Introduction to Gas-assisted Injection Molding - Makenica

Gas-assisted injection molding is a process that utilizes an inert gas (normally nitrogen) to create one or more hollow channels within an injection-molded plastic part. At the end of the filling stage, the gas (N2) is injected into the still liquid core of the molding. From there, the gas follows the path of the least resistance and replaces the thick molten sections with gas-filled channels. 

Next, gas pressure packs the plastic against the mold cavity surface, compensating for volumetric shrinkage until the part solidifies. Finally, the gas is vented to the atmosphere or recycled.

Gas-assist injection molding service has been around for well over two decades and many people have had concerns over patent, rights and royalty fees. Within the past few years, some of the original patents have expired. And now, gas-assisted injection molding service is widely practiced by many injection molding companies. 

Design engineers and processors of injection molding companies alike are discovering that this technology is an attractive option for certain applications and offers many benefits. It is the responsibility of the manufacturer to ascertain that their practice or technology of injection molding service is not covered by current patents.

Variants of the Gas-Assisted Injection Molding

The main two applications of Gas Assisted Injection Molding Service are to either inject the gas into the component cavity (internal gas injection), or to use the gas on the outside surface, but still within the mold cavity, to consolidate the component (external gas injection).

Internal Gas Injection – Most widely used process

• Substantial cost reductions resulting from:
  – Reduction in molded plastic weights, and therefore cost of material.
  – Reduction in molding time cycles, and therefore cost of production.
  – Reduced in-mold pressures, and therefore less wear on molds.
• The use of the gas as a means of transmitting pressure uniformly throughout the injection molding service.
• Elimination of sink marks.
• Avoidance of plastic packing from the molding machine.
• Reduced in-mold pressures by up to 70%, and therefore reduced press lock forces enabling larger moldings on smaller machines.
• Reduced power consumption.
• Reduced molded in stress, and therefore improved dimensional stability with no distortion.

External Gas Injection – used for enhanced surface definition

• Can eliminate sink marks.
• Virtually eliminates molded-in stress and therefore distortion.
• Improves dimensional stability.
• Applies pressure more efficiently, and therefore less pressure is required: 
  – reducing lock forces or machine size.
  – reducing wear on molds.
  – reducing power consumption.
• More design freedom:
  – thicker ribs with reduced wall thicknesses.
  – multi-rib components.
  – flat PP and PE products.

A number of variants of gas use are incorporated into the Internal gas injection process:

• Full Shot Internal Gas-Assisted Molding
• Short Shot Internal Gas-Assisted Molding
• Plastic Expulsion Process ∦ PEP
• Moving Core Gas-Assisted Molding
• Gas Cool for Internal and External Gas Molding

Why Gas-assisted Injection Molding?

Techniques have been developed whereby inert gas nitrogen is injected into the still molten plastic in the mold cavity. Acting from within the component shape, the gas inflates the component and counteracts the effects of the material shrinkage. The effect is to keep an internal pressure on the material until it solidifies and skins at the mold cavity surface. This is independent of any gate freezing. Raw Materials
Most thermoplastics can benefit from the use of gas-assisted injection molding service including Polypropylene (PP), ABS, HIPS, Polycarbonate (PC), PPC and Nylon (including glass filled grades). Tooling
Machined steel. Must be specially designed by the plastic injection molding companies with mold flow analysis to enhance the hollowing-out of thick areas Cost
Tooling costs are generally high. Part prices are generally higher than with plastic injection molding service.

• Material savings (weight, cost) for thick-walled parts up to 40%
  The combined benefits of not packing a molding are less material is used. By not having to pack the material, and in thicker components the resultant hollow core, can save as much as up to 40% on the material used.
• Reduced Cycle times by 50% or more when compared to standard injection molding of thick-walled parts 
  Another major benefit is the reduction in machine cycle times that can be achieved by the injection molding companies. With no molten core to solidify, the material in the muld cavity solidifies quicker thus enabling the component to be ejected sooner.
• Smooth surface in comparison with structural foam
  For the injection molding companies, external gas injection provides an enhanced surface definition of the component.
• Lower clamp forces
• Improved holding pressure effect
• High flexural stiffness and torsional rigidity
• Low internal stress level and low warpage for thick and thin wall combinations (uniform shrinkage and pressure)
• Reduction of sink marks
• Design freedom
• Fewer weld lines due to fewer injection points
• Longer flow lengths or lower number of injection points required for large thin-walled molded parts because gas channels act as flow leaders

Special care must be taken by the injection molding companies in designing parts. High cost of tooling and mold flow analysis.

Applications
Most plastic injection molding companies can benefit from the use of gas-assisted molding. Applications from consumer goods to automotive parts benefit from the process. The typical are: Toys, auto parts & anything with thick areas.

• Flat panels for office equipment.
• Computer enclosures.
• Furniture, i.e. tabletops.
• Automotive panels.
• Domestic appliances – e.g. fridges.

Gas vs Water Assisted Injection Molding

As someone who first became involved with Water Assisted Injection Molding in 1998 (and who initially actively tried to promote it) we soon realized that the principal benefit of the technology (fast cooling of the plastic in contact with the water bubble) is also its limiting factor for certain part geometries.

Parts with no attached wall sections (i.e. pipes and media ducts) can successfully be produced with water assisted injection molding at cycle times that are up to 40% shorter than gas-assisted injection molding. 

However, even with these types of parts, if the pipe wall section is sufficiently large you will tend to get “voiding” in the wall section. This is due to the water producing an instantaneous frozen skin when it comes into contact with the molten polymer. 

Thus, even if the water is at 300 BarG pressure, that pressure cannot be transmitted through the polymer melt as it is already contained within a “frozen” tube. If this tube (water channel) is attached to a wall section (chair molding) it also means that the surrounding plastic cannot be pressurized by the water channel, this can lead to dimensional issues and also sinkage if there are any nearby bosses or ribs.

With Gas-Assisted Injection Molding, the pressure of gas inside the gas channel can be transmitted to the surrounding plastic (wall section) as there is no instantaneous frozen skin formed as the gas bubble “tunnels through” the molten polymer. This is the reason why gas assisted injection molding is used by the injection molding companies for many plastic chairs / stadium seats. Unfortunately you can’t fight the Laws of Physics!

You first need to consider why you think you need the gas assist technology. While at first thought, you may think about weight reduction. However, this is rarely the end result if you attempt to retrofit a molded part designed as a solid part.  The advantage of the gas-assisted injection molding allows for a part designed for gas assist to be stiffer than a similar part made as a solid part. Essentially, you are replacing a solid part with a rib structure for a gas assist part without the ribs.

However your part is thicker, but with hollow channels. The design requirements for each process are unique to the process. The advantage or disadvantage depends upon the desired part design. However, the success rate of converting a solid part to a gas assist part is rather low. 

If you’re looking for overall material cost reduction you may consider talc or calcium carbonate filler to the PP. But be careful, the density of the filled material will go up, adding to part weight and shipping costs.

Can I use a nitrogen gas generator for injection molding?

There are many misconceptions across different industries when it comes to gas generation equipment and sources of gas. People are often surprised to know that gas generators are a better, more cost-effective solution to cylinder delivery service. The plastic injection molding industry is no different.

 Plastic injection is the manufacturing process of producing plastic injection molds by forcing chemical compounds into a mold and allowing them to cure and form a shape. The injection molded part cools and hardens to the configuration of the mold cavity which could be anything from a foam seat to automotive parts to Lego, really any plastic products available today. 

During gas-assisted injection molding, pressurized nitrogen is injected into the interior of the mold and flows through gas channels because it contains inert properties and high pressure & flow rates. The end result is a hollow mold that is light and inexpensive to make. This process offers many positive benefits like lower material usage, more complex shapes and structures can be made, and a faster cycle time.

 There are many misconceptions surrounding using a nitrogen generator for gas-assisted injection molding as an alternative to bulk supplied nitrogen. Below are some common misconceptions about using a nitrogen generator for gas assisted injection molding:

1. A nitrogen generator does not have enough flow.

A quick google search for a nitrogen generator will lead to many results for laboratory nitrogen generators. These generators produce much lower flow rates than is required by the injection molding industry which would lead many to think a nitrogen generator is not suitable. However, there are few companies making it more than capable of supplying nitrogen generators for gas-assisted injection molding applications.

2. Nitrogen generators are too big

Many injection molding companies have been innovating and reducing the size of their nitrogen generators so they are now more compact than ever, without compromising on performance. Certainly, compared with a large bulk tank outside a facility that is two stories tall, modern nitrogen generators are a far more compact and space-saving solution, freeing up much needed facility floor space. 

 It is true that some brands of nitrogen generator are of a similar size to bulk tanks. Many manufactures do have larger scale nitrogen generators that need to be put outside or in the warehouse but with some latest generators, that is not the case. The new ones can be installed within your existing space and using your current infrastructure. 

3. Nitrogen generators are complicated and expensive to maintain.

This is the largest misconception about nitrogen generators. Nitrogen gas generators take the hassle out of alternative sources of nitrogen gas. Nitrogen gas generators are plug and play, meaning they are operational once plugged into a power source and require only pre-planned annual maintenance.  

 With the latest nitrogen generators, you are given 24/7 gas production which ensures you are never left without nitrogen gas. You’ll never have to worry about actively monitoring your gas supply levels or need to worry about on-going delivery and rental costs.  

4. The pressures are not high enough 

With liquid-based nitrogen bulk systems, a boost is likely already installed so the pressure of your gas stream can be increased to your preferred pressure. The latest nitrogen generator is able to use that same infrastructure and equipment to deliver the high purity nitrogen you need at the pressure your application requires. 

Ultimately, nitrogen generator is the right choice for those in the gas-assisted injection molding industry. 

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