Step-by-step molding

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Well-Known Member
Jan 20, 2009
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Dear gentlemen,

I thought I could share with you my common tech for making composites molds that will last a lifetime.

I hope you don't mind if the following tutorial doesn't refer to rocketry-related parts but that's the one process I documented the best!

Let's get started!


Generally speaking mold-making deals with a sequence of known operations:

0. Plug finishing
1. Plug polishing
2. Plug waxing
3. Release agent (PVA)
4. Composites mold lay-up
5. De-moulding

Even if much of the work and materials belong to step .4 it is the precise and lovingly care exercised during .0 to .3 that leads to a satisying result!

The items needed to complete steps .0 to .3 are pictured below:


Polish (up left), PVA liquid release agent (up right) and Partall pasty parting wax, plus cloths and 'special' brushes for the PVA.

0. Plug Finishing

It may happen that some dust particles collect in your final coat of paint: it happens often as I paint in the outside so it could happen to anyone else as well.

Do not panic but do not leave any surface defect that will mirror into the molds and then into the finished part! The finished product should need little or no sanding & filling at all! This is why we invested time and patience and money in a good quality plug finish!

Inspect the plug surfaces carefully and find all the little defects: sand them down using 1200-grit WET sandpaper. Limit the sanding to those 'affected' areas as the scratches the 1200-grit leaves will fight with the overall glossy finish you gained with the painting.

Anyway do not dispear: the polish will clear those scratches later on...

1. Plug Polishing

Get a good quality polishing compound from your local automotive store or from your paint supplier. Usually it is a one-part viscous liquid that you rub onto surfaces with a soft cotton cloth. Apply the compound with a swirling motion and apply it with a generous pressure: insist on those areas you previously 'scratched' with the 1200-grit sandpaper until they shine as much as the rest of the plug.

The polishing will bring your already glossy piece to a mirror-like object! it amazes me everytime...

Consider the job done when you're satisfied with the overall appearence and remove any trace of the compound that got stuck in the recesses. The following pic doesn't do justice to the 'look' of the plug:


2. Plug Waxing

I hate waxes.

If you know what you're doing then go ahead and spread the Partall or similar pasty waxes.

If you're new to waxes then choose something such as R&G Priming wax: it is a lot easier to work with and it leads to consistent results without much experience (it is also referred to as beginner wax).

Generally speaking parting waxes may be liquid or pasty.

Liquid waxes are generally brushed on, let dry and then rubbed to a good shine with a soft cotton cloth.

Pasty waxes are spread with the same soft cotton cloth, let dry and rubbed to a good shine as well.

When you rub the wax to a good shine don't rub too much or you'll remove it!

Many coats are applied with long overnight drying times in between. That's time consuming on its own...

When you add coats be careful or the solvent of the wet coat you're applying may thin the dried coat and remove the wax you previously spread...

You know, you have to experiment on your own!!!

3. Release Agent

The liquid release agent, the PVA, is what really makes the difference between leaving the molds stuck to the plug or not.

It is a water-soluble liquid that you simply brush onto the parting surfaces: the above pictured brush (Moltopren sponge) should avoid 2 bad tendecies of the PVA coat: a lot of tiny air bubbles trapped in and the typical striping left by common brushes.

Even if the PVA is water-soluble it requires a night to dry. If you aren't in a hurry I suggest you apply 2 coats, the double coat builds up stronger and peels off easier.

Let dry overnight between coats!

Please remember that the PVA collects every possible dust particles in the air so brush it on in a 'clean' enviroment...

Here is a pic of the 2 plugs drying (darker areas are dried):

4. Composites Mold Lay-up

Here is the list of the materials required for the process:

1. Laminating resin of the brand you prefer (I ended up mixing 1.5 lbs in 4 batches)
2. Fiberglass cloth in the 6 to 7 oz./sq.yd. range
3. black coloured pigment compatible with the resin system you use
4. microballoons
5. core material, LANTOR Coremat XM (2mm thickness) HIGLY reccomended
6. disposable items such as latex gloves, mixing sticks and plastic dishes, brushes...

I usually plan in advance all the operations and the required materials for each step. I also keep a log on the resin batches mixed and times.

1a. - 'Amateur' Gel-Coat Layer = 1st epoxy batch

The very first step deals with mixing a sort of a homemade gel-coat:

- use the black pigment and mix it into the first batch of resin(*)


- add some microballoons until you get a thicker liquid than the resin alone BUT not too thick(**)


- paint the mixture in a thin layer on all the edges and corners of the plug (keep the layer thin as it will be easier to avoid trapping air bubbles in it)


- paint it on the complete plug surface (again keep the layer thin!)


(*) Pigments may cointain an epoxy binder (pastes) or not (liquids or powders): if they contain the binder as mine did they must be mixed accordingly!

i.e. my color paste is mixed up to 10% weight: if resin to hardener ratio is 50:10 without the pigment it should be (45+5):10 WITH the pigment, where 5 is 10% of 50 since they both are resins.

(**) The ideal thickness is such that the thickened mixture won't run down vertical surfaces; the thicker it is the higher odds are you'll get air bubbles in it and the harder it will be to reach all the recessed edges of the corners.
1b. - Filleting the corners = still 1st resin batch

Just add some more microballons to the black-pigmented resin batch t increase its thickness to a point it will stay where you put it:


Apply this mixtures to all the corners in order to increase their radius: that is very important for the following cloth lay-up as the cloth itself doesn't conform to tight corners very well.

I didn't mix enough microballoons into the resin: you can see the result, the mixture is 'flowing' away from where it belongs... avoid doing that!


After you're done with it leave the layer alone for a 'certain' time: it should gelify or get to that 'tacky' state before the next step.

How to determine if the elapsed time is enough? Start checking the curing after 30': press a finger into the layer and see if some residues stay onto your skin/glove. If they do wait another 15' and repeat the 'test'. If you only leave your fingerprint into the layer with no residues on your skin/glove, then it is 'tacky'.

It took me about 2 hours to get to this stage so it really depends on your resin system and on the temperature (it was hot in my workshop BTW).
2. Cloth Lay-Up = 2nd epoxy batch

Plan in advance how you'll cover the entire plug with the cloth: in case of a complex shape as this one it is better to cut stripes of cloth between 2 and 3'' wide.

Mix a new 'pure' batch of resin and paint it onto the gel-coat layer: NONE of it (the gel-coat layer) should be 'affected' by the new epoxy batch!!! in case it happens wait more time as you risk to remove the gel-coat layer or to thin it to a useless thickness!!!

Start laying down the cloth stripes: I started with the vertical striping...



... and I then continued with the horizontal ones...



... until I got 2 full layers:


Please note that the 'tacky' state of the gel-coat layer 'locks' the cloth in place and it will be not easy to move it around... plan its positioning carefully. You can still peel it off and re-do it without too much concern.

Once done, let this layer get 'tacky' before proceeding.
3. - Coremat Layer = 3rd epoxy batch

The LANTOR Coremat is a core material that builds up thickness in a quick and cheap way. Its thickness depends onto the curves it should conform to, I chose 2mm for this project and I found it a good compromise.

Why a core material? The core material is what really makes the difference in a mold: the core material increases the thickness thus the stiffness of the entire mold. The consequence is a greatly improved de-moulding experience.

I can assure you about it by first hand experience as I was used to make flexible or non-stiff f/g molds in the past and they were a real PIA to separate from the plugs.

If you insert a wedge to separate the mold and the mold itself is stiff it will deform (flex) a little thus popping the entire piece apart from the plug.

Enough, the Coremat absorb a huge amount of resin so mix the 3rd batch accordingly. I opted for 2 flat layers of Coremat and a single layer around the float.

The first layer in (you see it is white = no resin yet):


The single layer 'around' the float (the flat layer turns yellowish when soaked...):


The second flat layer helps to seal everything down:


Leave this layer until it gets 'tacky' again.
4. - Final f/g layers = 4th epoxy batch <OPTIONAL>

Seal the Coremat with 2 f/g layers: leave the cloth in one-piece, it should conform easily now.


Let it cure overnight.

5. De-moulding

Roughly trim the excess cloth around the perimeter of the plugs and collect all the de-moulding weapons you have!

I suggest a few PP wedges and a compressed-air wedge just in case things get really hard...


Separate the mold from the plug 'by hand' at a corner then insert the wedges working them around a little at the time:


The mold should pop out easily...


... and the PVA film should be stuck with it: you can both peel it away (in case it is thick enough = 2 layers) or wash it away with water.

Trim the mold and mount it onto a frame so you can work on them with ease.

Making the Parts

First let me say that making the parts involves a fraction of the time and effort we spent for making the molds themselves BUT you need to exercise care while handling the molds to avoid damaging them during any of the procedures involved.

0. Getting the molds ready

It happened that the PVA got stuck to the molds instead of the plugs: do not worry, PVA is easily removed with fresh water and some soap (dish-washing is fine). Wash and clean the molds throughly under the tap; a smooth sponge may help removing all the PVA from corners and sharper features.

I then mounted the molds onto some MDF framing: I covered the frame with some heat-shrinking covering material so the resin won't get stuck to it (fast and easy).

1. Waxing the Molds

2 important issues:

1. an epoxy-resin mold is a more chemically stable material on its own than a painted plug: its surfaces are harder and "glossier" than "the glossiest" of one-part paints!

2. polishing is not necessary BUT if you intend to polish the molds use a silicon-free polishing compound or it will be hard the get the wax to wet the molds properly.

I used a waxing compoud called 'Priming Wax': they claim it features superior 'wetting' properties and that it helps the PVA to spread in a more uniform layer, plus it doesn't need any polishing (=buffing to a good shine) between coats.

It is not the best wax available but it is suggested for beginners!

SO, I gave 5 coats of Priming Wax @ 24 hrs. intervals + 2 coats of PVA @ 24 hrs. intervals. I then let the PVA dry for another 24 hrs. before molding.

Picture shows the molds ready:


2. Homemade Gel-Coat Layer = 1st resin batch

I started with a ketchup-thick mixture of (resin+hardener + grey colour paste + microballoons) and I painted it onto the mold surfaces keeping the layer thin; please remember:

1. this layer fills the cloth

2. this layer should stay onto vertical surfaces in a thin layer (the thicker the more mass the more difficult to prevent it from running down on its own weight)

3. this layer should avoid trapping air bubbles <=> its thickness should be less than the radius of the bubbles themselves

4. color selection between molds and layer should enhance the contrast so that you know where you painted it and where you didn't (and it should help you determine its thickness at a glance)


Now thicken the mixture with more balloons and smooth all the tight-radius corners so that the cloth will conform to them more easily:



Now let this layer gelify to a state where it will be 'tacky' (if you press a finger into it NO resin should remain onto your finger): it usually takes 45 minutes but it depends strongly on many parameters so check every 15 minutes: it took about 2 hours in my case.
3. Cloth Lay-Up = 2nd Resin Batch

I chose to lay 4 layers of cloth: the first 2 should be light so they conform to the mold while the other 2 could be heavier BUT open-weave (not tight).

Paint the mold with a liberal amount of resin and then start working each cloth layer in place so that no bubbles get trapped in!


Work each layer at a time, remove all the bubbles and wrinkles from it before adding the next one. Easier done than said...

Once all the layers are in place remove the excess resin and let the epoxy cure overnight:

4. De-molding

I was surprised by how easily I got the parts away from the molds: I used some PP wedges and I worked them around the perimeter of the parts...


... until they popped out on their own:


I left you with the very first pair of halves de-molded: I then trimmed them a little bit more precisely.


Please note that it is not really necessary to cut them perfect since the final trimming is achieved by sanding them on a flat surface:


Note that @ this stage the PVA film hasn't been peeled off yet: you can peel it off by hand and/or wash it off with tap water and a sponge.


The same applies to the molds that are depicted here just after the parts have been removed:

Next came the weight-check: basically I averaged a 1 oz. weight per half.


Here they are once joined together:



They weight a little bit more/less than 2 oz. because of the gluing mixture (epoxy + microballoons):


Their surface finish is highly gloss and it will need some light sanding to promote paint adhesion (400-grit).

That's it gentlemen, thank you for passing by!