If you are looking to consider gas-assisted injection molding for your next manufacturing project, you should familiarize yourself with the nuances of this method.
It has recently gained considerable momentum in medical device manufacturing due to its unique versatility and cost-effectiveness.
This guide will give you the necessary insights into gas-assisted injection molding to make an informed decision for your project.
Let’s get started!
Gas Assisted Injection Molding
Gas-assisted injection molding (GAM) is an ingenious plastic molding process that combines conventional injection molding principles and pressurized inert gas application. It involves packing a part using an inert gas, most commonly ‘nitrogen’, due to its availability and cost.
Here’s a quick breakdown of the procedure for your understanding:
- The process commences with a mold cavity that mimics your product’s shape. The cavity is partially filled with about 70% – 80% of molten plastic raw material.
- Following the injection of the resin, nitrogen gas is introduced. This gas, under the pressure of 0.5 to 300 MPa, fills the remaining volume and pushes the plastic material against the mold’s walls.
It’s a reliable method for various applications, promising cost savings and improved operational efficiency.
Nitrogen assists in hollowing out thicker sections within the part for fast cooling. Consequently, this accelerates the overall process, increasing productivity and efficiency.
Gas-assisted injection molding is especially effective in creating large, hollow plastic parts with intricate designs and thick geometry.
Types of Gas-Assisted Injection Molding Processes
Two types of Gas-assisted injection molding (GAM) exist: Internal and External.
Internal Gas-Assisted Molding (Most Widely Used)
In internal gas-assisted molding, molten plastic resin enters the mold through spruce channels. Following this, pressurized gas (usually nitrogen) is introduced via a nozzle, forming a bubble within the molten resin.
This gas bubble then expands, exerting pressure on the resin, pushing it against the mold walls, and defining the shape of the part.
Due to the constant pressure from the gas, it effectively minimizes shrinkage and warpage, typically responsible for reduced dimensional accuracy in thin-walled components.
Moreover, the gas creates a hollow channel in the center for parts with thicker sections, making the components lighter without compromising their strength.
Key Advantages
- Substantial cost savings due to reduced plastic weights and shorter molding time cycles.
- Less wear on molds due to decreased in-mold pressures.
- Uniform pressure transmission throughout the injection molding service, courtesy of the gas.
- Elimination of sink marks and packing from the molding machine.
- Potential reduction of in-mold pressures by up to 70%, leading to lower press lock forces, enabling larger moldings on smaller machines.
- Less power consumption, and lower molded-in stress, resulting in improved dimensional stability and negligible distortion.
External Gas-Assisted Molding (For Better Surface Definition)
Unlike the internal method, the gas in external molding doesn’t penetrate the material to create hollow sections.
Instead, it enters the mold through microchannels from one side only, typically the non-aesthetic side. Applying pressure from the backside packs the molten resin against the mold’s walls.
As the gas pressure acts uniformly on the non-aesthetic side, the resin is pushed firmly into the mold extremities, accurately mimicking its shape.
This includes fine textures and patterns. As such, if your part requires surface patterns and imprints, external gas-assisted molding could be your best bet.
It’s also suitable for parts with large surfaces, curved profiles, and thicker sections, especially ribbed features.
Key Advantages
- Superior surface finish, replicating the intricate details of the mold.
- The potential elimination of sink marks and virtually no molded-in stress reduce distortion.
- Enhanced dimensional stability due to efficient pressure application.
- Lower pressure requirements, reducing wear on molds and power consumption, and decreasing lock forces.
- More design freedom, allowing for thicker ribs with reduced wall thicknesses, multi-rib components, and flat PP and PE products.
Notably, the surfaces in contact with the gas may have little to no surface texture. This could be a trade-off depending on your specific requirements.
Three Stages of Gas-Assisted Injection Molding
Gas-assisted molding is a three-stage process, each critical to the final product’s quality and design.
- Resin Injection: The process begins with the raw resin being heated and pressurized, then injected into the mold. The material fills about 60-90% volume of the cavity; the exact amount varies depending on the design of the part and the volume that needs to be cored out by gas.
- Primary Gas Injection: Pressurized nitrogen is introduced into the resin, forming a bubble that pushes the resin throughout the mold, ensuring a thorough fill.
- Secondary Gas Injection: As the resin cools and contracts, more gas is added to fill any resulting gaps, maintaining consistent pressure and ensuring optimal part formation.
Design Considerations for Gas-Assisted Injection Molding
- Adapting Part Design: The part must be designed to control the gas introduction location, timing, and method on the plastic surfaces.
- Including Gas Zones and Modified Thicknesses: Areas, where gas application takes place, require modified thicknesses and specific gas zones.
- Incorporating Overspill and Gas Pins for Internal Gas Assist: Design adaptations need to include overspill pins to remove excess resin and gas pins to act as channels for gas insertion.
- Programming Specialized Gas Control Equipment: The equipment that controls gas flow needs programming to ensure gas insertion happens at the right time and pressure during molding.
- Managing Aesthetic Impact with External Gas Assist: A common issue with external gas assist is the potential for wavy markings on one side due to the gas pushing against the plastic. To manage this, engineers should use external gas assist only for parts with hidden non-aesthetic sides in the final assembly.
Plastic Materials Used for Gas-Assisted Injection Molding
Gas-assisted molding can utilize various plastic materials to deliver versatile, high-quality parts.
Here are some commonly used materials:
- Polypropylene (PP): Known for its strength, versatility, and cost-effectiveness, PP is ideal for manufacturing durable, lightweight parts such as food packaging or kitchen utensils.
- Low-Density Polyethylene (LDPE): LDPE offers excellent flexibility, making it well-suited for gas-assisted molding of parts requiring complex shapes.
- High-Density Polyethylene (HDPE): HDPE is chosen for parts demanding superior durability and strength (in temperatures up to minus 60 °C) due to its high strength-to-density ratio.
- Polyamide (Nylon): Valued for its strength, heat resistance, and chemical stability, Nylon is often used for parts requiring high mechanical and thermal performance.
- Acrylonitrile Butadiene Styrene (ABS): ABS combines toughness with excellent impact resistance, ideal for parts that must withstand robust use.
- Polystyrene (PS): Known for its easy moldability, PS is commonly used to produce lightweight parts, mostly used for protective equipment for consumer goods.
- Polycarbonate (PC): Chosen for its exceptional transparency, impact resistance, and high-temperature resistance, PC is ideal for manufacturing parts with optical requirements.
- Polyvinyl Chloride (PVC): Combining flexibility, flame resistance, and affordability, PVC is versatile enough for various applications in the plumbing industry.
Two Types of Injection of Gas
There are two prominent methods in gas assist molding: Nozzle Injection and Gas Pin Injection. Both have their unique advantages and potential drawbacks.
Nozzle Injection (A Tried-and-True Approach)
Nozzle Injection, being the more traditional method, is straightforward to implement. Despite the variety of nozzle designs and hardware on the market, it’s important to note that not all are effective, and some can come at a steep cost. This method often requires a sprue break as the mold must vent out the gas from the component.
However, using self-venting check valves can render the sprue break unnecessary. Implementing gas assist via the nozzle can be achieved effortlessly, even with tools specifically designed for this process. The nozzle performs a dual role: it shuts off and acts as a gas nozzle.
It can be threaded onto any existing molding machine, with the gas connected via the check valve housed in the device, ready for gas injection.
A key drawback of the nozzle injection method is the resultant hole at the gate and less control over the gas.
To avoid gas backflow into the barrel, resin flow must be stopped, which can induce a stalling effect and leave a visible trace of the stop-start resin flow, also known as a hesitation line.
Gas Pin Injection (For Precise Control and Versatility)
On the other hand, Gas Pin Injection, which involves introducing a gas through a small device like a pin inside the tool, has been gaining popularity. This is largely due to its enhanced control, allowing simultaneous injection of resin and gas.
This precision can be advantageous in a range of applications. There’s no need for tool modifications when using gas pins, but minimal costs might be incurred. Using gas pins for injecting and pressurizing a plastic part has become increasingly user-friendly.
The gas pin injection process involves connecting a gas line to the side of the mold, which is then linked via a channel to the pin.
The pin is affixed to the mold face, and the gas is introduced into the part. Gas pins can be strategically placed in various locations, requiring less pressure to infiltrate the part than the nozzle injection method.
However, using gas pins will inevitably leave a hole in the part.
Applications of Gas-Assisted Injection Molded Plastic Parts
Gas-assisted molding yields products with high strength and excellent surface finishes while considerably reducing the amount of material used, making it a preferred choice across various industries.
- Automotive Industry: In this domain, gas-assisted molding is widely employed to produce various components, including steering wheels, door panels, and handles. These parts demand high strength, temperature resistance, and specific shapes for structural integrity, all achieved through this technique. The end products are lightweight, making elements like steering wheels easy to maneuver.
- Consumer Goods: Toys and many items with thick sections are also made using this method. Given the process’s capability to maintain strength while hollowing out the insides of the parts, it results in lighter yet robust products.
- Protective Housings: Another significant application of GAM is in creating protective housings for devices like medical equipment, computers, and eyeglasses. These items require an aesthetic appeal and toughness for protection, achieved seamlessly through this method.
- Sports Equipment: Basketball backboard frames and hoops, known for their sturdy nature, get superior properties from this technique. Long tubular shapes, such as shovel handles, also fall under this category, benefiting from the process’s ability to produce hollow yet robust structures.
- Office Equipment: Large covers and panels, a crucial component of office equipment, are also made using gas-assisted molding, ensuring the items’ durability and pleasing appearance.
- Garden Tools: Elements like lawnmower handles are other notable applications that leverage this method for producing lightweight yet sturdy parts.
Advantages of Gas-Assisted Injection Molding
Here are the advantages of using gas-assisted molding for manufacturing your parts:
Improved Efficiency
Gas-assisted injection molding is renowned for superior efficiency because a gas channel transmits a low-pressure nitrogen fluid. Any product manufactured with GAM will be higher quality than a traditional injection molding process.
With this method, the gas can permeate all areas of the mold quickly and easily. This improves the molding process by ensuring a consistent pressure distribution across the mold, resulting in less warping and lower internal stresses in the final part.
The efficient distribution of gas also aids in completing the holding phase of the molding process faster, further streamlining the production cycle.
Reduced Material Consumption
Molding pellets are expensive, i.e., $5 per kilogram, but gas-assisted injection molding minimizes consumption.
Because the mold is only partially filled with resin, it reduces the amount of plastic during production. This translates into lower material and handling costs, contributing to the overall cost-effectiveness of the manufacturing process.
The final product is hollow, using less polymer, leading to up to 40% potential material savings.
Opting for the gas-assist process means consuming fewer materials, letting manufacturers save surplus resin pellets for future projects without hurting product quality. If you’re a client seeking to create a prototype, this method is cost-effective and budget-friendly.
Lightweight Yet Strong Products
Despite being lightweight, products manufactured through gas-assisted molding maintain impressive strength. It’s possible to use lightweight plastics such as HDPE and polyethylene to manufacture products using GAM.
The technique’s ability to partially fill the mold cavity, coupled with the inherent lightness of plastic, results in lightweight yet robust products. The high strength-to-weight ratio ensures these products can handle heavy use without sacrificing portability and ease of handling.
High Dimensional Accuracy
Gas-assisted injection molding ensures high geometric precision and tight tolerances, essential for parts that adhere strictly to specified dimensions.
The mold shapes the outer surface, while the inner surface takes form due to the uniform pressure distribution of the gas. This high accuracy reduces waste, optimizes resources, and ensures the production of parts that fit seamlessly into their intended applications.
Products produced by gas-assisted molding are often called ‘ribbed plastic products’. For example, bottle caps, panels, and other packaging parts.
Reduced Manufacturing Defects
If you are looking for a manufacturing process that produces fewer defects and less stress, you should opt for gas-assisted injection molding. It significantly minimizes common manufacturing defects like flow marks, sink marks, and internal stresses.
The secondary gas penetration phase compensates for shrinkage during cooling, leading to a virtually defect-free product. This quality control can boost consumer satisfaction and enhance brand reputation.
Greater Design Flexibility
It facilitates the creation of parts with uneven wall thicknesses and complex shapes that traditionally require separate molding.
This flexibility simplifies the assembly process and expands the scope of products that can be efficiently manufactured using a single molding step.
This way, manufacturers can design complex parts without compromising strength or surface finish.
The process permits longer flow lengths, fewer weld lines, and a reduced number of injection points. This allows for the efficient creation of large, thin-walled molded parts, such as tabletops, doors, and boards, without compromising quality or aesthetics.
Disadvantages of Gas-Assisted Injection Molding
While gas-assisted injection molding provides significant advantages, it’s essential to be aware of the potential challenges associated with this process. It’s not a one-size-fits-all solution; in some cases, other methods may be more suitable.
High Initial and Maintenance Costs
Getting started with gas-assisted injection molding can require a substantial capital investment. The initial setup, along with mold manufacturing, can be costly.
Additionally, regular maintenance of the molds, which could be due to normal wear and tear or part malfunctions, can also be a considerable ongoing expense.
Unsuited for Resins with High Flow Indices
Not all resins are suitable for this technique. If the resin material has a high flow index, it may be too fluid to interact with the gas properly.
This issue can prevent the gas from penetrating effectively into the thinner sections of the mold cavity.
Complex Mold Design and Size
The mold design for gas-assisted injection molding can be quite complex due to the additional features such as gas channels.
The molds used in this process are also often larger than those in traditional plastic molding techniques, adding an extra layer of complexity to the design and manufacturing process.
Connect with Kemal MFG for Your Injection Molding Needs
Are you seeking a reliable, experienced, and innovative partner for your plastic injection molding projects? Look no further.
At Kemal, we combine our technical expertise with advanced machinery to deliver top-quality plastic parts that match your specifications and exceed your expectations.
We can guide you if you’re interested in gas-assisted injection molding or exploring other methods. Our team of skilled engineers is ready to assist you from the initial design phase through to the production of the final product, ensuring high-quality results every time.
Don’t let less experienced manufacturers compromise your product’s potential. Contact us today to discuss your project needs, learn about our comprehensive services, and discover how we can help transform your ideas into tangible products.
Conclusion
It’s evident that gas-assisted injection molding offers many advantages for certain plastic part manufacturing applications.
Whether gas-assisted injection molding is the right fit for your project depends on your specific needs and circumstances. Factors like the type of part, its design complexity, your budget, and required lead time all play a significant role in determining the most suitable molding process.
At the end of the day, partnering with a reliable and experienced manufacturer, such as Kemal, is crucial in navigating these choices and ensuring your project’s success.
