I have a really hard time finding a D-Limolene supplier in Europe (D-Limolene is a solvent that dissolves HIPS plastic used as 3D printing material ) so my next best option is ABS . I want to print a mold core and after lamination to dissolve it , if i print in ABS i have to use acetone to dissolve it . I  must soak my core in acetone for an hour max , would this be enough to ruin my part ? I dipped a small part i made in acetone for couple of hours and i notice some small softening of the surface part but nothing major ,as far as i can see. Since i want my part to take loads i don't feel really comfortable using acetone . What is your view on this?

If PLA deforms at 55, you might consider a similar approach to what Rob described but at lower temperatures. Most epoxies will take up to 70 without much issue, so if you can get the thing into an oven at 60 or 70 for a few hours then you may have a solution. 

I dont think hot water would work, it is too localised. You need to heat through the whole core, otherwise you'll be pulling out tiny pieces and it will take hours.

I think we have a winner here its PLA plain , easy printed , cost efficient . I have to find a big enough oven since one of my parts doesn't fit my kitchen oven (when my wife is absent of course) . I was thinking about water since the solar heater here in Greece provides big amounts of super hot water around 75C.