Water-Assist Injection Molding (WAIM)

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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.