Problem-solving of Injection Molding – Weld Line

Problem description:

When two different melt flows fill and converge in the part cavity of a mold, it forms a weld line on the molded part, as illustrated in Figure1. In fact, beneath a weld line is a weld interface across the molding part thickness, as shown in Figure 2. Weld line is considered a defect of injection molded part not only from a cosmetic point of view but also mechanical strength.

Figure 1: Formation of weld line

Figure 2: Formation of weld interface

Root cause analysis:

Understanding the microstructure of thermoplastic material at weld line helps know how and why it becomes a concern.

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As illustrated in Figure 3, a single melt flow fills in the direction across part thickness with the phenomenon called fountain flow, with that plastic melt moves forwards and fills the part thickness section by pushing the polymers at the central region forward and those at the melt front region outwards. It renders polymer chains of the thermoplastic material locally oriented along the shape of the melt front.

Figure 3: Fountain flow and polymer chain orientation

In addition, as illustrated in Figure 4, when two different melt flows meet, the two melts stop flowing forward immediately, and so do their fountain flow behaviors. It results that the polymer chains at the two melt fronts keep their orientation at the instant when the two melt flows meet. In such a situation, the polymer chains near part surfaces are likely to orient more perpendicularly to part surfaces and those near the thickness center in a more random pattern. On top of that, it generates micro V-notch grooves on the part surfaces right on the weld line.

Figure 4: Polymer chains orientation and V notch at weld line

Cosmetic concern:

Therefore, there are several causes as follows that might make weld line visible and become a cosmetic concern.

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<1> As illustrated in Figure 4, the deeper the V-notch groove is formed, the more visible a weld line.

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<2> If a weld line is formed at the last-filling location, as illustrated in Figure 1- (b), and the trapped air in the cavity has no way to escape, the eventual high pressure of the trapped air at the weld line would be an adverse effect on molding a shallower V notch. It makes the weld line more visible.?

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<3> The compressed high-pressure cavity air at the last-filling location mentioned above often goes with a high air temperature caused by the adiabatic compression process or so-called diesel effect. The compressed air’s temperature might become so high to cause thermal degradation of the polymers at the melt front, leading to a deviation in the molded part’s color and a more visible weld line.

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<4> When using mold release agent, some amount of it on the part cavity surfaces would be pushed forwards by the two melt flows and eventually cumulate at the V-notch groove of the generated weld line. The existence of contaminants like release agents at the weld line obstructs the V notch to be shallower and might differ in color from the part material; both make the weld line more visible.??????????

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Mechanical strength concern:

On the other hand, the mechanical strength at the weld line area of an injection molding part is always a concern. The following are the reasons.

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<1> As illustrated in Figure 4, the existence of the V notch itself has been an adverse factor to the molding part’s strength from the mechanical point of view, especially when it must resist impact or bending force under an application. In such an application, very possible a crack happens to start at the V notch and then propagates across the part thickness, leading to fracture in the end.

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<2> As illustrated in Figure 4, on top of the existence of the V notch, the orientation of local polymer chains in the surface region next to the V notch is more likely to be perpendicular to part surfaces because of the fountain flow effect. It is unfortunate that such a polymer chain orientation facilitates the inward propagation of a crack at the V notch.?

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<3> As illustrated in Figure 4, the central region of part thickness at the weld line is where the randomly oriented polymer chains from the two individual melt flows converge. When the two melt flows converge, the polymer chains at the two melt fronts collide and become packed closer to each other. Upon the convergence of the two melt flows, those polymer chains are still in their original melt flows along and next to the weld interface. If the two melt flows converge by touch instead of collision, a thin air gap (or layer) might exist where the weld interface is supposed to be. No matter how strongly the two melt flows converge, the initial situation that the non-integrated part structure across the weld interface is an adverse factor to the weld line strength. The boundary between the two polymer chain groups across the part thickness would be the place and path where the part breaks once the crack propagates.

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<4> The two melt flows stop advancing right after they converge, and then the cooling work from the mold starts taking a significant effect on the part across the thickness. Before the material freezes, the polymer chains next to the weld interface creep over the weld interface into the other side, creating a more or less diffusive and entangled mixing state and eliminating the original weld interface, as illustrated in Figure 5-(b) and Figure 5-(c). How strongly the polymer chains diffuse into the other side and entangle with other polymer chains over there influences the strength of the weld line. As the amount of diffusion and entanglement decreases, the generated weld line gets weaker.?

Figure 5: Polymer chains diffusion and entanglement crossing weld interface

<5> If there is no way for the trapped air in the part cavity to escape during the melt filling stage, the air pressure would become higher and higher with less and less compressed air volume between the two melt flows. It might prevent the two melt flows from strong colliding convergence. That provides the polymer chains adjacent to the weld interface with a worse initial condition for their subsequent diffusion and entanglement, resulting in a less integrated structure crossing the weld interface.

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<6> Compared to using other grades of material in the same resin family, using a glass-fiber-reinforced material would improve most mechanical properties of an injection molding part; the higher content of glass fiber in the material, the more improvement in the molded part’s mechanical properties. However, it is not the case for weld line strength. On the contrary, the higher content of glass fiber in the material, the worse performance in the molding part’s weld line strength. Same as how polymer chains orient at the melt fronts, the orientation of glass fibers at the weld line section results from the fountain flow effect. It makes glass fibers at the two melt fronts orient perpendicularly to part surfaces next to the V notches and across part thickness where the weld line is, as illustrated in Figure 6. Unlike the polymer chains, which can flexibly diffuse crossing weld interface to the other side and wherein entangle other polymer chains, glass fibers are rigid bodies and stand still. They have an adverse effect on the molding part’s weld line strength because they act as a barrier to retard the diffusion and entanglement of polymer chains crossing the weld interface. The more glass fiber content in a material, the more negative effect brought on its molding part’s weld line strength.?

Figure 6: Glass fiber orientation at weld line

Corrective/Preventive solution:

Do your best to avoid the formation of the weld line; otherwise, make it invisible and strengthen it.

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In most situations, the more invisible a weld line is, the better its mechanical strength. The knowledge of the microstructure and behavior of thermoplastic material at a weld line underpins the root cause analysis of the weld line problem. By thinking about how to avoid a weld line, eliminate the micro V notch at the weld line, and achieve the diffusion and entanglement of polymer chains crossing the weld interface as more amount as possible, corrective/preventive solutions for a weld line problem might be developed from the five aspects of which an injection molding engineering system comprises - process condition, mold design/manufacture/construction, part design, plastic material, and injection molding machine, respectively.

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Process condition:

<1> Increase melt temperature –

For cosmetic concerns, increasing melt temperature contributes because the time required to freeze the material is longer, and the melt viscosity is lower. So, it’s more likely that the pack/hold pressure effect can reach the weld line area before the material freezes, pushing the molten-still material under the V notch outwards to paste onto the mold surface and eliminating the V notch.

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For mechanical strength concerns, increasing melt temperature contributes, in the same way, rendering the time required to freeze the material longer, which facilitates more amount of polymer chains’ diffusion and entanglement crossing the weld interface, leading to a more integrated microstructure therein. Meanwhile, if the micro V notch on the part surface is flattened, the mechanical properties at the weld line would be even stronger.

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Note: Do not increase melt temperature over the limit that the material manufacturer suggests.

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<2> Increase mold temperature –

Basically, mold temperature influences cosmetic and mechanical strength concerns of a weld line under a similar physical mechanism to melt temperature but with an even more significant effect.

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For cosmetic concerns, increasing mold temperature prolongs the time required to freeze the material. Therefore, the pack/hold pressure effect is more likely to reach the weld line area before the material freezes, pushing the molten-still material under the V notch outwards to paste onto the mold surface and flattening the V notch.

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For mechanical strength concerns, increasing mold temperature contributes in the same way by prolonging the time required to freeze the material, which facilitates more amount of polymer chains’ diffusion and entanglement crossing the weld interface, leading to a more integrated microstructure therein. Additionally, it would be easier for the pack/hold work to make the micro V notch on the part surface flat, resulting in better mechanical properties at the weld line area.

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Note: Do not increase mold temperature over the limit that the material manufacturer suggests.

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<3> Increase pack/hold pressure and time –

For cosmetic concerns, as pack/hold pressure and time increase, the chance increases that the material under the V notch of a weld line is packed outwards to paste onto the mold surface, making the V notch blunter, shallower, and more invisible.

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For mechanical strength concerns, as pack/hold pressure and time increase, its effect is more likely to reach the weld line area and pack the polymer chains at the melt fronts of the two flows closer to each other. It facilitates more polymer chains’ diffusion and entanglement crossing the weld interface, leading to a more integrated microstructure. If the micro V notch on the part surface is flattened in the meantime, the mechanical properties at the weld line area would be even better.

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Furthermore, once the pack/hold work takes effect on a weld interface, it’s unlikely that the pressures from the opposite sides of the two melt flows would be equal head-to-head. Therefore, it’s likely that the partial polymer chains and glass fibers near the part thickness center are stretched and oriented over the weld interface, as shown in Figure 7. As a result, it facilitates more polymer chains’ diffusion and entanglement crossing the weld interface. Such orientation of polymer chains and glass fibers in the part thickness center area would retard the propagation of the V-notch-induced surface crack across the part thickness.

Figure 7: Polymer chain and glass fiber orientation at weld line with effective pack/hold pressure and time

<4> Avoid using mold release agent –

The spreading mold release agent on the part cavity surface might eventually cumulate at the V-notch groove of the weld line after each injection molding shot. Besides, possibly it would be accompanied by a degraded and discolored state and make the weld line more visible and weaker because the cumulated contaminants obstruct the V notch from being packed blunter, shallower, and even flattened.???

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Mold design:

<1> Runner and gate system design vs. melt filling pattern –

For both cosmetic and mechanical strength concerns, design the injection mold’s runner and gate system in such a way that the resulting melt filling pattern makes no weld line on the part or forms the weld line at the place where cosmetic quality and mechanical strength performance are not concerned.

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Theoretically, a single gate design is the only possible way for a cold runner system to avoid weld line formation on the molding part. However, when a multi-gate one is required resulting in an unavoidable weld line concern, the gate number decision becomes critical, which should depend on the part size and thickness. The gate number should make all the individual melt flow lengths, from their source gates to the corresponding weld line positions, short enough in order that the melt temperature at weld line positions can remain high and the pack/hold work can effectively reach the weld line areas.?

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<2> Air vent system –

For cosmetic concerns, design an effective air vent system on the injection mold so that the enclosed air in the part cavity has ways to escape and is not eventually trapped at the weld line to cause the V notch and stop its elimination. An effective air vent system would make the V notch at the weld line as shallow and invisible as possible.

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For mechanical strength concerns, design an effective air vent system on the injection mold so that the enclosed air in the part cavity has ways to escape and is not eventually trapped at the weld interface retarding polymer chains’ diffusion and entanglement. Achieving so also facilitates the V notch at the weld line to become blunter and shallower; thus, the molded part is more resistant to mechanical loading at the weld line position.

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For example, place ejector pins or split mold inserts with air vent channels under the weld line position.

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<3> Runner length –

For cosmetic concerns, a shorter runner design is helpful for the melt temperature to keep high at the weld line. It also helps the pack/hold work from the machine nozzle reach the weld line area easier. Both facilitate the material under the V notch being packed outwards to paste onto the mold surface and thus render the V notch blunter, shallower, and more invisible.

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For mechanical strength concerns, design the injection mold’s runner short so that the melt temperature at the weld line is more likely to keep high. The high melt temperature gives the melt-front polymer chains more energy to achieve more diffusion and entanglement. A shorter runner also helps the pack/hold work from the machine nozzle reach the weld line area easier. It makes the melt-front polymer chains of the two flows packed closer to each other, which is good for the polymer chains to diffuse and entangle more and thus form a more integrated and loading-resistant structure at the weld line position of the molded part.

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<4> Runner and gate size –

Designing larger cross-section areas of an injection mold’s runner and gate creates the same benefits as a shorter runner by keeping the melt temperature high at the weld line and facilitating the pack/hold work to reach the weld line on the molding part. With the similar physical mechanism explained for the runner length design, it reasons that the larger runner and gate sizes also bring a positive effect on coping with both cosmetic and mechanical strength concerns.

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<5> Ejection –

Design an injection mold with larger draft angles and larger-and-balanced total ejection area. By doing so, the molders can avoid using mold release agents while applying a stronger pack/hold work to bring the positive effect above-mentioned on the weld line’s cosmetic and mechanical strength quality.

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Part design:

Let the part design rule the first priority that one should always design thicknesses as uniform as possible across the entire part, and then think about how a part design can go with the mold design to seek the elimination of the weld line. If the weld line is unavoidable, consider how a part design can help move it to a non-critical position from both cosmetic and mechanical strength points of view, not leave the V notch at the weld line, and enhance the polymer chains’ diffusion and entanglement crossing the weld interface. For example:

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<1> For single gate mold design, is it possible to add a part feature, such as a rib, acting as a melt flow leader to eliminate the weld line or divert it to a non-critical area of the part?

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<2> For multi-gate mold design, is it possible to create better conditions at weld line positions to form more invisible and stronger weld lines by slightly increasing part thickness locally or overall?

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Plastic material:

<1> Select a material with the reinforced glass fiber content as low as possible –

Generally, using a material with higher glass fiber content improves the mechanical properties of an injection molding part. However, it’s not the case for the weld line strength.

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As illustrated in Figure 6, as the glass fiber content of a material increases, more glass fibers adversely orient at the weld interface, resulting in a stronger barrier for the polymer chains to diffuse and entangle over the weld interface. Thus, as the material’s glass fiber content increases, the molded part’s weld line strength drops significantly. Additionally, the weld line might become more apparent because it’s likely that more heterogeneous glass fibers gather at the V notch.????

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<2> Select a material that releases residual powder and precipitates the least during a process –

It is advisable to work closely with the plastic material manufacturers/suppliers on this consideration because the related information is rarely revealed in the material’s datasheet.

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If a material releases a significant amount of residual powder and precipitate in front of the melt fronts during its injection molding process, those micro by-products might eventually cumulate at the V notch, rendering the weld line more noticeable and weaker. Besides, those micro by-products might form a deposit on the lands of air vent channels, block the air vent system, and make the air vent system in vain soon, which then leave the air trapped again at the V notch and between the two melt fronts, rendering the weld line more visible and weaker as well.

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Machine selection:

Changing injection molding machines might do little in solving a weld line problem. However, it might be helpful by equipping the injection molding machine with either of the following two devices.

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<1> Vacuum pump system –

Integrate a vacuum pump system to withdraw the enclosed air out of the mold before the melt injection starts. It would work by removing the trapped air and its resistance to the elimination of the V notch, resulting in a more invisible and stronger weld line. In such an application, the air vent system of the mold becomes unnecessary. On the contrary, a seal mechanism must be designed and built on the parting line surfaces of the mold. It renders the part cavity a completely closed space, and when the vacuum pump system is working, only the enclosed air is withdrawn out without sucking in the air in the environment at the same time.

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<2> Sequential-valve-gate hot runner system –

Integrate a sequential-valve-gate hot runner system to avoid weld line formation. It would definitely work for those weld lines caused by a multi-gate mold design. In such an application, only one valve gate opens at a time for the melt to flow through it and fill the part cavity while all the other valve gates keep closed and stop the melt from flowing through them until the melt from the neighboring valve gate has passed over them sequentially.

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Note: The discussion in this article is specifically for solving a weld line problem. If other quality issues are involved and must be solved at the same time, further cross-issue and case-by-case consideration might be required.

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~ Bridge between plastics injection molding knowledge and practice @ Effinno Technologies ~