Plastic Part Analysis: The Future of Design and Manufacturing

Plastic Part Analysis: The Future of Design and Manufacturing

Analyzing plastic parts helps identify weaknesses early on, improves performance, reduces costs, and enhances reliability. This tool is vital for creating plastic parts that meet application requirements and perform reliably throughout their lifetime.

In our product development stage, we prioritize analyzing plastic parts to minimize manufacturing errors by 95% during the design phase, resulting in the production of cost-effective and high-quality parts. The following provides an explanation of the process and terminology involved in this analysis. To illustrate, we analyzed a standard stool model made of ABS material using Solidworks Plastic Simulation.

Plastic part analysis is a crucial step in ensuring the safety and reliability of plastic parts. By evaluating their performance under various loading conditions, potential weaknesses can be identified early in the design process, saving time and money. This valuable tool provides engineers and designers with the means to improve safety, reduce costs, enhance performance, and increase reliability. With plastic part analysis, we can create plastic parts that meet the application's needs and perform safely and reliably over their entire lifetime.

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.

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).

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 minimize 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 minimize 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 utilize 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.

Analyzing plastic parts is a crucial step in their design and manufacture, ensuring they meet specifications and are safe and dependable. Various methods are utilized for plastic part analysis, including Finite Element Analysis (FEA), which utilizes 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 utilizing plastic part analysis, designers and manufacturers can optimize their processes and create high-quality, safe plastic parts.


If your business is in need of a custom design drafting and or plastic part analysis project, please don't hesitate to reach out to us. We would be thrilled to discuss your requirements and collaborate with you to create a machine that meets your unique needs. Our team of specialists is always available to offer the necessary guidance and support for your projects. Feel free to contact us anytime at experts@appliedproject.com.

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