Solving the problem of cracking during injection molding (一)
In injection molding, the cracking of plastic products is a headache.
Because the material is melted through the screw and the hot melt is injected into the cold mold, thermal expansion and contraction will occur, and the volume, molecular orientation, internal stress, and residual stress will change, and whether the product cracks or not is directly related to the mold design (structure), the selection of materials, the stability of the machine, and the process conditions.
Cracking is usually caused mainly by residual stresses and external stresses, and occurs when the stress exceeds the toughness coefficient of the material itself.
In order to solve the problem of product cracking, the generation of stress should be reduced, so that the material reaches the limit of ductile deformation, and cracking will not occur.
一、 Cracking of mold inserts
It mainly occurs when the insert is just started up, because the insert is not flowing water, there is a difference between the temperature of the insert and the temperature of the mold core, and the temperature can only be driven by the material, resulting in the temperature cooling rate of the insert and the mold core is inconsistent, and cracking will occur when the product is taken out.
Therefore, when the machine is started, the mold should be connected to the mold temperature controller for preheating, or the water flow should be turned off to speed up the production beat, which has good results in solving this problem.
二、Installation of metal inserts cracking
Especially for rigid plastics, cracking of metal inserts is a common problem because of the difference in the coefficient of thermal expansion between metal inserts and plastics, and the temperature of the insert is much lower than the temperature of the material.
Inserts can only transfer temperature through the material, but they cool down very quickly, and when the cooling rate exceeds the limits of the plastic, cracking can occur when the product is removed.
Therefore, when the machine is turned on, the metal insert should be preheated to adjust the difference in the thermal expansion coefficient between the insert and the plastic.
三、 Hole cracking
There are dissolution marks in the product with holes, which is inevitable in injection molding, and the position of the dissolution line is the place with the lowest strength, especially for the products that need to nail the screw holes, special attention should be paid to the products, which will crack when the force is large.
In the process, the hole should be filled full, and the reused material should not be used as much as possible, or the proportion of reused material should be reduced, so as not to destroy the strength of the material.
Categories
injection molding
breaking
Featured Article
For more details, please visit our other website： Injection Molding