Design Focus: Plain sailing with water injection molding

Tag : Plastic Injection Process

Materials are also an issue. “Amorphous resins or materialswith low crystallization speeds are recommended for WIT,”says Marcel Op de Laak, managing director of WIT providerTiK-Technologie in Kunststoff GmbH (Freiburg, Germany). With somehighly crystalline polymers, resin set-up may lead to problems informing channels and an unrepeatable process, according toPME’s Herzog. Crystalline engineering plastics such aspolyamide (PA) typically are molded with a modified WIT process, inwhich a nitrogen “cushion” acts as a thermal buffer toprevent the abrupt hardening of the material that would occur withdirect contact with water.

Special grades of polyamide used in the WIT process cost roughly50% more than the standard grades suitable for GIT, according to Opde Laak. Therefore, if the greater cooling capacity of water, whichtypically halves the cycle time versus GIT, is the only reason forapplication of WIT, the cost equation needs to be assessed.
Plastics supplier BASF (Ludwigshafen, Germany) notes that WITtypically enables 25-40% cycle time savings, while PME fluidtecsays cooling time is reduced by up to 70% and cycle time by up to60%. BASF offers a series of grades specifically for WITapplications.

One example is a hydrolysis-resistant PA 66 type (Ultramid A3HG6WIT). This material also meets the requirements for a smoothchannel surface with favorable flow properties, exhibits highresistance to media, is chemically resistant, and preventsleaching. BASF also claims, however, that several of its standardgrades also are well suited for WIT molding. One example isUltramid A3WGM53 for applications exposed to hot oil. BASF alsooffers the special WIT grades Ultramid B3G10 SI (SI stands for“surface improved”) PA 6 and Ultradur B 4040 G10 WITPBT, reinforced with 50% glass fiber.

Another key consideration is the ease with which water can bepurged from a part after molding. This is especially true if thepart must be completely dry for painting. “GIT could be analternative here if the cross section is not too big and the longercycle time is within reason,” says Herzog. One‘hard-and-fast’ design rule is to purge water from thebottom of the part, because water flows down.

One other key consideration is pre-filling with resin. This isbecause water does not necessarily flow back to a common channel ifit forms several smaller channels when initially injected.“Laminar flow without jetting and a minimum of seam lines canaddress this problem, and this needs to be addressed by the partdesigner,” says Herzog. “The tool maker can only tweakthe gate.” Flow analysis can be helpful here.

In general, tubular parts with medium to large diameters areperfect for WIT. “Parts with diameters of 35-40 mm will notget sufficient cooling with GIT and a hard skin will not beformed,” says Herzog. “This can cause the material toflow down, eventually decreasing the diameter, and in theworst-case scenario, flow over the injection and prevent furtherpressure release.”
Flat parts with large channels are also suited to WIT on account ofdecreased warpage, shorter cycle time, and because telltale shinysurface strips that track the flow channels are minimized,especially when PP and ABS are processed.

Whenever WIT is applied, the designer must also ensure that anychange in diameter follows a smooth transition. Otherwise, sinkmarks or bad channel shapes will result.