How to Prevent Knit Lines in Injection Molding

How to Prevent Knit Lines in Injection Molding

Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. They are formed when two or more flow fronts of molten plastic meet and solidify without merging completely. This creates a visible or invisible line on the surface or inside the part, where the strength and durability may be compromised.

Table of Content

  • What are knit lines and why do they occur?
  • How to avoid or minimize knit lines in injection molding?
  • What factors affect knit line formation and severity?
  • Tips and best practices for preventing knit lines

What are knit lines and why do they occur?

Knit lines are formed in injection molding when two or more flow fronts of molten plastic meet and solidify without merging completely. This can happen when the plastic flows around an obstacle, such as a core, a hole, a rib, or a boss, or when multiple gates are used to fill the mold cavity.

Imagine a river splitting to go around a rock, then meeting up again on the other side. If the water is moving fast enough and has enough momentum, it will rejoin smoothly and form a continuous stream. But if the water is moving slowly or has lost its momentum, it will form a visible line where the two streams meet.

The same principle applies to plastic flow in injection molding. If the plastic is hot enough, has enough pressure, and flows fast enough, it will weld together seamlessly when it meets another flow front. But if the plastic is too cold, has low pressure, or flows too slowly, it will form a line where the two flows meet.

Knit lines can affect both the cosmetic and functional aspects of plastic parts. Depending on the resin type, color, surface finish, and part geometry, knit lines can range from barely noticeable to clearly visible. They can also create weak spots in the part, where stress or impact can cause cracks or fractures.

How to avoid or minimize knit lines in injection molding?

The best way to avoid knit lines is to design parts that do not have features that cause flow separation or multiple flow fronts. For example, avoid unnecessary holes, ribs, bosses, or cores that interrupt the flow path of the plastic. If possible, use a single gate location that allows for uniform filling of the mold cavity.

However, sometimes these features are necessary for structural or functional reasons, or multiple gates are required for large or complex parts. In these cases, there are some strategies that can help minimize knit lines or make them less noticeable:

  • Increase the mold temperature: A higher mold temperature can keep the plastic hotter and more fluid for longer, allowing for better welding of flow fronts.
  • Increase the injection speed: A faster injection speed can increase the pressure and momentum of the plastic flow, improving its ability to merge with another flow front.
  • Increase the injection pressure: A higher injection pressure can also help push the plastic together and create a stronger bond between flow fronts.
  • Increase the melt temperature: A higher melt temperature can reduce the viscosity and increase the fluidity of the plastic, making it easier to fill thin sections and weld flow fronts.
  • Optimize the gate location: The gate location should be chosen to minimize flow separation and maximize flow uniformity. Ideally, the gate should be placed near

What factors affect knit line formation and severity?

Knit line formation and severity are influenced by several factors, such as material properties, mold design, process parameters and part geometry. Knit lines are the result of two or more melt fronts meeting after flowing around an obstacle in the mold cavity. The strength and appearance of the knit line depend on how well the melt fronts fuse together. Some of the factors that affect knit line formation and severity are:

  • Material properties: The viscosity, melt temperature, molecular weight and crystallinity of the material affect the flow behavior and the ability to fuse with other melt fronts. Higher viscosity and lower melt temperature tend to reduce knit line strength, while higher molecular weight and crystallinity tend to improve it.
  • Mold design: The location, size and shape of the gates, runners and vents affect the flow pattern and pressure distribution in the mold cavity. The gates should be positioned to minimize the distance between the melt fronts and to avoid sharp corners or thin sections that can cause flow hesitation or freeze-off. The runners and vents should be designed to ensure adequate filling and venting of the mold cavity.
  • Process parameters: The injection speed, pressure, time and temperature affect the flow rate and pressure gradient in the mold cavity. The injection speed and pressure should be high enough to overcome the resistance of the mold and to fill the cavity before the melt cools down. The injection time and temperature should be optimized to achieve a balance between filling, packing and cooling of the part.
  • Part geometry: The shape, size and thickness of the part affect the flow direction and length in the mold cavity. The part geometry should be designed to avoid or minimize areas where knit lines can form, such as holes, ribs, bosses or other features that create obstacles for the melt flow. The part thickness should be uniform or gradually varied to avoid sudden changes in flow rate or pressure.

Tips and best practices for preventing knit lines

Knit lines are visible marks on the surface of a molded part that occur when two or more flow fronts of molten plastic meet and solidify. They can affect the appearance, strength and functionality of the part, and should be avoided whenever possible. Here are some tips and best practices for preventing knit lines:

  • Increase the mold temperature and injection speed to ensure a smooth and continuous flow of plastic into the mold cavity.
  • Reduce the wall thickness and gate size of the part to minimize the resistance and pressure drop of the plastic flow.
  • Optimize the gate location and orientation to avoid flow fronts meeting at right angles or in areas with high stress or cosmetic importance.
  • Use a material with high melt strength and low viscosity that can withstand high injection pressures and temperatures without degrading or freezing off.
  • Add mold vents or vacuum systems to remove any air or gas trapped in the mold cavity that can cause flow interruption or hesitation.
  • Design the part with rounded corners, fillets and ribs to reduce stress concentration and improve the strength of knit lines.