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. In water assisted injection molding (WAIM) liquid water is injected during molding which expands to hollow out a plastic part. The thermal conductivity of the water leads to 50% faster cooling cycles and 25% thinner wall sections.Because of similarities between the two processes, both provide several of the same benefits:
The differences between water and gas, however, enable WAIM to introduce significant improvements of its own:
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:
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.
- Lower material costs
- Lower tool cost
- More part consolidation and less finishing than with un-assisted injection molding or metals.
- 50% faster cooling cycle time than with gas-assist molding
- Up to 25% thinner wall sections, based on resin / composition
| Water Injection Molding | |
| Water Assisted Injection Molding - new methods of using water instead of gas to further reduce time cycles of thick section tubular products. The mold cavity is filled and partially packed with plastic gas. Gas or water injection creates the initial bubble, followed by water injection to pack the cavity and then sequencing the opening of a shut-off valve to enable the expulsion of molten plastic to the secondary cavity. The secondary cavity is then pierced to allow water to flow through the gas channel(s) to cool the plastic. Water is then drained from the cavity by gas pressure or by air injection to dry the part before opening the mold and ejecting the part. | |
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.
Gas-assisted process
The gas-assisted injection molding process begins with a partial or full injection of polymer melt into the mold cavity. Compressed gas is then injected into the core of the polymer melt to help fill and pack the mold. This process is illustrated below.
FIGURE 1. Gas-assisted injection molding: (a) the electrical system, (b) the hydraulic system, (c) the control panel, and (d) the gas cylinder.
Benefits of the gas-assist process
The gas-assisted injection molding process is capable of producing hollow, light-weight, rigid parts that are free of sink marks and less likely to warp. Other advantages include:
The gas-assisted injection molding process is capable of producing hollow, light-weight, rigid parts that are free of sink marks and less likely to warp. Other advantages include:
- Reduced cycle time
- Reduced pressure and clamp force tonnage
- Part consolidation with both thick and thin sections.
Typical applications
Typical applications for the gas-assisted injection molding process can be classified into three categories, or some combination of them:
Typical applications for the gas-assisted injection molding process can be classified into three categories, or some combination of them:
- Tube-and rod-like parts, where the process is used primarily for saving material, reducing the cycle time by coring out the part, and incorporating the hollowed section with product function. Examples are clothes hangers, grab handles, chair armrests, shower heads, and water faucet spouts.
- Large, sheet-like, structural parts with a built-in gas-channel network, where the process is used primarily for reducing part warpage and clamp tonnage as well as to enhance rigidity and surface quality. Examples are automotive panels, business machine housings, outdoor furniture, and satellite dishes.
- Complex parts consisting of both thin and thick sections, where the process is used primarily for decreasing manufacturing cost by consolidating several assembled parts into one single design. Examples are television cabinets, computer printer housing bezels, and automotive parts.
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.
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