For cycle time reduction, just add water

It would seem that just as gas-assist molding is becoming widely used, and even coming to be considered standard, a new technology wave is breaking on the beach. Instead of using gas to create hollow spaces in parts, this process uses water to core out the designated areas. 

You might wonder if you really need or want to climb the learning curve of another process technology. Water, gas, what's the difference as long as it hollows out the part? Well, would cutting as much as 70 percent out of molding cycle times motivate you? 

Water-assist technology is just coming out of the development stage, so there is still much refinement to be done. However, the process is already a reality in the market. Commercial applications are in production. Several machine manufacturers have systems for sale now and more are indicating they will be in this market soon. 

Instant Benefits 
Many of the machine makers offering, or preparing to offer, water-assist molding are using technology developed over the past two years at the Institute for Plastics Processing (IKV) in Aachen, Germany (see "Molding 2000 Spotlights Leading-edge Technologies," May 2000 IMM, pp. 216-217). IMM spoke with André Brunswick, the scientist who started and ran the development program, not just about how the technology works, but why it is needed. 

For instance, is it an error to think, as many do, that gas-assist molding itself cuts cycle time? Gas does cut cycle time, says Brunswick, compared with compact molding for parts with thick-walled areas. But beyond that simple fact, you must consider the geometry and complexity of the part itself. 

One example is a uniformly thick rod-shaped part. In practice, such a part would almost always be designed as a ribbed thin-wall piece. If you compare the gas-assist cycle time of a rod-shaped part with compact molding using ribbed construction, says Brunswick, the gas time will be quite long. However, better mechanical properties, reduced material consumption, design freedom, and reduced tool complexity justify the longer cycle in a number of cases. 

If the part also has highly integrated channels, then the larger the diameter, the greater the residual wall thickness with gas, which further extends cooling time. With gas the tool steel must remove enough heat so that the part can be ejected, and the tool steel is only on one side of the part—the outside. 

The beauty of IKV's water-assist molding is twofold: The part is cooled from the inside and the outside simultaneously, and the noncompressible water core does not evaporate in the process. While the tool steel does its customary job on the skin side, water absorbs heat from the inner walls. Further, that heat is carried away as the water is purged. 

Inside the Water Works 
The water-assist molding technology IKV developed, called WIT (water injection technology), pushes the melt out of the inner channel like a piston. Water, unlike gas, is incompressible. The temperature of the water used is colder than the melt, about the same as the mold, and ranges from 10 to 80C, depending on part size and material. The melt forms a high-viscosity membrane or skin across the front of the colder water, which is used to push the still-molten core material forward. 

Coring takes place quickly and the water does not heat up immediately. Because the water flows at a high enough volume and pressure, it does not evaporate. This lack of evaporation, says Brunswick, is at the very heart of IKV's technology and enables every benefit of the process. If the volume flow is not high enough, he adds, one has a gas-assist process with steam. 

When the polymer skin solidifies on contact with the cold water, it creates a seal between the molten polymer and the water that prevents polymer degradation by hydrolysis. Put another way, IKV's WIT process is too fast for degradation to take place. 

After coring out the part, the water is held at pressure long enough to prevent voids within the wall as the material shrinks. During this pressure holding time the water becomes heated. When the water pressure is released, some or all of the water evaporates, which produces steam. The increased volume of the steam pushes any residual water back though the water injector, where a bypass leads to a tank in which the water is filtered and recooled. 

Water Delivery 
To achieve the required high-volume water flows, the IKV WIT injection device uses a three-piston pump and pressure accumulator that yields nonpulsating flow. At the start of the cycle, pressure is low. When water is injected into the melt, a valve opens a bypass to the injector. Pressure control is performed by a simple valve, and flow volume is managed by a governor that controls the pump rpms. This simple, low-cost solution has been proven in prototype and series production with a number of parts, including the Sulo shopping cart (See right). 

Since water must reach the melt in high volumes, the injector is an important component. Because orifices of gas-assist injectors normally are too small for this kind of volume, IKV developed a number of injectors using different mechanisms, sized for a variety of applications. 

As with gas, melt is injected first, and holding pressure is applied with a full shot. Then the water enters and pushes the molten center of the part either into open areas of the cavity, into spillover areas, or back into the screw antechamber. A full-shot process is highly recommended for complex parts with high-quality surface requirements. 

In complex and/or curved parts, some residual water needs to be released outside the mold. An air injector at the other end of the part can be used to blow water out of the channels. If a second injector cannot be placed in the part, water can be released in parallel to the injection molding process. This extra step is justified by the advantages of the water injection process. Optionally, the water can be left inside the part either for decorative effect, to create a noncompressible core for subsequent overmolding, or to add weight that gives the heft and feel expected by the product's buyers. 

More than Cycle Times 
Brunswick says the cooling time with WIT can be as little as 25 percent that of gas, and this has been already proven in applications. Yet, even at that speed, there is no chance of the material foaming, as often occurs with gas. Instead, the water creates a very smooth inner surface and the wall cross section is more compact and solid than it would be with gas. Even if foaming is acceptable in a gas-assist part, the water-assist cycle time will be between 25 and 50 percent that of gas. 

From the project's beginning, Brunswick's IKV team worked closely with A. Schulman and later with other polymer suppliers. To date there have been successful tests and/or actual production using PA, PP, PE, POM, HIPS, and ABS. PS and PC are less suitable due to a tendency toward stress cracking. Brunswick says not all materials produce a good coring effect and that different grades of polymers, with and without fillers, need to be evaluated for use with water injection. Melt viscosity is a critical factor in processing any material using this technology. 

Water-assist molding, asserts Brunswick, is not the answer for every part. Many people had similar thoughts with gas assist, tried to use it with the wrong parts, and were disappointed. The application has to be right for the process. If you're interested in exploring or implementing the WIT process, Brunswick advises consulting with equipment manufacturers.