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Injection Molding Flow Lines

Injection molding is a precise process, meaning many things can go wrong. Everything from operator error to minor issues with the mold design can cause problems for the part’s aesthetic or structural integrity.

Injection molding flow lines are one such issue. This article explains what they are, how they occur, and the techniques manufacturers can use to avoid them.

Injection molding flow lines

What Are Injection Molding Flow Lines?

Flow lines often appear as a wavy pattern on the surface of a plastic part, though several other patterns may occur. They usually display a slightly different color to the rest of the part and are more likely to appear on narrow sections of the item.

There are four main types of injection molding flow lines:

Type No. 1 – Snake Lines

Snake lines occur when a jet effect is created as the melt enters through a gate and into the mold cavity. The resulting line looks like a snake and appears on the product’s surface.

Type No. 2 – Wave Lines

Inconsistent melt flowing speeds tend to cause wave lines. The melt slows down or speeds up, leading to the melt wandering and causing wavy lines.

Type No. 3 – Radiation Lines

If the melt sprays as it enters the cavity via a gate, it becomes radial on the part’s surface. A radial line is usually the result.

Type No. 4 – Fluorescent Lines

Stress and pressure created by the melt’s flow results in a luster appearing on the product. This defect looks similar to a firefly, hence the name fluorescent lines.

What Is the Difference Between Weld Lines and Flow Lines?

Weld lines tend to occur at spots where flows meet at different temperatures. This results in inconsistent cooling that leaves a line on the part’s surface where the two flows met. Injection molding flow lines also often occur as a result of inconsistent cooling. However, in this case, manufacturers can have a single consistent flow of hot melt that reaches cooler melt inside the mold, leading to the creation of lines.

What Causes Injection Molding Flow Lines?

The causes of flow lines break down into four key categories, each of which has various issues that can affect it:

  1. The Injection Molding Process

  2. The Mold

  3. The Material

  4. The Operator

The Injection Molding Process

Plastic part injection molded

Faults with the injection molding process can cause an array of issues.

  • Either the holding pressure or injection pressure isn’t high enough to press the solidified melt against the mold’s surface, leading to flow lines that match the melt’s flow direction.

  • If the cycle time is too short, the melt may not be heated sufficiently within the barrel. With a low melt temperature, the material can’t be compacted during pressure holding and flow lines occur. This issue is often linked to improper residence time, which also leads to the melt being held in the barrel for too short a time.

  • A low barrel temperature leads to a low melt temperature. These temperatures affect the holding and injection pressure by ensuring it won’t be high enough to hold solidified layers of the melt against the mold’s surface.

  • The nozzle is the final heating zone inside the barrel. It passes heat to the melt. If the nozzle isn’t heated appropriately, the melt temperature decreases and causes the previously mentioned pressure issues.

The Mold

Injection molding tool - mold

Mold design is a key aspect of injection molding. Flow lines can occur if any of the following affect the mold:

  • A runner, gate, or sprue that is too small for purpose creates more flow resistance. When combined with low injection pressure, this problem leads to decreasing melt speeds and can cause flow lines.

  • The cooler the mold, the faster the material temperature drops as the melt is injected. Rapid temperature decreases lead to flow lines occurring as hot melt is poured on top of cool solidified material.

  • Blockages can occur if the mold isn’t adequately vented. Flow lines occur because the melt front can’t push the solid material layer against the mold.

The Material

Injection molding materials - polymers

Issues with the material used can also lead to injection molding flow lines.

  • If a mold cavity has a large flow length to thickness ratio, the material should have a low enough viscosity to ensure a consistent flow. If it doesn’t, the material's lack of fluidity leads to a slow melt flow that causes the previously mentioned cooling and pressure issues.

  • Failure to increase the material’s lubricant content in line with the flow length to wall thickness ratio can cause flow lines. The larger this ratio becomes, the more lubricant content is required.

The Operator

Operator errors can lead to flow lines occurring in a part. For example, if an operator mistimes the door switching process for the injection molding machine, this creates irregular heat loss that the machine has to try and compensate for. A lack of temperature uniformity occurs, creating cold spots in the mold that causes flow lines.

How to Prevent Injection Molding Flow Lines

The methods to prevent injection molding flow lines vary depending on the type of lines occurring in a part.

Snake Lines

For snake lines, the following techniques may help to reduce or prevent their appearance:

  • Reducing the melt injection rate can prevent the jet effect and expand the flow of the melt, leading to superior surface quality.

  • Ensuring the gate depth is equal to the cavity depth allows an expanding flow that prevents snake lines.

  • Setting the gate close to the cavity wall can allow a manufacturer to use the wall as a barrier that prevents a jet from forming. Adjusting the mold gate angle to between 40 and 50 degrees can also achieve this cavity wall barrier.

Wave Lines

Several changes to the mold design or its temperature can prevent wave lines from forming:

  • Melt fluidity increases as the mold’s temperature rises. This is beneficial for crystalline polymers, as it leads to a more uniform flow that reduces wave patterns.

  • Maintaining uniformity in the product’s thickness prevents wave lines.

  • Prominent edges and corners in a mold core lead to melt flow resistance. Changing these prominent edges and corners may aid flow stability.

  • Combining low-speed injection with the maintenance of high pressure levels ensures melt flow stability.

Radiation Lines

Preventing spray issues is the key cause for concern with radial lines. These techniques can help a manufacturer reduce spray:

  • Changing the gate to a fan shape may restore the melt’s elasticity prior to it entering the mold cavity. This prevents melt fracturing, which reduces radiation lines.

  • Extending either the main runner or nozzle lengths the melt’s flow path before it reaches the mold cavity. This increases the melt’s degree of elastic failure, preventing spray and the formation of radiation lines.

  • Slowing the injection speed also increases the elastic melt’s flow time, again increasing the degree of elastic failure.

Fluorescent Lines

Fluorescent lines tend to occur at the narrowest points of the part. These narrow points stretch the flow, creating internal stress that leads to flow lines. These solutions may prevent fluorescent lines:

  • Increasing the mold temperature reduces internal stress by relaxing macromolecules and reducing the flow’s molecular orientation. This reduces the appearance of fluorescent lines on the product’s surface.

  • Maintaining consistent part thickness reduces fluorescent lines. Kucukoglu, A, et al, emphasize this in their paper A Study for Detecting Flow Lines on The Aesthetic Plastic Parts During Design Phases as Using Material Flow Analysis Programs. They note that uniform part thickness prevents warpage and the shrinkage balance effect.

  • Combining medium-speed injection with medium pressure slows the solidification of the melt per unit volume. Internal stress decreases to reduce the appearance of fluorescent lines.

  • Applying heat treatment to a part, such as baking in an oven or boiling the part in hot water, intensifies macromolecule movement. This shortens the part’s relaxation time and reduces fluorescent lines.

Tackle Injection Molding Flow Lines

Injection molding flow lines are a key concern because there are several types, each with a variety of potential causes. Manufacturers can prevent flow line formation by tweaking their molds, processes, and machines based on the part’s requirements.


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