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It takes a lot of knowledge and expertise to ensure that every product that comes out of the injection molding process is a success. Some of the injection molding mistakes are unavoidable, irritating, and sometimes very costly.
Small improvements to your injection equipment and methods, on the other hand, can help you reduce the amount of flow mark defects that may result from the whole injection molding process. This article will provide you with details on the precautions that have been put in place to prevent this.
What Are Flow Marks?
The wavy lines or patterns that form on the surface of the molded object are what we refer to as flow marks. They are also known as flow lines. Flow marks mainly occur in the form of circles or lines. These marks will be evident after the product has cooled. They show nonuniformity in the flow pattern as it enters the mold. The temperature gradient within the melt is what causes this nonuniformity in the flow.
Because of light refraction, there is a tiny variance in the color tone of these flow marks. Pigment redistribution is also to blame for such. When an engineer or a plastic injection mold service provider identifies such a flaw, the appropriate steps are taken to correct it. A well-designed process and mold system will prevent this from happening in the first place. Early recognition of the flow mark can save a lot of time and money in the long run.
What Causes Flow Marks?
Flow marks are caused by a variety of factors in the injection molding making technology, and they happen at the gate. The problem could be the result of machine settings made before the gate was installed. Flow marks can be caused by any or all of the following factors:
- Slow melt injection speed. The rate at which the melt is injected should keep it going until it reaches the mold. The melt must also reach the mold quickly enough before losing all of its stored heat. If the temperature is too low, part of the melt will have solidified by the time it reaches the mold. This happens when it is pushed through the gate. A flow mark will be created by the change in the flow pattern.
- Very low melt injection pressure. The rate of melting is proportional to the pressure. It does not have enough compaction if the pressure is not high enough. Melting homogeneity is aided by compaction. Flow patterns are evened out with enough injection pressure. Meaning with low injection pressure, flow marks will be created.
- Too low temperature inside the mold. The mold is not heated because cooling takes place there. Some molds are even pre-chilled before use. However, if the mold's temperature is too low, it will definitely cause complications. It is possible that the melt will cool too quickly after it enters the mold. Flow markings appear when areas of the mold cool too early around the gate. Over the cooled melt, the hotter fluid flows. Flow markings will form as a result of this.
- Too low melt temperature. The temperature of the melt is an important element in determining the viscosity of the melt. The primary goal of heating plastic is to enable it to distort and flow. If you see flow markings, you're probably running at a lower-than-ideal temperature. Heat necessitates the use of some form of fuel, which incurs a cost. As a result, the operator must carry out the operation at the proper temperature.
- Wrong gate or runner size. The flow will be restricted if the runner and gates are too thin. The melting process is slowed when the flow is restricted, and portions of the melt are also exposed to temperature loss. Because the flow diameter is smaller, the melt's center is closer to the runner's walls. This is especially the case for cold runner systems.
In case your injection molding machine has any of the above problems. Flow markings are more likely to occur. The section below will look at how to avoid getting flow marks.
How To Prevent Flow Marks In Injection Molding
Unfortunately, there is nothing that you can do after the problem of flow marks occurs. The part will get rejected and sent to recycle. Putting measures in place for flow mark troubleshooting in injection molding is very important.
You can also prevent such marks from occurring in two main ways. Changing the mold design is one option and changing the injection molding conditions is the other.
- Using runners and gates that are sufficiently wide. Mould gate design also matters a lot. Melt flow is restricted when the gate and runner's diameters are too small. Generally, the smaller the channel the quicker it cools. Premature cooling will prevent enough fluid from entering the mold. More fluid entering the fluid helps to generate pressure, resulting in turbulence. This helps to prevent the formation of flow markings. The gate can always be widened if necessary.
- Increasing the backpressure. Increasing the backpressure in the injection chamber can help to enhance melt uniformity. As it is injected, the pressure at which the melt is stored is also conveyed. As the fluid moves through the runners, it will be subjected to additional pressure due to the backpressure. The backpressure also guarantees that the melt is compacted properly. Doing this will help to reduce the odds of an uneven flow pattern from forming.
- Reducing the distance between the hot nozzle and the mold. The longer the melt has to travel, the more likely it is to cool too quickly. The melt should not cool before being poured into the mold. Another approach that plastic injection molding companies take is employing a hot runner when the distance cannot be reduced any further. This will keep the melt hot until when it's poured into the mold.
- Increasing the injection rate. The injection speed is always proportional to the injection pressure. Therefore increasing the pressure supplied to the screw as it melts increases the speed. Injection speed is increased to help get the melt into the mold before it cools. Where a hot runner isn't possible and you want the melt to go into the mold as quickly as possible, maintain injection speed to a minimum. This is because excessive speed might result in other issues such as jet.
- Working on Fillet Corners and Edges. Non-uniform flow patterns can be caused by sharp edges in the mold design. Flow markings are formed as a result of these factors. rounding these corners or addressing this fillet will lessen the impact of the flow direction change. It also avoids the formation of unbalanced flow patterns. Another issue with mold release will be avoided as a result of this.
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Observing the right positioning of the gate. The gate's location is quite crucial. There are certain guidelines on where the gate should be placed for particular products for instance in the Injection molding of gears. In the case of optimal filling, the gate is positioned in the center of the gear.
However, if there are no clear guidelines, just place the gate on the mold's side where the wall is thinnest. Because thinner will result in a decrease in temperature and that prevents the mold's flow patterns from being disrupted. The temperature differences will even out as the melt progresses from the thinnest to the thickest wall. -
Increasing the temperature. You should always be cautious when considering the possibility of raising the melt's temperature. This will be accomplished by altering the barrel's temperature settings. Many of the parameters in injection molding are influenced by temperature, pressure, and viscosity, in particular.
When the temperature is raised, the melt will flow more easily. However, keep in mind that you don't want to approach too close to the plastic's breakdown temperature. Increasing the temperature in areas where flow homogeneity is a problem will have negative consequences. Improper mixing might result in hot patches and some parts overheating.
Overheating causes degeneration, and leads to other flaws. The temperature of the mold can be raised in addition to the barrel temperature. A pre-cooled mold is usually utilized to aid in the cooling of the item. However, if this results in premature cooling, consider raising the mold temperature.
How To Prevent Flow Marks In Injection Molding
As discussed above, one of the things that contribute to plastic injection molding defects like sink marks and flow lines is the type of injection molding machine used. It is therefore important to work with reliable injection mold makers for quality services and products.
FOW Mould is one of the best companies that use injection molding technology.
They provide comprehensive injection molding services. If you require product design or a prototype to verify viability, they have the expertise to suit your requirements. The following are some of the advantages of working with FOW Mould.
- They are cost-effective. A single injection mold can produce hundreds of pieces. Instead of engaging several service providers, working with FOW Mould will provide you with access to a broad range of injection molding services at affordable costs. Their accuracy and quick turnaround times also contribute to lower production costs.
- They are highly efficient. FOW will deploy numerous injection machines based on your product amount and delivery date to satisfy your needs. Their injection molding capabilities ensure that your contract manufacturing for plastic parts can exceed your clients' standards while reducing time to market.
- Volume production. FOW Mould is capable of producing approximately 600 set molds per year. In addition, they have built a validated network of partners that they collaborate with to help them expand their manufacturing capacity. As a result, no order is too huge for them to handle. Because FOW Mould understands that their clients serve a variety of market sizes. They also accommodate low-volume orders.
- FOW Mould has the experience needed in any serious field. For over 40 years, FOW Mould has been offering injection molding services to businesses. They combine their extensive experience with cutting-edge technology to provide clients with dynamic and cost-effective injection molding solutions. They also have a broad staff of mold designers and engineers who specialize in many fields.
Conclusion
Injection molding flow marks are a problem that can always be avoided. However, once it appears on a product, it may be rejected and your injection molding company may lose clients. As much as flow marks may not impair the product’s operation, it does have a big impact on its appeal. Get in touch with us for more information.