Flash of parts may occur due to various reasons, from changes in process or materials to tooling failures. Burrs will appear on the edge of the part along the parting line of the mold or anywhere where the metal forms the boundary of the part. For example, plastic electrical shell, pipe joint, plastic food container and other daily injection molding products.
Tools are often the culprit, so identifying the type of flash you are acquiring and when it occurs can point you in the right direction.
A common first reaction to reduce spillage is to slow down the injection rate. Reducing the injection speed can eliminate the burr by increasing the material viscosity, but it also increases the cycle time, and still cannot solve the initial cause of the burr. Worse still, flash may occur again during the packing / holding phase.
For thin-walled parts, even a short shot may generate enough pressure to blow the clamp open. However, if flash occurs in parts with similar wall thickness after the short shooting in the first stage, the most likely reason is that the parting lines in the tool do not match. Remove all plastic, dust or contaminants that may cause the mold to fail to close properly. Check the mold, especially check whether there are plastic chips behind the slip form and in the guide pin recess. After such finishing, if there is still flash, please use pressure-sensitive paper to check whether the parting line does not match, which can show whether the mold is clamped evenly along the parting line. Suitable pressure sensitive paper is rated at 1400 to 7000 psi or 7000 to 18000 psi.
In multi-cavity mould, flash is usually caused by improper balance of melt flow. This is why in the same injection process, the multi cavity mold may see flash in one cavity and dent in the other cavity.
Insufficient mold support can also lead to flash. The shaper should consider whether the machine is equipped with sufficient support columns for the cavity and core plate in the correct position.
The runner bushing is another possible source of flicker. The contact force of the nozzle ranges from 5 to 15 tons. If thermal expansion causes the bushing to “grow” to a sufficient distance from the parting line, the contact force of the nozzle may be sufficient to push the moving side of the mold in an attempt to open it. For non gate parts, the shaper should check the length of the gate bushing when it becomes hot.
Post time: Aug-30-2022