ABSTRACT: This article provides an introduction to the benefits of water-assist injection molding (WAIM), as well as to some of the key differences between WAIM and gas-assist injection molding. A detailed animation quickly introduces the reader to a typical WAIM process, its equipment and configuration. The article also contains links to WAIM-optimized materials and opportunities for more detailed education on WAIM.
Water-assist injection molding (WAIM) is one of the latest and most promising developments in “assisted” injection molding. As in the established gas-assist injection molding process, WAIM technology uses a fluid under pressure to core out a hollow plastic part in the mold. Because of similarities between the two processes, both provide several of the same benefits:
Lower material costs
Lower tool cost
More part consolidation and less finishing than with un-assisted injection
molding or metals.
The differences between water and gas, however, enable WAIM to introduce significant improvements of its own:
50% faster cooling cycle time than with gas-assist molding
Up to 25% thinner wall sections, based on resin / composition
Molders may also combine gas-assist and water-assist technology in sequence to achieve further benefits in certain applications.
Advantages of WAIM Technology
Cooling
WAIM reduces cooling cycle time to as little as half that of gas-assist molding both because of the properties of water and the structure of the process. To begin with, the thermal conductivity of water is 40 times greater thanthat of gas; its heat capacity, four times greater. After injection, the continuous flow of water coring out the hollow section cools plastic from the inside at the same time that the mold tool’s metal cools the plastic from the outside. Although the mold tool performs the same cooling function in gas-assist, injected gas simply cannot cool as well as water can. In fact, studies performed by BASF demonstrate that the temperature of parts molded with gas-assist continues to rise after demolding. The temperature of WAIM-molded parts drops. In addition, WAIM creates thinner walls that cool faster.
Thinner walls, less material.
Compared to gas, water has higher viscosity and is incompressible. In WAIM, water compresses plastic uniformly into thinner walls – approximately 25% thinner than is typical in gas-assist molding. Uniformity of wall thickness around bends and other geometric shapes is aparticular advantage of WAIM. Thinner walls directly correlate to materials savings; uniform walls, directly to uniform strength.
Material surface and performance.
Even with hygroscopic resins, such as polyamides, the water injection process does not have a negative effect on material characteristics – probably because the rapid cooling with WAIM prevents water ingress. The molecular weights of WAIM-molded resins are comparable to those measured in gas-assist-molded resins. In studies conducted by BASF, both polyamides and polyesters at 25% regrind levels exhibit expected molecular weight retention over five heat histories. And the water-assist-molded parts can be successfully powder-coated or painted.
Proven process technology, materials.
WAIM has been under development for some 10 to 15 years and is already successfully commercialized in Europe. In North America, BASF Corporation has built upon its European experience and established WAIM technology and application development capabilities. At its Budd Lake, New Jersey, location, the company can perform pilot-scale WAIM, gas-assist injection molding, or the combination of the two processes (GAS-WAIM).
BASF has already optimized several resin grades* (LINK) specifically for use in WAIM or GAS-WAIM, and in several product applications.
Applications.
WAIM is especially well suited for a number of hollow-parts applications:
• Automotive fluid handling tubes for oils and coolants
• Automotive door handles
• Oven and refrigerator handles
• Chain saw handles
• Office furniture chair arms
• Structural components in many industries.
Learn More About WAIM.
Every new processing technology comes with a learning curve. Because WAIM resembles gas-assist injection molding, molders already familiar with gas-assist should grasp the intricacies of WAIM most quickly. Before launching a successful WAIM operation, however, some details regarding equipment, process variations, materials and applications (and investment) require explanation and mastering.
Fortunately, several companies in Europe – and now North America – have already done much of the research and development work to ensure this technology’s value and viability. In addition, courses are available to help molders master WAIM.
About
Randy Fleck Randy Fleck is a Sr. Process Engineer with BASF Engineering Plastics, a global supplier of engineering plastics. He is a member of BASF’s Advanced Process Technology Group, where the focus is on new process technologies. Prior to that he was a field Technical Development Engineer. Before joining BASF in 1991 he worked for 12 years in the injection molding industry in supervision, process engineering, and project management.
