Injection Molding Defects: Jetting – What It Is and How to Prevent It

In precision injection molding, certain defects can manifest during the process, with one of the more notable being "jetting." While it may appear as a simple surface blemish, jetting can signal significant issues within the molding process or tool design. For many industries, this defect can compromise the part's structural integrity and aesthetic quality. Addressing and eliminating jetting is crucial for upholding production standards, minimizing material waste, and delivering dependable, high-quality components. This article will explore what jetting is, investigate its fundamental causes, and detail the effective strategies used to prevent it.

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What Exactly Is Jetting?

Jetting is a type of defect that appears as a serpentine or squiggly line on the surface of a molded part, typically originating from the injection gate. It happens when a stream of molten plastic is injected at a high velocity into the mold cavity without making contact with the mold walls. Instead of the material flowing evenly and filling the cavity from the outside in, it shoots straight through the open space.

This initial "jet" of material begins to cool and solidify before the rest of the cavity is filled. As subsequent molten plastic fills the mold, it flows around this solidified jet, resulting in poor fusion between the initial and later material. The presence of jetting is a clear indication that the flow of molten plastic is uncontrolled, which can lead to weakened, aesthetically flawed, and functionally unreliable parts.

What Are the Root Causes of Jetting?

The emergence of jetting can generally be attributed to several key factors, primarily related to injection mold design services and the process parameters used during injection. Pinpointing the specific cause is the first step toward implementing a successful and lasting solution.

Mold Design

The design of the injection mold is a critical factor. The most significant cause of jetting is related to the gate's location and size. A gate is the opening through which the molten plastic enters the final part cavity. If the gate is positioned in a way that the material flows directly into a wide, open area without obstruction, jetting is likely to occur. The material needs to impinge on a mold wall or a core feature immediately upon entering the cavity to stop the jetting effect and promote a uniform flow front.

Furthermore, a gate that is too small for the part's wall thickness can cause the material to be injected at an excessively high velocity, much like water being forced through a narrow nozzle. This high-speed injection leads directly to the jetting phenomenon. Abrupt changes in wall thickness within the part design can also disrupt the flow of material and cause jetting.

That’s why Design for Injection Molding (DfIM) and Plastic Part Design Optimization play a foundational role in preventing such flow-related defects early in the design for manufacturing services stage.

Process Parameters

Even a well-designed mold can yield parts with jetting if the processing parameters are not correctly set. The most frequent process-related cause is an overly high injection speed. Injecting the material too quickly propels the plastic into the cavity as a free-standing jet rather than a progressive flow front.

The temperature of the molten material and the mold itself also play a significant part. A melt temperature that is too low increases the viscosity of the plastic. This thicker, more viscous material requires higher pressure to inject, which can lead to the high velocities that cause jetting. Conversely, a mold temperature that is too low can cause the initial jet of material to cool too rapidly, preventing it from properly fusing with the subsequent flow of plastic.

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How Can Jetting Be Prevented and Eliminated?

Addressing a jetting problem involves a methodical approach that starts with the most straightforward potential solutions and progresses to more involved adjustments. The fix almost always involves modifying the mold design or optimizing the injection molding services.

The Importance of Mold Design and Gate Location

Since gate design and location are primary contributors, the first area to evaluate is the mold itself. The most effective solution is to reposition the gate so that the incoming molten plastic immediately hits a wall or pin within the cavity. This obstructs the free jet and forces the material to spread out in a more controlled, uniform manner.

If relocating the gate is not feasible, increasing the gate size can help. A larger gate will reduce the injection velocity for the same fill time, slowing the material's entry into the cavity and mitigating the jetting effect. Designing the flow path to have a consistent wall thickness can also promote a more even fill.

Adjusting Processing Parameters for a Flawless Part

If the mold design is sound, attention should be turned to the injection molding process settings. The following adjustments are commonly made:

• Reduce Injection Speed: Lowering the injection speed is one of the most direct ways to combat jetting. This should be done carefully to avoid other defects like short shots, where the mold doesn't fill completely.
• Increase Melt Temperature: Raising the temperature of the molten plastic will lower its viscosity, allowing it to flow more easily and at a lower pressure, which can help prevent the high-speed injection that leads to jetting.
• Increase Mold Temperature: A higher mold temperature prevents the initial jet of material from solidifying too quickly, allowing for better fusion with the rest of the plastic as it fills the cavity.

At a design and manufacturing company like Aprios, we combine injection molding quality control with custom injection molding solutions to ensure every part meets functional and aesthetic specifications. From Medical Device Prototypes to additive manufacturing solutions, process optimization is integrated at every stage.

Why Is Preventing Jetting a Core Aspect of Quality Control?

Jetting is more than a superficial flaw; it is a sign that the molding process is not stable or controlled. If not corrected, it can result in parts with significant structural weaknesses, dimensional inaccuracies, and increased production costs due to scrap. A properly designed mold combined with meticulously optimized processing parameters is the key to producing parts consistently and accurately, free from defects. Successfully preventing defects like jetting requires a thorough understanding of material behavior, tool design principles, and processing dynamics to deliver clean, robust parts for even the most critical applications.

At Aprios, we leverage our comprehensive expertise to address these challenges, delivering clean, high-integrity parts for even the most demanding fields.