Guidelines for trouble-free gas-assist molding.
By now, most molders know that gas-assist injection molding offers a host of benefits: It makes parts with selectively hollowed-out sections, thereby reducing weight and resin consumption. It produces uniformly filled and packed parts that have lower molded-in stresses, less warpage, and improved surface appearance. It is a short-shot process, and thus can lower clamp tonnages, injection pressures, and tool costs. And by hollowing out thick sections, it often makes for speedier cycle times than comparable solid parts.
Yet despite all of gas-assist molding's well-known benefits, many molders still don't know how to avoid some common design and processing pitfalls that can make this valuable process far more difficult than it should be.
Recognizing gas-assist molding's growing importance to users of engineering thermoplastics, GE Plastics set out four years ago to investigate the full spectrum of commercial processes - that is, systems based on introducing gas at the machine nozzle, at the runner, and at the mold cavity. We performed dozens of experiments on gas-capable injection machines encompassing a wide range of tonnages, capabilities, and ages. We put all of GE's engineering resins through the gas-assist process. We employed a specially instrumented test tool that revealed information about gas-channel formation. We also helped develop or evaluate many production tools that now produce gas-assist parts every day.
All this work, which was carried out at GE's Polymer Processing Development Center in Pittsfield, Mass., has now resulted in a systematic approach to gas-assist molding that covers everything from part design to process control. Our recommendations, selections from which are excerpted here, appear in their entirety in a new 20-page booklet from GE Plastics, Gas Assisted Injection Molding.
WHICH PROCESS IS BEST?
One thing our investigations did not do was come down squarely on the side of any one commercial gas-assist process. To be sure, there are cases where one system has an advantage over another. Some parts process best when gas is introduced at the mold cavity; others, when gas enters at the nozzle. But for economic reasons, most molders must apply a single licensed process to all their gas-assist applications.
Fortunately, the commercial gas-assist systems have much in common in those areas that influence performance the most. For one thing, all the processes obtain packing pressure from the gas itself, not from the machine screw as in traditional injection molding. After delivering a short shot of polymer, all the systems have two stages of gas delivery. In stage one, gas displaces a molten core in thick sections, while in stage two the gas expands into the shrinking part, providing the packing.
The most important thing to remember is that gas behaves the same way regardless of the commercial process you're using. Gas bubbles always move through the molten resin along the path of least resistance. This path is defined by both lower-pressure and higher-temperature (low-viscosity) regions in the melt. Lower-pressure areas are determined by melt-front location, the part's cross-sectional area, and the polymer gating position. Higher-temperature areas occur in centers of thick part sections, in high-shear regions, and as a result of mold-temperature variations. Keep gas behavior in mind when designing a part and controlling the process.
REFINING PART & TOOL DESIGN
Perhaps more than in other injection molding processes, part design plays a crucial role in gas-assist molding. When gas assist is only an afterthought, don't be surprised if it doesn't produce optimum parts - or doesn't work at all.
Open-channel parts present the biggest challenges. These parts, such as panels or access covers, consist of a thin wall with rib-like traversing gas channels. Because gas can penetrate into the thin-wall regions - an effect called "fingering" - the design and processing of these parts can be tricky. Poor design can result in a significant penalty in structural performance [ILLUSTRATION FOR FIGURE 1 OMITTED].
In general, contained-channel parts, such as an arm rest or handle, are easier to process because the gas propagates through a clearly defined path, and the part has no thin-walled areas that must remain gas-free. These parts show the greatest gains in thickness-to-strength ratios - 40% or more relative to solid injection molded parts.
Design strategies for any gas-assist part and tool should stem from three objectives: optimizing the gas-channel layout, selecting the correct channel size, and balancing the fill.
OPTIMIZING CHANNEL LAYOUT
Optimizing gas-channel layout means picking the best locations for the gas channels and nozzle relative to the sprue or gates. With engineering resins, which tend to exhibit low shrinkage, this step is more crucial because the primary gas-penetration stage, governed by design parameters, dominates the secondary gas-penetration stage, which is driven by part shrinkage.
The main design goal in selecting a channel layout is to set up a cavity-filling pattern in which the lowest pressure exists near the end of each channel after the short shot is delivered. You can often create this condition by positioning the channels so that they end near the last areas of the cavity to fill. This tactic will help define the path of least resistance along the gas channels - called a "flow-leader" effect. The challenge, however, is making sure that the addition of gas channels won't disrupt the cavity-filling pattern and affect the location of the last area to fill.
Here are some other helpful layout rules.
* Flow around corners: Gas traveling through a curved channel tends to follow the shortest path through the curve, which means that the bubble will cut to the inside of sharp corners and create an uneven thickness distribution. Using a more generous radius can avoid thin inside corners.
* Large, thin walls: Parts that have large thin-walled regions should have the melt gated directly into the thin Gas wall [ILLUSTRATION FOR FIGURE 2 OMITTED]. Otherwise, the gas channels could result in undesirable flow-leader effects, causing the resin to flow where you don't want it to.
* 'Closed-loop' channels: Wherever two gas bubbles converge there will remain a slug of plastic material in the gas channel between the bubbles. Such solid portions may be acceptable in corners or other areas; just don't expect the kind of rapid cycle times that a completely cored-out part will leave [ILLUSTRATION FOR FIGURE 3 OMITTED].
* Channel orientation: Try to orient the gas channels in the direction of melt flow. Channels that cross in front of the melt flow will likely lead to fingering [ILLUSTRATION FOR FIGURE 4 OMITTED].
Gas-channel sizes can vary significantly with part design, but a 2:1 ratio of channel dimension to nominal wall thickness is typically used as a lower boundary for channel size.
The upper limit, however, depends on the geometry of the part and positioning of the channel. Large channels present an especially thorny problem in that the polymer will "racetrack" through, leaving the adjacent thin-walled areas untilled. We suggest that you start with channels that are relatively small - 2.5 times nominal wall thickness - to minimize this effect.
Also, don't try to increase structural performance by increasing the channel size. Instead, parts needing a boost in strength or rigidity should use ribs on the gas channels or nominal wall. A rib is more efficient than a gas channel in adding structure to the part and will not contribute to an undesirable flow-leader effect the way larger gas channels will. And ribs on top of gas channels can add even greater strength than traditional ribs since their thickness can be a full 100% of the nominal wall without producing sink marks on the surface. Traditional solid ribs are limited to 60% of nominal wall thickness if you want to avoid sinks.
Because gas bubbles displace resin from the gas channels to untilled parts of the tool, parts with multiple gas channels must have a balanced gas fill. If some gas channels fill sooner than others, poor gas penetration in the gas-deprived channels will result - as will poor resin filling.
One way to properly balance the filling is through the sizing of the gas channels. Gas channels near the gate should be smaller in diameter since they fill first; channels farthest from the gate should be larger in order to balance filling. A common design mistake is holding a uniform channel size in spite of the balancing problems that approach can cause.
Another balancing action, though one of last resort, is to use multiple polymer gates to balance the flow. Obviously, tooling costs will rise, but multiple gates are sometimes the only way to fill some parts.
Our best advice for solving many problems is to do so up front by investing in a computerized mold-filling analysis - even if only to check the balancing before the tool is cut.
In tooling design, the good news is that gas-assist molds are very similar to conventional injection molds. So follow standard injection molding rules of thumb. There are, however, a few small differences to remember:
* Gate size: In-runner and in-nozzle gas-injection techniques often demand larger gate sizes than conventional molding to prevent resin freeze-off before the gas injection begins. Edge- or fan-style gates should have an integrated channel to give the gas a clear path into the cavity.
* Hot-manifold systems: Use them only with in-cavity gas injection, not with the other two types of gas-assist systems. We've found that the gas often makes its way back into a hot-manifold system, pushing the hot resin back into the machine barrel. Shot-size variations and increased reject rates follow. Even with the in-cavity method, a valve-gate system may be required to keep the gas at bay, depending on gas nozzle location.
* Nozzle location: When selecting a gas nozzle location for in-cavity and in-runner systems, choose one that allows the polymer to cover the gas nozzle prior to gas introduction. Otherwise, the gas will choke off the polymer flow.
* Shut-off nozzles: These are a good idea when using the in-cavity or in-runner techniques without valve gates. These nozzles can prevent the gas from entering the barrel and pushing the screw back.
If the polymer gate and gas channel are not in close proximity, you can use an extended hold time to fulfill the shut-off nozzle's function.
Beyond standard injection molding control considerations, gas-assist molding requires careful attention to controlling the channel wall thickness - i.e., the thickness of the material surrounding the hollow core. Wall thickness influences the extent of gas penetration and ultimately the part performance.
During processing, the channel wall has two components: a solidified skin layer and a molten inner layer. While both layers can be controlled through selection of processing conditions, the thicker molten wall is subject to a more limited range of control options.
For the solid layer, which forms on the mold walls as hot polymer contacts the colder mold surface, a range of options exist. It can be thinned out by decreasing gas delay time, increasing melt temperature, increasing mold temperature, or by changing materials to one with a lower thermal conductivity and specific heat.
Meanwhile, the molten layer's thickness is determined by the velocity of the gas bubble through the molten core and the rheological properties of the resin. With non-Newtonian melts - such as an engineering thermoplastic - faster bubble velocities usually result in a thinner molten layer until a limit is reached [ILLUSTRATION FOR FIGURE 6 OMITTED]. The same goes for melt viscosities: As the polymer viscosity increases, the gas-channel wall thickness will increase.
Thus, to reduce the thickness of the molten layer, a combination of increased gas temperature, increased melt temperature, and decreased shot size will often do the trick in creating the desirable velocity and viscosity conditions. An alternative would be to change materials to one with lower viscosity or more shear thinning. For example, even changing between two grades in the Lexan polycarbonate family can have a substantial effect on wall thickness.
Another aspect of gas-assist process control relates to the interaction of wall thickness, shot size, and gas penetration. Shot size for a given part and tool design must be adjusted in lockstep with the process conditions that influence wall thickness. The reason is that as wall thickness increases, it becomes more likely that gas will penetrate farther than is desired. At the same time, increasing shot size will often inhibit adequate gas penetration.
Michael Caropreso and Peter Zuber work at GE Plastics' Polymer Processing Development Center in Pittsfield, Mass. Michael Caropreso is a senior process development specialist and Peter Zuber is a senior process development engineer.
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|Title Annotation:||Special Report; injection molding of plastics|
|Date:||Mar 1, 1995|
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