A common method that allows molders to produce large injection molded parts with high quality surface finishes is sequential valve gating. Yet, this creates defects such as reflection marks on painted parts, pressure flow marks on unpainted parts, and hot spot marks opposite of direct gated nozzles (Fig. 1).
Synventive’s activeGate Technologies are a line of control systems that permit molders who are utilizing valve gated hot runner systems more control, better than the on/off capability of conventional valve gates.
The molder can control the acceleration, velocity, and stroke of the valve pins. Moldex3D can simulate this high-level control and enables molders to more accurately predict and prevent common defects in injection molded parts.
Figure 1. Common molding defects on standard sequential valve gating system
- To eliminate defects without retrofitting an existing mold for testing or rebuilding a mold
- Common molding defects for injection molded parts by sequential valve gating system
Moldex3D Advanced was employed to model the system with standard sequential valve gating system to identify the molding defects, and additionally to simulate valve pin movement control to optimize the process (controlled sequential valve gating system) and remove the defects discovered in the injection molded parts.
- Saved time and money by scrapping less parts
- Successfully identified defects that can be eliminated with activeGate controls through pin movement simulation
- Early detection of common defects in injection molded parts
The aims of this study are:
- To accurately identify the common molding defects in sequentially molded parts
- To construct a simulation that highlights an area that has high possibility of producing those defects
- To simulate the use of valve pin movement technology (controlled sequential valve gating system)
activeGate Technologies, were used as the solution to resolve the defects.
The complete model is a two-cavity mold that employs a two valve-gated hot runner system per cavity (Fig. 2). After the melt front from the first set of valve pins (Drop 1 and 3) passes Drop 2 and 4, the second set of valve pins (Drop 2 and 4) is opened at full speed. When Drop 2 and 4 have opened, the material inside (which has been compressed under high pressure) is released into the cavity. At this stage, there were some points to note.
- The melt front is pushed forward at a much quicker speed than the first nozzle. This is shown as a large melt front advancement, which can lead to pressure transition marks and can be seen from large distances between iso-contour lines.
- Some material can flow backwards and create dense areas, which can be seen from small distances between iso-contour lines.
These two suggest large uncontrolled melt front stagnation and melt front advancements on the side of parts (Fig. 3). When the parts are removed, they may still look okay, but the dense areas will try to relax and generate reflection marks if they are painted and then put in a dryer.
Additionally, the material which was deposited on the wall can be re-melted by the high-pressure melt that later enters the cavity, resulting in a hot spot mark.
Figure 2. Hot runner layout design
Figure 3. Uncontrolled melt front advancements are produced in original sequential valve pin movement setting
The change that was made was to manage the opening speed and acceleration of the valve pin in the second nozzle. The second valve pin (the valve pin represents both Drop 2 and 4 in the valve pin movement setting) is no longer opened at full speed, but at controlled speed of 6.35 s (Fig. 4).
This produces a small gap which allows the pressure in the second nozzle to equalize to the system. The material will not advance the melt front too much from the second nozzle, re-melt the material opposite of the gate, or push material backwards to create the dense areas because of the equal pressure. The simulation result exhibits that the iso-contour lines look uniform throughout the entire part (Fig. 5).
Figure 4. Fast and slow opening speeds between original and optimized sequential valve pin movement settings
Figure 5. Controlled melt front advancements occur after adopting optimized sequential valve pin movement setting
Three injection molded products were created by utilizing the controlled sequential valve gating system (to control the release of plastics into the cavity the pin was opened slowly) for the verification that the controlled process could enhance the conventional/standard process. The three products were door panel, glove box, and seat back. The results exhibit that the defects that happen under conventional/standard process can be eliminated (Fig. 6).
Figure 6. The validation results show that the controlled process can effectively eliminate defects in injection molded parts
Synventive succeeded in identifying the injection molded parts that could have potential defects under standard valve pin movement setting with Moldex3D. Just as important, Moldex3D could simulate the advanced valve pin movement control that aided in altering the characteristics of the melt front and ultimately eliminate any defects that could have occurred.
Furthermore, the molders could save time and money by scrapping fewer parts from the optimized sequential valve pin movement setting.
This information has been sourced, reviewed and adapted from materials provided by Moldex3D.
For more information on this source, please visit Moldex3D.