Gas Assist Injection Molding Technology

Frequently Asked Questions

Q Do I Need a License for Gas Assist?

A Gas Assist Does Not Require a License If Modern Technology is Followed.

The first commercial use of the gas assist process is nearly 30 years old. The art of performing gas assist injection molding has been in the public domain since 1996. It is mission to educate people on the methods machinery to perform gas assist in a modern technological environment to prevent litigation. No BAUER PTG customer has been forced to purchase a license...EVER!

Q What is Gas Assist Injection Molding?

A Gas Assist Injection Molding is the Most Effective Method of Applying Pressure to an Injection Molded Part During Cooling.

It is a fact that packing force must be applied and maintained to an injection molded part as it cools. With conventional injection molding, hydraulic pressure is exerted on the screw in the injection barrel, forcing molten resin through a runner system and material feed gates into the part. This generally results in relatively high pressure within the mold cavity, sometimes causing flash and premature tool wear.

With this conventional method of pressurizing a part, packing force will only be applied until the material feed gates freeze off. Once this occurs, additional packing pressure cannot be applied to the part. This is the major cause of sink marks in conventional injection molded parts.

Another cause of defects occurs when packing force is applied through the gates and freeze-off occurs in the part itself, usually in the areas of the part furthest from the gates. As material is forced into the cavity to replace volumetric shrinkage, areas nearest the gate will have more resin packed into them. As the part cools, areas of the part will shrink at different rates due to varying density of resin across the part. This causes defects such as warpage and stresses and can result in longer cycle times.

Gas assist injection molding is a low pressure process, reducing the clamp tonnage required for molding. Nitrogen gas is applied to the part internally, either directly into thicker sections of the part or via a network of gas channels that are added to the part. Through proper design, this lower gas pressure is applied evenly throughout the part and can be sustained during the entire cooling phase. High pressure differential within the mold cavity is avoided, thereby eliminating the associated defects.

Q What Can Gas Assist Do for Me?

A Gas Assist Injection Molding will Lower Your Production Costs.

There are many benefits associated with the gas assist process. These benefits include:

1 Clamp Tonnage Reduction
The gas assist molding requires less clamp tonnage from your injection molding press because the cavity of the mold is rarely completely filled. Unlike conventional molding, you are not pressurizing a local gradient area of the machine platen attempting to pack out the part. You will apply the lower gas pressure evenly throughout the entire part reducing the required clamp tonnage. In most cases, gas assist will lower the clamp tonnage requirements by 30% to 70%. Even in cases of what is referred to as full shot gas assist molding, there is a vacancy somewhere in the part that allows gas to penetrate, even if it is only to take up volumetric shrinkage within the cavity.

When clamp tonnage is reduced, significant cost savings can be realized. It is not uncommon for gas assist molders to place tools that would normally run in 1500 ton press into a 50 ton press. The machinery cost difference of a 1500 ton press ($700,000) versus a 750 ton press ($350,000) can make any project interesting. Because of the substantial reduction in clamp requirements, it is also possible to quote jobs that are beyond your current tonnage capacity. Physical tool size is usually the limiting factor due to tie bar clearances. Imagine being at capacity in your single 750 ton press and your largest customer awarding you another 750 ton job. You can buy a new press, juggle capacity schedules, and work more days a week or go into your 600 ton or even your 450 ton with open capacity by using gas assist.

2 Cycle Time Reduction
Another important benefit that gas assist technology offers is cycle time reduction. Anytime you can make a quality product faster you reduce production costs and increase the capacity of your plant.

Thick walled parts are naturals for reductions in cycle times. The short shot process indicates that you will be reducing the amount of resin that is introduced to the part.

Gas travels very easily through these thick walls.
As it travels through the part, it extends the flow of resin, completing fill, while evacuating resin from the thick areas. Without having the mass of material in thick areas of the part, cooling time and therefore cycle time, is reduced. Some cycle times of thick walled parts have been reduced by more than 50%. When reducing the amount of material introduced to a mold, less time is required to cool the part internally so stresses are at a minimum.

On thin wall parts, cycle times can be reduced as the pack and hold phase of the process is replaced with gas pressure allowing screw recovery immediately after resin injection.

With all gas assist molding, cooling occurs inside the part as well as outside. This is due to the void in the part created by the displacement of resin within the gas channel or thick areas of the part.

In conventional molding, additional cycle time is added if parts have a tendency to warp, allowing the part to fully set-up. Warpage is not an issue with gas assist molding as gas pressurizes the part equally and eliminates stress when proper tool design is employed.

3 Resin Savings
With the cost of resins today, especially engineered grades, weight reduction of part is important to molders.

Most people think thick parts are the only gas assist candidates. It is true, handles and similarly thick-sectioned parts are naturally conducive to the process and offer more than 50% weight reductions in some cases, but significant resin savings are possible in thin walled parts as well. In many instances, nominal wall thickness can be reduced when using gas assist and the part will actually be stronger.

Another major contributor to the resin savings is scrap reduction. With proper tool design, gas assist will allow you to experience scrap free start-ups and production runs. The process is precisely controlled and the processing window is increased.

4 Improved Part Quality
Gas assist technology can improve the quality of products having certain dimensional criteria and appearance requirements. Many times a tool design limits the capability and quality of the finished product. More and more new designs have reduced nominal wall thickness and increased boss and mounting configurations. This typically results in added stresses and shrinkage, which can cause surface defects and imperfections. Thin walled parts with heavy bosses are susceptible to sink. When properly applied, gas assist will eliminate these quality concerns.

5 Design Freedom
The gas assist process will also offer certain design freedoms to part designers that are unavailable in conventional injection molding. Heavy wall sections can intersect thinner ones, ribs and bosses can actually be designed thicker than the nominal wall without fear of sinks, mechanical tool movements used for coring can be eliminated, and parts may be designed with thick sections that will become hollow.

Another benefit gas assist can offer a designer is the possible elimination of a hot runner system. When the runner system is designed within the part, there is no longer a need for the added expense and complications associated with hot manifold systems. This will also eliminate weld lines on the surface of the finished part.

Q How Quick is My Payback?

A Shorter Than You Think!

If it were going to cost you money to use the gas assist process there would be no reason to do it. The fact is that gas assist will save you money. The cost savings in production are real and immediate. True there is a capital expenditure that will allow you to start a gas assist program, but with a BAUER gas assist system the payback will usually take six months to a year. Then you will enjoy the cost savings for years to come.