Efficient way to fabricate a mold ?

it is about hardness and abrasion resistance of the surface coat, regular PE needs additives to increase those, but epoxy is already sufficiently hard and abrasion-resistant, at least for good part of applications. from what i read epoxy tooling surface coats are mainly more thixotropic than regular resins.
seems like PE wins mainly by cost and availability and as many say by lesser health risks (my best argument so far when the family complains about the smell)

 

But epoxy resin isn't ever UV stabilized is it.? , yet epoxy paint is of course...by tooling surface you are referring to the mold ,or form..?

 

epoxy can be UV stabilized, but the above is in the context of a mold, so UV is not a concern. tooling surface coat is epoxy version of gelcoat, surface layer of the mold

 

Ok thanks, I was assuming that UV stabilising would also reduce the thixotropic property of epoxy, but only of the surface.

 

The surface coat, whatever it is, provides a resin rich uniform surface for release agents.

Thixitropes, fillers, pigments, etc, add impurities to the resin which can cause a reduction in gloss and poor release.

Polyester gel coats don't typically have any ingredients to increase hardness and abrasion resistant. The base resin they're made from is chosen for those properties. When pigments fillers and other stuff is added the hardness normally goes down.

In a typical black tooling gel coat they use the pigment to control the viscosity and thix, this results in a much better mold surface than normal gel coat.

Epoxy surface coats are thick and pasty because epoxy doesn't respond to thixitropes in the same way polyester does, and due to health concerns it's rarely sprayed.


Epoxies tend to sag and flow for an extended period of time, so it complicates applying a uniform thin surface. Because of this they're made much thicker so they don't sag.

Imagine trying to spread Vaseline in a uniform layer on the entire surface of the mold you're making. On small parts it can be done, as the mold gets larger its far more difficult.

The thick and uneven, epoxy surface coating tends to shrink and crack after a while.

The reason for having a surface coat of any type is to achieve a high gloss, non porous, hard surface. Any porosity or imperfections create sites where the release will be poor, and a build up forms that turns the mold dull. The more build up there is the harder it is to pull parts.

Without a surface coat the fibers are right at the mold surface, these fibers cause sticking and can start pulling chunks out of either the part or the mold.

Infusion creates a situation where far more fibers are forced to be right at the surface, without a surface coat this results in a terrible mold finish.

 

Epoxy resins tend to start life out with high viscosities, and reducing the viscosity without affecting the physical properties is difficult.

Polyesters start out with relatively low viscosities and need to have the viscosity increased for normal use.

So polyester and VE infusion resins are typically much lower in viscosity and easier to use.

Reducing the viscosity of epoxy for easy infusion can reduce the physical properties significantly, even lower than some VEs.

 

Negative pressure is required for good evacuation of the fumes. Mounting the exhaust fans low and blowing out works well.

 

let's zoom in and try to understand the objectives of a mould and how PE system suits them from a rational point of view:

1) we want a good release
The fibres on the surface have a negative effect on release and hence need to be covered. But in this sense covered just so there is no physical contact with the release layer, not necessarily by 0.5mm thick resin.
The problem is that if PE is too thin, it will not cure properly, that's because there are two processes happening at the same time: polymerization and solvent evaporation. So the layer can't be too thin. It also cannot be too thin because it will wrinkle from the next layer solvent attack.
At the same time, the resin layer can't be too thick as with no reinforcement it will be the weakest part of the laminate. Also, the layer cannot be uneven as after shrinking it will develop internal stresses.

2) we want a good retention of the plug surface, high gloss and low porosity
This is said to be in properties of the resin. And it's all about the surface film, inside the backing laminate it can be porous and anything as long as it supports this surface film. Again, such surface film in PE cannot be thin and hence the compromise with additives that are designed to help with correct application and reinforcement without fibres (i.e. gelcoat).

3) we don't want backing laminate to affect the surface and the finish
Because of PE shrinkage, the thinner the resin coat the more likely the print. The compromise is to have a resin-rich layer next to the surface coat, preferably with CSM or better yet with special surface mat made with polyester fibres.

4) we want a feasible solution
if budget wasn't a factor, it would be easier to CNC machine the mould from an aluminum block and not worry about everything above. PE happens to be the cheapest material to make a mould and this seems to outweigh all related compromises.


But what is there was something that can be applied and cured in a thin layer? Thin enough that it can hold on verticals without thixotropic thickening. Cured in a way that would allow further build up, if one desires a thicker resin coat. What if it didn't shrink, so there were no issues with print-through and internal stress? What if it was solvent-free, so there is no need for a special facility with specific exhaust? What if it could be infused to save on labour? What if the total cost of tools, facility and materials was comparable or lesser to the PE system? You might have guessed where I am going with this.

 

Few thoughts on the mould function
Let's look at two extremes.
1) They build one-off boats or prototypes without moulds and arrive at a fair and glossy surface although, labour intensive
2) They invest in costly and labour intensive moulds but then have lighter and less costly final products.

If considering the total effort and resources, I believe there is room for multiple scenarios in between. What if a mould was used to get the shape and fair surface but not the final finish? What if a mould was designed to last half of the production run at a quarter of the cost? What if the production quantity is not a fixed figure? What if down the road one would wish to slightly change the shape?

As someone who is transitioning from one-off builds into the moulds, I want to learn from the best but apply only what is relevant to the task and hence the subject of the optimal way

 

The most popular one-off mold method these days;

EPS foam is rough machined with a large format mill. the foam is encapsulated with a thin layer of EPS compatible resin and lightweight woven fiberglass. Machineable fairing putty is extruded on the surface, such as Scott Bader/ATC T29. Then, the T29 putty is rough machined, then fine machined. It is then lightly sanded and sprayed with a surfacing system such a polyester or Vinylester primer system, which is then sanded and buffed to the desired gloss level. If small imperfections are likely in the final part, the P400 is usually the final gloss level.

Using low shrink tooling systems, I sand to P1200 or P1500 then buff.

popular cheap primer ; Duratec 707-061 EZ Sand Primer

popular common primer ; Duratec 707-002

my fav; 1902-045 VE Black Topcoat applied on top P220 sanded -061 ez sand.

 

Wow all that and then such a one-off mold would work only one time?

Personally my favorite one-off mold system is developable surfaces and melamine boards and modelling clay(?) to fillet corners.

Thanks ondarvr, great information to understand the complexities of mold making better!

 

for a 40 ft yacht one-off with exact geometry requirements?

The machining process takes maybe 3 days with 1 technician overseeing, 2 days for spray, sand up, buff, wax with a team.

 

KD8NPB, are you referring to a female disposable mold or to the plug to make the tooling from? The process you describe is familiar to me but only for plugs. When the plug is brought to the desired finish level, a mold is layed up as you described in a previous post (gelcoat, chopper gun, etc.) and then used for the final article.

 

The process can be used for a male plug or female mold. It’s typically only good for one pull though. Usually, the putty suffers some heat distortion and the foam partially fails once the part or mold is removed.

I’ve reused a machined plug before, it took 1.5 weeks to repair the damage from the first pull with 11 people.

 

Thank you, I did not know it works for disposable female molds too. I can imagine that sanding and buffing inside the mold is hard work, more so then on the plug.