How to: Glue and assemble latex

This video summarises the process for cleaning, gluing and assembly.

Cleaning

The first step is to prepare the surface using the cleaner.  This removes the powder that was applied by the manufacturer and also any trace skin oils from handling.   XXX what does it really do molecularly?

The cleaner is very volatile and flammable.  The fumes, as with most solvents are harmful.  You should work in a well ventilated area.   I decant a small amount into a single portion jam jar, this helps keeps the fumes down and reduces wastage (through evaporation); always keep the lid on the jar whenever you can.   It is also a good idea to wear disposable vinyl gloves (XXX link).   Prolonged skin contact with the cleaner should be avoided as with most chemicals.  I find that even moderate contact leads to drying and desensitisation of the fingertips.

Cut a J Cloth into strips no larger than a playing card.  A single J Cloth gives me about 16 pieces.  If you use a piece of cloth that is bigger than this, it will soak up more solvent than you need and this will then evaporate into your environment.

Replace the bits of J Cloth regularly as the powder will build up on it and it will become less effective.

If you use too much cleaner the latex will buckle.  This will settle down again after a few minutes and does no harm – other than to slow you down.

Gluing

The glue is also volatile.  As it dries out it becomes more viscous and harder to work with.  Therefore I also decant a small amount into a small jar.

The glue spreaders are, before modification, about 13mm across.  Cut this down to 7mm with scissors.   You will probably also find uses for thinner and wider spreaders; 7mm seems a useful size for normal seams.

The trick is to get just the right amount of glue onto the spreader: too much and it will spread too widely and be untidy;  too little and you’ll only go an inch before dipping into the pot again, or not get proper coverage.

The glue starts to thicken immediately as the solvent evaporates.  You’ve only got 5-10 seconds to use the glue on your spreader before it is useless.  Regularly clean the thickening glue from the spreader with a piece of kitchen towel.

Avoid brushing the spreader on the edge of the jar (as you would a paintbrush) – you’ll just end up with strands of glue.

Aim for perfection and be as neat and consistent as you can.   You must have no gaps in the application of the glue, and it should go all the way to the edge.  If you have gaps then you’ll have a weak seam that could burst and rip when stretched.

You need to get all the way to the cut edge of the latex, but not over it.  If you do go over the edge, then just move your piece slightly before the glue has a chance to dry, otherwise you’ll get a pizza cheese effect when you move the latex and as the strands break you’ll end up with little balls of glue stuck to your seam.  If you do get a glueball then use tweezers to pick it off.

The amount of glue that is ideal, is the minimum amount when you can see that there is complete coverage.

When you’ve used the idea amount it will dry almost instantly.  If you’ve use too much the seam will not be as tidy.  If you use way too much then the latex may buckle and will take a minute or to settle down.  If you don’t use have any gaps then the seam will be weakened.

Protect the glued surface with cling film.  Carefully cut a 1/2″ from a roll of cling film using a fresh scalpel blade – this provides a continuous 1/2″ clingfilm tape 50m long.  Ensure that the glued section is not right at the edge of the cling film otherwise it can be hard to remove (the clingfilm can tear and can then be hard to pick off).

The cling film allows you to prepare all the seams before starting assembly.  Cleaning and applying the glue is much easier when the latex lies completely flat on your work surface.

When the glue in your working pot has become more viscous it will be harder to work with.  Decant fresh glue into a new jar.  (The jars can be recycled be leaving the glue to dry – outdoors – at which point it can be peeled off of the glass.)

Assembly

Always check that you are about to join the correct pieces together and in the correct direction.  I mark my patterns with a numbered arrow and transfer this to the latex when I cut the pieces out.

Arrange the pieces so that you can clearly see what you are doing and ensure you have good lighting.

Remove the cling film from both pieces – a little at a time.  Unprotected glued surfaces will stick to each other instantly, and will pick up talk, powder or other contaminants very readily.

By applying the glue to a consistent width you can use the glued area as a guide to how much the pieces are overlapping.  E.g. having used a 7mm wide spreader, then you can align the latex so that 2mm of glue remains visible, and you’ll achieve a consistent 5mm overlap.

Ensure that you prevent air bubbles by smoothing the surfaces together carefully. Always work along the seam, don’t let the surfaces ahead of you touch.

It is easy to inadvertently stretch the pieces as you line them up, and if you don’t realise this is happening you can be several cm out at the end of a long seam.  I put marks every 5 cm along both pieces – these come directly from the pattern so I know that if the marks line up then the ends will line up.  (In iDraw just use a line with a custom dash length of 140pt, 2pt.  For tighter curves use 2cm dashes instead.)

Edges will normally have a curve to them in order to form a 3D shape from a 2D surface.  This means that the latex must stretch/deform slightly in order to achieve a constant overlap.  The tighter the curve, the harder it is to achieve a constant overlap and have a constant stretch.  If the latex is tensioned unevenly then, despite having a constant overlap, you may have dimpling along the seam.  This dimpling represents the internal tensions that have been created

The trick is to hold the pieces in such a way that the overlap of where the latex is about to join wants to remain constant.  It if often helpful to hold the already-joined section off of the working surface in an approximation of the radius being formed.

The latex will adhere immediately, but it is very easy at this stage to unpeel it. If you have gone wrong, then carefully unpeel the seam and try again.  So long as you are not repeatedly touching the glued surface and getting powder on it or oil from your fingers, then no harm seems to come from repeatedly re-attempts.

Sometimes when you unpeel a seam without enough care the latex rips.  If you unpeel a seam very slowly you can see when this is going to happen and prevent it.

Roll the seam firmly with the roller.  Ensure before you start that there is nothing underneath, ideally no other layers of latex, just the cutting board.

You will see now the quality of your seam.  There should be no air bubbles, no bumps from glueballs, a consistent overlap, no untidy excess glue and no dimples.   Excess glue can be removed with cleaner.   The look of dimples seems to improve a little overnight.

After firm rolling, the seam should be now be at full strength.

 

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