How to Solve the Issue of Burn Marks on Injection-Molded Parts?
Injection Molds often experience burn marks during production due to various reasons.
The solutions to these burn marks differ based on the causes, which can stem from mechanical issues, mold-related factors, or molding conditions.
Causes and Solutions for Burn Marks on Injection Molded Parts
1. Mechanical Causes
For instance, over-heating of the barrel due to abnormal conditions can cause resin to decompose at high temperatures and burn before injection into the product. Or, resin stagnation can occur due to thread issues in the nozzle, screw, check valve, or other parts within the barrel, leading to discoloration and subsequent incorporation of burnt, dark brown traces into the product.
Clean the nozzle, screw, and barrel.
2. Mold Defects
This is primarily due to poor venting, and this type of burn generally occurs in specific areas, making it easy to distinguish from the first scenario.
Mold vent holes can become blocked by solidified residues of mold release agents and materials. The mold venting is insufficient or improperly positioned, and the injection speed is too fast, causing adiabatic compression of air inside the mold, resulting in the generation of high-temperature gases. All of these factors can lead to the decomposition and charring of the resin.
(1) Pay attention to the addition of vent grooves and venting pins, and consider the flow status of the melt and the venting performance of the mold.
(2) Remove blockages, reduce mold clamping force, and improve mold venting.
(3) Do not use excessive mold release agents, and ensure a high degree of gloss on the mold cavity surface.
3. Improper Molding Conditions
In terms of molding conditions, excessive back pressure above 300 MPa can lead to overheating of the barrel, resulting in burn marks.
Excessive screw rotation speeds can also cause overheating, typically within the range of 40 to 90 r/min.
When there is insufficient venting or small venting slots, high injection speeds can generate overheated gases causing burn marks.
(1) Generally, the screw speed of injection molding machines should be kept below 90 r/min, with back pressure below 2 MPa, to avoid excessive frictional heat in the barrel.
(2) Reduce the injection speed appropriately, ensuring that the material flows into the mold in a laminar manner.
(3) Periodically clean the screw system, particularly around the nozzle.
4. Inadequate Raw Materials
If the raw materials contain too much moisture or volatile components, have a high melt flow index, or excessive use of lubricants, it can lead to scorching and burn marks.
Raw materials should be treated with a material hopper dryer or other pre-drying methods to remove moisture. Consider switching to resins with a lower melt flow index and reducing the amount of lubricants used.
The above are the four main reasons for scorching in injection-molded products and their corresponding solutions. We hope this information can be of help in practical applications.