How to Avoid Flow Lines in Injection Molding

Rapid injection molding may be cost-effective for low-volume end-use manufacturing, especially when the service provider uses production-grade materials and follows the best practices. However, it is not without flaws. One of the most common defects on injection molded parts is the flow line.

Flow lines resemble ripples on plastic parts after formation, and they follow the molten resin’s flow from the gate through the finished component. Although their cause can depend on molding conditions and the type of material, human error can also be a factor.

Flow lines won’t impact the part’s structural integrity. However, they can affect the product’s aesthetic qualities. Plus, they indicate a problem with material flow. The lines are often near the injection nozzle or gate and radiate outwards along the part. Sometimes, they appear as circles and repeated patterns.

To prevent them, it helps to know what causes them in the first place. Usually, it’s due to uneven cooling throughout the plastic material because of these reasons:

Material temperature

It’s important to monitor the material temperature while making injection molded parts to reduce flow lines. Consider referring to the melt flow index (MFI) of the thermoplastic to identify the material’s flow characteristics. A low MFI is more prone to these marks, and higher MFIs can resist them. In addition, MFI is closely linked to temperatures. If it’s too hot, the plastic will degrade, but keeping it too low can increase the occurrence of flow lines.

Machine parameters

Flow lines can also occur because of machine parameters. A low injection pressure can result in the material being unable to melt uniformly and uneven cooling. Likewise, low barrel and nozzle temperatures cause flow issues as they cannot heat the plastic well enough. In addition, improper timings can cause lines and uneven cooling times, particularly short cycling times and residence.

Mold design

Low-quality and poorly designed molds can also result in flow lines on injection molded parts, particularly those with uneven wall thickness, insufficient lubrication, and poor venting. Mold makers should also pay attention to sprue, runner, or gate design. If it’s too thin or small, material flow, pressure, and temperature will be reduced, leading to flow lines and other flaws.

Work with a seasoned injection molding partner

Injection molding is one of our specialties here at HLH Proto. We’ll work with you to reduce any flaws in injection molded parts. Contact us now!


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