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Processing: gas-assisted injection molding - a new technology is commercialized.

PROCESSING Gas-Assisted Injection Molding--A New Technology is Commercialized

Gas-Assisted INjection molding, GAIN, is a revolutionary plastic molding process that presents many of the advantages of structural foam molding but avoids the surface problems and long-cycle-time and thin-section limitations generally experienced with structural foam. In addition, gas-assisted injection molding will substantially reduce operating expenses through reductions in tool cost, in required clamp tonnage, and in cycle times for thick sections.

Gas-assisted molding has been under evaluation, mostly in Europe, for about 10 years. Only within the past few years has its potential been brought to the attention of the North American market. With this knowledge, utilization of the process is expanding at an unprecedented rate for a new molding technique, and major applications are developing in the automotive, business machine, and consumer product markets. The limiting factor at this point is the lack of engineering and manufacturing knowhow required to fully utilize the true potential of gas-assisted molding.

The principles of gas-assisted molding are described in this article, its advantages and disadvantages outlined, and examples of successful production applications presented.

Process Description

In the process, shown schematically in Fig. 1, the mold is partially filled with plastic (short shot). The plastic flow is then stopped, and a controlled volume of inert gas (usually nitrogen) is injected into the center of the flow at the point at which it entered the mold. The combination of high surface tension and the lower viscosity of the hotter plastic melt in the center of a thick section, such as a rib, confines the gas, forming hollow areas in the thicker sections of the part. The gas pressure is held constant as the plastic cools in the mold, and then is relieved (vented) just prior to mold opening.

Controlling the quantity of plastic injected, the gas pressure, and the gas injection and venting times allows a predetermined network of hollow interconnecting channels to be formed. The molten plastic that is displaced from the thicker sections by the gas is pushed into the extremities of the tool, packing out the molded part. Thus, the gas channels that are designed into the part act as internal flow runners, replacing external hot and cold runners. This network also provides constant gas pressure throughout the part, thereby creating a more uniform pressure on all surfaces of the mold than can normally be achieved conventionally.

The hollow gas channels compensate for the tendency of plastic to form sink marks at thicker cross sections, and the constant gas pressure during solidification minimizes or eliminates part warpage and reduces "molded-in" stress. GAIN has many of the advantages of structural foam molding, but unlike foam, the gas remains in specified channels within the part and the surface splay that occurs with structural foam is totally avoided. The dramatic result is a rigid, class A finish with virtually no sink marks.

Molding Cycles

The typical injection pressure vs. time curve for conventional molding, given in Fig. 2, shows that the majority of the mold volume (typically 90-95%) is filled in the time period t1. The injection pressure then rises sharply as the tool is "packed out" in the time period t1 to t2. The maximum pressure needed to completely fill the mold at t2 determines the machine clamp tonnage required to mold the part. The cavity pressure decreases as the plastic cools during the time period t2 to t3. Mold opening is at some time equal to or greater than t3.

In the gas-assisted molding process (Fig. 2), the short shot is injected during the period t1. Plastic flow is stopped at t1 with a positive shut-off valve. The cavity pressure at t1 is lower than in conventional molding because of the relatively large internal flow channels designed into the part. The gas is then injected during the time period t2 to t3. At this point, the tool is completely packed out by the displaced plastic, and there is no high second-stage packing pressure. The "dwell time" prior to gas injection, t1 to t2, is an important parameter that must be accurately controlled. Depending on other variables, it may be set between zero and several seconds. During the period t3 to t4, gas pressure is held constant as the plastic is cooling. At t4, the gas is vented by retracting the nozzle ("sprue break"), and mold opening is at some time equal to or greater than t5.

The initial gas injection pressure at t2 is a function of the plastic material characteristics and melt viscosity. Typical values are between 14 and 70 MPa. The optimum hold pressure is generally 10% to 80% of the initial pressure and depends upon the geometry of the molded part. Melt temperatures are generally the same as in conventional molding.

The overall molding cycle is not significantly different for gas-assisted than for conventional molding. The times required to fill out the mold and cool the plastic are similar. If there is a change, the GAIN cycle will usually be slightly shorter. IF GAIN allows designing the part with a thinner wall, as is frequently the case, or if the thicker section can be made hollow, significant reductions in cycle time will be realized.

Conversion Equipment

Detroit Plastic Molding has designed and built a gas injection nozzle that can be adapted to a conventional machine. Together with a gas-system control panel and add-on conversion equipment for precise gas pressure, timing, and volume control, the typical molding machine can be set up for GAIN in one day. The injection molding machine must be capable of delivering a controlled short shot and of retracting the barrel and breaking the plastic sprue to vent the gas. Shot size control of [plus-or-minus]0.5% is desirable and is achievable on most machines.

Advantages and

Benefits

Reduced molding stress and warpage. In conventional molding, a high injection pressure may be applied at the sprue in order to achieve the few MPa at the edges necessary to fill out the part. The stress profile thus generated is "frozen" on solidification, and because plastic has a "memory," warpage may result after part removal. Distortion is particularly evident after secondary operations that heat the part, e.g., painting, chrome plating, or bonding.

In GAIN, the gas pressure is equal everywhere, and with a properly designed gas channel network providing an internal runner system, the applied pressure and hence the stress gradients will be markedly reduced. Moreover, the gas pressure in the channels is held constant during cooling, further reducing the tendency towards part distortion.

Elimination of sink marks. Surface distortions, or sink marks, resulting from ribs or bosses on the back side of a part, are caused by the volume contraction of plastic during cooling and are a traditional plastic molding problem. With GAIN, sink marks can be minimized or eliminated by directing a hollow gas channel between the surface of the part and the back-side detail, as shown in Fig. 3. Note that the base of the rib is made somewhat thicker to help direct the channel. This is the opposite of normal design practice, where ribs are made as thin as possible to avoid sink marks. Because the material is hottest at the center, it will shrink away from the inside surface of the channel as the part cools, leaving no sink mark on the visible outside surface.

Reduced clamp tonnage. The injection pressures are highest during the packing-out of the mold, and this normally determines the force required on the clamp ("tonnage") to keep the mold from "flashing." The lower pressures used in GAIN can result in a 25% to 90% reduction in clamp tonnage. Parts as large as 1.2 m.sup.2 projected area have been easily molded on machines with only an 8000-KN clamping force. Generally, the mold clamping force required for GAIN is below 6500 KN/m.sup.2., based on projected area, even for plastics with relatively low melt flow indices. This is only slightly higher than the clamping force used with structural foam molding. The relatively low pressures used in GAIN also reduce problems associated with flash, tool wear, and "lifter read-through."

Elimination of external runners. Because flow runners can be designed into the part, even on a large complex one, all external runners (hot or cold) can frequently be eliminated, and the plastic injected through a single gate. This can reduce tool cost, reduce regrind, and improve temperature control of the melt. Use of internal flow runners can improve the flow pattern and eliminate or control knit-line locations resulting from multiple gates. Figure 4 shows the underside of a tabletop where the plastic is injected at the center. The ribs provide internal flow channels while also furnishing the necessary structural rigidity, and the surface is free of sink marks.

Possibility of different wall thicknesses. A standard guideline for injection molded plastic parts is "maintain a constant wall thickness." With GAIN, different wall thicknesses are possible if a gas flow channel is designed into the part at the transition point (Fig. 5). The gas channel permits uniform material flow and avoids the high stresses that normally result from this design geometry. The gas channel may also be used to form tubular sections, particularly at the edge of the part, as shown in Fig. 5. This adds strength and rigidity without increasing part weight.

The design flexibility allowed with GAIN may also permit the elimination of lifters or core pulls in the mold or the redesign of a multipiece assembly into a single molded component. Note that the overall molding cycle is normally not increased when a localized thick section is designed into the part as part of the gas flow channel.

Disadvantages

All processes have their disadvantages, but those of GAIN appear relatively minor compared with the significant advantages.

Vent hole. The gas channels must be vented prior to opening the mold, leaving a hole, or holes, somewhere on the part. Normally this can be placed on a non-visible portion, but if appearance or function are affected or secondary operations such as chrome plating or painting are required, it may be necessary to seal or finish the hole.

Surface blush. The gas channel may leave a surface blush, which arises primarily from differences in surface gloss level. This is partially masked by a surface grain, or by placing the channel behind a surface feature such as a groove or character line, and is not visible if the part is painted. The tendency to blush is a function of processing conditions and type of plastic.

Multicavity molds. Because the gas cannot push material from one cavity to another and the same short shot must be in each cavity, precise short shot control may be extremely difficult to achieve. A four-cavity tool for four identical automotive parts is being used, but process capability beyond four cavities has yet to be established.

Mold temperature control. Because the wall thickness along the gas flow channel is a function of cooling rate, consistent wall thicknesses require good temperature control. Mold temperature is particularly significant, and the plumbing must be designed to avoid irregular wall formation arising from mold temperature gradients.

Blisters. Failure to vent the high-pressure gas will generally result in a blister. Permeation of the gas into wall sections so as to cause discontinuous bubbles in the part will not only lead to blisters, but will also result in loss of process control due to the mold's not filling out as required.

Unique design. The unique part design and mold design required in most cases to fully utilize GAIN may be considered by some to be a disadvantage. While conversion of a conventional machine is relatively simple, conversion of a mold may be time-consuming, and the modified injection molding tool may not yield an optimum process or design. Likewise, a GAIN mold will not produce an acceptable part on a conventional machine. Therefore, an "upfront" commitment is necessary to fully realize the benefits from GAIN.

Large hollow sections. Despite surface similarities, GAIN is not a substitute for extrusion or injection blow-molding. GAIN is not well suited for thin-walled hollow parts, such as a bottle or tank, and attempts to make such parts have generally led to irregular wall thicknesses and/or gas "blow-through."

Comparison With

Structural Foam

Many of the advantages of structural foam are also found in gas-assisted molding. Therefore, many structural applications are candidates for GAIN, e.g., business machine housings and furniture. Generally, GAIN provides improved surface quality, reduced wall thicknesses and hence, lower part weight, and shorter molding cycles. For some business machine applications, part weight was reduced 30% to 40%, cycle time cut by 50%, and part quality improved.

Material Selection

GAIN has been found to work with any thermoplastic; no exceptions have been reported. However, because process control demands that thick sections remain fluid while the plastic at the mold surface is solidifying, the process does not work with thermosets. For well-controlled gas channel formation and to prevent gas blow-through, the plastic must possess a certain level of hot melt strength, a problem generally only with very-low-molecular-weight resins. GAIN works exceptionally well with low-melt-flow materials such as high-MW resins, highly filled resins, and alloys and blends. The large internal flow runners designed into the part assist in filling the tool, while the high melt viscosity helps control the formation of the hollow gas channels. The large flow channels minimize fiber breakup during injection of fiber-filled materials, further improving part performance.

Part Design

Design guidelines will be treated in detail in future presentations. A few general design principles are as follows:

* The intended path for the gas channels must be clearly defined in the part.

* The thickness must be distinctly larger along the intended gas channel to confine the gas flow and avoid permeation of the gas into the thinner walls; the most effective gas channels have approximately circular cross sections.

* The plastic flow into the mold must be analyzed for the proper gate location.

* The material displaced from the gas flow channels must have somewhere to go and must be sufficient to pack-out the mold.

* Normally only one injection gate is used, and it must be located so that the short shot will fill the mold uniformly.

* The usual gas channel geometry is either symmetrical or unidirectional relative to the injection gate.

* The gas channel must be continuous but should not loop back upon itself.

* In most cases the volume of the gas channel will be less than 10% of the total volume of the part.

Production

Applications

Numerous applications are being put into production as the significant advantages of GAIN are becoming apparent. The following is a general listing of representative applications now in production with a brief summary of the reasons for selecting GAIN.

1. Automotive radiator grille--dimensional stability, avoids use of cooling fixtures, reduced cycle time.

2. Exterior automotive moldings--dimensional stability, sink marks eliminated.

3. Chrome-plated bumper trim--dimensional stability, sink marks and lifter line eliminated.

4. Chrome-plated automotive wheel hub--sink marks eliminated, reduced cycle time.

5. Automotive console door--replaced structural foam for cost savings, sink marks avoided.

6. Automotive interior trim support--dimensional stability, permitted design with differing wall thicknesses.

7. Business machine housings--replaced structural foam for cost savings, improved surface quality, dimensional stability, sink marks avoided.

8. Tabletops--reduced clamp tonnage, sink marks avoided, improved structural rigidity.

9. One-piece chairs--reduced clamp tonnage, improved structural rigidity, controlled knit-line location.

10. Wheelbarrow bucket--improved rigidity at lip, dimensional stability.

Future applications will include automotive interior trim panels, automotive bumper fascias and reinforcements, and automotive fenders and exterior body panels. An additional benefit realized through GAIN in fenders and body panels is a cost savings through simplified mold construction.

The gas-assisted process is protected under U.S. and Canadian patents with other U.S. and foreign patents pending. A licensing program is available from Detroit Plastic Molding.
COPYRIGHT 1989 Society of Plastics Engineers, Inc.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Rusch, Ken C.
Publication:Plastics Engineering
Date:Jul 1, 1989
Words:2637
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