Burn Marks

Burn marks are normally black discolorations found on a workpiece surface. Dieseling on the object takes the form of a burn mark and poor surface finish. However, depending on the material, this discoloration can range in color from black to tan to white. There are two basic causes of burn marks. lists causes and solutions for burn marks.

Cause:

1. Contamination is one possible cause of burn marks. This can occur when the melt temperature of the

contaminant is lower than the plastic material being molded. The burned contaminant can take the form of burn marks

2. Excessive temperature is caused primarily by high melt temperature, and as discussed previously under brittleness, excessive residence time

3. A BSR(barrel-to-shot ratio) value below 30% can cause the potential of overheating a material, leading to gas evolution from the material

4. Material can hang up in areas such as check rings or in hot manifolds and become exposed to excessive heating

5. Heater bands can malfunction, causing a nearby zone to overheat and overcompensate by overheating

6. Too small a gate diameter leads to high shear conditions, which produce burn marks in the workple

7. The source of burn marks in the gate is burred or rough gate orifices resulting from rough machining or gate wear

8. Burn mark occurs when gas is trapped in a mold cavity, pically in the last section of the part to be filled. The pressure of the melt compresses this trapped air, and the combination of pressure and heat burns the material at the melt front, leading to the blemish

9. Gate locations can cause dieseling, especially if flowing around a hole or at a weld line

Solutions:

1. It is recommended that any contaminated material be eliminated from the material feed system

2. Reducing melt temperatures to the midrange of the melt

temperature recommended. Reducing the injection speed that can reduce the chance of overheating the material

3. Decreasing back pressure and screw speed will lower melt temperature

4. Check rings should be checked for hang-up areas and areas of wear, and hot runner manifolds should be designed to eliminate hang-up

5. Calibrating and checking heater band function need to be checked locations to avoid potential overheating and burn marks on objects

6. Gate diameter is critical to prevent part burn marks. Gate size should be designed according to recommendations from the material supplier

7. Gate areas should be polished and checked for wear, and repaired. One option is to use a gate in an insert and replace the worm gate with an insert instead of remachining a new gate

8. Reducing injection fill time will allow more time for gases to

escape into the vents without being trapped, assuming that the vents are all clear of clogged material. Cleaning and maintaining vent areas will also help in eliminating the dieseling effect

9. An insufficient number of vents can create dieseling since there are not enough passages for the trapped gas to escape. It is recommended to use as many vents as possible without causing flashing of the tool. Vents can be added into near a hole or where the two melt fronts converge toform the weld line

10. Porous metal inserts can be placed at weld lines and around holes to remove trapped gas