By Dallas Cada. DCC Consulting
Gas assist injection molding is a process with a high speed and low-pressure injection method. It involves the injection of high-pressure nitrogen gas into the melt stream instantly. You’ll use a short shot method of resin fill and utilize the gas to fill and pack out the remainder of the part. There are four main considerations for a successful application of the technology:
- A repeatable and precise shot control on the injection-molding machine.
- Precise control of time, pressure, and speed of the nitrogen being injected.
- Control of the nitrogen dispersion within the molded part.
- Appropriate tool design.
It is important to note that the flow path that the nitrogen gas takes is mainly controlled by the resin flow within the cavity. The volume and viscosity of the resin determine flow and not by the nitrogen
gas injection.
Advantages
The primary advantage of gas assist is cost reduction by improving productivity. Using lower clamp tonnage means using a smaller machine thus improving the quoting process, not to mention the eliminating the cost of a larger machine. Cycle time reductions save costs as you do not need pack and hold times and pressures. This of course saves time. In gas assist you never completely fill the part. By using a short shot process there is less plastic injected and less plastic to cool. The gas will also promote cooling which slashes cycle time. New methods of tool design flow and fill is improved again saving time. In very thick parts, gas follows the path of least resistance which promotes low pressure fill without packing out the outward features of the pat. This means reductions in part weight; even thin walled applications. Thick walled parts can have up to seventy percent less part weight. While using less plastic may weaken the part it actually forms a tube configuration with less stress which actually improves strength. You also have to remember that by eliminating resin especially engineering grades, is quite cost effective. This of course depends on the end use of the part.
Design Options
Design of the tool is critical whether using gas assist or any other molding process. With gas assist it’s possible to design a part based on concept for use, rather than limitations of conventional molding capabilities. Features in wall thickness can now be designed directly into the part, rather than limitations using second mold operations. These features can eliminate problems with the part. Gas channels or pins can be used to help fill the cavity and strengthen a feature of an area of the part based on design. The use of mold filling analysis can be used to indicate the outcome of a particular design. Weld lines can be eliminated by design which will help in over-all part appearance.
Quality Improvements
Large concerns of the molder and also cost production is the overall quality of the part. As mentioned, weld lines of the part can be minimized or even eliminated. Sink marks can also be eliminated by utilizing design or placement of the gas channel or pin. Utilizing design options can reduce stress which could result in reductions of dimensional stability. By reducing molded in stress, parts will have less shrinkage much less than a conventional molded part. This leads to more repeatability and less distortion within a part. Weight can become a quality measure, rather than actual measurement of the part. This is due to a repeatable process.
Long Flow Lengths
Gas injection accommodates very long flow lengths by using correct tool design. Even parts that are in excess of five feet long have been molded using one gate. This usually can’t be done with conventional molding. Multiple gates or hot runner systems are required to fill very long parts. Not only is this an easy design with gas assist but it eliminates cost of extra tool considerations.
Structural Foam Parts
The easiest tool to convert to gas assist is usually one designed for structural foam. Structural foam parts usually have thick wall sections. This makes it naturally conductive to gas assist. The structural foam process mixes a certain gas agent such as nitrogen. This agent will usually leave a swirl effect on the part surface. Gas assist injects the gas after the resin injection leaving a smooth surface on the part. Structural foam parts usually require secondary operations and several coats of paint. Elimination of paint, surface defects, secondary operations and long cycle times are a huge advantage to convert structural foam tools.
Disadvantages
As with any project there are certain disadvantages. With gas assist, there will be a hole in the part where the gas enters and exits. You usually can’t use a tool that has a hot runner system that does not have valve gates. The gas wants to take the path of least resistance. The gas would prefer to go into the manifold of 500°F, rather than go into the part that is under 200°F in most cases. Another disadvantage is that there will be read out if the design is not correct. Read through of the gas will appear on the wall of the part. Placing gas channels and injector pins correctly will help eliminate this problem.
Two Types of Injection of Gas
Nozzle Injection
Nozzle injection is oldest and easiest way to implement gas assist molding. There are many designs of nozzles and hardware, but some are expensive and simply do not work. It is necessary to use sprue brake with this method, as the mold needs to vent the gas out of the part. There are self-venting check valves that can eliminate the need for sprue break. Gas assist through the nozzle is easy to do, even on tools designed for this process. The nozzle acts as a shut off and gas nozzle both. They simply thread on an existing molding machine. Gas is connected to the check valve located in the device and you are ready to shoot using gas. The only setback in using nozzle injection is you will have a hole at the gate. It is not as controllable using the gas. You must shut off the resin flow to prevent any gas from going back into the barrel. This can cause a stalling effect and can leave a trace of stop and start of the resin flow known as a hesitation line.
Gas Pin Injection
Injection of gas through a small device such as a pin located within the tool has become more renowned as of late. This is because one has precise control while you shoot resin and gas simultaneously. This makes it very beneficial in many applications. You do not have to do tool modifications with gas pins but there will be minimal cost involved. The use of gas pins has become very user friendly by way of injecting and pressurizing a plastic part. Gas pin injection works by connecting a gas line to the side of the mold and then connected via channel to the pin. The pin is mounted to the mold face and the gas is injected into the part. Gas pins can be placed in multiple locations and will require less pressure to penetrate into the part compared to nozzle injection. However, there will always be a hole within the part.
Summary
• Contact an expert when considering gas assist injection molding.
• Control of the short shot is critical. Make sure the barrel and screw tolerances are within specifications.
• Use a highly controllable gas system.
• Gas assist is not always the correct process.
• There will always be a hole in the part.
• Using a mold flow simulation will be very valuable before deciding about gas assist.
• Gas assist will save you money in many ways.
• Reduced tonnage, cycle time and tooling costs.
• Increased part design measures.
• Improved quality, marketability and startup of molds.
About the Author
Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting). His experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies. For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at
dallascada@charter.net. Contact Dallas by phone (507) 458-5785.
