Gas-assist on the outside

The gas injection technique has undergone rapid development ever sinceit was first introduced more than 10 years ago, with particular advancesbeing achieved during the past five years. The insufficient reproducibilityof the process, however, has led to the failure of all efforts to developgas-assisted injection moulding to the point where it could be marketedfor thin-walled moulded parts with one side that is subject to stringentsurface requirements.

In the classic internal gas pressure process, nitrogen is injected intothe center of the melt giving rise to local channels. The internal gaspressure in the moulded part counters the volumetric contraction of themelt during cooling, which makes it possible for thick-walled injectionmouldings to be produced without sink marks. This technique is thus usedfor parts that display thicker walls in certain areas, which the gas isable to penetrate. In other words, the gas is selectively distributed withinthe thick-walled areas through the formation of gas channels. In the caseof thin-walled parts with pronounced ribbing, which are similarly proneto sink marks, it is not always possible to employ this process.

The Process

A new variant of gas-assisted injection moulding has been introducedby Battenfeld of Meinerzhagen, Germany, called the Airmould Contour process.The opposite of using gas internally, this process introduces nitrogenbetween the mould wall and the surface of the part or the melt. The gaspressure acting on one side of the moulded part pushes additional meltinto the areas that are prone to sink marks in the plastic on the visibleside (Figure 1). This is designed to give a high surface quality on thefront side of the moulded part.

Applications

On parts that are to be manufactured using exterior gas pressure, therear side, which has the cushion of gas acting against it, cannot be employedas a visible surface. The top section of a housing in ABS that was producedfor demonstration had a mixture of glossy and rough patches on its underside.The process is suitable not only for moulded parts that incorporate ribson one side but also for large-area components that tend to warp when producedby the conventional process. According to Battenfeld, further typical applicationsinclude moulded parts incorporating different wall thicknesses, or high-glosssurfaces on one side, and also screw closures with their thick-walled areasand ribbing.

The exterior pressure gas process can help to avoid sink marks in caseswhere conventional methods (modifying the holding pressure, increasingcooling times, or adding blowing agent) no longer work for a given design.One positive side effect is the lower clamping forces required in comparisonwith solid injection moulding. According to Battenfeld, the insulatingproperties of the layer of gas between the mould wall and the molten plasticdo not have any significant negative impact on cooling time or cycle duration.

Battenfeld is offering this process as the exclusive licensee of thepatent rights, which are held by an English company. The process can becontrolled with such standard gas-assist components as pressure generationunit, gas pressure control system, nitrogen generation unit, and so forth.It is basically possible for the gas internal pressure and gas exteriorpressure processes to be employed at the same time.

The injection modules for the gas exterior pressure process close flushwith the surface of the mould and do not protrude into the melt as theydo in the gas internal pressure process. Special know-how is required herein the design of the mould, so that the gas spreads out in a uniform, 2-Dmanner along the mould wall, thereby guaranteeing that the process is reproducible(Figure 2). No experience is available on industrial-scale production asyet.

Contact Information:

Battenfeld GmbH
Mr. Helmut Eckardt
Scherl 10
D-58540 Meinerzhagen
Germany
Tel: +49 (2354) 72 326
Fax: +49 (2354) 72 503