Last updated: 2026-09-04 · 5 min read
Water-Assist Injection Molding — How Does the Process Work, What Parts Benefit From It, and How Does It Compare to Gas-Assist Molding?
Water-assist injection molding (WAIM) injects water at 20–30 MPa into a short shot fill — the water pushes the melt to fill the cavity and creates a hollow channel. Unlike gas-assist (finger-like penetration), water produces a smooth inner surface and cools from the inside — cutting cycle time 20–40%. Water is drained, recovered and reused (less than 0.5 liters per cycle). Best for thick-walled tubular parts (3–12mm) in water-resistant resins (PP, PA, POM, TPE).
Key Facts
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Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.
How Does the Water-Assist Process Work?
Four stages:
- Short shot fill (60–85% of cavity volume). The melt is injected at a reduced volume — the cavity is 60–85% full (compared to 95–100% for conventional molding). Too little melt: water breaks through the melt front (leakage). Too much melt: hollow channel is too small (reduced weight savings).
- Water injection (20–30 MPa). Pressurized water is injected through a pin or nozzle into the center of the melt front. The water pushes the melt to complete the cavity fill and creates a hollow channel. Water pressure is 3–5× higher than gas-assist — smoother inner surface, more uniform channel. Water also cools the inner surface, reducing cycle time by 20–40%.
- Hold pressure and cooling. Water pressure is maintained during cooling to prevent the hollow channel from collapsing. Cooling time is reduced because water extracts heat from both the outer surface (mold cooling channels) and the inner surface (water channel).
- Water drainage and recovery. After the part is rigid enough to eject, water is drained through a drain pin and recovered into a closed-loop system. Filtered, re-pressurized and reused. Water consumption: less than 0.5 liters per cycle.
WAIM is best suited for tubular or rod-shaped parts with thick cross-sections: automotive door handles, grab rails, steering wheel frames, furniture components, appliance handles. The combination of 20–40% weight savings and 20–40% cycle time reduction produces a significantly lower cost per part at high volumes.
How Does Water-Assist Compare to Gas-Assist?
Five comparison points:
| Factor | Water-Assist (WAIM) | Gas-Assist (GAIM) |
|---|---|---|
| Inner surface | Smooth (water cools uniformly) | Rough, finger-like (gas penetrates unevenly) |
| Cycle time reduction | 20–40% (water cools from inside) | 10–20% (gas does not cool) |
| Pressure | 20–30 MPa (3–5× higher) | 5–15 MPa |
| Equipment cost | $30,000–80,000 (water pump + recovery) | $15,000–40,000 (nitrogen generator) |
| Material restriction | No water-sensitive resins (PC, PET, PBT) | Any resin |
Five design rules for WAIM:
| Rule | Requirement | Reason |
|---|---|---|
| Wall thickness | 3–12mm (thicker than gas-assist) | Water needs a thick melt core to penetrate — below 3mm, water breaks through |
| Channel diameter | 40–70% of wall thickness | Larger channels = more weight savings but risk collapse |
| Flow path length | Up to 2m (longer than gas-assist) | Water has higher pressure and momentum than gas |
| Part geometry | Straight or gently curved channels | Sharp corners block water penetration |
| Resin compatibility | PP, PA, POM, TPE (water-resistant) | Water-sensitive resins (PC, PET, PBT) degrade on contact |
If your part requires PC or PET, gas-assist is the only option — WAIM cannot process water-sensitive resins. The water contact causes hydrolysis, degrading the polymer chains and weakening the part.
Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.
WAIM is a cycle time process, not just a weight reduction process. The water cools the part from the inside — cutting cycle time by 20–40% compared to solid molding. For high-volume programs, the cycle time savings alone justify the equipment investment.
Frequently Asked Questions
What are the most common defects in water-assist molding?
Three defects. Water breakthrough: the water penetrates through the melt front and leaks out — caused by insufficient short shot volume or excessive water pressure. Fix: increase the short shot volume or reduce water pressure.
Channel collapse: the hollow channel collapses on one side — caused by insufficient water pressure during cooling or uneven wall thickness. Fix: maintain water pressure during the entire cooling phase and balance the wall thickness. Bubbles on the outer surface: trapped air or steam creates bubbles — caused by inadequate venting or excessive water temperature. Fix: add vents at the last fill area and maintain water temperature below 60°C.
What is the payback period for a water-assist system?
The system (water pump, injection unit, recovery, controls) costs $30,000–80,000. Mold modification for water injection pins and drainage: $5,000–15,000. Total investment: $35,000–95,000.
The payback comes from cycle time reduction (20–40%) and material savings (20–40% weight reduction). For a part with a 30-second conventional cycle vs. 20-second WAIM cycle, savings are $0.08 per part (material) + $0.005 per part (cycle time) = $0.085 per part. At 1 million pieces per year: $85,000 annual saving — payback in 5–13 months.
Can you switch between water-assist and conventional molding in the same mold?
Yes — a WAIM mold can run conventional molding by disabling the water injection system. The water injection pins are built in but can be retracted or blocked when running conventional. This flexibility allows the mold to produce solid parts (low-volume or prototype runs) and hollow parts (high-volume production) without changing molds.
The trade-off: the WAIM mold costs 15–25% more upfront than a conventional mold. The flexibility is worth it only if the program has both low-volume and high-volume production phases.
Key Takeaways
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gas-assist: smoother inner surface, faster cycle, higher equipment cost, no PC/PET.
If your part has thick cross-sections and could benefit from weight reduction or cycle time savings, send the drawing and the resin specification for a water-assist feasibility review. The review checks the part geometry, estimates the weight and cycle time savings, and calculates the payback period.
Sources
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Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet