SOLIDWORKS Mold Flow Analysis

The $2.3 Million Question That Changed Everything

Picture this: It’s 3 AM on a Friday, and Sarah Chen, a senior design engineer at a leading consumer electronics company, is staring at her computer screen in disbelief. The latest batch of plastic housings for their flagship product—1,000 units that took weeks to manufacture—had just failed quality inspection. Sink marks marred the surfaces, weld lines created weak points, and warpage made assembly impossible. The cost? A staggering $2.3 million in wasted materials, production delays, and potential customer dissatisfaction.

Sound familiar? If you’re nodding your head, you’re not alone. This scenario plays out countless times across manufacturing floors worldwide, where traditional trial-and-error approaches to plastic part development continue to drain budgets and test patience. But what if Sarah had possessed a crystal ball—a way to peer into the future and see exactly how molten plastic would behave in her mold before a single pellet was heated?

Enter SOLIDWORKS Mold Flow Analysis—the closest thing we have to that crystal ball.

At the heart of our product development process lies a crucial step that often determines the difference between manufacturing success and costly failure: analyzing plastic parts before they’re born. This isn’t just another checkbox in the quality assurance process—it’s a game-changing methodology that helps us identify weaknesses early, boost performance, cut costs by up to 95%, and ensure reliability that extends throughout a product’s entire lifecycle.

In today’s hyper-competitive manufacturing landscape, where companies in the electronics sector alone are achieving average cost reductions of 5.8% through advanced analysis techniques, the question isn’t whether you can afford to implement mold flow analysis—it’s whether you can afford not to. With manufacturing costs exceeding $2.3 trillion annually in the U.S. alone and 78% of manufacturing executives citing cost management as their top priority, the stakes have never been higher.

The Hidden Cost of Getting It Wrong

Consider the ripple effect of that 3 AM revelation Sarah experienced. Beyond the immediate $2.3 million loss, her company faced:

Time-to-market delays that could cost millions in lost revenue
Supply chain disruptions affecting downstream assembly operations
Customer confidence erosion from potential quality issues
Engineering team morale impacts from having to restart the development process
Competitive disadvantage as rivals gained market share during the delay

Industry data reveals that injection molding defects affect approximately 15-20% of plastic parts in traditional development processes, with each defect potentially costing anywhere from $50,000 to $5 million depending on the complexity and scale of the project. More alarming still, 60% of these defects could have been prevented through proper flow analysis during the design phase.

The Science Behind the Magic

SOLIDWORKS Plastic Simulation operates on a sophisticated understanding of polymer physics, fluid dynamics, and thermodynamics. When we inject molten plastic into a mold, we’re orchestrating a complex dance of variables:

Material viscosity changes as temperature fluctuates
Pressure distribution affects how completely the mold fills
Cooling rates determine dimensional stability and stress patterns
Flow velocity influences molecular orientation and final part strength

By leveraging computational fluid dynamics and finite element analysis, SOLIDWORKS Plastic Simulation can predict these interactions with remarkable accuracy. The software doesn’t just show us what will happen—it shows us why it happens and, more importantly, how to prevent problems before they occur.

A Journey Through Polymer Transformation

In this comprehensive exploration, we’ll embark on a detailed journey through the intricate world of plastic part analysis, breaking down complex processes and terminology using a standard stool model made of ABS material as our practical example. Whether you’re a seasoned engineer looking to refine your approach, a designer seeking to understand the manufacturing implications of your choices, or a project manager trying to grasp why mold flow analysis deserves a place in your budget, this guide will illuminate the science and strategy behind creating plastic parts that don’t just meet specifications—they exceed expectations.

We’ll walk through every critical stage of the analysis process:

Fill Time Analysis: Understanding how molten plastic flows through mold cavities and identifying optimal flow patterns
Injection Location Optimization: Strategic gate placement for uniform filling and pressure distribution
Air Trap Prevention: Eliminating voids that can cause burn marks and incomplete filling
Weld Line Management: Predicting and optimizing locations where flow fronts merge
Temperature Dynamics: Managing heat distribution for consistent part quality
Pressure Mapping: Ensuring adequate packing without over-pressurization
Cooling Optimization: Minimizing cycle times while maintaining dimensional accuracy
Defect Prevention: Proactively addressing sink marks, warpage, and other common issues

Through real-world examples, detailed visualizations, and practical insights gained from thousands of successful analyses, you’ll discover not just the technical mechanics of mold flow analysis, but the strategic thinking that transforms good designers into great ones.

The mold flow analysis process involves sophisticated computational fluid dynamics that predict how molten plastic behaves within mold cavities 2 3. Modern SOLIDWORKS Plastic Simulation capabilities have evolved significantly, with 2024 enhancements including automated injection location advisors and batch processing capabilities that streamline the analysis workflow

Quantified Impact and ROI Metrics

Quantified benefits of SOLIDWORKS Mold Flow Analysis across different manufacturing metrics and industries

The financial benefits of implementing comprehensive mold flow analysis are substantial and measurable across multiple industries 6 21 24. Companies utilizing SOLIDWORKS Plastics report significant improvements in manufacturing efficiency, with electronics manufacturers achieving average cost reductions of 5.8% and industrial equipment manufacturers seeing 6.2% cost savings 21. The 95% reduction in manufacturing errors during the design phase represents a transformative shift from reactive to proactive manufacturing approaches 18.

FILL TIME

The diagram presented above illustrates the manner in which molten plastic courses through a cavity in a mold during the initial stage of injection molding. The areas shaded in blue denote the point at which the flow front commences, while those shaded in red indicate the position of the flow front during the filling phase animation or the end of the filling process when a “short shot” occurs. The filling pattern begins at the center (blue) and culminates at each corner of the part (red). The color spectrum on the left-hand side of the image indicates how long it takes for the flow front to reach each cavity region. Notably, the duration of the filling process hinges on the flow length and other related factors; hence, the corners farthest from the injection site fill last.

INJECTION LOCATIONS

The injection location is the point where melted plastic enters a mold cavity. The “end of fill” is the last area of the cavity to be filled, typically located in the thinnest wall sections or furthest from the injection location. When the flow length is too long for an injection location on either end of the part to fill the mold cavity, it’s best to move the injection location to a central point. This will reduce injection pressure requirements and result in a more uniform filling pattern from the injection location to the end of fill. However, changing the injection location can cause a non-uniform filling of the cavity, where the melt reaches one end before the other.

To find the best injection location, review the fill time plot to ensure the extremities of the cavity are filled simultaneously. For example, the injection location located at the middle of the handle’s left edge fills the lower section before the head of the drill casing. An uneven filling pattern like this can lead to non-uniform packing and volumetric shrinkage, and could cause post-molding problems like warpage in the part.

AIR TRAPS

When plastic material is being molded, it’s important to make sure that any air in the mold cavity is vented out during the filling stage. If the air is trapped, it can prevent the plastic from filling the area where the air is trapped, causing incomplete filling and packing. In some cases, the trapped air can even become compressed, ignite, and cause burn marks on the molded part or damage to the mold core and cavity surfaces. To reduce or prevent air traps, it’s helpful to use a parting line vent, ejector pin, cavity insert, or a porous metal insert at these locations. However, it’s best to avoid air traps altogether if possible.

WELD LINES

When two or more plastic melt fronts merge, they form weld lines. These lines can arise due to various factors such as mold shut-off surfaces, mold core features, multiple injection locations, or wall thickness variations that cause flow front promotion or hesitation. Weld lines tend to be weaker than other areas, and they often lead to visual defects. Furthermore, they can serve as stress concentrators in the molded part.

Weld lines usually form 180 degrees opposite to the point where the melt front meets the standing core of a shut-off surface. It is not possible to eliminate weld lines entirely, and they are inevitable in parts that have through-holes or multiple injection locations. One can only change the injection location to modify the weld line.

VELOCITY VECTOR AT THE END OF FILL

The fill plot’s velocity vector shows the molecular orientation of melted plastic as it flows through the mold part cavity. Spherical fillers create a more consistent distribution of mechanical properties in both the flow direction and perpendicular to it. When using fillers with high aspect ratios, the mechanical properties differ in the flow direction compared to the perpendicular direction. Materials with high aspect ratio fillers typically have better properties in the flow direction and lower mechanical properties perpendicular to it.

PRESSURE AT THE END OF FILL

When filling the cavity with molten plastic, the forward injection velocity of the screw is controlled to achieve the necessary pressure. As the injection pressure travels through the plastic, there is a drop in pressure along the flow length, which can be measured at the end of the fill to determine if the cavity has been evenly filled. The pressure drop is affected by the length of the flow, the thickness of the part’s walls, and the viscosity of the molten plastic. Thin-walled injection molded parts require high pressure due to greater flow resistance through the smaller cross-sectional area. If a short shot is detected, the injection location should be placed near the middle of the part to reduce the flow length by about half and decrease the injection pressure requirements. If the injection location is near the end of the part, the flow length is essentially the entire length of the part. Placing the injection location in the middle reduces flow lengths and injection pressure requirements, even though the plastic flow must travel in two directions.

TEMPERATURE AT END OF FILL

After the filling process, a thin layer of frozen plastic is formed on the cavity wall that has been cooled to the mold’s temperature. This layer’s thickness is not affected by the part wall thickness, but rather by the temperature difference between the melt and mold, as well as the thermal conductivity of the material.

BULK TEMPERATURE AT THE END OF FILL

In the fill stage, the melt temperature changes are determined based on various factors such as time, distance from the cavity wall, and part wall thickness. Once the fill is complete, the Bulk Temperature plot shows how much the melt temperature has changed from the initial set temperature. The blue color on the plot indicates stagnant material which has significantly cooled by the end of fill, while the red color represents plastic material with a velocity just before filling that retains heat.

TEMPERATURE GROWTH AT END OF FILL

During the injection moulding process, the polymer melt experiences shear heating during the fill stage. This causes an increase in temperature due to elevated shear rates, which can lead to the melt temperature exceeding the set melt temperature within the cavity. The temperature increase may be caused by short filling times, small injection locations, or material flow characteristics. If the conditions become extreme, the material may degrade.

SHEAR STRESS AT END OF FILL

Shear stress refers to the amount of force applied per unit area in a direction parallel to the plane of the force. This force acts as a push towards the direction of flow, rather than pushing outward from the wall. The formula for calculating shear stress is T = F/A, where t represents shear stress, F represents applied force, and A represents the cross-sectional area of the material parallel to the applied force vector.

Stationary Wall.webp
In the context of a moving wall sliding past a stationary fluid, the wall drags the fluid along with it, applying more shear stress to the liquid in contact and minimal stress to the fluid furthest away from the stationary wall. This is different from plastic flow through a cavity, where the wall remains stationary and the plastic melt moves along the cavity wall. Think of the stationary wall in the diagram as the center of flow through a cavity, with the material in the center of flow moving with less resistance than the material along the cavity wall, which experiences greater flow resistance. Ultimately, the extra force required to flow along the cavity wall relates to the higher shear stresses, with the material in the center of flow experiencing far less shear stress due to less resistance to flow.

SHEAR RATE AT END OF FILL

The shear rate is a measure of the speed of a fluid layer as it moves over another layer of fluid that is moving at a different velocity. When frozen plastic material comes into contact with the cavity wall, it doesn’t move in relation to the wall, resulting in a shear rate of zero (0.0 1/sec). In contrast, the molten plastic material inside the frozen layer moves over the frozen material, creating a positive shear rate (>0.0 1/sec).

As the shear rate increases, it reaches a maximum just inside the wall before decreasing towards the flow centre, where it experiences a local minimum. This minimum occurs because the polymer chains at the centre of the flow move at the same speed and don’t move relative to each other, resulting in a shear rate of zero (0.0 1/sec).
Cavity Wall.webp
The graph shows the movement of polymer chains as they slide past each other at different velocities, resulting in a positive shear rate. For instance, the polymer chains that freeze along the cavity wall don’t move (outer minimums), but the molten polymer chains that flow past them induce an extremely high shear rate (maximums). The two polymer chains at the flow centre move at the same velocity, which doesn’t produce any shear (centre minimum).

VOLUMETRIC SHRINKAGE AT END OF FILL

During the molding process, it’s important to pay attention to the volumetric shrinkage at the end of fill. If there are high shrinkage rates in thick sections of a plastic part, it may indicate potential concerns. This occurs when the part doesn’t undergo sufficient packing stages. If there’s no adequate packing stage, there will be high volumes of shrinkage in the areas marked in yellow and red by the volumetric shrinkage at the end of the fill plot.

Another type of shrinkage can occur when voids are formed. These voids appear as bubbles in the wall of transparent molded parts. They aren’t air bubbles, but rather vacuum voids. A void forms when the part surface is rigid enough to maintain its shape, and the molten core material separates from the inside, creating a vacuum void. Voids can also happen in opaque plastic parts, but they’re not visible from the outside. To see if voids are happening, the molded part must be cut open. Typically, voids occur in thicker sections and in areas where the wall thickness changes (like around the rim of a boss or along the transition from a part wall to a rib).

FREEZING TIME AT END OF FILL

The freezing time scale used at the end of the filling process refers to the time it takes for the molten plastic material to cool down to its glass transition temperature. The time required depends on the difference in temperature between the melt and mold, as well as the thermal conductivity of both materials.

It’s important to note that cooling the part to its ejection temperature doesn’t necessarily require the material to be reduced to its glass transition temperature. The material’s deflection temperature under flexural load is what determines the ejection temperature. This temperature is typically around 2/3 of the material’s glass transition or melt temperature, measured in degrees Kelvin.

COOLING TIME

During the cooling stage, the material’s temperature is lowered to its deflection temperature under flexural load, known as the ejection temperature. This stage usually takes up 70% of the total cycle time. Two main factors that impact the cooling time are the melt temperature and mold temperature. If either temperature is increased, the cooling time is usually prolonged. Plastics take a longer time to cool because they have low thermal conductivities and act as good insulators. The cooling time is proportional to the square of the part wall thickness, meaning that doubling the thickness results in a four-fold increase in cooling time. To minimise cooling times, it’s advisable to make the part wall thickness uniform and as thin as safely possible.

TEMPERATURE AT END OF COOLING

To determine the temperature at the end of the cooling process, we need to consider the ejection temperature. This is the point when 90% of the part volume is below the material deflection temperature under flexural load. If there are thick regions in the part with varying temperatures, it may lead to issues such as sink marks, internal voids, or warpage. To avoid this, it’s recommended to design the part with a uniform wall thickness.

SINK MARKS

Depressions on the surface of an injection-moulded plastic part are known as sink marks. The reason for sink marks is that there is not enough polymer molecules packed into a part to make up for the shrinkage. Thicker sections of a part take longer to cool than thinner sections, which results in significant shrinkage in the thicker branches. Once the outer plastic material has cooled and solidified, the molten core material needs to transfer heat through the solidified plastic surface to the cavity wall. However, plastic materials are poor heat conductors, which slows down the cooling rate of the thicker core volumes. As a result, the more time a plastic material takes to cool, the more it will shrink. The high degree of shrinkage in the core volume pulls the part’s surface inward, causing depression on the part surface.

To minimise sink marks, you can follow these design rules:

When possible, design with uniform part wall thickness.
Place injection locations at thicker sections of the part to allow for higher pressures and better packing of the thicker areas.
Avoid using injection locations that are too small, as they prevent sufficient packing of the part cavity.
Ribs and bosses should be around 60 to 80 per cent of the nominal wall thickness.

INJECTION LOCATION FILLING CONTRIBUTION

If you use only one injection location, the cavity will be filled by material from that location alone. However, if you use multiple injection locations, the cavity will be partially filled by material entering from each location. This will result in a significant weld line at the interface of the green and blue regions, where the flow fronts merge.

EASE OF FILL

To determine the success of cavity filling, you can utilise the ease of fill plot. The green areas indicate that the cavity can be filled using normal injection pressures. The yellow regions indicate that the injection pressure has exceeded 70% of the machine’s maximum injection pressure. The red regions indicate that the injection pressure has exceeded 85% of the machine’s maximum injection pressure.

If the ease of fill plot shows yellow or red areas when simulating a part cavity (without runners), you may need to adjust certain factors to decrease the pressure required for filling. Try increasing the wall thickness, changing the injection location, adding more injection locations, modifying the material, or adjusting processing parameters. It is important to do this to ensure successful cavity filling.

Analysing plastic parts is a crucial step in their design and manufacture, ensuring they meet specifications and are safe and dependable. Various methods are utilised for plastic part analysis, including Finite Element Analysis (FEA), which utilises computer-aided engineering to simulate physical systems, material testing to establish properties such as strength, stiffness and toughness, and failure analysis to investigate the cause of plastic part failure. Implementing these techniques helps engineers to improve plastic part design and manufacturing, guaranteeing their safety, reliability, and adherence to regulations. Plastic part analysis also benefits the industry in several ways, including identifying weaknesses for improvement, reducing costs, and eliminating defects to improve quality. By utilising plastic part analysis, designers and manufacturers can optimise their processes and create high-quality, safe plastic parts.

Conclusion: From Costly Mistakes to Manufacturing Mastery

As we reach the end of our comprehensive journey through SOLIDWORKS Mold Flow Analysis, let’s return to Sarah Chen’s story—but with a different ending.

Armed with the knowledge and tools we’ve explored throughout this analysis, Sarah’s Friday night looked very different. Instead of staring at failed parts in disbelief, she was reviewing the final validation results from her mold flow simulation, confident that Monday’s production run would yield perfect parts on the first try. The simulation had predicted potential weld line issues in the original design, allowing her team to relocate the injection points and optimize wall thickness distribution. Temperature analysis revealed optimal cooling channel placement, reducing cycle time by 18% while ensuring dimensional stability. Most importantly, the volumetric shrinkage analysis guided material distribution adjustments that eliminated the sink marks that would have plagued the original design.

The result? Sarah’s project delivered:

Zero defective parts in the first production run
$2.3 million saved in avoided waste and rework
Three weeks accelerated time-to-market
18% cycle time reduction for ongoing production efficiency
Enhanced team confidence in design decisions

This transformation from reactive problem-solving to proactive design optimization exemplifies the power of SOLIDWORKS Mold Flow Analysis. But Sarah’s success story represents just one of thousands happening across industries worldwide.

The Compounding Value of Analytical Excellence

The true value of mold flow analysis extends far beyond preventing individual failures. Companies implementing comprehensive plastic simulation strategies report:

95% reduction in manufacturing errors during design phases
10-30% optimization in overall cycle times
5.8% average cost reduction in electronics manufacturing
Improved part quality with enhanced structural integrity
Faster design iterations with virtual validation capabilities
Enhanced competitive positioning through superior product reliability

Industry leaders like Minebea AccessSolutions have documented how Autodesk Moldflow simulations resolved critical warpage issues that traditional parameter adjustments couldn’t address, ultimately meeting specifications and preventing costly mold rework. Similarly, companies utilizing gas-assisted molding have optimized complex part geometries through simulation, eliminating welding lines and shrinkage marks before ever cutting steel.

The Strategic Imperative

In an era where manufacturing agility determines market success, the question isn’t whether mold flow analysis provides value—it’s whether your organization can afford to compete without it. As global supply chains face increasing pressure, material costs continue to fluctuate, and customer expectations for quality reach new heights, the companies that thrive will be those that leverage predictive analysis to make informed decisions before problems occur.

Consider the broader implications:

For Design Engineers: Mold flow analysis transforms uncertainty into confidence. No longer do you need to rely on experience and intuition alone—you have data-driven insights that validate design decisions and optimize performance parameters.

For Project Managers: Budget predictability becomes achievable when you can identify and resolve issues in the virtual world rather than on the production floor. Timeline compression through first-time-right manufacturing delivers competitive advantages that compound over multiple product cycles.

For Manufacturing Leaders: Operational efficiency gains through optimized cycle times, reduced scrap rates, and predictable quality metrics directly impact profitability and customer satisfaction. The ability to guarantee production outcomes builds trust throughout the supply chain.

For Organizations: The cultural shift from reactive firefighting to proactive optimization creates learning organizations that continuously improve and innovate. This capability becomes a sustainable competitive advantage that compounds over time.

The Path Forward

As we conclude this exploration of SOLIDWORKS Mold Flow Analysis, remember that mastery comes through application. The concepts we’ve discussed—from fill time optimization to temperature management, from air trap prevention to weld line control—represent tools in a comprehensive toolkit for manufacturing excellence.

Start with your next project. Begin with simple analyses—perhaps a fill time study or injection location optimization. Build your understanding incrementally, letting each successful simulation reinforce the value of the methodology. Gradually expand to more complex analyses as your confidence and expertise grow.

Most importantly, embrace the mindset shift from “How can we fix this problem?” to “How can we prevent this problem?” This transition from reactive to proactive thinking is where the true transformation occurs—not just in your parts, but in your entire approach to product development.

The technology exists. The knowledge is available. The only question remaining is: Will you be the engineer who discovers problems at 3 AM on Friday, or the one who prevents them on Tuesday afternoon during design review?

The choice—and the future of your manufacturing success—is in your hands.

Ready to transform your plastic part development process? The journey from costly mistakes to manufacturing mastery begins with your next mold flow analysis. Don’t let another Sarah Chen moment happen on your watch.