Injection molding has the potential to achieve uniform identical parts. Thousands to million parts can get made using injection molding. With the high capital investment in machinery and tooling, efficiency is a watch ward. Inefficiencies in any aspect of injection molding lead to defects. These defects lead to losses in time and finance. Countless things can go wrong in injection molding. And one of such is burn marks appearing on the product. Because the burn mark is indicative of the presence of soot or degraded material. Recycling is not an option. Or at least it is not a good idea to recycle the whole part as this means introducing contamination. This means not only have you lost the time put into creating the now discarded part. You have also lost some plastic material. The quality of your mold also depreciates over time. The mold gets graded by the number of cycles it has left in its lifetime. You have molds that can run up to a million cycles and those that can only run a few thousand. So the more parts you are having to reject due to defects also mean you are wasting mold lifetime. The quicker you detect them too the better. Early detection saves a lot of downtimes and potential loss. What’s better than early detection is prevention. Preventing these defects from occurring in the first place should be a watchword. Here we discuss burn marks, we look at the different causes and how to solve them. The recommendation in this article is general guidance. To get a professional service that is specific to your need contact us today. You can send an email or call. If you are in the area feel free to drop by at our offices to talk to our specialists.
What is Burn Marks and Why You Must Avoid them in Plastic Injection Molding
Burn marks affect both the mold and the product. You can identify it as a visible formation of soot on the product surface. It is also called gas traps or dieseling. It is a common problem in injection molding. Once it occurs, the part has to get rejected. Because burn marks also mean a degradation occurred this can also affect function. So the problem is not only the visible burn mark. It is also that the burn mark is indicative of weakness on the part. The burnt part of the plastic will not have the physical property of the intact plastic. This means the loss of its properties like elasticity and impact strength. The chemical properties of the plastic relate to the physical property. When burned these plastics have their chemical structure altered. Hence their physical properties also get altered
Detecting Burn Marks
Burn marks are, as the name implies, are burns appearing on the product surface. So you are pretty much looking for signs of burns on the product. From visual inspection, they are black-brown soot formation on the product surface. Might get accompanied by physical deformation or roughness in the affected part. You can also have burn marks where the part surface remains smooth by texture. But the burn mark will be visible on the surface as soot formation. This is where either the burn occurs before the product forms in the mold or as it does so. The image below shows some examples of burn marks from across the web.
Upon visual inspection, you can do some diagnosis on the spot. Some possible causes of burn marks are obvious based on the appearance of the burn mark. The table below summarizes some of them.
| Type of Burn mark | Most possible cause |
| Black streaks on the molded part | Gate or nozzle not wide enough |
| Dark specs on product accompanied with striation of color | The screw and Barrel have worn out and must get replaced. |
| Burn marks appearing on mold or the product after mold filling. | Poor venting in the mold. |
| Burning within resin before mold filling | Melt temperature too high. |
Causes
Any defect in the product is a result of inefficiencies. This can be in either the process, machinery, or the material used. Similarly, burn marks can be a result of any or a combination of these. Particular inefficiencies can lead to more than one kind of defect on a product. For example, you can have a warped part as well as weld lines on the same product. H Because burn marks also means a degradation occurred this can also affect function. So the problem is not only the visible burn mark. It is also that the burn mark is indicative of weakness on the part. The burnt part of the plastic will not have the physical property of the intact plastic. This means the loss of its properties like elasticity and impact strength. The chemical properties of the plastic relate to the physical property. When burned these plastics have their chemical structure altered. Hence their physical properties also get altered. Here we look at some of the factors that tend to lead to or contribute to burn marks.
The causes of burn marks can get grouped into the machine, material, or mold-related causes.
Gas trapped within the melt
This is the number one cause of burn marks in injection molding. It also leads to other problems. These lead to burn marks or other product defects. When gas gets into the melt, this gas is then pressurized. A high-pressure, the temperature to cause ignition gets lowered. So the gas can ignite faster. With the increased temperature the gas can include volatiles. These get released from the partly degraded plastic. These are more flammable than air. Hot spots develop causing degradation but burns are not only due to degradation. The actual burning is what leads to the burn marks and degradation is also a result of that.
Uneven Temperature Distribution
This can be a result of different factors. It could be that the screw is not doing a great job of mixing and melting. This attributes to low injection torque or rpm. It could also be that the melt is too viscous thus making mixing less effective. The uneven temperature distribution can be in the mold. This is a result of poor flow pattern as the mold fills.
Melt Temperature is Too High
If the melt temperature is too high this not only leads to degradation. It also sets the right condition for other things to go wrong. The plastic can get degraded and other problems like trapped gas get compounded. Excessive melt temperature can also cause the release of volatile from the plastic. This then led to burn marks. The problem of trapped air leading to burning marks as explained in the above section.
Pretreatment of pellets
Proper drying is very important. More so when working with plastics that tend to absorb moisture. Ensure pellets get dried to very low or negligible moisture content. This should be less than 0.2% moisture. The dried plastic should get sent to the hopper immediately or sealed tight. This prevents it from reabsorbing moisture.
Mold Filling Velocity
If the filling velocity of the melt into the mold is too high this can cause some problems. High velocity introduces turbulence into the system. This increased turbulence gets gas to mix in with the melt. This is more particular for melts with high viscosity. The higher the viscosity of the melt, the more difficult it is for the trapped air to escape. Gas remains in the stream and ends up causing burn marks.
Contamination
This can be contamination of the feed. Or can be contaminants getting into the system from the environment. The contaminants most often have a lower degradation temperature than plastic. They will thus burn during processing and this soot from their burning will get on the plastic. The contaminants could also be volatiles which forms gases that get trapped. All the same, the result is a burn mark on the product.
Back Pressure
This is the pressure exerted on the melt in the injection chamber. This pressure gets retained as the screw accumulates more melt in the chamber. This in itself might not be a direct cause of burn marks. Backpressure is necessary for the formation of even compact products. This I discussed in previous blog posts on www.cavitymold.com. But it is a key contributor in different ways. One way is that if backpressure is too high. From thermodynamic principles, pressure increases, and the temperature of the fluid increases. So when the back pressure is high the melt gets hotter than the set point. This leads to overheating as discussed in the above section. Not only the melt but any trapped gas will also get overheated. Both cases can lead to burn marks.
Additives
Many products make use of additives to tune their properties. They also get used to aid the injection molding process. Examples are plasticizers, antioxidants, and flame retardants. Mold release agents also get used to aid the release of the product from the mold. It is important to use good quality additives. When choosing additives also give proper consideration to compatibility. That is compatibility with the plastic, the inserts, mold, and machine. Where the additive is not compatible this leads to different problems and burn marks are one. If for example, the additive degrades at the melt temperature of the plastic. This can lead to burn marks. Also if the additive when heated releases volatiles, these can lead to burning marks too.
Machine Build Up
Over time oils, moisture, and dust particles can build up in the machine. This in particular where there is not a stringent maintenance schedule. These buildups find their way into the melt and cause problems. Some of these problems include trapped gas, contamination, and degradation. All of which lead to the formation of burn marks on the product.
Solutions
- As we discuss the causes some of the solutions have gotten pointed out within the section. Below gives a list of solutions to consider when you detect burn marks on your product.
- Trace gas buildup. Find where the gas gets into the melt to know how to address the problem.
- Improve venting in the barrel, gates, and mold.
- Lower melt temperature
- Check screw speed is at the best setting for proper mixing and melting.
- Check the compatibility of all additives used with process, mold, and plastic.
- Add a well-designed decompression zone
- The screw design must match the plastic-type. For example, screw suited for temperature-sensitive plastics. This has a narrower feed zone and a shorter compression and metering zone.
- Use high-quality additives and check compatibility with product and process. As well as the compatibility with the mold.
- Avoid excessive back pressure
- Keep the mold filling rate moderate. Carry out analysis and/or simulation to ensure flow remains in the low turbulence range.
- Where possible used sealed systems to prevent contamination
- Do regular cleaning of the production environment to prevent dust and contaminant accumulation.
- Measure and control humidity in the factory
- Check all temperature sensors and heating barrels are functioning right. Also that the readers are not giving false readings.
- Keep a stringent maintenance schedule. The planned time for maintenance is better than unplanned downtime. Time for diagnosing and repair is often much longer than maintenance time.
Conclusion
Burn marks cause visible part defects and result in part rejection. The causes vary from direct burns to trapped gas within the melt. Indirect causes occurring as a result of backpressure or improper mold filling. There are many approaches to solving the problem of burn marks. This can be by addressing the machine, material, or mold-related inefficiencies.
Contact www.Cavitymold.com today for high-quality mold making service. We design molds that avoid burn mark problems from your products.
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