Injection molding has the potential to achieve efficient and fast plastic part manufacturing. Identical parts get made in repeated cycles without defects. Thus leading to very good profit margins. When everything goes well injection mold can be a very rewarding and pleasant process to run. Emphasis on “when everything goes well”. Getting the full benefit of injection molding requires skill and experience. Good time and process management. As well as technical skill and experience with injection molding. So your investment does not end with purchasing machines and tools. You must also invest expertise to get a good return on invested time and finance. One of the key priorities in injection molding is to prevent defects. Any defect in the product could lead to loss of functionality or aesthetics. This often leads to part rejection. Even where the part gets recycled it still means loss of process time and energy. All this counts towards the cost of production. There seems to be an endless list of things that can go wrong in injection molding. With proper setup, operation, and management, you can avoid them all. One of such defects is jetting. This article discusses the problem of jetting in injection molding. We look at what leads to jetting and how you can avoid it. Thanks to the expertise at www.cavitymold.com, this is another off your list of mold concerns.
What is Jetting?
Jetting is a defect in the formation of the product. It occurs during the molding process. This is not to mean that it gets caused by the mold. It results in the product not having the desired surface detail. Rather it forms what looks like a frozen curled jet of melt on the product surface. It is often described as snake-like because of their appearance. Another name used for jetting is worm tracks. Jetting problems do not only affect the appearance of the product. They can also render the product useless. The mechanical properties and structural integrity of the product get altered by jetting. It is not a mark on the surface or discoloration or speck. The pattern in most cases protrudes from the surface. So where you should get a smooth surface finish or particular texture. You instead have this curled snake looking pattern. The image below shows an example of jetting.
Example of jetting on a product
How can you detect jetting in a product?
Good thing is that jetting gets spotted by visual inspection with ease. You can also use process performance and settings to predict jetting. Knowing what values to set for parameters like temperature and pressure helps. Once these parameters deviate from the setpoint and the machine gives certain signs. A third factor is an informed instinct. You can tell you are about to have a problem. Experience with injection molding, in general, helps develop such skills. Having experience with a specific machine and plastic also gives such knowledge. It is like having a bike for a long time. You can tell from how the brakes sound and feel of the pedals when it needs oil or something is about to break. This is like how an operator has instincts for when something is about to go wrong with a machine. Of course, the instincts only support the technical approach which is more effective.
What to do when you detect jetting?
When jetting has already occurred on a product, the only option for that product is to scrap it. If it is a plastic that gets recycled it gets sent to grind and recycle. This saves plastic but does not replace the time and resources spent to make that part. The next thing you want to do is halt the process. This prevents more defective parts from getting made. The sooner you find the source of the problem, the better. Time translates to money in manufacturing. Every second the machine is not in operation is a waste. The manpower and production time all translate to money. Once you detect the problem you then record the incident and fix it. You also take preventive measures to ensure the problem does not repeat. The next question is then what causes jetting and how to prevent it.
Causes of Jetting
In the ideal case, the melt enters the mold and contacts the mold cavity walls first. This melt in contact with the walls begins to cool as more melt fills in. This mode of mold filling ensures even product formation. The problem occurs when, as the name suggests, the melt comes in as a thin jet. This jet forms a snake looking pattern and hardens in this form. The rest of the melt fills the mold but the curled jet remains at the wall. There are different factors that lead to this undesirable event occurring. To help you detect where the problem is coming from we put together a checklist of possible causes.
- Excessive melt injection speed
- Gate size too small
- Wrong gate positioning
- Runner diameter too small
- Nozzle size too small
- Cooling rate too fast
If you have a sophisticated injection molding machine, detection might be easier. Some injection molding machines get fitted with diagnostics software. These are able to detect problems and show what the problem is and where it occurs. You might also be able to run a simulation of the process. This helps you detect potential problems before running in real life. This saves time and cost. But where this does not exist or fail. It is always good to have good knowledge of detecting and troubleshooting problems. Detection and troubleshooting are important parts of injection molding. This is also where having the proper sensors in the right location is important. Taking measurements of temperature, pressure, flow, and other parameters are key. Also important is having these sensors in the right location. For example, the temperature on the surface of the barrel. This is not the same as the temperature of the melt inside the barrel. The temperature reading either comes from inside the barrel. Or accounts for the thermal conductivity of the metal. You must also take a temperature reading at strategic points. The heat applied to the plastic varies at different points. This accounts for the heat generated from friction as the screw shears the plastic. The right sensors in the right position give a good idea of what is happening at different locations. This makes for more effective troubleshooting of jetting and other defects.
How Do you prevent Jetting in the first place?
So now you know the factors that could lead to jetting how do you prevent it from happening in the first place?.
Mold design and material selection
The design of the mold goes beyond getting the cavity that forms the shape of the product. Other parameters of the mold affect how the plastic replicates the intended design. You might find that although your mold cavity has been well machined with the right design. For some reason, the plastic isn’t forming into these design details. Do not overlook factors like the thickness at different parts of the mold and the surface finish. Flow restriction due to poor surface finish makes jetting more likely to occur. A problem or product defect is most often a combination of inefficiencies. Where slight inefficiency in other aspects might not lead to jetting. When you combine that slight inefficiency with poor mold design, it becomes inevitable. Using the right material for the mold also goes a long way in product formation. The mold material must take into consideration factors like corrosion and texture. If you are running a plastic-like PVC. you must use the right quality of steel to ensure that the mold is not getting corroded. Corrosion leads to surface imperfections which make problems like jetting more likely. The mold maker thus needs to have information on the type of plastic you are using. The mold maker then applies this information in designing features like cooling lines. This information is also used to select the right material for the mold
Location and type of gate
The placement of the gate is important in injection molding. The location of the gate affects the flow pattern of the melt into the mold. The distribution of the melt as it enters the mold determines how cooling occurs. You want a good spread on entry such that the walls get filled in an even way. Once you have a cool shell formed the rest of the mold entering absorbs heat from the layer next to the wall. There is a level of insulation from the plastic layer on the wall. This allows the melt to keep enough velocity to ensure proper filing. The melt holds enough temperature to maintain a low enough viscosity. Enough to allow flow and fill all corners of the mold well. This ensures good product formation. It also generates enough entropy to have turbulence. This in turn allows former mixing and uniformity. The location of the gate depends on the product design. The mold maker decides the best placement of the gate based on the specific product design. This is where you need the experience and skill of the mold maker in mold manufacturing.
The type of gate also affects the melt flow pattern. The choice of the gate should match the type of plastic and mold. Fan and tab gates help to reduce shear and achieve better flow distribution. Submarine gates or overlapping gates allow for better contact with the mold surface to prevent the jetting patterns from developing.
Nozzle, runner, and gate size
One of the challenges in injection molding is getting the melt to make the journey from the nozzle to the mold. The injection pressure is a factor that contributes to this. Another is the size of the channels through which the melt flows. These channels are the nozzle, sprue, runner, and gate. Before it finally enters the mold. The thinner the channels the more surface area of the melt exposed. Thinner jets cool faster. Thinner streams of fluid also mean less of the fluid volume is entering the mold per time. So heat gets transferred to the mold faster. This increases the chance of jetting occurring. Thus a preventive measure is to make the channel as wide as the melt property and process allows. Flow rate calculations, simulations, and trial run. These all aid in determining the right gate size. For example, using software with thermomechanical analysis of flow rate. This helps predict flow patterns for different channel diameters.
Mold temperature
Yes, precooled molds help with ensuring a favorable temperature distribution during cooling. But where the temperature is too low, this causes problems. One of such problems is jetting. Even where the gates and runners cannot get made any wider, you can regulate the mold temperature. The goal is to have a temperature gradient between the mold and the melt. If the temperature gradient is too sharp the melt touching the mold cools too fast. It cools faster so that it can receive heat from the next layer. This makes jetting more likely. Even where the stream forms, if the stream can transfer heat fast enough, this can prevent jetting.
The plastic-type also determines the chances of getting occurring. If you have plastic with a sharp glass transition temperature for example. The stream of the melt is more likely to solidify at the slightest drop in temperature. So when setting the mold temperature you should consider the plastic melt property. For plastics with sharp glass transition temperature, avoid very low mold temperature. This ensures the plastic maintains enough temperature to allow it to flow. This ensures proper mold filling.
Image illustrating temperature and velocity distribution. A gradient exists from the mold wall across the melt thickness. The melt touching the wall is much cooler. This hardens first. This layer of plastic receives heat from the layers above it. The hotter melt has high viscosity so the flow to fill the mold. The slight insulation from the cooled layer of plastic is important. It ensures the plastics maintain enough flow to fill up the mold. The flow pattern hence temperature distribution depends a lot on the mold temperature.
Cavity Mold is here to help
At www.cavitymold.com we have over a decade of experience in mold manufacturing. We also provide injection molding services for small and large companies. Working with Cavity Mold you have a team of professionals dedicated to your project. We design and produce high-performance mold and injection molding processes. Contact us today to discuss how we can help you achieve high efficiency in injection molding.
