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Typical Flow Patterns and Analysis in Injection Molding
2025-11-20
During the Injection Molding melt flow process, some flow patterns have been proven to be close to the actual flow state. These flow patterns form a fixed flow state within the mold, and the quality state formed under this flow state can be predicted. Therefore, some typical flow patterns have been listed to guide on-site process debugging andmold modification.
The flow pattern is the macroscopic manifestation of the melt filling the cavity, which directly determines the product's internal quality, appearance quality, and dimensional stability.
I. Unidirectional Flow / Sequential Flow
Flow characteristics: After the melt enters the cavity from the gate, it moves forward like an ever-expanding bubble with a flat flow front, advancing at a constant speed and in the same direction until it fills the entire cavity. This is the most ideal and healthiest flow pattern.
Impact on product quality:
1. Uniform molecular orientation: The molecular chains and fibers (if containing glass fiber) inside the product are uniformly oriented, resulting in low internal stress.
2. Dimensionally stable: Uniform shrinkage minimizes the risk of deformation (warping). Excellent appearance: Free from flow marks, air bubbles, and other appearance defects.
3. Effective pressure holding: Pressure is transmitted evenly, product density is consistent, and over-pressure holding or under-pressure holding is avoided.
Improvement measures:
Mold design stage: This is the core stage. This flow pattern is achieved as much as possible through reasonable gate location and quantity design. Verification is then performed using mold flow analysis software (such as Moldflow).
Process adjustments:
If this can't be fully achieved, the flow should be made as close to unidirectional as possible by adjusting the injection speed and switching the holding pressure point.
II. Runway Effect
Flow characteristics: When the melt flows through regions with different wall thicknesses, it will preferentially fill the thick-walled regions with less resistance, and then fill the thin-walled regions. Just like water flows, it will first fill deep trenches (thick walls) and then overflow shallow beaches (thin walls).
Negative impact on product quality:
1. Trapped gas: At the junction of thick and thin sections or in the last thin-walled area to be filled, trapped gas can easily form, leading to burning.
2. Flow marks/shrinkage marks: In thick-walled areas, the melt flow rate and cooling are slower, making it easy for flow marks to form on the surface, and shrinkage marks to occur due to uneven shrinkage.
3. High internal stress and warping: The different cooling rates and shrinkage rates in thick-walled and thin-walled areas lead to huge internal stress, which is the main cause of product warping and deformation.
Mold design improvement measures
Optimize product design to make the wall thickness as uniform as possible.
Add exhaust channels near the thick-walled areas.
Consider placing the gate in a thin-walled region to force the melt to fill from thin to thick.
Process adjustments:
A "slow-fast-slow" injection speed curve is adopted: the speed is appropriately slowed down when flowing through thick-walled areas to reduce turbulence; the speed is accelerated before entering thin-walled areas to ensure full filling. The mold temperature is appropriately increased to delay surface cooling in thick-walled areas, which is beneficial for pressure transmission and shrinkage compensation.
III. Hesitation Flow
Flow characteristics: When there are significantly different flow paths within the cavity, the melt will preferentially fill the path with low resistance and short flow, while almost "stagnates" at the front end of the path with high resistance and long flow. The melt seems to be "hesitating" whether to continue.
Negative impact on product quality:
1. Short shot: The flow path may not be filled due to the cooling and solidification of the melt front.
2. Surface flow marks/cold material: The melt flow rate in the stagnant area is extremely slow, and the temperature at the leading edge drops rapidly, forming cold material marks or obvious flow marks.
3. Polymer orientation and weak bonding lines: If the stagnant flow path is eventually filled, it will form a bonding line with the melt in the main flow path, and the bonding strength is very poor due to the low melt temperature.
4. Uneven density and warping: The material density is low in the stagnant area, and the shrinkage is different from other areas, which leads to warping.
Mold design improvement measures:
A balanced runner system ensures that all cavities or flow paths are filled simultaneously. Modify the product design to reduce wall thickness variations, or increase the wall thickness of stagnant flow paths/add runner channels. Adjust the gate location to reduce resistance to flow into stagnant areas.
Process adjustments:
Significantly increase injection speed: This is the most effective method. It allows the melt to rush through stagnant areas before cooling and solidifying. This increases both melt and mold temperatures, delaying cooling.
IV. Jetting
Flow characteristics: When the gate size is too small and faces a wide cavity, the high-speed injected melt will act like a "snake," spraying directly into the depths of the cavity without contacting the cavity wall. Subsequently, the following melt will push it back, fold and fill it.
Negative impact on product quality:1. Serpentine flow pattern: Obvious, folded jet patterns are left on the product surface, which seriously affects the appearance.
2. Disordered molecular orientation: The ejected melt is highly oriented and doesn't fuse well with the subsequent filling melt.
3. Decreased mechanical properties: The sprayed area has a loose structure and low strength. Air entrapment: It easily traps air within the folded melt.
Mold design improvement measures:
Change the gate design: Use a fan-shaped gate, a submarine gate, or adjust the gate direction so that the melt impacts the cavity wall or pin as soon as it exits the gate (impact injection), thus expanding into a smooth flow front. Increase the gate size.
Process adjustments:
Use low-speed injection through the gate: Initially, use a very slow speed to allow the melt to "guzzle" out of the gate, forming a sprue, before increasing the injection speed. Appropriately increase the mold temperature.
V. Weld Line
Flow characteristics: When the leading edges of two or more melt flows meet, they merge to form a line. The strength of the weld line varies depending on the temperature, pressure, and angle at the time of the meeting.
Negative impact on product quality:
1. Structural weakness: The weld line area is the weakest point of the product, with poor strength and appearance.
2. Appearance defect: A visible line forms on the surface, affecting the aesthetics.
3. Cavitation: If two molten materials trap air at their point of contact, cavitation will form, leading to scorching or short-circuiting.
Mold design improvement measures:
1. Optimize gate location: By adjusting the gate location, the position of the weld line can be changed and moved to a non-visual surface or non-stress area.
2. Add venting channels: Venting channels are opened at the locations where the weld lines are expected to be formed to promptly discharge gas and cold material.
3. Use hot runner sequence valves: By controlling the opening sequence of each gate, eliminate weld lines or change their positions.
Process adjustments:
Increasing melt temperature, mold temperature, and injection speed: The core is to increase the temperature and fluidity when the melts meet, promoting the mutual diffusion and entanglement of molecular chains, thereby improving the weld strength.
Increase holding pressure and time: fully compress the weld line area.
On-site process debugging guidance procedure: When product quality problems occur, process personnel can follow the following logic for diagnosis and rectification:
1. Observation and Analysis: First, conduct a short-shot test (injecting 90%-95% of the mold cavity) to clearly observe the melt flow pattern. Identify which defective pattern(s) are dominant.
2. Identify the root cause: Mold-related causes: unreasonable gate location/type, uneven wall thickness, unbalanced runners, poor venting. Process-related causes: improper injection speed, temperature, and pressure settings.
3. Develop countermeasures: Prioritize process adjustments as this is the most cost-effective method. For example, if stagnation occurs, try significantly increasing the speed; if jetting occurs, try decreasing the speed; if weld lines occur, try increasing the temperature and speed. If the process can't resolve the issue, propose mold modification solutions: clearly relay the analysis results (e.g., "Severe stagnation exists in area XX; it is recommended to increase venting or modify the wall thickness") to the mold department. The mold flow analysis report is the most compelling evidence at this stage.