Moulds That Work: A Gas-Assisted Clothes Hanger

The clothes hanger (Figure 1) is essentially a bent rod 16 mm in diameter with a hook at one end. The two ends of the rod are connected by a cross member having an I-beam-shaped cross section and a wall thickness of 2.2 mm. Two knob-shaped projections are provided on the horizontal rod to hang women's skirts.
In order to save material, reduce the cycle time, and avoid sink marks, the mould has been designed to permit gas-assisted injection moulding. Dimensions of the mould: 546 mm by 346 mm by 297 mm shut height. The cavity for the moulded part and the runner are located between the two mould plates (1, 2). The runner enters the cavity near the cross member. The gas injection nozzle, consisting of nozzle body (4) and gas injection pin (5), is located directly adjacent to the gate.

The clothes hanger is formed by initially injecting a specified amount of material. The injection pressure required for this is relatively low, since the mould is filled only partially and the large cross section offers little resistance to flow. As a result, no moulding compound is forced into the air gap between the nozzle body (4) and gas injection pin (5).

Next, nitrogen gas at a pressure of about 150 bar is injected through the gas injection nozzle into the runner and, from there, into the melt that partially fills the mould cavity. An ever-growing bubble (or void) forms in the melt, while the molten core advances via fountain flow toward the ends of the flow path. The result is a tubular cross section 16 mm in diameter, and with a wall thickness of approximately 2.5 mm.

Part Release/Ejection

As the mould opens, the two mould inserts (6) are actuated by springs (7) and release the cross member of the hanger from the stationary-side mould plate (2). The sprue is pulled out of the sprue bushing; the hollow stub is pulled out of the gas injection nozzle. Once the mould has opened completely, the profiled ejectors (8) and the runner ejectors (9, 10) eject the complete shot (part and runner). After the moulded part has been degated, a hole about 3 mm in diameter remains where the gas was injected.

Contact Information
Arburg GmbH & Co.
Injection Moulding Machines
Dr. Christoph Schumacher,
  Public Relations
Post Box 1109
D-72286 Lossburg
Germany
Tel: +49 (7446) 33 3149
Fax: +49 (7446) 33 3413