There are many defects that can occur during the injection molding process. One that can often occur for many reasons is a flow line. If you’re not familiar with flow lines, they can often appear like ripples in the plastic after the part is formed. They follow the flow of the molten resin from the gate along the finished part. The cause can vary depending on the material, the molding conditions, and the people involved. If you’re trying to troubleshoot your way out of flow lines in your parts, these causes may help diagnose the root cause quickly.
How To Detect Flow Lines in the Injection Molding Process?
The molding process can have many challenges concerning flow lines. There are five main problems that we monitor in our daily operations:
- Incorrect Cycle Time – There are multiple parts to a complete cycle time but having too low of a melting time will cause flow lines to occur. The plastic pellets must be sufficiently melted to a molten state in the barrel of the machine before the injection process can be fully completed. If you’re seeing flow lines, try extending the time the plastic pellets are being melted.
- Barrel Temperature Too Low – The barrel on the injection molding machine incorporates heaters that melt the resin pellets as they travel down the barrel to the mold. The barrel has three zones (rear center, and front), and then the nozzle at the end. The three zones should increase in temperature along its length to ensure the resin in sufficiently melts for injection. If the resin isn’t melted, it indicates the barrel temperature is most likely too low for the resin. The resin won’t be pressed against the mold surface with injection and holding pressure, and it will show flow lines. The best countermeasure for this occurrence is to increase the material temperature, the injection pressure and/or the holding pressure to fill the injection mold. The material melting temperature is normally supplied by the material supplier’s recommendations.
- Nozzle Temperature Too Low – Once your material is sufficiently heated in the machine barrel, it will be pushed through the nozzle and into the injection mold. The nozzle must be correctly heated to ensure the material doesn’t lose temperature. Your nozzle temperature should be set slightly higher than the barrel to keep your material molten. If the material can cool through the nozzle, it will start to cool and harden before the injection mold is filled. The nozzle temperature is typically ~42°F (~6°C) above the barrel temperature.
- Melting Time is Low – Time and temperature are required to melt your resin completely. Even if the resin is subjected to the correct melt temperature, not allowing enough time for the resin to melt with cause injection issue further in the process. If you find the temperature to meet the manufacturer or supplier recommended settings, also check the time the resin is allowed to melt.
- Insufficient Injection or Holding Pressure – Temperature isn’t the only setting on the injection molding machine that can cause flow lines. The injection and holding pressure are equally important for filling the injection mold. If the temperature is correct for the material, and it is completely molten when it enters the mold, the pressure must push it into every cavity and hold it until the cooling process is complete. If the pressure is too low, the mold won’t fill correctly and can show flow lines.
How Does the Injection Mold Influence Flow Lines?
The molding process is clearly dependent on temperature to melt and fill the injection mold, but that doesn’t leave the mold itself out of the equation. The mold must be at the correct temperature to keep the molten resin warm during the injection process. If the mold temperature is too low, it can start to cool the resin before the cycle is complete. That can allow for flow lines to form as the front of the molten resin begins to cool before it reaches deep into the mold cavity. If the front half of the process is at the correct temperatures, the mold temperature may be the culprit for your flow lines. You can increase the mold temperature and complete a handful of injection cycles until the parts are complete formed with no flow lines to verify settings.
Mold design can also cause flow lines is the sprue, runners, and/or gates are incorrectly sized for the material or calculated flow rate. Each can cause backpressure that resists the flow of the molten resin. Restricted flow can cool too quickly in the mold to form flow lines, and it will require increased pressures to overcome a poor mold design. If the initial trial runs of a new mold struggle with resin flow, it’s time to check the design and adjust as necessary to remove the restrictions.
Filling the mold takes pressure to push the molten resin into the mold, but improper venting can block the flow. Venting allows trapped gases to escape the mold, and there are different ways venting can be incorporated in the mold design. A vent can be added at a parting line directly opposite from the gate into the mold. If that doesn’t completely remove all trapped gases, a vacuum system can help draw out trapped gases too. That may be a requirement for some textured finishes that inherently cause gas issues inside the mold.
What Part Does Resin Play in Causing Flow Lines?
The resin involved in the injection process doesn’t usually cause flow lines, but don’t let it completely off the hook just yet. Resin does have a ‘fluidity’ that indicates how well it can flow during the injection process. The mold in question may have long runner lengths that require a resin with great fluidity. That means it will flow well under the proper pressure. A resin with low fluidity properties won’t flow well, and it must be paired with a mold that has shorter runner lengths. Mixing the two will cause colder and slower resin flows leading to flow lines in your products.
One way to increase fluidity is increasing the molding lubricant added by the resin manufacturer. The lubricant content in resin is usually below 1% for most resins, but a challenging mold design may require the content to be increased. Changing the resin mixture may require trials to find the best lubrication content percentage, so use it only as a last resort. It can take too much time and investment to adjust mixtures.
Why Is Employee Training Important?
The last cause of flow lines may be the operators running your daily operations. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature fluctuations with the mold. Something as simple as leaving a door open on a cold day can allow a draft to cause cold spots on an injection mold. Cold spots can affect resin flow, and then the flow lines start to form. Employee training on proper environment and equipment is one way to ensure smooth operations, but automation can also be added to remove the potential problems under the right conditions.
Flow lines are just one of many defects that can wreck production line efficiency. Temperature, pressure, material selection, mold design and employee training all play a part in world-class operations. Finding a partner with elite levels of all factors is hard to find, but SEA-LECT Plastics prides itself at being at the top of the charts. SEA-LECT Plastics has an elite team that produces world-class prototypes and products, and we have decades of experience with plastic injection molding operations and creating tooling for perfect parts. We can offer support to determine what type of mold you need, what resin to choose, and how to best invest in your future. Give us a call at (425) 339-0288 or email us at firstname.lastname@example.org. We’ll help to determine the best manufacturing for your next project.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.