Injection molding is a popular manufacturing method for plastic parts. This technology is efficient for high-volume production, maintaining repeatability and cost-effectiveness. However, any deviation in tooling, process variables, and raw material quality causes various defects in molded parts. A flow mark in injection molding is one major defect that influences the finish and aesthetic appeal.
Understanding the nature of flow marks can help you produce defect-free parts exact to your design specifications. So, let’s discuss types of flow marks, their causes, and prevention strategies in detail.
What is a Flow Mark in Injection Molding?
Flow marks, also known as flow lines, are the wavy lines, streaks, or patterns developed on the surface of the plastic material during the injection molding process. Although it does not affect the structural integrity of the material, it is a cosmetic defect that can reduce the aesthetic appeal of the product, resulting in lower perceived quality and consumer acceptability.
The flow lines usually follow the direction of melt flow inside the cavity and are commonly found near gates or around sharp corners. It indicates poor process control, as it is caused by various factors leading to uneven flow patterns of the melt plastic, resulting in inconsistent cooling.
Types of Injection Molding Flow Marks
These marks on the surface of molded parts/products can be categorized into different types based on their appearance and formation mechanism.
Let’s discuss wave, jetting, radiation lines, periodic bands, weld lines, and other types of flow marks in injection molding.
Wave Flow Marks
It is a wavy streak along the flow direction of the feeding charge. Faster cooling and solidification of the outer layer while the inner layer flows inside the mold, resulting from sudden cooling at the walls of the mold or non-uniform flow velocity, cause this type of flow mark to appear. These types of flow marks are commonly found on large, flat, and glossy surfaces, including electronic casing and automotive interior panels.
Jetting Marks
It is a small, irregular snake-like streak leading from the injection gate. It happens when the flow of the fluid is too high and gets turbulent, where the charge does not get enough time to spread evenly. The marks resemble the squirting of the molten charge from the gate. It is typically found in components with narrow gates followed by wide openings of the cavity, such as connectors, automotive bumpers, and precision components.
Radiation Lines
These ring-shaped lines appear radiating outwards from the injection gate like pebbles dropped in a pond. This is caused primarily due to uneven cooling of different layers of the charge as the layer touching the walls cools and solidifies faster than the inner layer.
Lower mold temperature, along with slower speed and pressure gradient due to the runner, can trigger the creation of radiation lines during injection molding. These lines are created on parts with a central gate with plastic flow radiating outwards, which comprises circular or radial parts such as bottle lids, round housings, and gear hubs.
Cloud-Like Flow Marks
These are streaky lines or cloudy or milky patches formed primarily linked to inconsistent and contaminated material properties, poor mix, and temperature variation in the mold. These patches are mostly found on transparent parts such as lenses, medical devices, light diffusers, and display covers.
Periodic Bands or Tiger Stripes
Periodic bands or tiger stripes refer to the cyclic pattern of alternating bands that are perpendicular to the flow direction. The bands may be different in transparency, color intensity, or gloss. It occurs commonly due to melt flow instability, cyclic fluctuation in injection parameters, and interaction of the charge with additives, including fillers and colorants. It is common in parts having a longer flow path or a thickness variation and using a filled or pigmented charge. This includes components such as thin-wall containers, long automotive parts, and coloured housings.
Weld Line Like Flow Marks
Weld lines, also known as mend lines or knit lines, are common types of surface defects appearing as a thin visible seam or line at the junction of two flow fronts. It is more common in parts where the plastic flow has gone around any obstacle, causing the flow to split and rejoin on the other side. Too low temperature or pressure can also hinder the proper fusion between the two flow fronts, leading to weak bonding and the creation of weld lines.
The Common Causes of Flow Marks
The causes of flow marks in injection molding are related to errors in design, feed temperature, process variables, gate & runner location, and raw material quality. You must consider these causes while setting up the injection molding machine and handling the process.
Low Mold Temperature
The low mold temperature is related to the viscosity of molten plastic; it reduces the flowability. As the temperature varies, the material at the surface solidifies faster than the material in the core. This uneven cooling causes the formation of flow marks.
The section that cooled earlier is more prone to marks, like the area near the gate. That’s why molds are often heated before and after the injection process to a certain temperature.
Low Injection Speed and Pressure
Low injection speed leads to differences in initial flow rate and the rate at which it reaches the mold, which changes the flow patterns of molten layers and forms flow marks. Additionally, it is important to maintain the hold pressure once plastic is injected for the homogeneity of flow.
Furthermore, low machine injection pressure leads to non-uniform filling of the mold cavity, and the injected feed can not be compact or dense.
Incorrect Gate and Runner Size
If the gate and runner are thinner than required, they cause material flow issues. Small sizes increase the mold filling time, and some amount of material loses heat. Consequently, a narrow runner reduces the speed and pressure, eventually forming the flow lines.
The incorrect placement of the mold gate can also form wavy patterns on the surface. Because it affects the uniform distribution of material inside the mold cavity.
Material Quality Issues
Quality factors of raw molten resins, such as low melting temperatures, material contamination, moisture, inadequate additive dispersion, and the presence of volatile gases, can also result in flow marks.
Impurities or degraded particles disrupt uniform flow and cooling. These irregularities create streaks or lines on the surface of the molded part.
Mold Lubrication
Not having enough lubrication on the surface of the mold cavities affects the flowability of the material. It slows down the melt flow and spreads different patterns. Subsequently, the injection molding machine needs to apply higher injection pressure due to a lack of lubrication.
Equipment Related Causes
Setting and variables of the injection molding equipment also causes the flow lines, such as low nozzle & barrel temperature, improper cooling system, and short cycle times. Therefore, you must check the settings of the machine and optimize for defect-free molding.
How to Prevent Injection Molding Flow Marks?
To prevent the flow marks in injection molding, you must take a proactive approach. It requires optimization of an injection mold for flowability, use of correct tooling & molding parameters, and process improvement.
Let’s look at the common strategies to prevent injection molding flow marks.
- Increase Mold Temperature: Maintain a high and stable mold temperature, which improves the flowability of the molten charge and restricts the flow line formation.
- Use Cold Sludge: Place a cold sludge in front of the gate so it can collect cool plastic and restrict its entry to the cavity.
- Correct Injection Speed: High-speed injection helps to fill the mold faster and solidify uniformly. Therefore, increase it after analyzing the scenario.
- High Injection Pressure: Set a high injection pressure
- Place Nozzle & Gate Close: A Short distance between the plastic injection nozzle and the gate does not allow the molten plastic to cool quickly. So, place them close to each other.
- Apply Back Pressure: Applying backpressure during the process maintains a consistent flow and compactness of molten material. So, increase it as per the requirement.
- Correct Mold Design: Optimize mold design parameters like gate location and size, runner system, and edges & corners for flowability of injected plastic.
Risk of Flow Marks and Mold Design Considerations
The size of the runner and gate, their location, sharp cavity edges, part thickness, venting, texture of cavity wall, and many other mold design features influence the flow characteristics and formation of visible lines or marks. Thus, designing is a critical stage that requires the above considerations to ensure the stable flow.
The list below shows the common design practices & considerations to avoid flow lines injection molding defects.
Choose the Right Type of Mold and Material
There are many types of injection molds, such as single, multi-family, cold runner, and hot runner molds. Choose the right one based on the raw plastic type and production needs. Additionally, the mold material should be strong enough to withstand the high thermal stress.
Select Correct Gate Type, Size, and Location
Position the gate at the thicker section of the part and choose the right gate type that can promote the flowability. For example, a valve gate can offer precise control. Meanwhile, makes the gate wider to avoid any restriction in the desired flow rate.
Uniformity in Part Thickness
Uniform wall thickness in part design enhances the smooth flow without any sudden transition. Therefore, it is recommended to limit the thickness variation within ±15%, unless it is absolutely necessary.
Incorporate a Proper Venting System
Place vents along a long flow path or where air is likely to be trapped. You also need to use multiple vents with the correct depth and sizes. Incorporating micro-vents into ejector pins can also help gaseous to escape easily.
Use Cooling Channels
The distribution of cooling channels must be uniform across the mold cross-section. The size of the channels must be selected based on part size and cooling requirement. You can run thermal simulations on mold channels while designing for optimization.
Choose the Right Texture for Cavity Walls
Since the exterior of the molded part is directly related to the cavity wall texture. The mirror finish makes marks more visible, even though there is a low flow line frequency. But matte texture can hide their appearance.
If you are further interested in mold design, read our detailed “injection mold design guide”, which covers its various aspects in detail.
Get Your Defect Free Injection Molding Parts from RapidDirect
As we discussed in this article, producing plastic parts free from flow marks or any other defects requires careful handling of every molding stage. Only advanced injection molding capabilities and professional expertise can deliver this. Therefore, we have advanced equipment run by injection molding experts at RapidDirect.
We consider the possibility of marks, jetting, flash, wrapping, and all other injection molding defects from the very beginning of mold design to the end of production. Additionally, our experience in this industry can also help you to find the solutions to any challenges that might come during process execution.
Our injection molding services are also flexible in production volume and shipping locations. For pricing, lead time, and any further information, upload your design and request a quote. We will provide you with an instant and accurate estimation.
FAQS
You can identify flow lines injection molding parts by visual inspection; they appear as wavy, streaky, or ring-shaped patterns on the surface.
To reduce flow marks, optimize the mold design, mold temperature, injection speed, pressure, and cooling system. Ensure back-pressure, holding pressure, and good quality of molten feed.
To fix lines in molding, you need to address the errors in design, molding equipment setup, tooling, injection variables, mold temperature, and raw material quality.
The root cause of flow marks in molding parts is instability in filling and solidification, which changes the flow patterns and form marks or lines.
