Water-assisted injection technology is an advanced injection molding process in which a portion of the melt is injected into the mold cavity and then high-pressure water is injected into the melt through equipment to finally mold the workpiece.

Due to the incompressibility of water, thus forming a solid interface at the front end of the water, the inner wall of the product is extruded into a cavity, and the front end of the water also plays the role of rapid cooling.

Therefore, water-assisted has many advantages that cannot be compared with gas-assisted. Studies and applications have shown that water-assisted can generate thinner and more uniform cavity walls, and the inner wall surface of the runner is very smooth.

Especially for thick-walled workpieces, the cooling time can be significantly reduced by water-assisted compared to gas-assisted.

The principle of water-assisted injection molding

Water-assisted injection molding is a development of gas injection technology in which the gas that blows away the melt (usually nitrogen) is replaced by water.

The hot melt can flow into an injection mold. This causes the thicker melt to cool slower because it is insulated by the outside wall. As the inner melt front cools at a different rate from the outside surface, it shrinks and can pull on the outside surface, creating warping and sink marks. temperature regulator hot oil.

By reducing artifacts like sink marks, warping, and clamping force through the use of WIT, you can ensure the highest utilization of material. A by-product of higher material utilization is lower weight because less material is going to waste and unwanted artifacts.

Although both methods can be used to make plastic parts with functional cavities, water-assisted injection molding has proven to be the most suitable for the economical production of parts with large closed sections.

Water-assisted injection molding comes in different forms. During blowing, the mold cavity is partially filled with melt, and then the gas expands (blows up) until the cavity is filled.

In contrast, in a blow-out or counterflow process, the cavity is filled with melt and then the fluid core is blown out into the overflow cavity or blown back into the material tube.

Typical problems in water-assisted injection molding

The defects that can occur in water-assisted injection molding have so far not appeared in gas-assisted injection molding. All defects can be compensated by a proper choice of processing parameters.

As a rule, it is better to aim for a high volumetric flow rate with a low injection pressure, which can be achieved by minimizing the back pressure and thus obtaining a sufficient water holding time.

To suppress swirls near the injection nozzle and ripples in the wall, the water must be injected at low pressure, to begin with, and then the pressure should be increased to the actual injection pressure as quickly as possible.

When the volume flow rate of water is too low, local molding caused by steam will occur. If pores appear, the water pressure builds up slowly to the point where the thin surface layer already formed at low pressure can be broken by the water pressure.

The diffusion process causes water bubble, and porosity occurs when the melt solidifies at low pressure on both sides of the mold and fluid, and the material between the outer layers of the solid crumples causes the formation of vacuoles.

To eliminate or reduce the generation of porosity, high volume flow rates are additionally required during the injection phase and high water pressure during the holding or cooling phase. The slow solidification of the material can counteract the formation of porosity.

Media lines with bifurcations present a particular challenge. If the bifurcation is also blown out, then control of the overflow mold cavities is tight.

Especially if a fast-setting material is used, a thin layer will solidify unwanted at the bifurcation and it will then have to be torn apart again. Cracks in the outer layer are the result.

Water-assisted injection molding is composed of five steps

(1) Injection of the melt

(2) Injection of water and core change

(3) Water pressure maintenance in the pressure-holding section (optionally including the flushing process)

(4) Pressure release and water evacuation

(5) Demolding

Characteristics of water-assisted injection molding

Because of the need for a fully formed waterway, process parameters are more important in water-assisted injection molding than in gas-assisted injection molding.

The effects of different plastics processing parameters on the lengths of water penetration were determined. It was found that the shrinkage rate and the viscosity of the polymeric materials, and the void shapes of the hollowed cores mainly determined the water-penetration lengths in molded products.

The incompressibility of water provides better process control but places greater demands on the water injection technology unit, which has to continuously provide the necessary volume flow rate.

One advantage of water over gas is in the holding and cooling stages: its excellent cooling properties allow the melt to be cooled internally and the cooling time to be significantly reduced. Because of the better cooling effect of water compared to gas, the cooling time and therefore the cycle time can be reduced significantly when parts with bigger diameters are processed.

Water evacuation can be achieved in several different ways. For materials with high processing temperatures, such as polyamides, steam pressure is sufficient. Gravity also plays a further role.

Another method is to inject compressed gas through another syringe, which blows the water out and causes a drying effect. Regardless of the method chosen, the water emerging from the plastic part flows back into the tank via the injector. To ensure good evacuation, the water injector must be placed at the lowest point of the mold.

Comparison of water-assisted and gas-assisted injection molding

Although the principles of water-assisted injection molding and gas-assisted injection molding are the same, most observers believe that water-assisted will not replace gas assist and that the process to be adopted depends on the application and the mold.

Water-assisted injection molding is the same as the gas-assisted process. Water injection technology starts with a short section of melt injected into the mold cavity, followed by the injection of water, which squeezes the resin melt to mold the part.

In some applications, compressed gas is used to squeeze the water out of the runner to completely de-moisten the structural components.

Studies and applications have shown that water assist produces thinner and more uniform cavity walls, which means material savings. In addition, water injection nozzles are typically larger than gas nozzles, water-assisted molding produces larger runners with smoother walls than gas-assisted molding.

The main benefit of water-assisted molding over gas-assisted molding with nitrogen is the rapid cooling efficiency of water. The thermal conductivity of water is 40 times higher than that of nitrogen, and the heat capacity of water is 4 times greater than that of gas. For thick-walled workpieces, water-assisted can reduce the cooling time by 30-70% compared to gas-assisted.

The main difference between gas and water is that gas can be compressed while water cannot. The higher viscosity and incompressibility of water make the front end of water form a solid interface, which acts as a squeezing hammer to hollow out the workpiece. The front end of the water also plays the role of cooling the melt filled into the mold cavity.

The advantages of water-assisted injection molding compared with gas-assisted injection molding:

(1) Significantly shorten the cooling time of the workpiece

(2) Larger workpiece cross-sections are possible

(3) Smooth inner wall

(4) Less deformation of the workpiece due to uniform cooling

(5) Uniform wall cross-section

(6) Lower cost and easy access to water as a pressure medium

Potential disadvantages of water-assisted injection molding:

(1) Water leakage problem

(2) Workpiece needs to be dewatered

(3) Large size water injection machine

(4) Not suitable for all workpieces

Materials for water-assisted molding

The greatest progress has been made in replacing pipes and metal parts used for fluid transfer in the engine compartment of automobiles with polyamide (nylon) type materials.

These special materials have been modified to obtain a slower crystallization rate and to avoid premature setting and perforation (or linting). The new polyamide materials are of the polyamide-6 or polyamide-6/6 type and contain mostly glass fibers or glass fiber-mineral fillers.

Materials based on polyamide-6/6 have better corrosion resistance to glycol coolants. Some processors are using polypropylene in water-assisted processes, while others are evaluating unfilled acrylonitrile-butadiene-styrene copolymers (ABS), acetal, and polybutylene terephthalate (PBT) as base materials for related applications.

Development and application prospects of water-assisted molding

Water-assisted injection molding technology is developing rapidly, and the nozzles have been improved to improve sealing performance and reduce water pump leakage. Nozzles used for water and air injection can also be used as an outlet for water accumulation in the workpiece cavity.

The new water supply container shape is more optimized to improve the performance of pressure, capacity, and timing control of the centering process.

Water-assisted injection molding technology has been developed mainly in Europe, which means that the commercial application of this technology is more advanced in Europe than in North America or Asia. Its applications involve automotive parts, consumer parts, and industrial parts.

WIT Process is great for most kinds of hollow or partly hollow parts like water pipes and door handles. Typical WIT applications include handles, top frames, rocker covers, door blocks, spatulas, brackets, chairs, and office furniture. Some of these tubular components used to be produced using gas-assisted molding technology, but water-assisted technology is more suitable.