In the early days plastics moulders tended to apply descriptive terms to moulding faults, such ‘gate splay’, ‘mica marks’, ‘orange peel’, ‘flash’ and ‘record grooves’. (Explain that to the younger generation). These terms do not really indicate the root cause. Nor do they suggest a possible solution, but diesel burning is an accurate description of what has happened.
When a rapidly approaching thermoplastic melt front chases air in the mould into a corner, the highly compressed air reacts with the hot melt, which is essentially a hydrocarbon fuel, to give a high temperature oxidation reaction. In other words this is combustion as in a diesel engine. If the combustion is inefficient the polymer will be converted to carbon. This gives a black spot at the extremity of the moulding.
To avoid diesel burning, or the associated fault, gas spotting, mould venting needs to be improved to release the compressed air. Reducing the clamp force can be a quick fix . I have even seen moulders adding a dummy ejector pin near the offending area. Prevention is always better than cure and injection moulding simulation during the part design can quickly identify possible problems and avert them by changing the gate position. Reducing the injection pressure can also help.