Defect analyses of welding marks
When molten plastic converges with different streams due to their coming across the areas where there are inserts, holes, discontinuous flow rates or interrupted filling materials, the welding marks like threads appear because of the incomplete fusion of molten plastic. The presence of the welding mark greatly reduces the mechanical strength of the plastic product. The method of overcoming the welding mark is basically the same as the method of reducing the concave of the plastic product.
1. The equipment
We can extend the molding cycle so as to make the molten plastic plastify completely if the molten plastic doesn't plastify well, or the molten temperature is nonuniform. If necessary, we can change machines with large plasticizing capacity.
2. The mold
(1) We should appropriately increase the mold temperature or purposefully improve the partial temperature of the welding part when the mold temperature is too low, which will cause the welding mark.
(2) We should increase the size of the runner, improve the efficiency of the runner as well as increase the volume of the slag well when the runner is too small, too narrow or too shallow, which will make the welding mark happen.
(3) Expand or narrow the pouring gate's section and change the position of the pouring gate when the welding mark happens. The molten plastic flowing over the insert and the surroundings of the hole should be avoided when the pouring gate is set up. Multi-pouring gates should not be used or be used less.
(4) Gas vents should be set up, expanded or unchoked, and inserts can be used to discharge gases when poor discharges happen or there are no gas vents.
3. The technology
(1) Increase the injection pressure and extend the injection time when the welding mark occurs.
(2) Adjust the injection speed when the welding mark happens. The high speed can make the molten plastic get to the meeting part without lowering the temperature; the low speed can let the gas in the cavity discharged within the time.
(3) Adjust the temperature of the machine barrel and the nozzle when the welding mark happens. High temperature can make plastic viscosity small, resulting in smooth flow and fine welding marks; low temperature can reduce decomposition of gaseous substances.
(4) The release agent should be used as less as possible, especially for the silicon containing release agent, when the welding mark occurs. Otherwise, the release agent will make the molten plastic not fuse.
(5) Reduce the mold clamping force to facilitate the discharge when the welding mark occurs.
(6) Improve the speed of the screw so as to decrease the viscosity of the molten plastic or increase the back pressure in order to improve the density of the molten plastic when the welding mark happens.
4. The raw material
(1) Raw materials should be dried, and liquid additives should be reduced in the formulations as much as possible when the welding mark happens.
(2) Appropriately add lubricants and stabilizers for plastics with poor mobility and high heat sensitivity when the welding mark appears; if necessary, use the plastic with good mobility and high heat sensitivity.
5. The product design
(1) Plastic products should be thickened to avoid too early solidification when wall thicknesses of plastic products are small, which will cause welding marks.
(2) Improper inserts' positions should be adjusted when the welding mark appears.