Gas-Assist Molding: Not just for hollow parts

Michael Hansen
Senior Technical Development
Engineer
Mack Molding Co.
Arlington, Vt.

Edited by Jean M. Hoffman

The first patents for gasassist injection molding came in the early 1970s. But the technology languished many years as patent holders and equipment manufacturers battled in the courtroom. Now the dust has settled. Equipment and licensing agreement costs have dropped. Gas-assist techniques are today positioned to handle design configurations or constraints well beyond the scope of conventional injection molding.

Many designers may still view the process as only for simple, hollow, tube-shaped parts. In practice, however, gas-assist is much more versatile, often solving many of the problems inherent to conventional injection molding.

Gas-assist lets designers use less material and reduce weight. Its benefits include the elimination of sink marks and warpage in heavily ribbed parts and significantly less internal or molded-in stress. It also improves dimensional stability and gives better surface finish and finer detail over a range of polymers that include those with glass reinforcement.

Gas-assist lets designers mold thin-walled parts with less clamp tonnage, thereby cutting the cost of tooling and the size of the molding machine. It also is said to be an ideal method for adding hollow sections to an otherwise flat part. Conventional injection molding can't easily produce such a configuration. Other plastic forming processes such as blow or rotational molding have a tough time with it as well.

The gas-assist process improves polymer fill or packing and boosts the melt flow length. This lets designers mold larger, more complex parts with fewer injection gates and may help eliminate the need for complicated runner systems. Finally, the coring out of thicker sections also makes parts cool faster, slashing cycle times.

The process
Gas-assist injects pressurized nitrogen into the interior of the polymer melt. The gas follows the path of least resistance, penetrating the part via carefully designed networks of thicker cross-sectioned gas channels. Nitrogen seeks the area in the mold cavity with the lowest pressure and travels most efficiently in the hotter core regions of the polymer melt. It has a viscosity much lower than its polymer host and remains isolated within the channel as it flows in the direction of the melt front. The displaced molten polymer is pushed towards the melt front by the advancing gas, filling and later packing the mold cavity.

There is almost constant pressure in all areas penetrated by the gas. The pressure pushes the polymer against the mold walls and helps keep the polymer from shrinking as the part begins to cool before ejection from the mold.

Conventional injection molding, in contrast, often uses increased polymer injection pressures at the nozzle to accomplish the same thing. But the method may not work well for large, complex designs with thin and thick transitions. The thin sections may solidify and keep molten polymer from flowing to mold extremities.

Gas-assist parts need not be completely hollow. They can also be solid with hollow sections such as cored-out ribs. Gas channels through rib sections offer designers two advantages. The first is the ability to add thicker, more robust reinforcements on structural parts without sink marks. These are well-known surface defects associated with conventional injection molding. The second is boosting strength without adding weight, thus improving the part stiffness-to-weight ratio.

Most gas-assist injection moldings fall into two categories. The first method injects the polymer and gas through the same injection nozzle. The polymer only partially fills the cavity in what's called a short-shot. Then gas injection pushes material from the core regions to fill out the cavity.

The second employs full-shot molding and fills the cavity completely with polymer. Gas then enters via gas pins at various locations in the mold. This packs the melt into difficult-to-fill areas and helps compensate for shrinkage as the polymer cools. Gas can be injected sequentially and at various pressures which are typically rather low. Injection pins are, however, prone to clog depending on the material used. The result can be variations in wall thickness and incomplete fill or differential shrinkage.

Gas injection through the nozzle is generally the simplest of the two methods to implement. There's no need for extra hardware such as gas injectors, nozzles, or pins when injecting nitrogen directly into the cavity. But gas-injection pressures must be high enough to make the gas break through the residual polymer left inside the nozzle. And designs are constrained by the need for all gas channels to fan out from the nozzle.

An alternative method is multinozzle gas-assist. It lets designers strategically position injection sites of both the polymer and gas to get optimum mold fill. Independent control of injection points and gas pressure is particularly helpful for tailoring the injection of large, complex parts.

Gas-assist designs
There are three categories of parts made by gas-assist. The first group includes tube and rodshaped clothes hangers, armrests, and a wide variety of handles. The second consists of large cover or panel-shaped parts containing ribs, usually for automotive or business machine applications. The third category are the designs where part consolidation creates complex structures containing both thin and thick walls.

Tube or rod-shaped parts have a one-dimensional flow path and the part effectively is the gas channel. The wall thickness is a function of viscosity and temperature distribution within the melt when nitrogen gas is injected. Changes in polymer viscosity, temperature, or gas pressure may play havoc on wall uniformity. So a close watch on these variables is imperative.

Corner radii are also design features that must be accounted for. The gas generally follows the shortest path through a curve near the inside edge. This can create uneven wall thickness at the corner. Use of a generous radius will help avoid the problem.

The classification of parts into thick or thin-walled depends on the width-to-height (W/H) or length-to-width (L/W) ratio. Thick-walled parts typically have a W/H &≤3 or 4 and an L/W ≤5. Conversely, thin-walled parts are classified as having W/H > 10 and L/W ≤3.

The only way to distribute gas within large, thin-walled parts is to design the part specifically to do so. Gas-assist parts require some ribbing to create a network of gas channels. The channels must be thicker than the areas surrounding them. This helps ensure that the gas won't penetrate the adjacent thinner-walled sections, a defect known as fingering. A channel too thick, however, may let the gas break through the melt front, filling the cavity with gas. The lack of material at the melt front will inhibit complete mold fill and cause a molding failure called a "blow out."

The gas network should be designed to put the lowest pressure inside the cavity near the end of each gas channel. These areas should be the last to fill during the initial polymer injection phase. This helps define the path of least resistance for the gas. Likewise, parts with large, thin-walled sections locate gates in the thin section leaving the area near the end of the gas channel that's last to fill.

Channels should also be oriented in the direction of the melt flow. Parts with multiple or branched gas channels require special measures to balance mold fill. One suggestion is to use the existing rib configuration and place the channels at the base of the ribs for guiding the gas to the mold extremities. Mold-fill analysis programs designed specifically for the gas-assist process can help determine gate, channel, and gasinjection locations for balancing mold fill.

Design strategies
There are some rules of thumb for optimum design of gas-assist parts.
• Never place gas channels where two melt fronts come together forming a weld line.
• Make runners and injection points slightly larger than those for conventional injection molding.
• Make sure mold walls are evenly heated and cooled. This is because the temperature of the mold wall can help influence the part wall thickness distribution.
• Sharp corners create thinwall sections so it may be advantageous to cool the mold separately in these locations.
• It's imperative to exactly balance melt flow into the cavities of multicavity tools. Mold-fill analysis software can help.
•Cavities should be arranged symmetrically in the mold and each should have its own separate gas needle.

After the polymer is injected into the mold, a skin layer next to the mold begins to solidify, but the inner core remains molten. Injected gas displaces the polymer from the core region and forces it into mold extremities. As the polymer cools, the gas pressure continues to force it against the cavity wall, helping prevent volumetric shrinkage of the part.

Three basic part categories are most suited for gas-assist injection molding.