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Weld Line (Knit Lines) in Injection Molding

Weld line around hole after injection molding

No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. Weld lines, which are also referred to as knit lines, are among the most common of these blemishes. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them.

Why You Need to Avoid Weld Lines

Weld lines lead to two significant issues with an injection molded product. The first is that they deform the part’s surface by leaving unsightly lines that the consumer may not want to see. Second, weld lines compromise a product’s structural integrity. The result is a more fragile piece that has a higher likelihood of breaking.

What Can Cause a Weld Line in Injection Molding

There is no singular cause of a weld line in injection molding. Instead, several issues can lead to the formation of knit lines that manufacturers should aim to manage.

Weld line in injection molding formation

Obstructions in the Melt Flow

Knit lines typically form around holes and obstructions in the melt flow. If the flow can’t properly access part of the mold, it enters in an inconsistent way that leads to weak areas.

Low Melt Temperatures

Temperature inconsistencies in key areas of the mold can lead to issues with material flow. In addition to the temperature of the mold itself, variations in the injection molding machine or its runners can also lead to knit lines. In the paper, Study of the Effects of Injection Molding Parameter on Weld Line Formation, Azieatul Azrin Dzulkipli and M.Azuddin concluded that decreases in melt temperature also raise the possibility of weld lines forming.

Material Composition

The same paper highlights that the material used for the melt also affects both the possibility of weld lines forming and the nature of those lines. For example, a composite material made using polypropylene, fiber, and glass that has a high filler weight will usually have shorter knit lines, though these lines have a wider angle.


Inconsistent application of pressure can lead to flow fronts failing to be pushed close enough together to allow the melt to merge properly. The result is often a broad melt line. Pressure issues tend to occur if the user creates incorrect settings or if the injection molding machinery is faulty.

Mold Design

Several mold design issues can lead to the formation of a weld line in injection molding. Examples include improperly placed gates, which affect the material flow, and incorrect wall thickness. This demonstrates the importance of design testing and prototyping to ensure a mold fulfills its purpose correctly.


If the material injected into the mold contains impurities, the melt can’t form properly. Weld lines tend to form around these impurities, leading to compromised structural integrity. Impurities can also lead to inconsistent flow, resulting in one part of a flow being faster than another.

Excess Mold Release

Mold release affects speed. If there is too much mold release, a manufacturer must find a way to create a higher pressure inside the mold. Otherwise, the melt isn’t pushed through the machine at the required speed to prevent knit lines from forming.

Techniques to Prevent Weld Line in Injection Molding

Just as several issues can cause a weld line in injection molding, there are also several solutions a manufacturer may use. Manufacturers may need to conduct tests with each solution to determine which solves the specific problem that leads to the weld lines in products.

Weld lines on clear plastic

Increase Mold Temperature

Increasing the temperature in the feed cylinder may solve the issue if the material entering the mold isn’t fully melted. Higher temperatures also lead to more efficient material drying, again preventing knit lines.

Change the Product Wall Thickness

Wall thickness affects the time taken to fill a mold. Adjusting wall thickness can slow down or speed up the melt at various points in the process, allowing the manufacturer to create a more consistent flow.

Move a Gate

Melt is injected into a cavity using an opening that’s called a gate. Many molds have several gates that determine how melt is inserted and how it fills the cavity. Adjusting the position of these gates can help manufacturers to limit weld lines by ensuring consistent flow throughout the mold.

Reduce Runner System Dimensions

Small runner systems conduct heat faster. By reducing the size of the runners, manufacturers can increase the temperature of the molten material at the front of each flow. This reduces the possibility of knit lines by keeping the flow heated for longer.

Adjust to a Single Flow Design

Having multiple gates can cause weld lines due to inconsistencies in flow speed and direction. By using a single flow source, the manufacturer avoids the issue of multiple flows coming together.

Faster Injection Speeds

Premature cooldown is a major issue in plastic injection molding. This cooldown often occurs if the melt flows into the mold too slowly. The melt inside the mold starts to cool even as the hotter melt is layering on top of it. Increasing flow speeds can prevent premature cooling. However, it can also lead to weld line’s simply moving to another location on the part if the flow isn’t managed properly.

Use Plastic With Lower Viscosity

In the research paper A Review Article on Measurement of Viscosity, M. Maheshwar discusses the importance of viscosity in material selection. He points out that viscosity affects a melt’s internal resistance, with higher viscosity leading to more friction. This friction is a form of resistance that can slow down the speed at which melted materials flow through a mold, resulting in knit lines if unmanaged.

Use Vibration-Assisted Injection Molding (VAIM)

With VAIM, a manufacturer introduces a vibrational movement to the injection screw used during the injection portion of the molding process. In the paper Vibration Assisted Injection Molding for PLA with Enhanced Mechanical Properties and Reduced Cycle Time, researchers from Lehigh University tested how VAIM affects the physical characteristics of polylactic acid (PLA) melt.

They discovered that using VAIM reduces injection cycle times by up to 25%. As discussed earlier, shorter cycle times reduce the possibility of melt cooling, leading to fewer knit lines.


Weld lines and melt flow examples

Though weld lines are a prominent challenge in injection molding, several solutions exist that can prevent or reduce the chances of their formation. The key challenge manufacturers face lies in identifying the cause behind the knit lines they observe so they can implement appropriate solutions.


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