Moulds That Work, Part 8: 8,000 cleanly formed perforations

Editor's note:This issue's featured mould is a two-cavity hot runnermould for the production of loudspeaker covers in polyacetal. Its particularchallenges were the thousands of perforations and the need to change colourseasily. The mould was designed and built by Ewikon Heisskanalsysteme, Frankenberg,Germany.

Honeycomb grilles that are 150 mm by 180 mm in size (Figure 1) are usedas covers for loudspeakers in car interiors. For acoustic reasons, theyare required to have a large number of perforations that will let soundthrough. The 8,000 or so holes have a diameter of 1.1 mm and are positionedwithin a hexagonal honeycomb structure that has an inside dimension of13 mm, which was selected for purposes of both strength and flow engineering.The structures are surrounded by a continuous wall around the outside.Wall thickness is approximately 1 mm for most of the part. The outer contourof the grille is a curved 3-D design. The range that is offered includesdifferent grilles that are injection moulded in various colours. The materialemployed is an acetal copolymer with an MFR 190/2.16 of 28g/10 minutes.

The Mould

The mould, which is 346 mm by 696 mm and 403 mm high, is a two-cavityall hot runner mould (Figures 2 and 3). The melt passes through a connectingnozzle (1), into the hot runner plate (2), and from there into each ofsix cavity nozzles (3). The hot runner system is fully controlled and isequipped with internal heating. In order to improve on the colour changebehaviour of the hot runner plate, flow-optimized packing boxes (4) wereemployed as deflector elements, with a polymeric coating offering highheat resistance. The gate diameter is .8 mm. The 3-D curvature of the grillestructure means that the cavity nozzles (3) have different shaft lengthsof 69 and 76 mm, respectively. The distance between two nozzles is minimal,42 mm. The torpedo ends protrude into the gate and thus form an annulargap. The tear-off height is <.1 mm. The system requires an installedheating capacity of only 1,700W overall. Mould wall temperatures are 80to 90 C, and the melt temperature corresponds to the temperature in thehot runner plate and the cavity nozzles, approximately 210 C. Cycle timesof between 35 and 40 seconds are attained. The heating and cooling of themould is performed via 2 x 2 independent heating/ cooling circuits. Themould halves are guided by four columns in the standard manner.

Demoulding

Since the surface of the moulded part has to fulfill stringent qualityrequirements on the visible side, the gates are positioned on the concaverear of the grille, which is demoulded on the gate side as well, with theejectors acting solely on the surrounding contour. The ejector unit isguided by ball-bearing guides (6) and driven by hydraulic cylinders (7).

This concept means that the hot runner plate is supported over its entirearea by just two support bolts. The moulded parts are removed by a handlingunit and deposited in defined position to cool to room temperature.

Contact Information
Ewikon Heisskanalsysteme GmbH & Co. KG
Mr. Helmut Vogel
Siegener Strasse 35
D-35066 Frankenberg
Germany
Tel: +49 (6451) 5010
Fax: +49 (6451) 501205
e-mail: [email protected]