When the injection molding process runs well, it can all seem so straightforward. In reality, to ensure that every product comes out of the process a success takes a lot of skill and knowledge. When in the hands of inadequate expertise, a lot can go wrong. Many defects can occur in injection molding. Minimizing the occurrence of these defects is key to getting the best out of your machine. The plastic product market is becoming more competitive. Consumers today are more informed and have more options. They demand spotless products which are no short of aesthetic perfection. Defects in plastic products are not acceptable. Other than aesthetics, these defects can also affect product functionality. For example, a plastic material used as a gear needs to have perfect smoothness. These defects occur as a result of improper processing conditions and settings. Defects such as flow marks are undesirable in injection molding. This article gives you an insight into the precautions taken to prevent such issues and optimize the injection molding process.
What is a Flow Mark?
Flow marks occur as circles or lines that appear around the gate. These marks remain and are visible in the cooled product. They are evidence of nonuniformity in the flow pattern upon entry into the mold. This nonuniformity is usually caused by the temperature gradient within the melt. The slight difference in the color tone of these flow marks is due in part to light refraction. It is also a result of pigment redistribution within the plastic material.
In the ideal case, such defects occur during the trial runs. Once the engineer notices such a defect, the proper corrective measures are put in place. This can be prevented from the outset by a well-designed process and mold system. The ability to recognize flow marks early can save a lot of trouble and expenses down the line, avoiding costly rework and material waste. You can recognize flow marks by the following characteristics:
- Occurs near the gate: Flow marks are most noticeable around the gate area where the material enters the mold.
- Sometimes has a slightly different color tone from the rest of the part: This can be caused by light refraction or pigment redistribution.
- Occurs as circles, lines, or patterns: The shape and pattern depend on the flow characteristics and mold design.
How to Fix Flow Mark on Plastic Parts
Unfortunately, once a flow mark appears, there is no simple solution to fix it on the affected part. The part must usually be rejected and sent for recycling. However, the key to managing flow marks is early detection and prevention. Early detection of flow marks during the manufacturing process can prevent them from recurring in future cycles, saving both time and resources. Process engineers must ensure that adequate measures are in place for defect detection.
In an ideal scenario, flow marks can be detected before they occur. This is possible by understanding and adjusting the key parameters that lead to this defect. By detecting the issue before it manifests, engineers can save both time and material costs. Parameters such as temperatures, melt flow rate, and injection speed can serve as important indicators for preventing defects like flow marks.
What Leads to Flow Marks?
A combination of factors contributes to the formation of flow marks. While they are typically observed near the gate, the root causes can stem from multiple areas in the injection molding process, even from the machine settings before the gate. Common causes of flow marks include:
Image illustrating flow marks in injection molding
Melt temperature is too low
The melt temperature is crucial for controlling the viscosity of the plastic. When the melt temperature is too low, it can cause flow marks due to insufficient flowability. The main purpose of heating the plastic is to make it deform and flow smoothly into the mold. If the temperature is suboptimal, the melt may not flow uniformly, causing issues like flow marks.
It’s essential to monitor and maintain the correct melt temperature to ensure that the material flows efficiently into the mold. Temperature sensors should be installed at key points of the molding machine to monitor the melt’s temperature. Control systems and alarms can alert the operator if the temperature falls below the set parameters.
The temperature inside the mold is too low
While the mold is typically cooled during the process, if the mold temperature is too low, it can lead to premature solidification of the material, particularly around the gate. This premature cooling can prevent the molten plastic from filling the cavity completely and evenly, leading to the formation of flow marks.
Proper mold temperature control is necessary to ensure that the melt flows evenly and doesn’t solidify too quickly upon entry into the mold.
Melt injection speed is not high enough
The injection speed must be high enough to ensure the melt reaches all areas of the mold before losing its heat. If the melt is injected too slowly, parts of it may begin to solidify before filling the mold completely, creating flow marks. Speed optimization is necessary to ensure the melt maintains sufficient heat and flowability to reach the mold.
Melt injection pressure is too low
Insufficient injection pressure results in poor compaction of the melt, which can cause uneven flow. Higher pressure helps maintain a consistent flow pattern, preventing the formation of flow marks. It’s essential to optimize injection pressure for uniform filling of the mold.
Wrong size of runner and/or gate
If the runners or gates are too narrow, the melt flow can become restricted. This slows down the flow and causes the melt to cool prematurely, increasing the likelihood of flow marks. Properly sized gates and runners help ensure smooth and uniform flow throughout the molding process.
If any of these issues are present in the injection molding process, the chances of flow marks forming are much higher. The following section will look at methods to prevent flow marks from occurring.
How to Prevent Flow Mark Problems
Preventing flow marks requires a combination of optimizing mold design and adjusting the injection molding process. The following sections discuss strategies to remedy flow marks and prevent them from recurring in future cycles.
Cold Sludge Well
A common cause of flow marks is the presence of cold sludge, which can get caught in the melt. This cooler plastic mixes with the hot melt but only partially melts, causing a drop in the temperature of the surrounding melt. This temperature difference increases viscosity, causing resistance to flow and leading to flow marks.
To address this, a cold sludge well can be placed right before the gate. This well collects any cooled or frozen plastic, preventing it from entering the mold and disrupting the melt flow. The sludge drops into the well by gravity before the next shot enters the mold.
Use Gates and Runners that are wide enough
If the gate and runner diameters are too small, the melt flow will be restricted, and the material may cool too quickly. Increasing the diameter of gates and runners helps maintain the proper flow speed, preventing premature cooling and ensuring an even melt distribution. A wider gate ensures better flow and helps prevent the development of flow marks.
Reduce distance from the hot nozzle to mold
Longer distances between the nozzle and the mold can lead to premature cooling, as the melt loses heat during its journey. This can cause parts of the melt to solidify before reaching the mold. To minimize this issue, the melt path should be as short as possible. If this isn’t feasible, using a hot runner system is an effective solution to keep the melt hot until it reaches the mold.
Increase Back Pressure
Increasing the back pressure during the injection process improves melt uniformity by ensuring a more consistent flow. This helps to reduce the formation of flow marks by providing better compaction and reducing the chances of uneven flow patterns.
Increase Temperature
Raising the melt temperature can help improve flowability and reduce the formation of flow marks. However, care must be taken not to exceed the degradation temperature of the plastic. Excessive temperature can cause thermal degradation, leading to other defects. In addition to melt temperature, the mold temperature can also be increased to prevent premature cooling.
Increase Injection Speed
Increasing injection speed can help the melt reach the mold faster, reducing the chances of premature cooling and flow marks. However, excessive speed can lead to other issues like jetting. It’s important to balance speed to ensure the melt flows smoothly without causing other defects.
Fillet Edges and Corners
Sharp edges in the mold design can cause uneven flow patterns, which contribute to flow marks. By filleting or rounding sharp corners and edges, you can reduce the impact of changes in flow direction and prevent the generation of flow marks. This also helps with mold release, making the entire process smoother.
Gate Location
The gate location plays a crucial role in ensuring uniform filling. In the case of gears or similar components, the gate should be placed at the center for optimal filling. For other parts, place the gate where the wall thickness is thinnest. Thinner walls allow for better heat distribution, which helps to avoid flow marks.
Conclusion
Flow marks in injection molding are preventable but, once they appear on a product, the part is often deemed unacceptable. While they may not always affect functionality, they significantly impact the aesthetic appeal of the product. Maintaining optimal conditions, having a well-designed mold, and ensuring appropriate injection parameters can help prevent flow marks and other defects, ensuring high-quality production outcomes.