Knit Lines in Injection Molding

The knit line is the location where the two flows of plastic material meet, whether it is visible enough.

Knit lines often occur: the plastic material flow splits to bypass obstacles (like different structures) then meet on the end side. Or when there are more than one gate, the different resin flow fronts meet together within the mold cavity.

The mechanical strength of the knit line area is lower than that of knit line areas. The primary cause of this phenomenon is that during cooling at the knit line location, trapped air forms between the two streams of plastic. This trapped air impedes the entanglement effect of polymer molecules, causing molecular chains to separate from one another.

Knit lines can’t be eliminated completely, but they can be minimized through design and process optimization to avoid affecting appearance or structural strength.

For some parts, knit lines can affect appearance, so manufacturers typically aim to avoid them. However, according to the part’s operating environment and requirements, knit lines are not always unacceptable.

This article explores the causes of knit line formation during the injection molding process and its effects, and provides corresponding prevention and improvement solutions.

What Causes Knit Lines in Injection Molding?

Injection Cooling Process

The plastic material is heated to a molten state and then injected into a mold to form the final thermoplastic part.

As the material flows into the mold, it continuously cools. Therefore, the leading edge of the material flow remains the coldest and closest to solidification. The further from the gate, the lower the resin temperature.

Ideally, two material flows merge smoothly into a solid bond; if cooling occurs too quickly before complete fusion, a knit line forms.

Insufficient Pressure

Insufficient injection pressure can prevent fusion at the melt front, leading to knit lines.

Plastic Materials

A key factor is the plastic material used, as different resins exhibit varying tendencies to form weld lines. ABS is one of the materials most prone to knit lines. In many cases, ABS knit lines remain structurally strong enough not to significantly weaken part strength, but they may appear as a crack on the finished surface.

Boss

A boss is a structural feature designed to accommodate threaded fasteners, typically containing a hole for installing screws or bolts.

Bosses are formed by raised core pins inside the injection mold, around which plastic material flows during injection molding. Where material flows combine behind the core structure, a knit line forms at that point.

Two factors are particularly critical in this situation. If the boss is located near the edge of the part, the knit line becomes very short, significantly reducing the effective area for the two melt streams to combine. When a screw is driven into the boss, the knit line in this region is more susceptible to developing into a crack due to the “wedge effect.”

Multiple Injection Gates

Knit lines may also appear between multiple gates on a part. Gates are the points where plastic material enters the mold cavity. When reviewing gate and ejector pin layouts, always inspect these locations. While we generally avoid multi-gate designs, if necessary, specifically verify whether critical cosmetic or strength requirements exist in the midpoint area between any pair of gates.

How to Minimize Knit Lines in Injection Molding?

You can minimize knit lines to make your parts stronger and more attractive.

Simulation and Mold Design

You can minimize them to make your parts stronger and more attractive.

Gate Location: Through proper mold design, avoiding flow bottlenecks, and appropriately adjusting the number or position of gates, you can significantly improve filling and molding results.

Mold flow simulation can determine the most suitable gate location, placing the knit line in an area that minimally impacts the part’s overall assembly and functionality.

Large parts often require multiple gates, which can create multiple knit lines and potentially cause surface defects. Designers often incorporate inconsistent wall thicknesses, bosses, ribs, and other features to facilitate assembly. These elements obstruct normal plastic flow, causing knit lines. Opportunities for flow disruption should be minimized. Ribs should align with the direction of plastic flow to aid filling and venting. While these appear as basic design principles, achieving them often requires compromise among customers, designers, and mold makers.

Runner Size: If the runner is too small, it increases flow resistance, making it easier for weld lines to form.

Wall Thickness: Thicker walls slow down resin cooling, improving knit line fusion quality and strength while enhancing appearance.

Poor Venting: Insufficient venting causes material retention or blockage, resulting in incomplete mold filling and knit line formation.

All knit lines trap air, making proper venting of the surrounding area a critical mold consideration. Venting is an essential mold issue that should not be overlooked; vacuum venting systems may be introduced if necessary.

Adjust the process

Adjusting injection mold temperature, filling speed and other injection molding process parameters is the most fundamental and effective first step, these adjustments can improve the appearance of kint lines.

Keeping the proper temperature of injection molding machine, runner and mold cavity is the key to ensuring the plastic fills the mold cavity with the correct flow rate, combined with other melt fronts and fills all areas of the mold cavity properly.

Optimize filling speed to ensure complete fusion of melt fronts; adjust barrel/nozzle temperatures appropriately to prevent premature cooling; and maintain stable melt temperatures throughout the mold using hot runner systems.

Set injection pressure to allow sufficient melt velocity into the cavity, enabling smooth spreading along mold walls and effective fusion with other melt fronts.

Switch up your materials

Different plastic materials have different knit lines strength. Some soft contact materials are shear-sensitive, even if the front flow is not interrupted, they still have a knit line. So we need to change the material to solve the knit line problem.

By using thermoplastic materials that stay in liquid for a longer time, it will be helpful to reduce the knit line.

Fillers: compared with plastic materials that have no fillers, plastic materials with fillers commonly have weaker strength knit lines. As the fillers quantity increases, the knit line strength will decrease more. For example, pure pp can have 90% of its inherent strength in well designed and properly formed knit line. But if it has 30% fillers, the knit line strength will decrease rapidly to only 34% of its inherent strength. Fillers will enhance the plastic strength, but it will reduce knit line strength.

Add Thickness to Slow Cooling

You can add part wall thickness to slow down the cooling rate of the plastic material flow temperature. Suciffient wall thickness will let plastic material stay longer time in the mold cavity, and decrease knit lines. But we should pay attention to sink marks may ocurr when the wall thickness is too large.

Choose the Suitable Manufacturer

If the experienced mold manufacturer knows your product demands, you can work with them to create the best result.