Secrets of water injection. (Processing Tips).

* Molding with water is still more an art than science. Water injection technology; or WIT, has been around only since 1998 (see p. 42 for details). The industry is still learning how to optimize water's ability to core out thick sections in parts more effectively than gas, while providing the added benefits of faster cooling, smoother surfaces, and thinner, more uniform walls. Here are some tips on the fine points of WIT:

Don't get steamed

Water flow rate and pressure are critical to the process, says Steven van Hoeck, v.p. of sales at Alliance Gas Systems, Inc., Chesterfield Township, Mich. Low flow and pressure could allow the water to turn into steam when it enters the hot part, creating turbulent flow and the possibility of a "foamy" inner surface of the part.

Keep the pressure on

"In theory, there will be areas where water is in the gas phase [steam]. Holding high pressure throughout the process can keep water in the liquid state," advises Jorg Dussow, manager of application technology at Ferromatik Milacron in Malterdingen, Germany.

Gas first, then water

Helmut Eckardt, technical director for low-pressure injection molding at Battenfeld Injection Molding Technology in Meinerzhagen, Germany, says it can help to combine WIT with gas-assist--especially when molding crystalline materials (like PP and nylon) that might cool too quickly in contact with water. Injecting gas first cores out the part and sets up the outer wall without cooling the inner melt.

Don't cross paths

Oliver Phannschmidt, head of the injection molding department at the Institute fur Kunststoffverarbeitung (IKV) in Aachen, Germany, says water flow can be split into different paths within the part, but the paths cannot intersect. If they do, you risk problems with excessive pressure drop and difficulties in water evacuation after the part is filled.

Beyond coring & cooling

Water can accomplish packing and holding, in addition to coring out and cooling the part. Ferromatik's Aquapress process applies pack and hold for a short time--0.3 to 0.1 sec for test parts.

Water should wait

A short delay between melt and water injection gives the outer surface of the part a chance to set up against the mold. Kai Jacobsen, manager of machine sales and technologies for Engel Canada Inc., Guelph, Ont., says the length of the delay for switchover from melt to injection will affect final wall thickness.