Water-assist injection molding is a technique that uses pressurized water to create hollow cavities in plastic parts. It is similar to gas-assist injection molding, but with some advantages and disadvantages. In this blog post, we will introduce the basic principles of water-assist injection molding, and discuss its benefits and challenges.
Water-assist injection molding works by injecting molten plastic into a mold, followed by a jet of water that displaces some of the plastic and forms a hollow channel. The water cools the plastic and helps it solidify faster, reducing cycle times and improving dimensional stability. The water is then drained out of the part, leaving a hollow structure.
Some of the advantages of water-assist injection molding are:
- Reduced material usage and weight, which can lower costs and improve performance.
- Improved strength and stiffness, as the hollow sections act as ribs that reinforce the part.
- Reduced warping and shrinkage, as the water cools the plastic uniformly and prevents differential shrinkage.
- Enhanced surface quality and appearance, as the water eliminates sink marks and other defects.
Some of the challenges of water-assist injection molding are:
- Higher complexity and cost of the equipment, as it requires a separate water injection unit and a water management system.
- Higher risk of water contamination and corrosion, as the water can introduce impurities and damage the mold or the part.
- Higher difficulty of design and optimization, as the water flow and pressure need to be carefully controlled to achieve the desired cavity shape and size.
Water-assist injection molding is a promising technique that can offer significant advantages for certain applications, such as automotive, medical, and packaging. However, it also requires careful consideration of its limitations and challenges. If you are interested in learning more about water-assist injection molding, or want to try it for your next project, contact us today. We have the expertise and experience to help you achieve your goals.
This blog post provides an introduction and overview of water-assist injection molding (WAIM), a technology that uses water under pressure to core out hollow plastic parts in the mold. WAIM has several advantages over conventional injection molding and gas-assist injection molding, such as faster cooling cycle time, thinner wall sections, uniform wall thickness, and improved material surface and performance. The post also covers some of the key aspects of the WAIM process, such as equipment, configuration, materials, and applications.
Table of Content
- What is Water-Assist Injection Molding?
- How Does Water-Assist Injection Molding Work?
- What are the Benefits of Water-Assist Injection Molding?
- What are the Challenges of Water-Assist Injection Molding?
- What are the Equipment and Configuration for Water-Assist Injection Molding?
- What are the Suitable Materials for Water-Assist Injection Molding?
- What are the Applications of Water-Assist Injection Molding?
What is Water-Assist Injection Molding?
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. The fluid used in WAIM is water, which has different properties and behaviors than gas. Water has higher viscosity and is incompressible, which means it can compress plastic uniformly into thinner walls and maintain a stable penetration front. Water also has higher thermal conductivity and heat capacity than gas, which means it can cool plastic faster and more efficiently from the inside.
How Does Water-Assist Injection Molding Work?
The WAIM process consists of four main steps: injection, water penetration, water evacuation, and part ejection.
- Injection: The molten plastic is injected into the mold cavity at high speed and pressure. The plastic fills the cavity partially or completely, depending on the part design and process parameters.
- Water penetration: After a short delay time, water is injected into the mold through a nozzle or a pin. The water penetrates into the molten plastic along a predetermined path, displacing the plastic outward and forming a hollow channel inside the part. The water pressure is controlled to balance the plastic pressure and prevent water leakage or plastic blowout.
- Water evacuation: After a sufficient cooling time, the water is evacuated from the mold through a valve or a vent. The water can be recycled or disposed of according to environmental regulations.
- Part ejection: The cooled part is ejected from the mold with a hollow structure and a smooth surface.
What are the Benefits of Water-Assist Injection Molding?
WAIM offers several benefits over conventional injection molding and gas-assist injection molding, such as:
- Faster cooling cycle time: WAIM reduces cooling cycle time to as little as half that of gas-assist molding because of the superior cooling effect of water. The continuous flow of water cools plastic from the inside at the same time that the mold tool’s metal cools the plastic from the outside. This results in shorter cycle times and higher productivity.
- Thinner wall sections: WAIM enables thinner wall sections than gas-assist molding because of the higher viscosity and incompressibility of water. Water compresses plastic uniformly into thinner walls – approximately 25% thinner than is typical in gas-assist molding. Thinner walls directly correlate to material savings and weight reduction.
- Uniform wall thickness: WAIM ensures uniform wall thickness around bends and other geometric shapes because of the stable penetration front of water. Uniformity of wall thickness around bends and other geometric shapes is a particular advantage of WAIM. Uniform walls directly correlate to uniform strength and quality.
- Improved material surface and performance: WAIM improves material surface and performance because of the rapid cooling and low residual stress of water. 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. WAIM also eliminates surface defects such as sink marks, warpage, or burn marks that are common in gas-assist molding.
What are the Challenges of Water-Assist Injection Molding?
WAIM also poses some challenges that need to be addressed by careful design and optimization, such as:
- Water leakage: Water leakage can occur if there is a gap between the water nozzle or pin and the plastic melt, or if there is a crack or hole in the plastic wall. Water leakage can cause water contamination, mold corrosion, or part quality issues. Water leakage can be prevented by using proper sealing devices, adjusting water pressure and timing, and selecting suitable materials and mold design.
- Water evacuation: Water evacuation can be difficult if the water channel is long, narrow, or complex. Water evacuation can affect the cooling efficiency, cycle time, and part quality. Water evacuation can be improved by using proper venting devices, optimizing water flow rate and direction, and designing appropriate water channel geometry and layout.
- Water quality: Water quality can affect the WAIM process and the part quality. Water quality can be influenced by factors such as temperature, purity, hardness, pH, additives, or contaminants. Water quality can be controlled by using proper water treatment systems, monitoring water parameters, and following water maintenance procedures.
What are the Equipment and Configuration for Water-Assist Injection Molding?
The equipment and configuration for WAIM are similar to those for gas-assist injection molding, with some modifications to accommodate water injection. The main components of a WAIM system are:
- Injection molding machine: The injection molding machine provides the molten plastic and the injection pressure for the WAIM process. The injection molding machine should have sufficient clamping force, injection speed, and injection pressure to handle the WAIM process. The injection molding machine should also have a compatible interface with the water injection unit.
- Water injection unit: The water injection unit provides the water and the water pressure for the WAIM process. The water injection unit consists of a water tank, a water pump, a water heater, a water filter, a water metering device, a water pressure regulator, a water injection controller, and a water nozzle or pin. The water injection unit should be able to deliver water at a precise flow rate, pressure, temperature, and timing to achieve optimal WAIM results.
- Mold: The mold defines the shape and size of the part and the location and direction of the water channel. The mold should have a suitable cavity design, runner system, gate type and size, venting system, cooling system, and ejection system for the WAIM process. The mold should also have a proper sealing mechanism to prevent water leakage and a compatible connection with the water nozzle or pin.
- Water evacuation unit: The water evacuation unit removes the water from the mold after the WAIM process. The water evacuation unit consists of a water valve or vent, a water collector, a water recycler or disposer, and a water evacuation controller. The water evacuation unit should be able to evacuate water quickly and efficiently without affecting the cooling performance or part quality.
What are the Suitable Materials for Water-Assist Injection Molding?
WAIM can be applied to various thermoplastic materials that have good melt strength, flowability, thermal stability, and compatibility with water. Some of the common materials used for WAIM are:
- Polyamides (PA): Polyamides are hygroscopic resins that have high strength, stiffness, toughness, heat resistance, chemical resistance, and abrasion resistance. Polyamides are suitable for WAIM because they have good melt strength and flowability that allow for thin wall sections and complex geometries. Polyamides also have good thermal stability that prevents degradation during rapid cooling with water.
- Polypropylene (PP): Polypropylene is a semi-crystalline resin that has high impact strength, stiffness, chemical resistance, and recyclability. Polypropylene is suitable for WAIM because it has low density and low cost that enable material savings and weight reduction. Polypropylene also has good flowability that allows for easy filling of long and narrow channels with water.
- Polyethylene (PE): Polyethylene is a semi-crystalline resin that has high impact strength, flexibility, chemical resistance, and environmental stress crack resistance. Polyethylene is suitable for WAIM because it has low density and low cost that enable material savings and weight reduction. Polyethylene also has good melt strength that allows for stable penetration of water into the plastic melt.
- Acrylonitrile butadiene styrene (ABS): Acrylonitrile butadiene styrene is an amorphous resin that has high impact strength, stiffness,
What are the Applications of Water-Assist Injection Molding?
Water-assist injection molding is a technique that uses pressurized water to create hollow cavities in plastic parts. This technique can improve the quality, strength, and appearance of plastic parts, as well as reduce the material and production costs. Water-assist injection molding can be applied to various industries and products, such as:
- Automotive: Water-assist injection molding can be used to produce lightweight and complex parts such as door handles, bumpers, fuel tanks, and air ducts. These parts can have better dimensional stability, impact resistance, and surface finish than conventional injection molding.
- Medical: Water-assist injection molding can be used to produce medical devices and components such as catheters, syringes, tubes, and valves. These parts can have smoother inner surfaces, reduced wall thickness, and enhanced biocompatibility than conventional injection molding.
- Furniture: Water-assist injection molding can be used to produce furniture parts such as chair legs, armrests, and frames. These parts can have improved stiffness, strength, and aesthetics than conventional injection molding.
- Consumer goods: Water-assist injection molding can be used to produce consumer goods such as bottles, containers, toys, and sports equipment. These products can have reduced weight, increased durability, and better functionality than conventional injection molding.