In vertical injection molding, improper control of injection conditions can indeed lead to product burn marks, which is one of the main causes of surface scorching and blemishes on molded parts. The injection speed plays a significant role in this issue. When the molten material is injected slowly into the cavity, the flow remains laminar, resulting in a smooth and shiny surface. However, when the injection speed increases beyond a certain point, the flow becomes turbulent, which can lead to burn marks and even cause air pockets inside the product.
Therefore, maintaining the injection speed within the laminar flow range is crucial for proper mold filling. If the molten material’s temperature is too high, it can decompose and scorch, causing surface defects. Typically, the screw speed of the injection molding machine should be kept below 90 r/min, and the back pressure should be less than 2 MPa to avoid excessive frictional heat in the barrel.
Additionally, during the molding process, if the screw’s rotation time during its retraction is too long, it can generate excessive frictional heat. To address this, adjustments can be made by slightly increasing the screw speed, extending the molding cycle, lowering the screw’s back pressure, increasing the temperature of the feeding zone, or using raw materials with lower lubrication properties.
Additionally, excessive backflow of molten material in the screw grooves or resin retention at the check ring can lead to polymer degradation. To prevent this, using resins with higher viscosity, reducing injection pressure, or switching to an injection molding machine with a higher length-to-diameter ratio is recommended. Common check rings often cause resin retention, leading to decomposition and discoloration, which results in brown or black burn marks after being injected into the cavity. Therefore, regular cleaning of the screw system and nozzle area is essential.
DAYIN Plastics Co., Ltd. specializes in home appliance mold manufacturing and offers OEM/ODM contract manufacturing services. As a comprehensive solution provider integrating research and manufacturing, we have accumulated 30 years of mold development experience since our founding in 1989. With over 50 R&D personnel and more than 100 injection molding machines, we are dedicated to delivering high-quality manufacturing services and creating greater value for our clients.
Burn Marks: Diagnosing the Scorch Before It Costs
Burn marks on vertical molding machines tell a gas story. The diagnosis ladder reads it:
| Burn signature | Gas source | Fix that addresses cause |
|---|---|---|
| Black streaks at flow ends | Trapped air, no vent path | Vent depth and placement review |
| Char specks through the part | Resin degradation upstream | Drying and residence-time audit |
| Silvering near thin sections | Moisture flashing to steam | Drying verification, not assumption |
| Scorch at weld fronts | Fronts meeting in blind pocket | Gate or vent relocation |
The residence-time row is the vertical-machine special: gravity-fed and taller barrel geometries create residence distributions that horizontal machines never see, and resin that sits past its degradation window chars quietly, delivering the evidence downstream one speck at a time. The venting row is where the diagnosis usually ends happily — most trapped-gas burning is a venting geometry problem from the tool’s original design, correctable in steel at modest cost and permanent in effect. The moisture row pays double, since dried resin also fixes the silvering that operators otherwise chase with barrel temperatures, heating their way toward degradation while the steam’s actual source sits in the dryer hopper. Burn marks are thus a readable language once the ladder is learned, and every mark diagnosed at the ladder’s first rung is money the scrap bin never sees. The short-shot cousin is in short-shot solutions, and the thermal context in mold temperature control. Reference: venting and defects and degradation chemistry.