HDPE or PP as mould material for a one- off female mould?

Hi. Just wondering if anyone has tried using HDPE (High Density Poly Ethylene) as a material for creating a mould for creating a fibreglass product? Gelcoat shouldn´t stick to it as far as I know, and it could perhaps be more practical than using something like MDF in combination with Melamine? On the other hand, the Gel-coat shouldn´t release prematurely, that is before all the fibreglass layers are laid. That´s why I´m asking if anyone has some experience. Thanks.

 

The heat from the resin curing will deform the HDPE. If all you need is a crude mold, it should be OK.

 

Thanks Gonzo. Well, the heat when laying down layers of fibreglass would be insignificant as you start with the Gel coat and a thin first layer of glass when the gel coat has become tacky. Subsequent layers are laid after the first layer is cured. It would be different if one was to use PEHD as a plug material for creating a mold I guess, the new type of mold making polyseters do emit a lot of heat, which is good in other ways. But in this case I would just use regular polyester. But did you have actual first hand experience with using PEHD in plugs? If so, did you experience pre- release of the product during lay-up? That would be my main concern, as the PEHD material does not create a strong bond, which is what you want, but only so much that it doesn´t release the product before it fully laid up to its final thickness. BR.

 

How do you propose to get "soft" corners in the mould using HDPE , presumably sheets ?

 

Good point. I was thinking Gelcoat with microballoon fillers. It would adhere to the Gelcoat, de-mould with the product and could be sanded to a rounded finish on the final product , then polished.

 

Well, that might work, if you can fill the corners smoothly, but that sounds difficult.

 

Trust me gel coat with any significant micro balloons will NOT polish

 

yes, it remains a rough surface.

 

Polyester resin is exothermic. It is possible to laminate letting each layer cure. However, that would mean sanding in between laminations and only having secondary bonding which is weaker.

 

You would obviously need to sand between the first an subsequent layers of glass, and yes this would become a secondary bond which is weaker. But that is the way that it is usually done when laying up by hand. If you put all the layers on to the gelcoat when it is still tacky, then you get pre- release from the mold for sure. I expect the Gelgoat and first layer of glass would be a barrier to heat dissipation onto the PEHD when laying the remaining layers.

 

It can be used, as mentioned, heat causes it to soften and change shape, and it doesn't take much of a temperature increase for this to happen.

It is also affected somewhat by the resin itself, it will tend to warp if used a few times. Couple this with some heat and it doesn't hold tight tolerances very well.

After saying that, it is used in many different ways, but the cost can become prohibitive when you start scaling up in size.

MDF and melamine are cheap and readily available anywhere, plus are easy to work with and hold their shape better.

 

I understand what you say. But that is not what I meant. The Gelcoat and microballoons would only be there to create a radius so that the fibreglass can be applied and conform to the sharp inner corners, then get sanded away when the product is released.

 

Thanks for the input from you and the others. It is a good advice to use melamine and MDF.

 

Micro balloons would add unneeded delays and complications in building the part.

One thing that HDPE does well is create a flexible shape for a radius or edge.

You can shape a long length to the radius you desire and lay it in place even around curves.

It can be used in the same way for the perimeter walls of flat stock and other shapes.

You can make square or rounded corners as needed, clay comes into play too.

 

Hi I have made about 20 moulds for GRP including a twin skin sea sled dinghy, Cruiser aft cabin, cockpit mouldings and cockpit roof . I have used hardboard ply MDF and in the case of sea sled GRP e.g. made plugs GRP moulds and then the dinghy.
Depends on shapes you want personly beware of two thin sheets as will not give nice form between frames abd need more frames. Dont use MDF is moisture dependant. Easiest moulds I have made were 1/2 inch ply varnished with resin (its thick and can sand nice gloss) use two coats or more Wax mirror glaze or similar then twocoats of PVA which will improve finish considerably is water solvent so can run water between mold and moulding if a problem. PVA is the solution for easy release with wax. Dont remove mouldings to quickly.
You will need some acetone and a lot of stiff brushes and plastic jugs.
Have fun.