is to trim excess HD foam from the bottom of the board.  You want to trim to the inner of the lines that you marked out on the bottom  (see photo).  Use a Stanley knife with its brand new blade dipped in Acetone.  Cut through the HD foam and fibreglass, but try not to cut into the EPS foam underneath.  Several shallow cuts are better than one deep one.  Once you have cut through, pull the excess HD foam from the board.  You will find that a few beads of EPS come away with the fibreglass, but that is unavoidable and to be expected. 

Spend a little time making sure that the centreline on the bottom of the board is aligned to the centreline on the top.  Re-mark the outer rail of the board on the EPS foam, and then sand the EPS foam (using your 10” sanding board) around the edge of the board until you get to the rail.  Be sure to keep the sanding board perpendicular to the deck of the board.  You’re now ready to shape the board!  Does it get anymore exciting?  Only when we get to deciding the paint job!