Injection molding is a vital manufacturing process for producing plastic components, but defects can disrupt production quality. Burn marks, one of the most common issues, affect both aesthetics and product durability. In this post, you'll learn about the causes, effects, and practical solutions for preventing burn marks to ensure better production outcomes.
What Are Burn Marks in Injection Molding?
Burn marks are a common defect that can occur during the injection molding process. They appear as discolorations on the surface of the molded part, typically in shades of black, brown, or rust.
These unsightly marks are caused by trapped air inside the mold cavity during the injection phase. As the molten plastic is injected at high pressure, it compresses the air, causing it to heat up rapidly.
If the air reaches a high enough temperature, it can actually burn the plastic, leaving behind a visible mark. This phenomenon is sometimes referred to as "diesel effect" or "dieseling."
How Do Burn Marks Affect Injection Molded Products?
Burn marks are more than just cosmetic blemishes on injection molded parts. They can have far-reaching consequences for product quality, performance, and production efficiency.
Aesthetic Impact
One of the most obvious effects of burn marks is their impact on the appearance of the final product. These unsightly discolorations can make parts look defective or low-quality, even if they are functionally sound.
In industries where aesthetics are crucial, such as consumer goods or automotive, burn marks can lead to rejected parts and dissatisfied customers. This is especially true for visible components like housings, covers, and trim pieces.
Performance Impact
Beyond looks, burn marks can also compromise the physical and chemical properties of injection molded parts. The high temperatures that cause burning can degrade the polymer structure, leading to weakened areas or brittleness.
This damage may not be immediately apparent but can affect key characteristics like:
Tensile strength
Impact resistance
Heat tolerance
Chemical resistance
Over time, parts with burn marks may be more prone to failure or premature wear, especially under demanding conditions. This could lead to product recalls or liability issues down the line.
Production Impact
From a manufacturing perspective, burn marks are a major source of waste and inefficiency. Parts with visible defects often need to be scrapped or reworked, which drives up costs and slows down production.
Troubleshooting burn mark issues can also be time-consuming, requiring adjustments to mold design, process parameters, or material selection. In some cases, it may even necessitate tooling changes or new mold construction.
All of this adds up to higher overhead costs, longer lead times, and reduced profitability for injection molders. That's why preventing burn marks through proper design and process optimization is so critical.
Causes of Burn Marks
Machine and Molding Conditions
Burn marks in injection molding can often be traced to problems with machine settings.
High Injection Speed or Pressure
When the injection speed or pressure is too high, the molten plastic flows too quickly. This leads to uncontrolled fill rates and results in burn marks at the end of the filling process.
Gate Size and Nozzle Issues
Improper gate size or positioning affects the resin flow. If the gate size is too small, the resin can't flow smoothly, causing burn streaks. Similarly, a small nozzle size restricts the flow and results in black streaks on the molded parts.
Equipment Maintenance Problems
Equipment wear and tear can also cause issues. Dirty or damaged screws and barrels create black specks or color striations in the molded parts. Regular maintenance is crucial to avoid these defects.
Mold-Related Issues
The design and condition of the mold can lead to burn marks if not properly managed.
Poor Venting in Mold Design
If the mold doesn't allow trapped air to escape, air gets trapped at the end of the filling process, causing burn marks. Proper vent holes and their placement are essential to prevent this issue.
Improper Mold Construction
Insufficient mold venting or poorly positioned vents can also trap air, which results in burns. Sometimes, mold release agents block these vents, worsening the problem.
Molding Process-Related Factors
The molding process itself can create conditions that lead to burn marks.
Melt Fracture
High injection speeds and pressures can cause the melt to fracture, creating burn marks and surface defects.
Excessive Injection Speed
When the injection speed is too high, it creates turbulent flow instead of laminar flow, leading to burn marks.
High Melt Temperature
If the melt temperature is too high, it can cause material decomposition, leaving charred resin in the mold.
Prolonged Residence Time
When material stays in the barrel too long during extended cycles, it overheats, which can also lead to burn marks.
Material-Related Issues
The materials used in injection molding also contribute to the appearance of burn marks.
High Moisture Content in Materials
Materials with excessive moisture content create steam pockets, which cause burn marks as they can't escape during molding.
High Melt Index Materials
Materials with a high melt index have good fluidity, but this makes it harder to expel gases, leading to burn marks.
Volatile Substances in Additives
Additives, colorants, or volatile substances in plastics decompose under high heat, creating burn marks on the surface.
Troubleshooting Burn Marks
Machine and Molding Adjustments
To minimize burn marks, adjusting machine settings is a key step.
Adjusting Injection Speed and Pressure
Reducing the injection speed and pressure helps to prevent air trapping. This control is especially important during the last 5-10% of the mold filling process. Slowing down at this stage gives trapped air more time to escape, reducing the chance of burn marks.
Cleaning Machine Components
Regular maintenance is essential to avoid material retention in the machine. This means keeping the screw, barrel, and nozzle clean. Over time, worn screws or barrels can cause black specks and streaks. Replacing these parts regularly is critical for long-term maintenance. Also, manage backpressure and screw speed to avoid overheating during molding.
Mold Design Solutions
Improving the mold itself can also help prevent burn marks.
Improving Mold Venting
Venting issues are a major cause of burn marks. Adding or enlarging vent holes allows trapped air to escape more easily. Proper vent placement near the end of the flow path is key. It’s important to ensure air is released before it gets compressed.
Reducing Clamping Force
Excessive clamping force can compress the mold so much that it closes off vents. Reducing this force helps prevent vents from getting pinched shut, improving air escape.
Enhancing Mold Structure
Some molds may need to be reworked to improve their venting system. Poorly designed molds can trap air easily. Using glossy mold surfaces and appropriate release agents also helps reduce the chance of air pockets.
Material Handling Solutions
Material-related adjustments can also reduce the likelihood of burn marks.
Proper Resin Drying
Before the injection process begins, resins should be adequately dried. Moisture in the resin can lead to steam pockets, which create burn marks. Resins with a lower melt index are generally easier to process and less prone to trapping gases.
Lowering Resin Temperature
If the resin temperature is too high, it can char and leave burn marks. Adjust the heater bands to maintain optimal temperatures. Reducing the melt temperature also prevents excessive heat buildup and helps avoid charring.
Advanced Troubleshooting Techniques for Persistent Burn Marks
Injection Process Optimization
To address persistent burn marks, optimizing the injection process is essential.
Multi-stage Injection Techniques
Multi-stage injection involves adjusting the injection speed at different phases. In particular, slowing down the injection rate in the final stages helps control air trapping. This method reduces the chance of burn marks by ensuring the air has time to escape.
Using Flow Rate Control
Fine-tuning the flow rate during the last phase of injection can make a big difference. By controlling the flow rate, you can better manage the pressure buildup, minimizing the risk of burn marks.
Real-Time Monitoring of Molding Conditions
Using real-time monitoring systems allows for faster adjustments when problems arise.
Implementing Sensors
Sensors can track mold pressure, temperature, and venting conditions in real time. With these measurements, it’s possible to pinpoint where air is being trapped or where overheating occurs. This data can be used to make adjustments before defects appear.
Benefits of Data-Driven Adjustments
With real-time data, manufacturers can quickly adjust mold settings based on actual performance. This helps optimize conditions during production, lowering the risk of burn marks and improving overall product quality.
Use of Simulation Tools
Before full-scale production begins, simulation tools can help predict potential issues.
Flow Simulations
Flow simulations allow engineers to visualize how molten plastic moves through the mold. By simulating the process, they can identify areas where air might get trapped, allowing them to adjust the mold design before production.
Computer-Aided Design (CAD)
Using CAD software helps manufacturers spot potential air pockets in the design. By identifying these problem areas early, teams can modify the mold to improve venting and reduce the likelihood of burn marks.
Improving Resin Selection
Choosing the right resin material is also crucial for preventing burn marks.
Choosing Low-Volatility Materials
Low-volatility materials are less likely to decompose at high temperatures, making them ideal for high-temperature processes. These resins produce fewer gases, reducing the chance of air pockets that lead to burn marks.
Avoiding Burn-Prone Resins
Certain resin types are more prone to burn marks based on their mold geometry. Avoiding these materials or selecting ones with a lower melt index helps improve processing and lowers the risk of defects.
Case Study: Solving Burn Marks in a PBT Connector Product
To illustrate how these troubleshooting techniques can be applied in practice, let's look at a real-world case study involving burn marks on a PBT connector product.
Product Description and Quality Requirements
The product in question was a thick-walled connector made from PBT material. It had a secondary appearance surface that needed to meet high aesthetic standards.
The customer had strict requirements for surface quality, so any visible defects like burn marks were unacceptable. The molding conditions were as follows:
Mold temperature: 60-80°C
Material temperature: 240-260°C
Injection speed (3 stages): 180mm/s, 135mm/s, 40mm/s
Injection time: 1.5-3s
Injection pressure: 1300-1500kgf/cm²
Initial Analysis and Identification of the Root Cause
During the initial mold trial, a flow analysis was conducted on the product. It revealed that burn marks were appearing at the end of the flow path, where melt fronts were converging.
The root cause was identified as trapped air that couldn't evacuate from the marked areas of the mold cavity. This trapped air was getting compressed and heated, causing localized burning of the PBT material.
Implemented Solutions
Since the product and mold designs were already approved by the customer, the only options were to improve mold venting and adjust the injection molding process.
Expanding Vent Slots
The first step was to add and enlarge vent slots wherever possible along the melt flow path, at the parting surface, and around ejector pins. This helped trapped air escape more easily and reduced the severity of burn marks.
However, venting alone wasn't enough to completely eliminate the problem, so further process adjustments were needed.
Adjusting Injection Molding Process
The thick walls of the connector meant that the melt was spending a long time in the mold cavity, increasing the risk of burning. To address this, the injection molding process was switched to a multi-stage approach:
Initial injection stage at high speed to quickly fill the cavity
Reduced speed in the middle stage to allow air to evacuate
Very low speed in the final stage to pack out the part without trapping air
By carefully controlling the injection speed profile and giving the air more time to escape, the severity of burn marks was significantly reduced.
Results and Lessons Learned
Through a combination of improved mold venting and optimized process parameters, the burn marks on the PBT connector were eliminated, and the product met the customer's strict quality requirements.
Conclusion
Burn marks in injection molding can be a frustrating and costly problem, but by understanding their causes and implementing the right solutions, you can minimize their impact on your products and bottom line.
Addressing burn marks early in the design phase is crucial. Work closely with your mold designers and material suppliers to optimize venting, gating, and resin selection.
Proper mold maintenance, machine upkeep, and material handling are also essential for preventing burn marks. Regular cleaning, drying, and parameter adjustments can go a long way.
For the best results, collaborate with experienced injection molders who can help you troubleshoot issues and implement long-term preventive measures. With the right expertise and proactive approach, you can achieve consistent, high-quality production free of unsightly burn marks.
