Talk Composites - The Forum for Advanced Composites

moulding warping

http://www.talkcomposites.com/Topic21308.aspx

By FLD - 5/27/2016 11:52:10 AM

I went round to a friends house who's also a hobby moulder.  He's been having trouble with his mouldings warping.  They are CSM polyester and have moulded fine.  Over time (up to a year) they are warping really badly.  One example is a panel that has a 90 degree return / lip which has curled up beyond the 90 degree!  I've never had this and all I could come up with was demoulding too early.  Are there any other reasons he could be seeing this?
By Zebra - 6/13/2016 6:23:28 PM

I typically get warping or bowing on flat surfaces for one of two reasons:

The first is if I am using a flexible mold like one made from silicone and it doesn't have a hard shell to make it hold it's shape. The same applies if the hard shell is not properly adhered to the mold so it doesn't support it sufficiently. The weight of the resin is enough to make it sink in areas where the mold is thinnest. If you are using an unsupported flexible mold then try putting the original pattern back in and coat the outside with something hard like epoxy putty or gypsum. This same thing could also happen with a semi-rigid mold like one made from fiberglass that is too thin (espiecially under a vacuum).

The second reason is if the part is not sufficiently reinforced along the areas that are warping or bowing. You can use a layer of Kevlar or a stiffer carbon fiber on the rear side of the problem areas. I find that a 4x4 twill carbon fiber is usually enough for medium sized products (like 20" long). I would try and use a fast setting epoxy for this section and maybe use a little extra hardener. It will be more rigid and brittle but it won't matter in just that one section. 

For parts with particularly challenging angles where getting an exact shape is essential, I prefer to cure the part with a heavy rigid bladder on the rear side with weighs on top to hold it in shape and flat against the mold surface. This sometimes works better than a vacuum with problems like this. A vacuum can often be the cause of warping or bowing on it's own. If the vacuum is too strong the mold then....