When designers consider using finite-element analysis, they often want a tool to simulate the effects of loading or heat. There are other, fairly well-known benefits of the finite-element method for plastics, including mold filling and cooling simulations. But there are not a lot of people modeling the gas-assist process this way. If the results of a recent study are any indication, more designers may use FEA to optimize gas-assist designs.

Tom Kowalske of DuPont Automotive (Troy, MI) realized it would make sense to simulate the effect of process variables on gas penetration so that customers could cut down on blow-through and related defects normally requiring a good deal of trial and error to correct. To this end, Kowalske contacted Shrikant Oak and Ross Nordin at another Troy-based company, Feamold Inc., that specializes in injection molding process simulation and design optimization for the automotive industry.

Oak used a finite-element/finite-difference, or hybrid solver, to analyze how the following variables would affect gas penetration: volume fraction, melt temperature, gas pressure, packing time, delay time, and channel dimensions. In performing the simulations, Feamold measured gas penetration as the length of primary and secondary penetrations, change in volumetric fill time, and uniformity of gas/skin distribution.

Both Kowalske and Oak shared the results of their experiments with IMM, adding the caveat that similar studies performed by other researchers have yielded contradictory results. "The results of this study are specific to the combination of geometry, material, and processing window we used," notes Oak. Geometries consisted of various lengths of a long, flat plate similar to an "A" pillar with a gas channel running along one long edge of the part, Figure 1. Material properties used for all the analyses were those for PET (DuPont Rynite RE5309 BK515). Processing conditions changed for each variable analyzed. And now for the findings.

Volume Fraction
Primary gas penetration length decreased with increasing volume fraction, or prefill percentage, in all cases. At 70 percent prefill, the gas raced beyond the melt front and blew through, as illustrated in Figure 2. Check the straight lines in that graph, and you'll see that gas penetration decreased in a linear fashion as the percentage of prefill increased.

For part one of this experiment, fixed inputs included a melt temperature of 585F, mold temperature of 200F, gas pressure of 1000 psi, gas injection of 10 seconds, and delay time of 0 seconds. The part being modeled was 33 inches long, with a 1/2-by-1/2-inch gas channel. For part two, the only changes involved were a 2000-psi gas pressure and a 66-inch-long part.

Results of both experiments showed that there are two factors that affect primary gas penetration more than any other parameter. They are volume fraction and melt-front advancement pattern. Volume fraction can be used to control the amount of gas penetration, while melt-front advancement tailors gas direction.

Melt Temperature
Results indicated that secondary gas penetration and total gas penetration increase linearly with melt temperature, Figure 3. This makes sense intuitively, because a higher melt temperature will induce greater part shrinkage; thus, more secondary gas penetration occurs to compensate for the increased shrinkage.

While this analysis showed melt temperature having little effect on primary penetration, Oak cautions that highly temperature sensitive materials may behave differently: "They can experience a greater change in viscosity as temperature rises, and this in turn can reduce primary penetration and affect volumetric fill times."

One area that showed distinct changes as melt temperatures went up was skin thickness distribution, which becomes more uneven with increasing melt temperature, Figure 4.

Gas Pressure
For a 100-inch-long part with a 1-by-3/4-inch gas channel, gas pressure was set at 1000, 2000, 4000, and 6000 psi with all other inputs remaining the same as in the volume fraction experiment. Experiments were run at 80 percent prefill with a resin injection time of 3.2 seconds, after which gas was injected.

As gas pressure rose, primary and total gas penetration decreased along with volumetric fill times, while secondary gas penetration went up. To explain this phenomenon, Oak says that the polymer-gas interface can be illustrated as made up of multiple layers of plastic, as in Figure 5. The layer next to the mold wall is coldest, while that in the center is hottest. Colder layers have higher viscosity, needing greater pressure before they will flow. So when gas pressure goes up, more layers begin to flow. As they become more mobile, says Oak, the cross-sectional area in front of the gas bubble grows, decreasing the length of penetration.

Gas pressures must be above a critical lowest value before any plastic can be pushed. Notice how there is no gas penetration below a certain gas pressure in Figure 6. This value, says Oak, depends on geometry, flow length, material, and process conditions.

As gas pressure rises above the minimum value, there is an increase in gas penetration similar to conventional molding in which flow lengths increase. But too often, if molders still see insufficient gas penetration at this point, their reaction is to increase gas pressure even further. Oak cautions that beyond a critical maximum pressure, boosting the gas only decreases the length of penetration.

If, at reasonable pressures, parts aren't getting enough gas penetration, the solution probably lies elsewhere. "Take a look at part design," says Oak, "and check the melt-front advancement pattern and end of fill."

Packing Time
As packing times go up, secondary penetration increases in linear proportion, corresponding to the rate of cooling related shrinkage. Both primary penetration and volumetric fill times are unaffected. Skin thickness, however, became more uniformly distributed as packing times were boosted.

Delay Time
Increasing delay time boosted primary gas penetration up to a critical value, in this case .5 second. Further raising delay time past this point caused a decrease. Oak explains: "Remember the layers (Figure 5) at the gas-polymer interface? As delay time increases, temperature across those layers goes down. This reduces the number of layers in front of the gas bubble that can be moved at that level of gas pressure, so the primary penetration length increases. Once you reach the critical value, however, the plastic freezes excessively, and there are fewer fluid areas remaining for the gas to push. So penetration length decreases."

Summing It Up
Feamold's Oak stresses the importance of analysis for gas-assist part and process designs. "An acceptable or target level of gas penetration must be defined. This process is extremely sensitive to process variables," he notes. For example, if gas is injected too early, it reduces the melt cushion available and increases the potential for gas blowthrough. If gas is injected too late, the plastic cushion reaches the end of the flow path before the gas has advanced far enough, leading to highly cored out areas close to the gas pin and solid sections away from it. This uneven thickness distribution means different cooling and shrinkage rates, thus highly stressed parts that can warp unacceptably. Table I summarizes the Feamold findings for PET.