What is the cause of the brittle product produced?
Problem Description: The product produced is brittle, what is the reason?
Cause analysis: The existence of internal stress in
products will seriously affect the mechanical properties and performance of the product, due to the existence and uneven distribution of internal stress of the product, the product will crack during use.
In the injection molding products, the local stress state is different, and the degree of deformation of the product will be determined by the stress distribution.
There is a temperature gradient when the product is cooled, and the uneven contraction results in stress.
When the melt enters the mold with a lower temperature, the melt near the wall of the mold cavity cools rapidly and solidifies, so the molecular chain segment is "frozen". Due to the solidified polymer layer, the thermal conductivity is very poor, resulting in a large temperature gradient in the direction of the thickness of the product.
The core of the product solidifies very slowly, so that when the gate is closed, the melt unit in the center of the product has not solidified, and the injection molding machine can not replenish the cooling shrinkage.
So the internal shrinkage of the product and the direction of the hard cortex is opposite; The core is in static stretching while the surface is in static compression.
If the mold cavity pressure is maintained before the gate is closed, it is beneficial to increase the product density and eliminate the cooling temperature stress, but there is a large stress concentration near the gate.
It appears that when the thermoplastic polymer is forming, the greater the pressure in the mold, the longer the pressure holding time, which is conducive to the reduction of the shrinkage stress generated by the temperature, and the opposite will increase the compression stress.
Improvement measures:
Increase mold temperature, reduce injection, pressure holding pressure, pressure holding time. The "hot water bath" can be carried out after the mold in the product to eliminate the residual stress.