In the production process of medical injection-molded products, burn mark defects are one of the key issues affecting product qualification rates. These defects not only mar the appearance of the products but may also generate toxic substances due to material degradation, directly threatening medical safety. This article systematically analyzes the causes of burn marks on medical injection-molded products from four dimensions: material characteristics, process parameters, mold design, and equipment status.

I. Chain Reactions Caused by Insufficient Thermal Stability of Materials

Medical-grade plastics (such as PPSU, PEEK, PC, etc.) have extremely high requirements for thermal stability. When the barrel temperature exceeds the decomposition critical point of the material, the polymer chains break, producing low-molecular volatiles. For example, PPSU releases toxic substances like tetrafluorobenzoquinone above 320°C. These gases undergo adiabatic compression in the mold cavity, generating localized high temperatures that cause material carbonization and form burn marks.

Improper pre-treatment of materials is another important contributing factor. Undried raw materials (e.g., PA66 with a moisture content > 0.2%) generate micro-explosions during heating due to water evaporation, intensifying material shear heating. A medical catheter manufacturer once experienced periodic burn marks on products due to excessive moisture content in the raw materials, and thermogravimetric analysis confirmed the presence of hydrolysis degradation products.

II. Thermodynamic Runaway Due to Imbalanced Process Parameters

There is a strong correlation between injection speed and melt fracture phenomena. When the flow rate exceeds the critical shear rate of the material (e.g., 10^4 s⁻¹ for PC), transverse cracks appear on the melt surface, and friction heating at these cracks forms burn spots. A syringe manufacturer discovered through high-speed photography that obvious melt fracture occurred at the gate when the injection speed reached 180 mm/s.

Improper back pressure control poses dual hazards. Low back pressure results in uneven melt plasticization, with local temperature fluctuations exceeding ±5°C; high back pressure causes repeated shear of the melt in the screw groove, generating degradation heat. An artificial joint manufacturer found through infrared thermometry that when the back pressure increased from 1.5 MPa to 3.0 MPa, the melt temperature fluctuation range expanded from ±3°C to ±8°C.

The mismatch between screw speed and metering time is also crucial. A case study by an infusion set manufacturer showed that when the screw speed increased from 60 rpm to 90 rpm, the melt residence time in the barrel shortened by 30%, but the shear heat increased by 45%, resulting in scorched stripes on the product surface.

III. Venting System Failure Caused by Mold Design Defects

Improper venting groove design is a common problem. Due to the stringent airtightness requirements for medical products, the depth of venting grooves is typically controlled between 0.02 - 0.05 mm. A blood collection tube manufacturer experienced melt seepage forming flash due to a venting groove depth of 0.1 mm, and the flash carbonized under high pressure, producing burn marks.

The position of the gate and the layout of the runner directly affect the mold filling pattern. When the gate is located in the thick-walled area of the product, the melt front forms a jet flow, creating serpentine burn marks on the mold cavity surface. By optimizing the gate position through mold flow analysis, a surgical instrument handle manufacturer reduced the burn mark incidence rate from 12% to 2%.

Insufficient temperature control accuracy of the hot runner system can cause localized overheating. A case study in the production of insulin pen needle holders showed that when the hot runner temperature fluctuation exceeded ±3°C, the material degradation rate at the nozzle tripled, resulting in annular burn marks at the end of the products.

IV. Hidden Risks Arising from Equipment Aging

Excessive clearance between the screw and the barrel is a typical sign of equipment aging. A blood dialyzer manufacturer detected that after 5 years of use, the clearance between the screw and the barrel of an injection molding machine increased from 0.1 mm to 0.3 mm, increasing the melt residence time by 200% and raising the product black spot defect rate to 8%.

Delayed response of the hydraulic system can cause abnormal pressure increase during the holding phase. In the production of implantable stents, wear on the hydraulic valve led to a pressure fluctuation of ±15 MPa during holding, compressing the gas in the mold cavity to generate high temperatures and forming carbonized spots at the sharp corners of the products.

Decreased efficiency of the cooling system prolongs the high-temperature exposure time. Data from the production of infusion pump housings showed that cooling efficiency decreased by 40% due to scale buildup in the cooling water channels, increasing the product residence time in the mold cavity by 5 seconds and resulting in thermal degradation burn marks on the surface.