Engaged in the FRP processing industry, there is always a topic that can not be bypassed, that is, shrinkage deformation. What is the reason for this?
There are two mechanisms that lead to shrinkage: curing shrinkage and cooling shrinkage.
The first mechanism is curing shrinkage.
It usually occurs in two stages: the curing process (before demoulding) and the post-curing period (after demoulding).
1. Curing shrinkage refers to the volume change of resin during curing. This curing shrinkage is inevitable, preferably before demoulding from the original mold.
2. Part of the curing will also be carried out after demoulding, and these additional shrinkage will cause appearance defects on the surface of the mold. This is often referred to as "post-curing", but the real deformation is caused by additional shrinkage during post-curing.
Thus it can be seen that the selection of suitable low shrinkage resin is the most effective way to solve the above problems.
The rule of thumb for curing polyester at room temperature is that when the weight ratio of reinforced glass fiber is 25%, the shrinkage of each foot of the straight line is 1 32 inches (1.975px). Compared with the fiber reinforced resin, the rubber coat shrinks more, which leads to more concave bending on the rubber surface of the parts. For this reason, any large, flat area should be slightly convex to avoid depressions in the opposite direction. Small panels often have a bulge of 1 inch (15.875px) / linear foot.
Second mechanism, cooling shrinkage.
It is caused by the thermal expansion (actually shrinkage) of the layer when it is cooled at the stress-free temperature, which is related to the temperature of the layer when the resin hardens. The greater the difference between stress-free temperature and room temperature, the greater the cooling shrinkage.
Therefore, the cooling shrinkage can be controlled by reducing the exothermic temperature during the curing period as far as possible. By fully curing to minimize post-curing shrinkage, which may require