Overcoming Issues with Multi-Shot Injection Molding

In this article, the filling analyses of ethylene propylene diene monomer, or EPDM, and PP+GF30 are investigated. Multi-shot injection molding, in particular, is challenging to work with, which makes it important that glass fibers are oriented to the flow direction, as this affects the part deformation. It would not be feasible to perform EPDM filling if the distorted piece is not fitted accurately while being inserted to the other compartment.

The inability to fill a fine-layered EPDM onto PP+GF30, in a completely balanced way, is another challenge. Moldex3D has been used to perform various analyses on a gasoline tank casing mold. These results helped to predict and identify potential problems and allow appropriate modifications to be made, saving time and cutting net production costs.

Challenges

  • Undesirable deformation after PP+GF30 filling
  • Accurate gate locations and a cross-section of the passage for EPDM filling
  • Sufficient EPDM filling amount, to compensate the quantity and also the interval of the hot runner

Solutions

Moldex3D can help achieve exact design modifications, which lead to smaller warpage in the primary filling (PP+GF30) and excellent filling behavior, without short shot in the second filling (EPDM). It also offers a high mesh level option that results in more accurate (i.e. close to 100%) results between the experiment and simulation.

Benefits

Product Quality Improvement:

  • Achieve close-to-100% accuracy between experimental and simulation results of filling behavior and warpage
  • Realize short-shot-free EPDM filling
  • Reduce the time needed to complete design modifications, as well as development costs and production cycle time
  • Reduce total displacement of PP+GF30 filling

Case Study

The main aim is to overcome the challenges of multi-shot injection molding of PP+GF30 and EPDM, wherein warpages often occur due to the primary filling (PP+GF30), which should be reduced to a specific level to avoid any mismatch when the part is inserted to the other compartment for subsequent filling. The cavity of the subsequent filling (EPDM) should also be correctly designed to complete filling.

In this case, Moldex3D was employed before starting the mold design of the primary filling to initially realize the correct design, with adequate part deformation. Subsequently, an analysis of the second filling was undertaken, while the first mold design was in progress.

Finally, the mold design of the second filling referred to the modified one after the short-shot problem had been solved using information from the simulation analysis. Moldex3D identified two crucial problems for this mold: warpage problems from the primary filling and incomplete filling from the subsequent filling. The design modifications for the cavity of the primary filling involved added ribs at specific regions to support part rigidity and eliminating specific regions to ensure a more uniform wall thickness (Figure 1). The warpage from the first filling was minimized due to the addition of the supporting ribs (Figure 2).

Compared to the original design for the cavity of the first filling (top), the final design (bottom) has more ribs, and some of its sections have been core out.

Compared to the original design for the cavity of the first filling (top), the final design (bottom) has more ribs, and some of its sections have been core out.

Figure 1. Compared to the original design for the cavity of the first filling (top), the final design (bottom) had more ribs, and some of its sections were cored out.

The original design without supporting ribs (top) results in larger warpage total displacement (max: 2.78 mm) than the modified design with supporting ribs (bottom) (max: 2.47 mm).

The original design without supporting ribs (top) results in larger warpage total displacement (max: 2.78 mm) than the modified design with supporting ribs (bottom) (max: 2.47 mm).

Figure 2. The original design without supporting ribs (top) results in larger warpage total displacement (max: 2.78 mm) than the modified design with supporting ribs (bottom) (max: 2.47 mm).

The design modifications for the cavity of the second filling included the geometry (Figure 3) and the thickness (Figure 4). These modifications enhanced the EPDM filling behavior such that the filling could be completed without any short shot (Figure 5).

The geometry of the original design for the cavity of the second filling (top) has been modified for its final design (bottom).

The geometry of the original design for the cavity of the second filling (top) has been modified for its final design (bottom).

Figure 3. The geometry of the original design for the cavity of the second filling (top) was modified for its final design (bottom).

The thickness of the EPDM passage in the final design (bottom) has been increased, compared to the original design (top).

The thickness of the EPDM passage in the final design (bottom) has been increased, compared to the original design (top).

Figure 4. The thickness of the EPDM passage in the final design (bottom) was increased, compared to the original design (top).

The short-shot problem for the original design (top) has been solved in the final design (bottom).

The short-shot problem for the original design (top) has been solved in the final design (bottom).

Figure 5. The short-shot problem for the original design (top) was solved in the final design (bottom).

The design modifications were validated by comparing their filling results with the results from their original designs, wherein the enhancements could be seen; consequently, the short shot had been solved and the warpage had been reduced. Additionally, the simulation results were also compared with the experimental results in which both simulation and experiment were in excellent agreement - the similarity was almost 100% accurate when the mesh level for the simulation analysis was altered from 3 to 5. One example includes the following short shot issue from EPDM filling (Figure 6 and Figure 7).

The short-shot location of the original design in the simulation (top) is similar to the one in the experiment (bottom).

The short-shot location of the original design in the simulation (top) is similar to the one in the experiment (bottom).

Figure 6. The short-shot location of the original design in the simulation (top) is similar to the one in the experiment (bottom).

Both simulation (top) and experiment (bottom) result in short-short free filling in the final design.

Both simulation (top) and experiment (bottom) result in short-short free filling in the final design.

Figure 7. Both simulation (top) and experiment (bottom) result in short-short free filling in the final design.

Results

Both warpage of the primary filling (PP+GF30) and the filling behavior of the subsequent filling (EPDM) could be understood properly using Moldex3D analyses. By providing a mesh level of 5 for the simulation model, the simulation results were close to 100% accurate when compared with experimental results.

These advantages could help to forecast potential manufacturing challenges before production processes are initiated, meaning that any required modifications could be made earlier, saving time during design and production stages. Using this method, FARPLAS A.S. was able to effectively solve crucial manufacturing problems in multi-shot injection molding.

This information has been sourced, reviewed and adapted from materials provided by Moldex3D.

For more information on this source, please visit Moldex3D.

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