Plastic Fantastic

Focus: Water Injection Molding

Water-assisted injection molding technology (WIT) remains an up-and-coming process, particularly in the automotive sector. But as with any processing technique, it is only as good as its weakest link.

Tubular components for fluid transportation are an ideal application for WIT.

There is plenty to link. In the words of Bernd Herzog, manager of application and process development at WIT technology provider PME fluidtec GmbH (Kappel-Grafenhausen, Germany), “WIT only achieves its full potential if the mold, injector technology, equipment, material, and engineering are fine tuned and adjusted to work as a unit.”

Herzog adds, “If you have some process flaws with gas injection technology (GIT), it is not the end of the world and additional cooling might solve the issue, but with WIT it’s either black or white. Complex parts are either good or bad.” Consequently, a properly designed and engineered part is paramount to the success of any project. Design rules must be followed more closely.

The first thing a processor needs to consider is whether the part in question is suitable for WIT. If compensation for shrinkage is the primary objective, then gas injection technology may be the best solution. “The greater cooling properties of WIT can mean that the material will solidify too quickly and the required pressure cannot be applied through the solidified layer,” explains Herzog.

Oval cross-section parts may also be better molded with GIT. This is because water, as well as gas, always tries to flow with a round profile, but the effect is more pronounced with WIT, resulting in uneven wall thickness.

Small radii and sudden changes of channel direction are also not friendly to WIT. The channel will move towards the inside, and result in increased warping and variable wall thickness.

Further, WIT injectors are larger than GIT injectors so it is crucial to think about this during the design process. One key issue is whether the injector can be positioned directly on the part or whether a feeder channel is required.

Water injection nozzles are typically larger than gas nozzles, a point well worth remembering.

Materials matter

Materials are also an issue. “Amorphous resins or materials with low crystallization speeds are recommended for WIT,” says Marcel Op de Laak, managing director of WIT provider TiK-Technologie in Kunststoff GmbH (Freiburg, Germany). With some highly crystalline polymers, resin set-up may lead to problems in forming channels and an unrepeatable process, according to PME’s Herzog. Crystalline engineering plastics such as polyamide (PA) typically are molded with a modified WIT process, in which a nitrogen “cushion” acts as a thermal buffer to prevent the abrupt hardening of the material that would occur with direct contact with water.

Special grades of polyamide used in the WIT process cost roughly 50% more than the standard grades suitable for GIT, according to Op de Laak. Therefore, if the greater cooling capacity of water, which typically halves the cycle time versus GIT, is the only reason for application of WIT, the cost equation needs to be assessed.

Plastics supplier BASF (Ludwigshafen, Germany) notes that WIT typically enables 25-40% cycle time savings, while PME fluidtec says cooling time is reduced by up to 70% and cycle time by up to 60%. BASF offers a series of grades specifically for WIT applications.

One example is a hydrolysis-resistant PA 66 type (Ultramid A3HG6 WIT). This material also meets the requirements for a smooth channel surface with favorable flow properties, exhibits high resistance to media, is chemically resistant, and prevents leaching. BASF also claims, however, that several of its standard grades also are well suited for WIT molding. One example is Ultramid A3WGM53 for applications exposed to hot oil. BASF also offers the special WIT grades Ultramid B3G10 SI (SI stands for “surface improved”) PA 6 and Ultradur B 4040 G10 WIT PBT, reinforced with 50% glass fiber.

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

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

In general, tubular parts with medium to large diameters are perfect for WIT. “Parts with diameters of 35-40 mm will not get sufficient cooling with GIT and a hard skin will not be formed,” says Herzog. “This can cause the material to flow down, eventually decreasing the diameter, and in the worst-case scenario, flow over the injection and prevent further pressure release.”

Flat parts with large channels are also suited to WIT on account of decreased warpage, shorter cycle time, and because telltale shiny surface 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 any change in diameter follows a smooth transition. Otherwise, sink marks or bad channel shapes will result.

Roof rails and mirror brackets (inset) are examples of commercial WIT applications.

Best of both

Parts with both tubular and flat sections can be good candidates for WIT as the cored-out sections contribute to shorter cycle time, especially for slow crystallizing materials such as ABS and PP. However, faster crystallizing materials such as PA, PBT, and PPS need to be checked on an individual basis. WIT can lead to sink marks and blemishes because of the uneven cross-section of the flow channel, but a combination of WIT and GIT can remedy this situation, according to Herzog. This solution is particularly applicable for parts with large channel cross-sections as well as areas where shrinkage occurs, such as ribs and thick walls.

WIT in the real world

Successful examples of WIT application abound in the auto sector. These include functional channels that serve to transport fluids. Functional channels are found in the case of coolant channels but also include cylinder head covers that have lubricant lines or continuous flow heaters.

A tool designed for WIT.

BASF’s first WIT application was the mirror bracket for the DAF XF105, with a shot weight of 290 grams. Since 2005, this component has been produced commercially with Ultramid B3WG6 GP, a PA reinforced with 30% glass fiber.

Together with Tier 1 supplier Polytec Automotive (Hörsching, Austria), BASF has also developed a cooling water pipe made of Ultramid A3HG6 WIT. That component went into serial production in August 2007.

The feasibility of a roof rail made completely of plastic was also studied with Decoma, part of the Magna Group (Aurora, ON). Following good, reproducible results in terms of channel formation and surface quality, Decoma manufactured roof rails out of Ultramid B3G10 SI and Ultradur B 4040 G10 WIT that passed mechanical component tests, especially the crash tests. Future WIT applications could be, for example, bus seats or automobile seat backrests in areas where the stiffness would be raised by means of the hollow cross sections without using ribs.