Specialty injection molding methods for high-quality and competitive products
Gas and water-assist injection (GIT and WIT)
Virtually every segment of industry is subject to some degree of cost pressure. It is therefore essential for thermoplastics processors to continuously review their options for increasing productivity. Intelligent specialty injection molding methods can help. Examples include gas-assist injection, whose development can be traced back to patents written in the 1970s and water-assist injection, which can be considered a further development of gas-assist injection. Both methods continue to be very important.
The difference between the Gas Assist (GIT) and Water Assist (WIT) Technology
Surface defects often occur in injection molded parts due to variations in thickness or the increase in cross-sectional thickness where features such as ribs, bosses and posts merge onto the main wall. Due to thickness-dependent shrinkage potential and differences in cooling behavior, thicker wall sections tend to shrink more than thinner wall sections. This promotes sink marks and partdistortion. Gas-assist injection addresses this. The process injects gas (typically nitrogen) into the thick sections where the internal gas pressure compensates for the shrinkage and keeps the part surface in contact with the cavity wall, thus preventing the sink marks typical of such areas. As a result, areas far from the gate can be produced free of sink marks at a relatively low holding pressure, and the injection pressure inside the mold can also be kept low. Particularly in parts woth large projected area, this reduces the required clamping force, which in turn permits the use of smaller machines.
Gas-assist injection is particularly suitable for rod-shaped components with large cross-sections, such as door handles, and for hollow bodies, such as tubes. Blowing out the hot core makes it possible to produce large-cross-section parts economically. This method provides greater overall design freedom since the design guidelines that typically apply to the processing of thermoplastics can be liberalized.
There are two common variants of the method: the short-shot process, in which the cavity is only partially filled and the melt then pushed outward by the injected gas to completely fill the cavity; and the expulsion process, in which the cavity is filled 100 % before the hot core is expelled into an overflow cavity. Which of the two is used depends primarily on the component geometry.
However, gas also has a significant disadvantage compared with other materials and media – its poor thermal conductivity and capacity, which have a negative effect on component cooling. This is addressed by water-assist injection, which offers advantages over gas-assist injection because it uses water instead of gas, thusachieving significantly better cooling and better cavity formation.