Newbie - First Time Mold Making

General Composites Discussions Showcase
#1

Hi all,

I’ve been reading up on many of the topics here and on other sites as I want to fabricate some CF parts for my track motorcycle. I’ve some experience of GRP fabrication, but none with molds and complex parts.

Progress so far… I have a few questions which I’ve highlighted at the bottom if anyone is able to advise.

I thought I would start with a small and simple bracket - after all, how hard can it be!!

So the bracket in question - it holds the front brake master cylinder and in this shot is my first attempt at adding modelling clay prior to forming the “MK I” mould!

Obviously, the mould created from this part was way too small and tight to be able to create a suitable CF part. In fact, it was too small and tight to even get the original part out again!!!

So quickly onto the “MK II” mold, this time with bigger flanges

Again, still not big enough! But I cut away some of the exterior mold in an attempt to make the mold useable and give it a try laying up a part. Having cut away part of the mold I thought it was ok for a trial.

The mold finish was ok and as the part wasnt going to be too visible I wasnt spending too much time on the finish, but it did get a light sanding and plenty of coats of polish!

It was good to be able to lay up a part and get some valuable practice and experience. However, the MK II mold was still incorrect as the flange on the very short section of the bracket was not big enough (bottom left hand side of the bracket in the image), the CF wouldnt ‘sit’ in the mold and retain the shape so it started lifting away whilst I tried to lay it up.

I didnt get any pictures of the trial CF part, but the finish on the mold side was very poor. I think that the first wet layer of resin was probably way too thick. I mixed up a very small amount but while I was waiting for it to become tacky it started ‘pooling’ in the corners of the mold. Also, once cured the part was quite tricky to remove from the mold, possibly not enough PVA and too much resin in the first pass.

Anyway, I moved onto the Mk III mold, much bigger flanges this time, particularly around the ‘very short’ section of bracket to hopefully overcome the ‘lifting’ issue in the Mk II mold! (See top right hand of bracket in this image)

I have only just released the original part from this mold and have yet to clean up the mold. I think this one should be better andI will give it a trial in glass fibre before another CF trial.

At the same time as making the Mk III bracket mold, I made a pretty quick “Mk I” rear fender mold using the tooling resin and then overlaying glass fibre and resin. Heres the original part and the formed mold

I’m really pleased with how this mold turned out, even though there are some obvious defects. It was a pretty ‘quick and dirty’ trial for a bigger molding and seems to have worked out pretty well in terms of finish and suitability. The original part came out without any real trouble which was a great relief, but I didnt apply enough glass fibre at a couple of the edge positions, resulting in the tooling mold breaking away when I released the part (see top right and top left of the mold in the image). When I made the glass fibre mold I used lots of pieces of small glass fibre mat which were torn apart rather than cut apart. In the main this approach seems to work very well, but even the smallest of pieces of glass cloth were a bit of a struggle to form around the tight radius edges and corners.

Any tips for this? Have I used the wrong type of glass fibre mat, perhaps its too ‘course’ ??

So to, some questions. The original part has a small fixing hole in the centre which is in the base of a recess - the hole is in the centre of this image

When I formed the mold, this section of the mold is a very thin part and I’m guessing is way too thin to be viable to fabricate by ‘laying in’ with CF. The image of the mold above (the green mold) shows this section as a pinch point in the middle of the mold.

So, some tips/questions:

  • for the fender mold how do I achieve a neat finished edge? At the moment the “Mk I” fender mold has very small flanges which if used would require me to trim and finish the exposed/finished edge of the completed part. I’m expecting that if/when I do this, the edge of the finished part will not be so good. This would be a very visible part so ultimately a much better edge finish is required. How do you achieve a better finished edge?
  • How is the centre fixing correctly achieved and is it feasible to fabricate using this mold?
  • Much of the reading up I’ve done indicates the use of a single piece of CF sheet laid into the mold to achieve the ‘completed’ finish. In the case of my fender is it feasible to use a single piece of CF sheet for the upper more exposed element of the part and then lay small pieces of CF into the tighter sections of the mold to create the complete part. (In this fender, part of the finished item is concealed from view so the finish is not as critical). Ideally though, I’m hoping to perfect my techniques to achieve an overall level of good quality.

Any tips, thoughts or views appreciated as this is all good practice for me.:slight_smile:

#2

Avoid quick and dirty. Practice doing every step perfectly, as that is what is required to make good parts. Quality quality quality, every step of the way. Crap molds will only ever produce crap parts. Avoid complex shapes until you’ve made some simple ones correctly from start to finish.

#3

Ahhh a gsxr hugger and mold.

Will you be vacuum bagging the part? It is best to make a flange all around the part.

  1. Rough sand the original part.
  2. Attach flange all around.
  3. make sure the transition from the part edge to the flange is seemless and perfectly smooth.

( possibly make your flange out of plywood or mdf board. attach the flange to the part from the backside where you will not be molding from ). Use some bondo to fill in the gap between the flange and the part edge so as not to change the part shape or edges. Make sure there is no bump going from the flange to the part or else making the part will be impossible. You need a smooth transition from flange to part.

Once you have your flange attached and bondo’d smooth and sanded smooth. Apply sanding primer by spraying it on… you will need a nice thick coat about 10 to 15 mils thickness. That is about the thickness of 5 sheets of notebook paper.
Allow primer to fully dry. Sand primer smooth with 320 wet, then 400 wet, then 600 wet, then 1000 wet. Now get some buffing compound and buff the sanded surface smooth as window glass.
Duratec makes a great polyester primer and gloss addative for this purpose. You can also spray gel coat onto the plug mixed with duratec gloss or just spray on gel coat to 15 mil thickness. After you spray on the gel coat then spray on 5 coats of PVA mold release. This is so the gel coat doesn’t stay sticky after it is cured. Sanding it will be much easier if it ain’t sticky.

Now if you used gel coat follow the same wet-sanding schedule as above but finish with 1500 grit sandpaper wet.

Buff your plug smooth to perfection or darn near perfection.

Now mold release wax your finished plug 12 times with special mold release wax. Buff each coat of wax after applying and wait 30 minutes to one hour in between each coat of wax. else you will just remove the prior coat.

After you have 12 coats of wax on there and it is smooth… not streaks of wax showing if you did a good job. Don’t buff the wax off hard! use minimal pressure to buff wax.

Now spray on a 20 mil thick coating of tooling gel coat. mix it with 2% MEKP catalyst prior to spraying.

let that set up to the point where you can touch it and it doesn’t come off on your finger but it is still sticky/tacky. usually about 2 to 4 hours.

Lay up the first fiberglass mat layer onto the mold/plug. Make sure you use a cheap paint brush to work ALL the tiny air bubbles out of the fiberglass and resin. No air bubbles bigger than 1mm or 1/16". Make sure you tap down all stringy bits sticking up so they are flat to the mold. You will see what i mean when you do it. If you have any sharp corners or radi fill them in before laying up your first layer of fiberglass. Right on top of the tooling gel coat. Mix up some tooling gel coat with fine cut fiberglass or cotton flock or similar filler. This is so the first layer of fiberglass mat will not pop up from the inside corners of your mold leaving air gaps… this is to prevent air gaps. now lay down your first layer of fiberglass mat ( 1 ounce mat preferably ). let it cure for at least 8 hours. The first layer is the most critical layer so you want it to be perfect. After 8 hours, can be the next day… scuff, sand the fiberglass with 80 or 120 grit sandpaper… wipe it clean with acetone to remove the grit dust. Now lay up another layer of fiberglass mat. Let it sit for at least 4 hours before laying up a third layer of fiberglass mat. Repeat this 6 times so you have 6 layers of fiberglass mat. Let the whole mold fully cure on the plug for at least 24 hours before demolding it.

After you demold the plug, trim the edges even and smooth with a bandsaw, dremel saw wheel, or cut off saw. Sand the edges smooth using: 80 grit paper, 120 grit, 220 grit, 320, 400, 600, 1000, 1500 wet. all wet grits makes sanding easier and gives better results.

As for the mold part surface… wet sand it with 1000 grit, then 1500 or 2000 grit wet paper. Then buff it with rubbing compound, then polish it with a polishing foam pad and polish compound. Wax the mold 12 times before using it… wait at least 30 minutes between each coat of wax.

This lay up technique is for polyester ISO resin or Vinylester resin. Using polyester tooling gel coat. All of these are catalized with 2% MEKP.

#4

For that bottom center tab on the hugger fender… i would simply cut trim it off the plastic part. You can fabricate a flat cf plate and bond it on after the part is molded. Just bond it on with clear resin from the underside so it is not seen.

This is how your finished part should look. We made it a month ago.

#5

We are near the end of making a run of molds for car parts. I will post some pics. problem is now these molds are in the buffing and waxing stage. We are starting a new pair of fender molds though that i will post pics of the process.

Also you may want to learn to modify original parts to create your own custom part, then make the mold. Add your own touch to the design. Do this by cutting areas away, adding shapes using bondo or filler and resin before you primer the plug.

#6

I would like to add some things to the perfect explanation of Fastrr:

Instead of spraying PVA over your gelcoat, you could also use topcoat as the last layer, or add some parafin wax to the gelcoat. As you will be sanding it, it is not a problem.

When the tooling gelcoat is on the plug, make yourself some paste by mixing tooling resin with filler (Cabosil or similar) and apply that in every corner. Your glass fabric will never like sharp corners, so even outer corners may be worked to get a nice radius.

Only after that apply your fist layer of glass. After cure, inspect the glass for airbubbles. Now you can still cut them open and fill them. Later they will need more work if they break open on the mold surface.

And last but not least:

Many parts are design so they can be made easily using a suitable material. Aluminium in case of the bracket, and a thermoplastic polymer (probably ABS) for the fender. They are not designed for wet layup composite. Copying them can be quite tedious. Best is to redesign the part, and go from there. You can use composites as a shell, and use gadgets like “Weld Mount Fasteners” to install them (combined with methacrylate glue (ITW Plexus)).

In other words: design for composites.

Which means: open shapes, no tight corners, to be made with fabric and perhaps core, no excessive peel forces, etc.

#7

Thanks FastRR for the comprehensive explanation.

A couple of stupid questions if you dont mind…:confused:

  1. your description ‘builds up’ the original part using a variety of methods to create the ‘plug’. What is the purpose of this method?
    In this instance a small increase in size over the original part isnt a particular issue, but why follow this process instead of using the original part with appropriate flanges as the plug?
    Is this process intended to create a ‘perfect’ finish for the mold?

  2. Advice generally seems to be to apply bigger flanges. Ultimately I want to recreate the original part, so adding bigger flanges may result in cutting off more ‘waste’ around the edge. I do however appreciate already that bigger flanges are needed to get the carbon to lay flat and create a goo edge.
    What size flanges do you recommend. I was thinking approx 1 inch.

  3. I’m not trying to short cut the process, just understand things in more detail. But I had in mind for my next attempt to apply 2 or maybe 3 coats of tooling gel to the fender to provide a much thicker layer prior to adding resin and fibre to the back.
    Will this not provide a suitable finish and composition for the inside of the mold?
    Testing on the ‘throwaway’ mold already made, reveals that the finish comes up well with a mild cutting polish.
    Would this approach not provide an alternative method to the build up of the plug described above?

#8

I can answer number 2. Bigger flanges make laying up much easier. You have more space to work with the tacky tape when you’re laying up your final part. The amount of waste you’ll have on the flanges depends on how much you overcut the material not how big the flanges are. I’d go with 4 inches at least and 6 if you’re planning on infusion.

#9

To answer Q: 1 - the oem hugger is textured plastic that you are going to sand with about 80 grit sandpaper to make it rough so gel coat or plug primer ( Duratec Primer ) will stick to it. You would want around 15 mil thickness of primer or gel coat ( build up) so there is enough to hand sand smooth and not break thru the primer during sanding. Also you polish the finished plug with an electric buffer so you don’t want to burn thru the primer or gel coat if it’s not applied thick enough. 15 mils just gives you some room for mistakes. 15 mils is nothing really and by the time you sand it smooth it will be more like 5 mils of coating on the plug.

Very true on the gel coat… add some liquid wax ( composite supply shop) and the surface won’t be sticky once cured. No sticky = easier to sand it smooth.

Yes you sure can use tooling gel coat in 2 or 3 coats for the plug surface. It is harder to sand tooling gel coat than it is normal gel coat. Normal gel coat will buff to a shine, so you can wax it a dozen times then build the mold from it.

The reason for Duratec Primer and Duratec High Gloss Addative is for building plugs. It sands easy, the gloss addative prevents a sticky surface, and it polishes to a mirror shine. Gel coat does the same… it’s just harder to sand it.

No problem asking questions and coming here to learn… that’s what we are here for… to help each other and learn new things about composites.

If you should decide to use PVA over the gel coat ( to prevent stickiness ) you can get a Preval Sprayer. They are inexpensive and work well for PVA’ing small plugs or molds. I personally do not use pva in my molds… I prefer semi perm mold release or mold release wax because it provides a better part finish.