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INJECTION MOLDING: Injection Molding Innovation: The Microcellular Foam Process.

This process does not require chemical blowing agents, hydrocarbon-based physical blowing agents, nucleating agents, or reactive components.

The microcellular foam molding method known as the MuCell process uses supercritical fluids (SCFs) of atmospheric gases to create evenly distributed and uniformly sized microscopic cells throughout a polymer (generally 5 to 100 microns in size, depending on the material and application). Suitable for injection molding, the microcellular foam process enhances product design, improves processing efficiency, and reduces product costs.

The Process

The microcellular foam process follows four basic steps:

1. Gas dissolution: An SCF of an atmospheric gas (e.g., [CO.sub.2] or [N.sub.2]) is injected into the polymer through the barrel to form a single-phase solution.

2. Homogeneous nucleation: A large number of nucleation sites are formed where cells will grow.

3. Cell growth: Cells are expanded by diffusion of gas into bubbles. Processing conditions provide cell growth control.

4. Shaping: Mold design controls part shape. No modifications are required for most molds.

All four steps must occur to successfully produce microcellular foam molded products. The temperature and pressure of the single phase solution must be maintained precisely to prevent the solution from prefoaming. The microcells nucleate, grow, and freeze during filling, and the final size and shape of the microcells are dependent on the molding conditions.

Capabilities of MuCell Molding Technology

Weight Reduction

This microcellular foam process is different from all other foaming or gas-assist technologies in that it provides controlled weight reductions in cross sections as thin as 0.25 mm (0.010 inch). The ability to foam thin cross sections is made possible because of the uniform, single-phase solution and the resulting homogeneous nucleation. Additionally, the technology permits almost all materials to be foamed, including polyphenylsulfone, acetal, PPO, PC/ABS, PBT, and, especially, glass-filled nylon.

Cycle Time Improvements

In a number of applications, cycle times have been reduced by up to 50% (see Applications section). The decrease in cycle time is caused by five factors:

* Internal gas pressure allows for the elimination of pack and hold time.

* The nucleation and growth of the cell is an endothermic reaction.

* Significantly less mass must be cooled.

* The uniform distribution of cells allows for improved dimensional stability and shorter cycle times.

* Decreased viscosity reduces shear heating, and thus cooling time can be reduced.

Reduced Injection Pressures and Clamp Tonnage

The creation of a single-phase solution between a very viscous polymer and an SCF significantly reduces the viscosity of the material to be injected. Figure 1 is a plot of the viscosity ratio (single-phase solution viscosity/polymer viscosity) vs. SCF concentration.

The reduced viscosity allows for significantly lower injection pressures and clamp tonnage. The microcellular process has demonstrated up to a 60% reduction in injection pressure (under the same conditions as the standard molding process).

Obviously, the lower injection pressure requirement leads to reduced clamp tonnage requirements, lower molded-in stresses, and less warpage. Combining these attributes with the elimination of hold pressure allows the molder to reduce the clamp tonnage by up to 80%. However, a 30% reduction in clamp tonnage is more typical (see Applications section).

Energy Savings

The microcellular foam process has reduced energy consumption per part by up to 36%. To demonstrate this, a part was molded as a solid, without the benefit of the SCF. The part required 298 watts per piece as a solid, whereas the MuCell part required only 190 watts per piece. This 36% reduction in energy consumption per piece is due mainly to the lower viscosity of the material and to the significantly lower amount of plastic mass being processed as a result of the part weight reduction.

Alternatively, energy consumption can be reduced by placing larger molds in molding machines with smaller platens. With MuCell, it is not uncommon to use a 500-ton mold in a 200-ton tiebarless machine and successfully mold parts. Typically, a 200-ton machine runs with a 40-HP motor, whereas a 500-ton machine might run with a 60-HP motor. This represents a 33% drop in peak energy usage.

Controlling Weight Reduction and Cell Size With Processing Conditions

The performance of the microcellular foam process depends on several factors, including mold design, material type, SCF type, concentration of the SCF injected into the melt, and processing variables. We define performance as the ability to produce high weight reductions in thin parts and microcellular foam with uniform cell distribution.

The following section describes the effects of the processing conditions on weight reduction and cellular structure. Although several studies have been performed (on ABS, PC, PC/ABS, PP, filled PP, TPEs, and polysulfone), only the PS study is discussed. Microcellular structures were demonstrated in all materials, and the effect of the processing conditions on the weight reduction and cellular structure is similar for all materials with this part shape.

Polystyrene Case Study

Initial investigations into the effect of processing variables on weight reduction and cellular structure began with more than ten variables. Screening DOEs quickly revealed five primary variables with some first-level interactions: injection speed, melt temperature, gas type, mold temperature, and gas level in weight percent.

For the following case study, gas type was separated out and will be addressed later in this paper. [CO.sub.2] was the gas of choice for this experiment.

Experimental equipment and procedures. A simple 140 mm x 280 mm x 1.5 mm center-gated, flat plaque mold was installed in a 150-ton Engel injection molding machine modified to produce microcellular foam. Cellular size was determined by capturing the structure on an SEM photomicrograph and averaging the cellular diameter through a straight-line method.

Case study results. A two-level DOE with center points was performed with a GPPS 11.0 MI material. Injection speed (50-200 mm/s), melt temperature (160[degrees]C-193[degrees]C), [CO.sub.2] weight percent (4--7), and mold temperature (49[degrees]C-82[degrees]C) were evaluated for their effect on weight reduction and cell size.

Figure 2 shows that the injection speed is the factor that most affects the weight reduction. The weight of the part changes by up to 5% by increasing the injection speed from 50 mm/s to 200 mm/s. However, there are minor interactions with melt temperature. The second most important factor in weight reduction is the [CO.sub.2] percent, as seen in Fig. 3. Increasing [CO.sub.2] from 4 wt% to 7 wt% improves weight reduction by almost 3.5%. This Figure also shows how the interaction between melt temperature and gas percent affects weight reduction.

Melt temperature and mold temperature were not significant factors in the overall weight reduction for these experiments. However, it has been seen that in thinner cross sections and higher-viscosity materials, weight reduction can be affected by increased melt temperature.

Figure 4 shows the effect of melt temperature on cell size. The average response shows that the cell size approximately doubles from 41 to 85 microns with an increase in melt temperature from 160[degrees]C to 190[degrees]C. The increased melt temperature reduces the melt strength of the polymer and thus allows for greater cell growth. Figure 4 also shows that increasing gas concentration reduces the average cell size for the lower melt temperature. The cell size is reduced because of increased nucleation with higher gas concentration. At higher melt temperatures, the inability to control cell growth dominates the final cell size. These results were not unexpected. For these experiments, cell size is not significantly affected by mold temperature or injection speed.

Figures 5 and 6 are photomicrographs of the PS cellular structure under inappropriate and appropriate processing conditions, respectively. One can see that it is possible and easy to control the cellular structure and size. [CO.sub.2] vs. [N.sub.2]

It is well known that [CO.sub.2] has different solubility parameters and diffusion rates than [N.sub.2]. In most materials, [CO.sub.2] diffuses through the polymer at a rate almost 20% faster than that of [N.sub.2], and the solubility level of [CO.sub.2] is much higher. For most SCF/polymer systems, it is possible to add up to 3.5 times more [CO.sub.2] (by weight) than N2. Once again, the overall concentration is dependent upon system temperature, pressure, and solubility parameters. This leads to the question of what is the performance difference in microcellular foaming between [CO.sub.2] and [N.sub.2].

The above experimental procedure was repeated on polypropylene using various levels of [N.sub.2] and [CO.sub.2], characterizing the performance on the basis of cellular structure, weight reduction, and viscosity of the polymer/SCF system. Shown in the Table are the differences between [N.sub.2] and [CO.sub.2]. [N.sub.2] tends to provide for better cell size control, and [CO.sub.2] tends to provide a much larger reduction in viscosity as measured by peak hydraulic injection pressure. No differences in weight reduction were seen during this experiment. Even more interesting is that these trends are also true in the real part applications tested so far.

Applications

We have discovered that this microcellular foam process is controllable and very valuable to the end user. Here, we review two applications in which this technology has been successfully demonstrated.

Filled Nylon Mirror Bracket

Figure 7 shows an automotive mirror bracket pro duced with the microcellular foam process. This application demonstrates both a reduced weight, by up to 28 wt%, and a 50% reduced cycle time, from 51 to 25 seconds. An ROI on this mirror bracket showed a savings of $0.42 per part. Of this, $0.13 is due to the reduced weight, and $0.29 is due to the reduced cycle time. Payback for the technology is approximately 4 months.

The reason that the cycle time could be reduced is that the MuCell process improves dimensional stability. Particular sections of the part tended to bow and were measured as out of specification when molded with cycle times below 51 seconds without the MuCell process.

Internal Printer Parts

Several internal printer parts for inkjet and laser printers have been molded with the MuCell microcellular foam process. Tolerances and dimensional stability are critical for these applications. The microcellular foam process reduces warpage by up to 50% and improves the shot-to-shot consistency. Typically, in glass-filled PPO applications, the weight is reduced by 8% to 12% and the cycle time by greater than 25%. Clamp tonnage is reduced by up to 50%.

Future Developments

As in all foaming methods, the microcellular foam process does not usually produce a class A surface. (However, perfect surfaces have been produced in thin-wall PP parts and glass-filled nylon parts.) The surface shows a swirl pattern caused by the release and drag of bubbles along the mold wall during injection. Techniques exist to overcome this deficiency.

Tecomelt is a process whereby high performance "skins" are inserted onto the mold face, and polymer is injected onto the skin. The result is a perfect surface on one side with high weight reductions, no sink marks, and fast cycle times. Additionally, the microcellular process allows lower melt temperatures and injection pressures, both of which reduce damage to the skin during injection.

Combimelt is a co-injection technology whereby a special co-injection head is designed to combine the solid non-gas-laden polymer with the gas-laden polymer. The materials are combined in such a way as to allow the solid material to be deposited on the surface of the mold and the foamed material deposited in the core. This process allows for a perfect surface on both sides of the part and a foamed core.

Conclusions

Microcellular foam molding is now possible and has the potential to dramatically decrease overall manufacturing costs through material reduction, improved cycle time, and fewer rejected parts as a result of improved dimensional stability and tighter tolerances.

Although much more work is needed to fully understand all the capabilities provided by the MuCell molding technology, it is apparent that the benefits are significant.

[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]
Effects of [N.sub.2] and [CO.sub.2] on Three Characteristics
SCF Cell Weight Maximum Injection
Type Size Reduction Pressure Reduction
[N.sub.2] 68 11.5 22%
[CO.sub.2] 95 11.6 64%
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Title Annotation:production technique
Comment:INJECTION MOLDING: Injection Molding Innovation: The Microcellular Foam Process.(production technique)
Author:Pierick, David; Jacobsen, Kai
Publication:Plastics Engineering
Article Type:Tutorial
Geographic Code:1USA
Date:May 1, 2001
Words:2040
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