Water-Assisted Injection Molding Techniques

Water-assisted injection molding (WAIM) is a specialised variant of conventional injection molding in which high-pressure water replaces or supplements polymer melt in the core of a part, yielding hollow or partially hollow components. By injecting water into a short-shot or overflow cavity, WAIM overcomes common challenges in producing thick-walled plastic parts—namely long cycle times, sink marks and uneven cooling. Key process parameters such as injection pressure, melt temperature, water injection delay time and mold design dictate the morphology of the hollow section, the residual wall thickness and the final mechanical properties. Variants include short-shot WAIM, in which water is introduced after a partial polymer fill, and overflow WAIM, which uses a dedicated overflow channel to promote uniform wall thickness. More advanced approaches integrate projectiles propelled by water to shape complex geometries, and co-injection schemes that combine different polymers or fibre-reinforced layers. Collectively, these techniques offer rapid cycle times, material savings and improved structural performance, making WAIM increasingly attractive for automotive, medical and consumer applications.

Research from Nature Portfolio

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Research from all publishers

Recent studies have explored the interplay of process parameters with wall thickness, fibre orientation and component quality. In one investigation, fibre orientation in short-glass-fibre-reinforced polypropylene parts produced by overflow WAIM was analysed via a viscoelastic constitutive equation coupled with an orientation tensor model. Results indicated that increased water pressure and reduced delay times enhanced alignment along the flow direction, while higher fibre content and melt temperature diminished orientation, highlighting the trade-offs critical for mechanical performance. Another experimental study compared short-shot WAIM and gas-assisted injection across pipe fittings with varying bend angles, revealing that WAIM produced multiple penetration fronts yielding more uniform residual walls but at the cost of surface shrinkage, whereas gas-assisted approaches produced narrower wall-thickness distributions at low bending angles. More recently, water-powered projectile-assisted co-injection molding of composite pipe components demonstrated that combining high-pressure water with a projectile improves inner surface quality and pressure resistance; overflow configurations yielded thinner, more uniform walls and enhanced axial fibre orientation, underscoring the potential of hybrid WAIM variants for advanced lightweight structures.

Water-Assisted Injection Molding Techniques publication trend

The graph below shows the total number of articles in water-assisted injection molding techniques across all publications each year (not limited to Nature Index journals).

Technical terms

Water-assisted injection molding (WAIM): An injection molding process in which pressurised water displaces polymer melt to form hollow cores within parts.

Short-shot method: A technique where only part of the cavity is filled with polymer before fluid injection, creating a hollowed section.

Overflow method: A variant employing an auxiliary overflow cavity that ensures consistent wall thickness by accommodating excess melt prior to fluid introduction.

Residual wall thickness: The remaining polymer thickness between the mould surface and the fluid-induced hollow core, critical for mechanical integrity.

Water injection delay time: The interval between completion of polymer filling and initiation of water injection, which influences penetration depth and wall uniformity.

Fibre orientation: The alignment of reinforcement fibres within the polymer matrix, affecting strength, stiffness and anisotropic behaviour.

References

1. Fiber Orientation Analysis of Overflow Water‐Assisted Injection Molding with Short Glass Fiber Reinforced Polypropylene. Advances in Polymer Technology (2022).
2. Experimental analysis of fluid‐assisted injection molding of pipe fittings with different angles by short‐shot method. Engineering Reports (2022).
3. Research on the Quality of Composite Pipe Components in Fluid-Powered Projectile-Assisted Injection Molding. Polymers (2025).