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The injection molded part defect known as a “burn marks” can be identified as a blackened edge, usually located at the last place in the cavities to fill. The cause of burn marks is trapped air in the cavities of the injection mold. During the injection phase, the air inside the mold cavities was compressed so high by the injected plastic that a diesel ignition occurred. The black color is the carbon residue from the burned front edge of the flowing plastic.
Sometimes burn marks can occur when the design of a plastic part, and the subsequent injection mold design, causes air to be trapped during the injection (or filling stage) of the molding cycle. The air compresses and is heated as a result. Once it reaches combustion temperature, it burns and the part is left with carbon residue. Those black marks are called burn marks.
Before the cavities in the mold are filled with plastic, they naturally have air in them. This air must have some means of escaping when the plastic starts flowing into the mold. When melted plastic fills a mold cavity at a high enough flow rate and at high enough pressure, either the air is forced out through vents design into the injection mold or, if it has nowhere to go, it is compressed. If the air is compressed high enough, auto ignition occurs. This is the same principle used by diesel engines – compression to the point of ignition. From the plastic’s point of view, the solution is quite simple – let the air escape. Molds typically have vents on their inside surfaces just for that purpose. But if the vents are too small or they get restricted by residue from previous molding cycles, the air escape can be restricted. So before you go to the mold maker and complain about the vents, try other solutions.:
- Use your fill rate control to slow down the fill rate during the last 5 or 10 percent of mold filling. On a machine without fill rate control, set the first stage injection time to end just before the mold is full. Then go to a lower second stage (holding pressure) for the final fill. Both of these methods will give the air more time to escape.
- You can also try reducing the clamp force. When the clamp force is set too high, it compresses the mold so much that vents are pinched closed.
From the plastic’s point of view, this is a simple problem to solve. There are only four basic processing variables in the injection molding process
- Melt temperature
- Plastic pressure
- Flow rate
- Cooling rate
For a burn marks problem, melt temperature and cooling rate are not involved. So the solution has to be plastic pressure and/or plastic flow rate.
Remember, always try to look at any injection molding part defect from the plastic’s point of view first. This makes the initial problem analysis much simpler than if you try to guess which injection molding machine controls to use. Find the primary cause of the molding problem, as the plastic “sees” it, then you’ll know which controls are involved. Result – faster problem solving. As an added bonus, once you build this troubleshooting technique into your standard injection molding part defect diagnosis, eventually fewer problems that need to be solved.