Quick mold analysis for the designer.
Design engineers routinely design injection molded plastic parts that are esthetically pleasing and functional, yet they are often unsure of their designs' manufacturability. Within a CAD solid modeler, new mold-filling-simulation technology can be utilized to quickly perform filling analysis using only resin type, melt-flow rate (MFR or MFI), and a gate location on the part as input data. This technology allows the designer to quickly determine the feasibility and manufacturability at the earliest stages of part design without the necessary input data and engineering knowledge needed for a conventional mold-filling analysis. If a more rigorous analysis is requested, the finite-element mesh can be directly exported to a mold-filling-analysis software package.
Mold-filling simulation is typically done somewhere near the end of the part-design cycle. At this stage it is not uncommon for the actual part design to be already set in stone and the tool steel to be partially cut. Therefore, the impact of the simulation on the manufacturability of the design is limited.
New technology allows the designer to perform a simple mold-filling simulation in the design stage without ever leaving the CAD environment. This simulation will allow the design engineer to determine if the part can be injection molded.
When the remote control housing shown in Fig. 1 was being designed, it was specified that, owing to the recent rise in cost and decrease in availability of polycarbonate (PC), this housing needed to be as thin as possible. The initial design had a nominal wall thickness of 1.52 mm except for the thin-side features that mate with the other half of the remote. This is approximately 25% thinner than the previously designed remote.
It was also specified that, since the housing is a cosmetic part, it cannot be gated on a visible surface. The two initial gating schemes used a cashew gate on one end and a single tunnel gate on the side.
Initial Gate Locations Part Geometry
The initial design should be a simple form of the final remote housing. This will allow the model to mesh easily and run quickly. For the remote, the initial design is just the outer shell with the button holes.
As each feature was added, the part was meshed and run through the quick-filling simulation. This procedure ensures that the final part will mesh simply. The polymer entrances are assigned to datum points. A model with the entrance at the end and one with the entrance on the side of the remote was run. The parting plane was defined using the default coordinate system and selecting the XZ plane. This is used for calculating the required clamp force to mold this part.
Small rounds or fillets can be left out or suppressed when creating the mesh. These features greatly increase the number of elements and do not improve the accuracy of the simulation.
The remote control housing is made of a PC with an MFR of 12 g/10 min. In the simulation, a generic resin class is selected, and then an MFR, temperature, and weight are entered (12g/10 min, 1.2 kg, 300 [degrees] C). The MFR value is used because it is readily available, commonly understood, and increases the simplicity of the material-data inputs.
The simulation will automatically select a maximum fill rate based on the volume of the part and characteristics of a typical molding machine. From this ideal fill rate, the simulation will then calculate a fill time. A maximum fill pressure of 180 MPa is fixed for all simulations.
The only process conditions needed for the simulation are the melt and mold temperatures. A good estimate can be obtained from a process engineer or a resin-supplier's data sheet. However, the simulation will default to a recommended value based on the type of resin selected. For the initial iteration, the default values of 305 [degrees] C melt temperature and 70 [degrees] C mold temperature were used.
The fill time for the remote was calculated automatically by the simulation program for each gate location. This calculation is based on the volume to be filled, maximum flow rate, flow length and part thickness.
Launching the quick-filling simulation requires one menu pick. The current part design has about 1700 elements and was launched on an HP712/60. On this particular hardware, the overall simulation CPU time was under three minutes.
Initial Results Initial End-Gate Design
The melt-front advancement of the initial design is shown in Fig. 2. The design did not fill because of insufficient injection pressure. At this point the simulation offered the following design advice: 1) Increase the polymer melt temperature; 2) Use a material with a higher MFR; and 3) Add more gates to reduce the flow length.
The melt temperature was increased to the upper limit of 330 [degrees] C and the simulation was rerun. Increasing the melt temperature did not eliminate the short shot, so the simulation suggested increasing the part thickness. A PC with a higher MFR (22) and the default melt temperature (305 [degrees] C) causes a short shot. With the highest melt temperature (330 [degrees] C), the part requires 142 MPa to fill in 0.98 sec.
Because no more gates can be added to the end of the part to reduce the flow length, the nominal wall was incrementally increased using the 12-MFR PC and the 305 [degrees] C melt temperature. The housing filled at 1.9 mm with a maximum pressure of 141 MPa and a fill time of 1.1 sec, but the remote's weight increased by 5.1 grams.
These initial seven or eight simulations have shown that an end gate is not realistic unless a different material is used or the remote's nominal wall thickness is increased.
Initial Side Gate Design
This design fills with 134 MPa in 0.92 sec. The melt-front advancement was not symmetrical, so the gate location was moved and another analysis was run. The revised gate location required 132 MPa with a 0.90-sec fill time. After running both gate locations, we determined that a tunnel gate into the side of the part would provide the largest process window.
Final Iterations Part Geometry
The final design should include everything that will mesh. This model will give the designer the best prediction of pressure, fill time, and clamp tonnage. Remember, this model does not have a gate or runner system, so the required pressure and clamp force will increase. Ten to fifteen percent of the maximum pressure to fill the part is a good estimate for the pressure requirement of the runner system, which is approximately 17 MPa for this part based on a maximum injection pressure of 132 MPa.
Final Side-Gate Design
This final design filled with 157.5 MPa at 1.05 sec. The simulation gave the following warning: "Injection pressure may be too high." Along with this, it gave three possible solutions: 1) Increase the melt temperature; 2) Use a material with a higher MFR; and 3) Add more gates to reduce the flow length.
With the 22 MFR and the 305 [degrees] C melt temperature, the required pressure is 134.5 MPa at 0.86 sec. To increase the process window and help reduce the pressure, a second gate was added on the side to reduce the flow length. This reduced the pressure to 92.5 MPa with a 0.75-sec fill time. The melt-front and pressure plots for the final design are shown in Figs. 3 and 4.
This quick-filling simulation does not replace the more robust simulation, but does help teach the part designer to think about meshing, gate location, nominal wall thickness and other plastics-processing problems. In minutes the designer can try multiple gate locations and different materials and thicknesses to compare melt front, pressure, temperature, clamp force, part weight, and other molding factors. Then the mesh and other process conditions can be passed to the analyst for a robust molding simulation.
The authors thank AC Technology's Learning Center for the use of their software and hardware.
1. Parametric Technology Corp., Release 15.0 Pro/ENGINEER Mold Design User's Guides, P#U00170795.
2. AC Technology, C-MOLD QuickFill Design Guide, P#D-785 V#406995.
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|Author:||Brickley, Jim; Johnson, Trevor|
|Date:||Oct 1, 1996|
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