Gas-Assist Moulding Replaces Mould Slides

December 31, 1996

The injection moulded internal panel of a car tailgate is a multifunctional part. Among other things, it has to include the lid with all the on-board tools and the warning triangle. Moreover, the panel needs two depressions for gripping, one on the left and the other on the right, in order to pull the tailgate shut easily. *

Because of the direction of flow, in a conventional mould these depressions must be made by hydraulically operated angled slides. On the reverse side, between the outline of the grip and the outside wall of the panel, an accumulation of excess material would occur unless additional large slides were used. However, there is no space for such slides in this mould. How did the mouldmaker redesign the mould to eliminate the slides? He turned to gas assist.

The stationary half of the mould was made by mould manufacturers Sauer & Sohn Formentechnik, Dieburg, Germany. The panel itself is underneath. By using the gas-assist process, the time-consuming and expensive separate manufacture of the grips and the subsequent welding was avoided. In addition, gas assist was used instead of slides to prevent the undesirable accumulation of material. The gas drives out the excess melt after injection into overflow cavities.

In order to introduce the pressurized gas into the mould, two small gas inlets separated by a narrow slide are located in the middle of the lower edge of the panel. This location was chosen because this area is also facing the moulding direction and would have required a moving core for ejection. However, the use of gas also eliminates this core. The appropriate overflow cavities are located near each of the grip depressions. This results in the following process sequence:

  • The closed mould's volume is completely filled.
  • At the beginning of the holding pressure, nitrogen is introduced at a pressure of approximately 300 bar. In the region where the excess material is to blow out, a sufficiently thick surface layer has already solidified at this point.
  • Nitrogen pushes the excess melt first from the thickened wall region of the panel, and then arrives at the large cross section between the grip depression and the panel wall. There it drives out the still-liquid melt and finally reaches the outlet next to the grip depression.
  • At the end of the holding pressure, the gas pressure is reduced and the panel is ejected as usual.

In order to ensure reliability of this process sequence in production, the mould is designed with several unique features. The greatest challenge is the variability of the cross section along the gas channel. As shown in Figure 4, the blowout channel starts with a very small cross section for the gas injection. The increased cross section on the edge of the panel is then followed by an appreciable enlargement in the region between the grip depression and the bottom of the panel. The gas and melt outlet has again a small cross section.

Mould Temperature Control

Because of this complex process sequence, it is possible to achieve trouble-free uniform melt removal only when the corresponding mould areas are kept at specific temperatures by precisely aimed temperature control. Two separate blowout channels are necessary in order to achieve optimum melt removal. Another requirement was to produce the panel without a sprue. Therefore, the hot runner and the ejector were placed completely on the stationary part of the injection moulding machine. The one-piece design of the panel made from high-impact ABS was required for stiffness reasons. While the process of welding on the grip cups would reduce the stiffness of the part, the gas channels actually increase it. This can resist the high stresses raised by the lid containing the on-board tools.

Gating: Hot Runner with Four Needle Valve Nozzle

In order to assure a proper mould design and moulded part, a mould flow analysis was performed, which determined that a hot runner system with eight nozzles was required. Based on experience with gas-assist technology and some flow improvements due to modifications of wall thicknesses approved by the customer, the mould manufacturer selected the quadruple hot runner system with needle valve nozzles made by Eurotool, Bensheim, Germany. These needle valve nozzles are reportedly capable of filling an injection mould by segments using the cascade process when required. In spite of the application of the gas-assist process, the mould still contains 14 slides needed for hinges, apertures, attachment elements, and so forth. It is used on an injection moulding machine with 20,000 kN clamping force and its own nitrogen supply system.

When starting the work on the mould design, a complete experimental mould was built, since the entire tailgate panel for this application was being made by injection moulding for the first time. In order to reduce costs, only the cavity and core were made from aluminum to evaluate the functionality of the parts. The actual mould structure and the hot runner system were used.

At the same time, the mould manufacturer and the processor evaluated the operational safety of the gas-assist system during the test runs. Meanwhile, the series production of the steel mould has started. The melt blowout extends the cycle time slightly. However, the part produced by this method is a stiff, functional product without needing any additional welded-on components. For these reasons the application of the gas-assist process is technically and economically the most sensible solution for this mould.

Contact Information: Sauer & Sohn Formentechnik, Mr. J

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