Injection molding is a tried-and-true production technology that for years has been used to make millions of parts, large and small. Injection molding does have some drawbacks, however, especially with larger parts, such as visible sink marks and other surface defects. To resolve those issues, the basic technology is being augmented with new equipment, materials, and processes.

Since the early 1990s, a molding method that used nitrogen gas has been used in the U.S. and Europe. Using the gas-assist method, parts can be made on a molding machine that typically couldn’t be made with traditional injection molding such as very thick parts. And, as compared to traditional injection molding, gas assist can minimize sink marks and other surface defects that could be created as large parts cool. Gas-assist also proved effective in reducing part weight and molded-in stresses.

Enlarge this picture Full-shot water-assist injection molding

But, a new technology has been developed that even improves on gas-assist’s impressive results. Today, the newest injection molding technology to hit the market is water-assist injection molding (WAIM). With this method, water is used in place of gas to hollow out a part and it does so quicker than gas-assist, and improves wall thickness and wall uniformity across a part. The technology is used most efficiently to make hollow molded parts, which fall into two general categories: components that move fluids through them, and structural components such as oven and refrigerator handles, chain saw handles, office furniture chair arms, and other structural components that need to be sturdy, lightweight, and attractive on the outside. Research at BASF, a resin supplier based Florham Park, N.J., has developed WAIM resins, has also been done on molded part housings that would have an integrated tube coiling through the part, according to Randy Fleck, senior process engineer for BASF.

One of the first applications for the water-assist technology was in the automotive industry making fluid-handling parts, but WAIM’s uses are evolving and may soon include any application that requires a hollow part, or could be redesigned to incorporate one to consolidate parts and reduce part weight.

Enlarge this picture Water-assist injection molding with the melt push-back method.

Resin suppliers such as BASF and DuPont Engineering Polymers, Wilmington, Del., have developed materials designed specifically for water-assist molding. The resins are made to create a smoother interior surface. Previously, non-modified resins could leave problems such as foaming and voids. Smoothness of the interior wall is an issue when the part is used to move fluids. If it does, these flaws could slow the fluid flow rates, or glass fibers and other materials may break loose and enter the fluid stream, says Rob Palmer, a marketing manager with DuPont.

At BASF’s R&D center, which is located in a separate facility in Budd Lake, N.J., researchers use water-assist and gas-assist equipment from Cinpres Gas Injection Ltd, which is based in the U.K. BASF has a joint working relationship with Cinpres to develop applications and implement water-assist technology. At the facility, they have a 400-ton clamp injection molder from Engel as well as Cinpres’ water- and gas-assist pumps equipment.

Enlarge this picture Parts molded with water assist methods do not undergo a post molding temperature increase as do parts made with only gas-assist methods, according to BASF studies.

Germany-based PME Fluidtec and Cinpres led the way to develop the equipment and processes for water-assist injection molding. Today, a number of other equipment manufacturers provide capability for  this technology including the Austrian supplier Engel, and German companies Battenfeld, Ferromatik Milacron, and Maximator. PME Fluidtec is considered one of the world’s most experienced makers of water-assist molding equipment with more than 50 applications around the world that are producing millions of parts. Some non-automotive applications from PME Fluidtec include a Bosch refrigerator handle made from 30 percent glass filled nylon 6 at a cycle time of 34 seconds, and a chain-saw handle from Sachs Dolmar that also used a 30-percent glass filled nylon 6 and had a 45-second cycle time. 

In many ways, WAIM is much like gas-assist technology, and the previous two applications could have been made with that technology, but with slower mold cycle times. In fact, gas-assist and water-assist are sometimes used in tandem and equipment suppliers are working to combine gas and water capabilities into one machine. WAIM experts say that if a company is going to use the water-assist method, then they should also be able to do gas-assist.

Enlarge this picture Wall thickness is thinnest using only water-assist methods. Graphic: BASF

In general, they both reduce material costs, cycle times, and the number of parts needed (as different functional features can be molded into a single piece, reducing the number of parts that need to be assembled). Both technologies use a fluid, water in one and nitrogen gas in the other. In either case, the fluid is injected into a resin-filled mold and the oncoming fluid displaces the resin, coring out the part.

To employ WAIM, the water-assist equipment should have up to 300 bar pressure capability along with 50 liters per minute of water volume. Depending on the manufacturer, water units are either pressure or volume controlled.  Most WAIM units will have the necessary hydraulic or pneumatic controls to operate the water nozzle and mold actuators.  The water nozzle is an additional component to the injection mold, and must be capable of supplying a high volume of water.  Correct placement of the water injector is critical for success, says Fleck.

Enlarge this picture In BASF testing, the interior of the part is more fully cored when the resin is optimized for use in water-assist injection molding.

WAIM also can be employed in many of the same ways as gas-assist so the learning curve should not be as great for the molders and part designers that are familiar with gas-assist molding methods. The WAIM processes include the short-shot method, full-shot with overflow, full-shot with water flow through, and the melt push back method.

With short-shot, the mold cavity is filled with 60 to 70 percent polymer resin and after it reaches that point, water is injected. The water cores out the part, and finishes filling out and packing it. After a hold time of a few seconds to let the resin solidify, the pressure is released off the water and it drains out of the hole where the nozzle was inserted. (Preferably, the hole in which the water is injected should be at the bottom of the mold to facilitate draining. In some cases where the nozzle is inserted in an area in which gravity will not purge the water, forced gas or forced air can be used. These fluids can also be used to ensure that the interior is completely free of moisture.) An advantage of the short shot method is that there is little or no regrind with which to contend, but a disadvantage is that it could cause hesitation lines on the exterior of the part caused by the  switchover from melt injection to water injection.

Enlarge this picture Wall thicknesses can be reduced by using water-assist, gas-assist and a gas/water-assist injection molding process, but water-assist creates the thinnest walls, according to BASF testing.

In a full-shot method, the mold cavity is packed and held for several seconds. The water is injected and the displaced resin goes into an overflow channel. After a holding time, the water is released. The resins in the overflow can be collected and reground in secondary operations. The advantage of the full shot method is that the best exterior surface finish is attainable, but it does require secondary operations to remove the overflow and deal with regrind.

The melt pushback process is similar to full-shot, accept with this method the displaced polymer is pushed back into the barrel. This method makes the process more economical by not having to deal with resin waste or regrinding.

A handle for a small appliance made with PME Fluidtec water-assist injection molding.

Using these methods, WAIM has a number of advantages. As compared to gas, WAIM can reduce cooling times by 50 percent or more, and create up to 25 percent thinner wall sections depending on the application, according to Bernd Herzog, manager of application and process development for PME Fluidtec. In addition, the wall thickness can be more uniform over the entire length of the channel, and molded parts can have larger cross sections. With gas, Herzog says, there can be a problem when the wall thickness gets bigger than 4 mm because the resin doesn’t crystallize quickly enough and the material can flow down. Water, on the other hand, cools the resin faster, and as a result the materials do not flow downward. The resulting wall is smoother and more consistent in terms of thickness, he says. The largest part molded by PME is a lid for a plastic trash bin that measured 43.5 in. x 50 in. x 7.5 in. and weighed about 22 lbs.

To verify a reduction in wall thickness, PME cored out 60 mm to 80 mm diameter glass filled nylon and polypropylene pipes using both the gas-assist and water-assist method. The company discovered that the wall thickness of the gas-assist part was two to three times thicker than the water-assist molded part, Herzog says.

One of the earliest and largest applications for water-assist injection molding is the lid to this trash receptacle. The polyethylene lid has been made with PME Fluidtec technology since 2001.

Other studies confirm these findings. Harold Colwell, BASF Engineering Plastics, technical development engineering manager, says that his company’s internal studies, in which they tested an automotive cooling pipe test mold, confirmed that water-assist can generate thinner walls than either gas-assist or a combination of gas-assist and water-assist. In the study, nylon 6 with 40 percent mineral and glass fill can create a wall thickness of 2.9 mm using water-assist, while gas-assist was 4.2 mm, and a combination of gas-assist and water-assist was 3.2 mm.

The water-assist method compares favorably to gas because of water’s innate properties. Water’s thermal conductivity is 40 times greater than that of gas, and water’s heat capacity is four times greater than gas. In addition, cooling times, and typically the resulting cycle times, are reduced in part because of the dual jobs that water is doing in the mold machine. When water is injected into the mold cavity, it is not just coring out the part, it is also cooling the part from the inside, according to Fleck. In his studies, Fleck says that parts molded with gas-assist continue to get hotter, while WAIM-molded parts dropped in temperature after the part was ejected from the mold.

A handle for a Bosch refrigerator that has been in production since 2004. It is made with PME Fluidtec technology using a 30 percent glass filled PA 6.

In tests, 15 seconds after mold ejection both the gas-assist and water-assist parts were around 280 DegF. After 30 seconds, however, the temperature of the gas-assist had increased while the water assist part had decreased. Most likely, Fleck says, this is because the gas-assist part was still cooling on the inside whereas with water-assist the water more quickly cooled the inside of the part. A potential problem with the gas-assist part only being cooled from the mold tool side, is that the differential cooling could cause molded in-stress and warping, he adds.

Faster cooling times have also been shown to reduce cycle times. In BASF’s 18-in. test handle mold, the company’s 15-percent glass filled PET resin was molded and cycle times tested.  The gas took 62 seconds to cycle, while water took 40 seconds, and water flow through, in which the water flows through the hollow part and exits at the end of the part, took about 30 seconds, Fleck says.

The chain saw handle is made from PA 6 GF 30 at a cycle time of 45 sec., using PME Fluidtec equipment.

But water can work too well. Without the right process or material, problems can occur. The water can freeze the resin too quickly and sometimes that can lead to parts that are not internally smooth or can create shrinkage voids in the part wall. The water can push aside the cooling melt front and create new channels in the part, or cause other internal defects. (The water’s temperature seemingly has no part to play in this crystallization process. Researchers have injected water at temperatures up to 140 DegF without it affecting the crystallization rate.)

The problem has been solved with both material and process changes. Resin suppliers have created several specially formulated materials for water assist molding. To date, most of the resin material has been in the polyamide (nylon) grades 6 and 6/6, although other materials such as ABS, and filled polyesters have been used. Nylons were the initial resin targeted for WAIM, but other materials, including lower cost resins, are under development. Fleck adds that even with hygroscopic resins, which nylon is one, the water injection process has no adverse effect on the material properties. Fleck surmises that because water solidifies the resins so quickly, water doesn’t penetrate the material.

The production of the kettle handle was made with water injector technology from PME Fluidtec. It has no flash on the handle, no sinkmarks, and an even appearance.

In addition to resins specifically formulated for WAIM applications, processing methods have been improved including the use of gas-assist and water-assist in tandem to get the benefits of smoother internal walls and faster cooling times, says Brian Brookshaw, deputy managing director, Cinpres Gas Injection Ltd. In this instance, a bubble of nitrogen would be injected first and begin the coring process, helping to create a smoother interior surface. Water is then injected into the cavity, compressing the gas against the resin, and the water finishes coring out the part. The gas bubble is still in front of the water, just compressed, and when the water is released the gas bubble wants to expand. This helps to force the water out of the cavity and the gas will follow it out of mold. Sequencing the gas and water can reduce defects such as fingering voids and still reap some cycle and cooling time benefits, Brookshaw says.

This is an example of how the technology is evolving. Additional improvements to WAIM’S technology are already continuing. Companies such as PME Fluidtec have taken the technology even further with the use of multi-cavity tooling that can core several parts simultaneously. Also, in its early use, water assist had a reputation for leaks and unreliability. Better seals and injectors have been developed that has eliminated those problems, Herzog says. Resin manufacturers are working on process and material improvements that will enable its use on a growing range of applications.

The polypropylene handle of this transport cart made with PME Fluidtec equipment.

Testing has also begun to answer some questions about the technology. Previously, using water as a core out medium was a cause of concern over its effects on the resins’ molecular weight. Studies have shown that there are no adverse affects from water contacting the melt, Fleck says. For instance, polyesters can be sensitive to moisture in the melt, and some had questioned whether the water would degrade the polymer when the water and resin came in contact. However, Fleck says that because the melt freezes so quickly, the water does not have a chance to create any kind of hydrolytic degradation (polymer chains do not break down).

To test this, he used an intrinsic viscosity (IV) test, which gives an IV value. Typically, virgin PET pellets have an IV value of around 0.52. The gas-assist molded part received a 0.47 IV, which Fleck says is a typical drop in IV value. Looking at the IV values for water-assist, and gas-assist and water-assist molding, the IV values were 0.47 and 0.48, respectively, which Fleck says is within acceptable limits.

While water-assist injection molding is in its relative infancy in the U.S., in Europe the molding method has been used in the auto industry for several years. Growth in the U.S. is anticipated as potential users learn more about the technology, new resins are developed, and equipment capacity and other advancements are made. Some of which are already in the pipeline and are scheduled for release by the end of the year.

For more information, email:
BASF:  randy.fleck@basf.com
DuPont:  carole.a.davies@usa.dupont.com
PME Fluidtec: B.Herzog@pme-fluidtec.de
Cinpres: Brian.Brookshaw@cinpres.com