Getting a Grip on Gas Assist

Editor's note: At the November 1995 SPE Automotive Retec in Dearborn, MI, Allan Knight from Cinpres Ltd. (Staffordshire, England) presented a paper on gas-assist injection molding that includes case studies from two automotive molders: Injex Industries' door handles for the Toyota Corolla and Chevy Prism, and Regal Plastics' handles for the Ford Contour and Mercury Mystique. These applications open a window on the processing problems and fixes for gas assist and detail the benefits of the process.

Gas-assisted molding has changed the thought process involved in designing injection molded parts, and in no other industry is this trend more evident than in the automotive sector. Vehicles in most areas of the world are now fitted with gas-assist parts, including interior and exterior trim, bumpers, door pockets, front grilles, and door handles.

Two door handles, one created for GM/Toyota in California and another for Ford in Michigan, illustrate both the payback and the challenges of molding gas-assist parts with relatively simple geometries.

In-runner Design

The first door pull ever to use the Cinpres process in North America is molded by Injex Industries (Hayward, CA) for the Nummi plant in Fremont, CA, a joint GM/Toyota venture that makes the Chevy Prism and Toyota Corolla. Four-cavity tool design (Figure 1), includes two gas nozzles placed at the points where the main runner branches so that the gas feeds two handles per nozzle (molders used a two-cylinder gas-injection unit).

The primary gate is a subgate feeding plastic directly into the handle cavity, while a secondary gate off a common runner helps fill adjacent tabs. When only the primary subgate was used, plastic flow across the integral hinge wasn't fast enough to fill the tabs.

At the time gas was injected, the tab area did not fill. This left two unfilled areas, one at the tab and the other at the end of the handle. This occurred because gas always flows from high to low pressure, or in other words, seeks out the last point of fill.

In addition to flowing into the handle section, gas penetrated through the hinge into the tabs, causing thin sections. The problems were solved by adding secondary gates. These allow tabs to be filled earlier in the cycle, leaving a single final fill point at the end of the handle.

The gas nozzle layout for the GM/Toyota handle is known as an in-runner layout-nozzles are located within the runner system itself. One drawback to this arrangement is the possibility of unbalanced gas flow. Instead of traveling equally to each cavity, gas flows first into the one with the lowest plastic pressure. Only when pressure increases will the gas begin to flow into remaining cavities.

Gas nozzles positioned this way make it challenging to get the right balance between cavities needed for perfect flow distribution. To optimize this design, the rate of plastic flow into each cavity must be balanced so that there is an equal volume of plastic in all cavities when gas is injected.

In-article Design

Regal Plastics also uses a two-cylinder gas injection unit to mold handles for the Ford plant in Kansas City. And like the previous handle, this one uses secondary gates to help fill the tabs. But the tool for this part differs because gas nozzles have been placed "in-article." Four gas nozzles are used, one per cavity. Introducing gas directly into each cavity reduces the possibility of a gas flow imbalance. Although it works well here, in some situations this fix may not be sufficient to overcome an imbalance problem.

Another benefit to this design involves eliminating hesitation effects, surface imperfections caused by a hesitation in plastic flow between the end of plastic fill and the beginning of gas flow. Allowing plastic to stop causes the flow front to relax momentarily, leaving a notch. When packed out, this can take on the appearance of a line or gloss variation. By placing a nozzle in each cavity, transition from plastic flow to gas flow is easier to control, and hesitation lines in the melt front can be processed out.

Although Regal used one cylinder to supply two nozzles, a highly recommended practice for gas-assist molding is to locate a nozzle within each cavity, then feed every nozzle with gas from a separate cylinder. This allows molders to fine tune the relationship between plastic and gas flow for each cavity.

Grasping the Differences

There is a significant difference between the two styles of handles-the size of the gas core, Figure 2. That's because the cross-sectional shapes are different. In the GM/Toyota handle, the section is more rounded and lends itself to more even plastic distribution around the gas core. This gives a near constant wall section, leading to shorter cooling times.

The Ford handle, however, is more rectangular. During filling, gas will flow only in the center of the rectangle, leaving areas of solid plastic on either side. Although packing expands the gas into the areas to compensate for shrinkage, it will still leave a thick section of solid material. These thicker areas require longer cooling times.

Other comparisons, as shown in Table I opposite, verify that designers made the right choice with gas assist. Reducing weight by 32 percent for the GM/Toyota version and by 25 percent for Ford's handle adds up to time and material savings that were the automakers' original goals.