Molded-in stress could easily be nicknamed Public Enemy Number One among those who design with plastics. When it’s not busy causing warpage right out of the mold, it focuses on causing cracks once a part has left the factory. More than a few designers have also caught it raising their blood pressure.

Design consultant Mark Howards, president of 3D Shapes Inc. (Sharon, MA), wants to stop the insanity, so to speak, by calling your attention to the gas-assist process. “Most designers don’t consider gas assist as a way to solve the stress demon, but in fact, it automatically corrects many of the problems that cause molded-in stress,” he says. “Designing for gas assist is now more precise, thanks to finite-element analysis that targets the process.”

Howards adds, “There has always been a certain class of gas-assist parts that are very difficult to analyze with traditional techniques. Handles and other parts with heavy sections or with complex transitions require 3-D analysis tools. Analysis for 3-D gas-assist parts is now available in MPI/3D.”

In fact, Howards’ company has been using the Moldflow software for just such projects, and the results have helped inform his contention that gas assist battles stress effectively. (See “Analyzing with Gas,” below, for a sample analysis.)

Defining the Devil

Howards addressed this issue in a presentation at the 2003 Structural Plastics Conference, along with Paul Dier of gas-assist equipment supplier Bauer Plastics Technology Group (Clinton Township, MI). His first admonition still holds true today, he believes. “Most of us think of stress in terms of the in-use loading of a part. Virtually all molded parts will be stressed in use, but more importantly, the manufacturing ‘prestress’ more often degrades the performance of the part. Molded-in stress is caused by differential shrinkage, and the effects are cumulative. They can ultimately lead to poor-quality parts.”

Howards explains that, typically, the structure of the part is sufficient to overcome molded-in stress when it first comes out of the mold, only to warp or crack over time or at elevated temperatures. In use, forces applied to a part cause stress. If the stress is low, the effects may not be evident. If the stresses are high enough, however, molded-in stress becomes a factor because it is in addition to the applied stress, and the combination can exceed maximum values for the material. End results are defects and breakage.

“Many molding parameters will cause differential shrinkage and the resultant molded-in stress. Taken together they can add up to large prestress in the part,” he adds.

Analyzing with gas

A. Improved melt-front advancement One of the first antistress aspects of gas assist involves the way it enhances flow. If the part design allows for large flow leaders, also known as in-part runners, a much more robust molding process window will result. These flow leaders become gas channels as gas is introduced to the part. Gas channels can often be added in inconspicuous areas such as at the base of ribs or at wall intersections. Unlike conventional flow leaders that often cannot be made large enough to significantly enhance flow, gas channels are commonly two to three times the nominal wall thickness. (See Figures 3 and 4 to compare melt-front advancement.) For larger parts, the thicker gas channels often improve the melt-front pattern as seen by the enhanced flow to the corners of the part. Gas assist can also reduce the number of gates, and thus weldlines. With fewer gates, the melt-front pattern tends to be much more uniform. This is also a factor for filled materials where fiber distribution and dimensional orientation affect the part as well as weldline strength.

B. Reduced filling pressure Gas assist also helps reduce the pressure needed for filling in several ways. First, the flow leaders provide a path with less resistance. For the test panel, the maximum injection pressures were 7800 psi conventionally and 5300 psi with gas. Second, the gas can be used to complete filling of the cavity. The part reaches both peak injection pressures and maximum projected area at the end of filling (Figure 6). With gas assist, the shot size can be decreased to reduce pressure and the projected area. The panel described by this graph was filled with a 95% short shot. The projected area was reduced because less of the part was filled with resin. Clamp tonnage is a product of injection pressure and projected area. Filling is completed with gas pressure often in the range of 25% of the maximum resin injection pressure. In this case, 1500 psi of gas pressure was used to complete the fill and for the packing phase. This had a dramatic effect on clamp tonnage, reducing it from 175 to 75 tons.

C. Decreased shear stress High shear stress results in greater polymer chain orientation. This can affect surface finish and increase the differential shrinkage that causes warpage. By providing an easier flow path, gas assist reduces shear stress in the part. This output is available directly from the analysis and can be compared against maximum recommended values. Figures 7 and 8 compare the shear stress.

D. Packing pressure For a conventionally molded part, there is often a wide distribution of packing pressure in the part (Figure 9). Pressure that might be required to pack difficult features can actually lead to overpacking others. With conventional molding, it is common to use packing pressures of 80% of the maximum injection pressure. With gas, the pressure is often in the range of 25% of the maximum resin injection pressure. Gas injection can start before the end of filling or post-filling. As the gas enters the part, it seeks the path of least resistance. Gas hollows out thick regions and advances towards the last place to fill. In doing so, the gas forms a pressure manifold in the part that decreases the distance between the points where packing pressure is applied and where it is required. Compare the distances shown in Figures 9 and 10. With a solid part, the only packing path is through the resin. As soon as critical areas of the part freeze off, packing is no longer possible. With gas assist, areas of the part between the gas inlet and area to be packed can be frozen, yet packing is still possible because the path may be through the gas. With gas assist, lower packing pressure is required, the distances are shorter, and it is possible to pack across thinner areas of the part for a longer time. The coring and packing that result from gas assist will also eliminate sink marks in thicker sections.

E. More uniform tempertatures When it comes to cooling, a gas-assist part can be filled faster than the same part without gas assist. The time difference, especially on a large part, can significantly affect the temperature differences in the part at the end of filling. In a conventionally molded part, hotter areas will have greater shrinkage, while cooler areas may impede packing. Gas assist will hollow heavy sections of a part, decreasing hot spots and cooling time. The heavy sections are not only cooled from the outside, but are also cooled via the gas on the inside of hollowed areas. The gas channels often feed the entire part more evenly, resulting in more uniform temperatures at the end of filling. And gas-assist analysis can include the actual cooling layout for the tool, which allows comparison of different layouts as well as prediction of cycle time.

Contact Information Moldflow Corp., Wayland, MA (508) 358-5848 www.moldflow.com 3D Shapes Inc., Sharon, MA (781) 789-3380 www.3dshapes.com