Gas and Injection Moulding – A Winning Combination

Rutland Plastics answers some of the most commonly asked questions about gas assisted injection moulding

Q) What is Gas Assisted Injection Moulding? A) It is the introduction of nitrogen under pressure to the mould cavity during the moulding cycle. The aim is to either hollow out thick section mouldings or eliminate sinking and/or warping in thin section injection mouldings. Q) When would Gas Assisted Injection Moulding be used? A) There are two different product groups where this process is used, defined by their generic shape. Firstly, rod shaped mouldings resemble the shape of a rod or handle. Here the gas is used either to force molten plastic out of the centre of the moulding, or the mould cavity is partially filled with molten plastic and then the gas is used to inflate the plastic to fill the mould. In both cases the result is a hollow moulding that appears to be solid. Examples include handles of various types. The other type is panel shaped mouldings. Ribs act as gas channels, and tending to be deeper than conventional ribs provide additional rigidity. Using gas allows the mould to be filled at lower pressure so reducing internal stresses that lead to warping and also counteracts sink marks caused by ribs. Typical applications include TV surrounds and automotive interior parts. Q) What are the main advantages of this process? A) For thick section mouldings by hollowing out the part weight savings can be made and, dependent upon the method used, less material is required. There is also the possibility of eliminating assembly costs by producing a traditional two-part moulding as a single piece. For both types of moulding, parts can be produced with no sink marks and minimal warping. Due to the lower pressures required a smaller moulding machine may be used. Variable sections can be included and rigidity can be achieved through hollow box sections. Q) Is Gas Assisted Injection Moulding more expensive? A) Although there are additional tooling costs, such as gas pins, these are often outweighed by cost savings not just in the part, but also with a simplified mould cavity. Hollow sections can be produced without the need for mechanical cores and assembly costs may be eliminated.