Views: 0 Author: Danny Luo Publish Time: 2026-06-01 Origin: Site
| 1. What Is Jetting in Injection Molding? |
| FAQ |
Jetting is a common injection molding defect that often appears near the gate and may indicate poor melt flow control. For manufacturers, it can affect part appearance, production stability, and delivery quality. This guide explains the main causes of jetting and how to prevent it through process and mold design optimization.
Jetting in Injection Molding is a common surface defect that occurs during the filling stage. It happens when molten plastic enters the mold cavity too quickly and shoots forward instead of flowing smoothly along the cavity wall.
During normal injection molding, the melt should spread evenly and fill the cavity in a controlled flow pattern. However, when the melt enters the cavity at high speed or without enough resistance, the first stream of plastic may move directly into the open space.
This fast-moving melt cools quickly before it bonds properly with the following material. As the later melt flows around the cooled stream, visible jetting marks are left on the part surface.
This injection molding jetting defect usually appears as snake-like lines, curved flow traces, or irregular marks near the gate area. Because the marks often form at the beginning of the melt flow path, they are commonly found close to the gate or in areas where material flow suddenly changes direction.
In some cases, jetting can be confused with other plastic injection molding defects, such as flow lines, weld lines, or silver streaks. The key difference is that jetting usually shows a clearer “spray-like” or “snake-like” pattern caused by unstable initial melt flow.
Jetting is not only an appearance problem. If the first injected melt cools too early and does not fully fuse with the later flow, it may also affect local bonding strength, surface consistency, and part quality.
For manufacturers, repeated jetting during mold trials or mass production can lead to unstable molding results, extra process adjustments, and possible delivery risks.
To understand what causes jetting in injection molding, both processing conditions and mold design need to be reviewed. High injection speed, improper gate location, low melt temperature, low mold temperature, sudden wall thickness changes, and poor material flowability may all increase the risk of jetting.
Therefore, how to fix jetting in injection molding should not rely only on simple parameter adjustment. A more stable solution usually requires optimized injection settings, better gate design, balanced filling, proper mold temperature control, and early DFM review before mold manufacturing.
To understand what causes jetting in injection molding, we need to look at how the molten plastic enters and fills the mold cavity. In a stable filling process, the plastic melt should flow smoothly along the cavity wall instead of shooting directly into the open space of the cavity. Once the flow becomes unstable, jetting marks may appear near the gate area, creating visible surface defects on the injection molded part.
A small gate is one of the common causes of Jetting in Injection Molding. When the gate size is too small, the molten plastic is forced to pass through a narrow opening at a higher speed. This can create a thin, fast-moving melt stream that shoots into the cavity before it has time to spread along the mold wall.
As the first stream cools quickly, the following melt wraps around it and forms visible jetting marks. This injection molding jetting defect is often seen near the gate area, especially when the gate direction points directly toward an open cavity space. In this case, increasing the gate size or adjusting the gate position may help the melt enter the cavity more smoothly.
2.2 Injection speed is fast
Fast injection speed, especially during the initial filling stage, can also lead to jetting. When the melt enters the cavity too quickly, it may lose contact with the mold wall and move forward like a free-flowing stream. This unstable flow pattern is one of the main reasons why jetting appears as snake-like or irregular marks on the surface.
Among many plastic injection molding defects, jetting is closely related to the first stage of filling. If the initial speed is too aggressive, the melt does not have enough time to form a stable flow front. To reduce this problem, the injection speed can be adjusted in stages, starting slower at the beginning and increasing only after the melt has filled the gate area more evenly.
2.3 High viscosity / low fluidity
Material flowability also plays an important role in jetting. Plastics with high viscosity or low fluidity are more difficult to flow smoothly inside the mold cavity. When the material cannot spread evenly, the melt stream may separate from the cavity wall and create flow instability.
This problem is more common with engineering plastics, glass fiber reinforced materials, or resins with a narrow processing window. If the melt temperature is too low or the material is not properly dried, the fluidity may become worse, making surface defects more visible. For this reason, material behavior, drying conditions, and processing temperature should be reviewed when analyzing what causes jetting in injection molding.
2.4 Low holding pressure
Low holding pressure may also contribute to jetting-related quality problems. Although jetting mainly happens during the filling stage, insufficient holding pressure can make the defect more obvious because the later melt may not fully pack and bond with the early cooled flow stream.
When holding pressure is too low, the part may also show poor surface consistency, weak local bonding, sink marks, or dimensional instability. In this situation, how to fix jetting in injection molding is not only about reducing injection speed or changing the gate design. The holding pressure, holding time, and pressure transition point should also be optimized to improve part appearance and molding stability.
Jetting in injection molding does not only affect the surface appearance of plastic parts. In many cases, this defect can also influence injection molded part quality, plastic part strength, production stability, and final delivery performance. For appearance-sensitive or functional parts, jetting should be evaluated carefully during mold trial and mass production.
Impact Area |
How Jetting Affects the Part |
Common Risk |
Surface appearance |
Creates snake-like lines, curved streaks, or visible flow marks near the gate area. |
Cosmetic defects, poor surface consistency, customer rejection |
High-appearance parts |
More obvious on electronic housings, automotive interior parts, transparent parts, high-gloss parts, and painted parts. |
Unqualified appearance, extra polishing, painting defects |
Plastic part strength |
The first melt stream may cool too early and fail to fully fuse with later material flow. |
Weak bonding, reduced impact resistance, cracking risk |
Structural performance |
Hidden weak areas may affect assembly performance or long-term use. |
Structural defects, poor durability |
Mass production stability |
Unstable melt flow may cause uneven filling or repeated process adjustment. |
Higher defect rate, unstable production |
Cost and delivery |
Sorting, rework, remolding, or extra inspection may be required. |
Higher production cost, material waste, delivery delays |
Jetting should not be treated only as a cosmetic defect. To maintain stable injection molded part quality, manufacturers need to identify the root cause early and control the problem through proper mold design, optimized process parameters, and stable production management.
To understand how to fix jetting in injection molding, manufacturers need to control the melt flow more smoothly during the filling stage. In most cases, jetting marks are caused by unstable melt movement, excessive flow velocity, or improper mold design. Therefore, solving this injection molding jetting defect usually requires a combination of process adjustment and mold optimization.
Reducing the initial injection speed is one of the most effective ways to reduce Jetting in Injection Molding. When the molten plastic enters the cavity too quickly, the melt may shoot forward before contacting the mold wall, creating unstable flow and visible jetting marks.
In many molding projects, using multi-stage injection speed control can improve flow stability significantly. A common approach is to use a slower injection speed during the first 10%–30% of the filling stage, allowing the melt to contact the cavity wall smoothly before increasing the speed for the remaining filling process.
For appearance-sensitive parts such as electronic housings, transparent parts, or automotive interior components, reducing the initial speed often helps reduce surface defects and improve injection molded part quality. However, the speed should not be reduced excessively, otherwise short shots or weld lines may appear.
Gate design has a direct influence on melt flow behavior. If the gate is too small or faces directly into a large open cavity area, the molten plastic can form a high-speed jet stream immediately after entering the mold. This is one of the most common causes of jetting in injection molding.
A better solution is to let the melt first impact a mold wall or nearby surface before spreading across the cavity. This helps stabilize the flow front and reduces the risk of jetting marks. In practical mold design, different gate types may be selected according to the product structure and appearance requirements.
Gate Type |
Application Feature |
Effect on Jetting |
Edge Gate |
Common for medium and large plastic parts |
Helps the melt flow along the cavity wall more smoothly |
Fan Gate |
Suitable for thin-wall or appearance parts |
Reduces flow concentration and improves filling balance |
Submarine Gate |
Common in automatic production |
Can reduce gate marks, but gate size must be controlled carefully |
In many injection molding projects, the gate thickness is usually recommended to be around 50%–80% of the nominal wall thickness to maintain stable melt flow and avoid excessive shear velocity.
Temperature control is another important factor when analyzing how to fix jetting in injection molding. If the melt temperature is too low, the plastic material may cool too quickly after entering the cavity, making the unstable flow marks more visible. Increasing the melt temperature properly can improve material fluidity and help the melt spread more evenly.
Mold temperature also affects flow stability. When the mold surface is too cold, the first melt stream may freeze before fully bonding with the following material flow. For many engineering plastics, increasing the mold temperature by 10°C–20°C can help reduce jetting marks and improve surface consistency.
However, temperature adjustment should remain within the recommended processing range of the material. For example:
Material |
Recommended Melt Temperature |
Recommended Mold Temperature |
ABS |
220–260°C |
40–80°C |
PC |
260–320°C |
80–120°C |
PA6+GF30 |
260–290°C |
70–100°C |
PMMA |
220–250°C |
60–90°C |
If the melt temperature is too high, other plastic injection molding defects such as flash, burn marks, or material degradation may occur. Therefore, temperature optimization should always be balanced with injection speed, holding pressure, and mold design.
At Alpine Mold, we help customers reduce plastic injection molding defects through professional DFM analysis, Moldflow analysis, mold design optimization, and stable mass production support. With more than 20 years of injection mold manufacturing experience, our engineering team focuses not only on making molds, but also on improving molding stability, surface quality, and long-term production performance. If you are developing a new plastic product or facing molding defects in production, feel free to contact Alpine Mold for technical support and a custom injection molding solution.
No. Jetting usually appears as snake-like or worm-like marks near the gate area, caused by unstable melt flow at the beginning of filling. Flow lines are more general surface marks that follow the material flow direction and may be related to uneven cooling, low temperature, or flow speed changes.
Yes. Transparent materials such as PC, PMMA, and clear ABS can show jetting marks more clearly because light passes through the part and makes internal flow traces easier to see. For transparent parts, gate design, melt temperature, mold temperature, and polishing quality need stricter control.
Not always. Jetting may come from process settings, material flowability, or mold design. However, if the same defect appears repeatedly in the same position, especially near the gate, the gate size, gate direction, or runner layout should be reviewed.
Sometimes it can be improved by adjusting injection speed, melt temperature, mold temperature, or holding pressure. But if the root cause is poor gate location or unsuitable product structure, mold modification may be required to solve the problem more completely.
Yes. T1 trial is the best stage to identify jetting, gate-related defects, filling imbalance, and other plastic injection molding defects. If jetting is found early, the engineering team can adjust parameters or modify the mold before mass production starts
Jetting injection molding defects can compromise the quality and appearance of molded plastic parts. By understanding the causes and implementing the right solutions, plasic injection molding manufacturers can achieve better results. Increasing gate size, adjusting injection speed, managing resin viscosity, and monitoring holding pressure are all effective ways to combat jetting and ensure a smoother, higher-quality injection molding process.
