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How To Fix Burn Marks On Your Injection Molded Parts

injection molded burn marksThe 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.:

  1. 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.
  2. 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

  1. Melt temperature
  2. Plastic pressure
  3. Flow rate
  4. 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.

This Post Has 5 Comments

  1. This is a very useful information but one thing I dont understand , you are rulling out cooling rate as one of causes of burn marks but when a nould is overheated either by an intentional increase in mould temperature controller or by a faulty temperature controller, it is highly likely that due to overheating both mould halves will expand and this expansion will increase mould compression once locked. Therefore this increased compression will become an obsticle to escaping air which will result in burn marks.. My question is, would it not be considered as a direct linkage between coolin and the burn marks? I will appreciate your thoughts on this. thanks.

  2. Saeed,

    Mold temperature can certainly be a factor. But if you look at this problem from “the plastic’s point of view”, the cause of the mold temperature increase doesn’t matter (to the plastic). We encourage our students to look at root causes of problems and then work their way out to possible reasons. So yes, an increase in the clamp force due to a mold temperature increase or any other cause of a clamp force increase would restrict the escape of air and could cause a burn mark. But the quickest way to a solution would be to view this burn mark problem the way the plastic see’s it (“help, air is trapped and it can’t get out”) then look at things like high mold temp, which may lead you to faulty cooling. It’s scientific systematic molding. Thank you for your question

    Don Paulson

    1. We have online training for individuals. If your injection molding company is interested in an plant-wide, long-term solution, we also offer that

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