When the injection molding process runs well, it can all seem so straight forward. 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 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.
What is a Flow Mark?
Flow marks occur as circles or lines that occur around the gate. These marks remain and are visible in the cooled product. They are evidence of nonuniformity in flow pattern upon entry into the mold. This nonuniformity in the flow is a result of 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. In the ideal case, such defects occur during the trial runs. Once the engineer notices such a defect, the right measures get put in place to address it. This of course gets prevented in the first place by a well-designed process and mold system. The ability to recognize the flow mark early can save a lot of trouble and expenses down the line. You can recognize flow marks by the following characteristics:
- Occurs near the gate
- Sometimes has a little different color tone from the rest of the part
- Occurs as circles or lines or patterns
How to Fix Flow Mark on Plastic Parts
Unfortunately, there is nothing that you can do after the problem occurs. The part must get rejected and sent to recycle. It is important to ensure that measures are in place to detect such problems. The best thing is to detect the flow mark early. Chances are this is not only a one-off occurrence and will happen in the next cycle. So the earlier it gets detected the less the damage done. So process engineers must ensure that measures are in place for defect detection. In the ideal case, the defect gets detected before it occurs. This is possible by knowing the parameters which lead to this defect occurring. By detecting the error before it occurs you can save time and resources. Parameters such as temperatures and melt flow rate can be indicators for defects.
What Leads to Flow Marks
There is a combination of factors that lead to flow marks. Although it occurs at the gate. The cause can be a result of the machine settings originating from before the gate. Causes of flow mark can be any or all the following:
Image illustrating flow marks in injection molding
Melt temperature is too low
The melt temperature is a key factor in controlling the melt viscosity. The main purpose of heating the plastic is to allow it to deform and flow. When you notice flow marks, chances are you are running at sub-optimal temperature. Heat requires some sort of fuel input and this is running cost. So the operator needs to run the process at an optimal temperature. But caution is necessary to run well below the degradation temperature. Have temperature sensors to track temperatures at different points of the injection molding. This ensures the melt gets to the mold at the right temperature to prevent a flow mark. Control systems and alarms can get installed. These give warnings where the temperatures fall below the set values. This way the problem of flow mark gets alerted before it happens.
The temperature inside the mold is too low
The mold is not heated as this is where cooling occurs. Some molds get precooled. But if the temperature in the mold is too low it causes problems. When the melt gets into the mold and upon entry premature cooling may occur. If parts of the mold cols too early around the gate, this leads to flow marks. The hotter fluid flows over the cooled melt. This leads to the formation of flow marks.
Melt injection speed is not high enough
The speed at which the melt gets injected should sustain it till it gets to the mold. The melt should also get to the mold fast enough before it loses all it’s stored heat. If the temperature is too low, by the time the melt reaches the mold part of it has begun to solidify. This occurs as it gets forced through the gate. This difference in flow pattern registers as a flow mark.
Melt injection pressure is too low
The melting speed relates to the pressure. If the pressure is not high enough then it does not have enough compaction. Compaction contributes to melting uniformity. With enough injection pressure-flow patterns get evened out.
Wrong size of runner and/or gate
If the runner and gates are too narrow this restricts flow. Restriction of flow slows down the melt. It also exposes more areas of the melt to temperature loss. The shorter flow diameter means the center of the melt is closer to the walls of the runner. This is particular in cold runner systems.
If you have any of the above situations in your injection molding machine. The chances of flow marks are higher. The following section looks at how to prevent flow marks from occurring.
How to Prevent Flow Mark Problems
There are two approaches to prevent flow marks from occurring. Or occurring in the next cycles. One approach is to change the mold design. The other approaches involve modifying the injection molding conditions. The following sections discuss the different measures taken to remedy the flow mark.
Cold Sludge Well
The flow irregularity that leads to the flow mark is sometimes caused by a cold sludge. This gets caught in the melt. This mixes with the hot meld and only about melts enough to get through the gate. The thermal energy of this cooler plastic results in a lowering of the temperature of the melt around it. The lower temperature of this part of melt gives it higher viscosity. This increased resistance to flow slows down the melt. Incoming hotter melt flows past it to fill the other parts of the mold. This uneven temperature and viscosity results in the flow mark seen in the formed part. So a cold sludge well can solve this problem. The cold sludge well gets placed right before the gate. This collects any cooled or frozen plastic and prevents it from getting into the mold. The sludge drops in the well by gravity before the next shot gets into the mold.
Use Gates and Runners that are wide enough
If the gate and runner diameter are too small, this restricts melt flow. Also the smaller the channel the faster it cools. This premature cooling prevents the optimal volume of fluid from getting in the mold. Having more fluid entering the fluid helps build up pressure hence turbulence. This helps prevent flow marks from developing. So to address flow marks. Widening of the gate is an option.
Reduce distance from the hot nozzle to mold
The longer the melt has to travel, the more chances of premature cooling. You don’t want the melt cooling before entering the mold. Where the distance cannot be any shorter, another option is to use a hot runner. This way the melt remains hot until it gets into the mold.
Increase Back Pressure
Increasing the back pressure can help improve melt uniformity within the injection chamber. This pressure then melt gets held at is too transferred as it gets injected. This back pressure gives the fluid added pressure as it travels through the runners. The backpressure also ensures good compaction of the melt. This contributes to minimizing the chances of uneven flow pattern build-up.
Always be careful before going for the option of increasing the temperature of the melt. This is achieved by changing the temperature settings on the barrel. Temperature relates to many of the parameters in injection molding. In particular pressure and viscosity. When you increase the temperature the melt flows better. But be mindful that you don’t want to get too close to the degradation temperature of the plastic. The increasing temperature where there are problems in flow uniformity can have detrimental impacts. If there is poor mixing, this can lead to hot spots and parts overheating. Such overheating leads to degradation and this causes other defects. You don’t want to solve one problem and then create another in the process. Other than the barrel temperature the temperature of the mold can also get increased. Usually, a pre-cooled mold gets used to help with the cooling of the part. But if this causes premature cooling then increase the mold temperature.
Increase injection speed
The speed of injection gets related to the pressure of injection. So if you increase the pressure applied to the screw as the melt, speed is also increased as a result. Increased injection speed helps get the melt into the mold before cooling occurs. So where you can’t have a hot runner, you want the melt to get in the mold as fast as possible. But at the same time, excessive speed can cause other problems like jetting. So keep injection speed within reasonable limits.
Fillet Edges and Corners
Sharp edges in the mold design can contribute to non-uniform flow patterns. These contribute to flow marks developing. To address this fillet or round these corners. This reduces the impact of the change of flow direction. It also prevents the generation of uneven flow patterns. This also prevents another problem in mold release.
The location of the gate is an important factor. Although for some products there are set rules for where to locate the gate. For example in the injection molding of gears. Here the gate gets located at the center of the gear for optimal filling. But where there are no clear rules then locate the gate at the side of the mold where the wall is thinnest. The thinner walls mean a lower temperature drop. This prevents the disruption of flow patterns in the mold. As the melt goes from the thinnest to the thicker walls, the temperature difference evens.
The flow mark in injection molding is a preventable problem. But once it occurs on a product, such a product is most often not acceptable. While it may not affect the functionality of the product, it has a significant impact on appeal. Maintaining the right conditions and having a mold, gate, and runner system fit to purpose. This helps prevent the occurrence of flow marks.