By Hank Tsai
What is a “burn mark”?
Herein burn mark refers to a defect of injection molded parts with the phenomenon of gray, brown, or black and tar-like local discoloration of the plastics that usually happens at the corners of end-filling position or deep ribs of thin-wall parts, as illustrated in Figure 1. As the term expresses itself, a burn mark looks like a residual mark after something is burnt for a few seconds at its end by a lighter and extinguished.
Why does “burn mark” happen?
But how comes the burn mark on an injection molded part since no one ever used a lighter to burn it? The culprit is the air in the cavity that is trapped after the mold closes. Before discussing further, let’s stray a bit from this subject to know more about a physical phenomenon of the ideal gas – the adiabatic process.
Figure 2 shows the pressure-volume-temperature relationship of the ideal gas from the thermostatic process point of view. On an isotherm (T1) where the condition of gas temperature is kept unchanged, the pressure of the gas in a space increases (P1 to P2) when the gas volume decreases (compressed, V1 to V2); oppositely, it drops (P2 to P1) when the gas volume increases (expansion, V2 to V1). With the gas volume kept unchanged (V1), the gas pressure increases (P1 to P2) when its temperature increases (heating, T1 to T2) and decreases (P2 to P1) when the temperature decreases
(cooling, T2 to T1)). With the gas pressure being the same (P2), its volume increases (expands, V2 to V1) when the gas temperature increases (T1 to T2) and decreases (contracts, V1 to V2) when the temperature decreases (T2 to T1).
In case that the volume of the gas is compressed dramatically and instantaneously by an external work, like the cylinder movement of a diesel engine, the process becomes thermodynamic. It approximates the so-called adiabatic process, meaning that the gas compression is so fast that the transformed energy to the system from the external work has no time to be transferred to the system’s surroundings. Instead, all the transformed energy is used only to heat the gas, making its temperature exponentially increase very high in the instant time. It’s how a diesel engine works to ignite the fuel by the compressed gas with the elevated temperature caused by the adiabatic process. Figure 3 illustrates how the thermodynamic adiabatic process works by mapping it on a thermostatic pressure-volume-temperature diagram of the ideal gas.
Now let’s return to the subject burn mark. Isn’t it that the trapped air in the cavity after the mold closes experiences a similar adiabatic process, as the gas does in the cylinder of a diesel engine, during the filling stage of the injection molding process? It is. The trapped air is compressed rapidly by the advancing melt, like the gas is compressed by the piston of a diesel engine, to the end-filling or deep ribs’ corners of the molded part (as shown in Figure 4) becoming a pinpoint-sized volume with much higher pressure than that before injection, and elevated temperature. That’s the reason why the burn mark defect is also described as the diesel effect. If the temperature of the compressed air increases to be high enough, it thermally degrades the molten plastic material next to it, resulting in various levels of discoloration therein at least or burn mark in the most serious situation, as the high-speed adiabatic compression process curve in Figure 5 shows. Even if the elevated temperature of the trapped air is not so high to cause a burn mark or any apparent discoloration, short shots are often found as well at those corners of the molded part because the increased pressure of the trapped air resists the melt and stop it from advancing further to complete the corners, as shown with the medium-speed adiabatic compression process curve in Figure 5. Simply put, it’s the air trapped in cavities that burns the polymers adjacent to it approximately at the end of the filling stage.
How to deal with “burn mark”?
Having known the root cause of the burn mark defect, it goes without saying that an adequate venting mechanism designed and tooled on the mold is essential for the trapped air to escape from the cavities. In such a way, the pressure of the air left in the cavities always remains the same as the atmospheric pressure of the room and the temperature between the room and mold temperatures. Sometimes reducing injection speed seems workable, but it can only be considered a temporary and expedient measure because it is at the expense of a broader process window.
About the Author: Hank Tsai is the owner and consultant of Effinno Technologies Co., Ltd. in Taiwan, an injection molding training and consulting service provider. He has been an SPE member since 1995 and has more than 25 years of experience in the injection molding industry. He has expertise in injection molding technologies and practices, production efficiency management, part cost structure analysis, troubleshooting, simulation, mold/process/machine performance evaluation, and process optimization by Taguchi DOE. He is also the co-developer of Light MES, a cloud-based software system and solution that aims to facilitate the digitalization of the industry’s machine utilization and production efficiency management. Contact: hank.tsai@effinno.com. https://www.linkedin.com/in/hank-tsai-effinno/
