High-Speed Injection Molding of Thin-Wall Polycarbonate Tubes

Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published July1997

TECHNICAL PAPER SERIES

Demands for low cost, light weight, and miniaturization, along with restrictive engineering parameters, have forced many medical device designs into product envelopes that push the limits of moldability. For example, in endoscopic surgery, thinner-wall cannulae (tubes) are now being required. Such designs can tax the flow characteristics of the polycarbonates typically specified. When we decided to develop a 100-mm-long tube with ribs and 0.68-mm-thick walls, mold-flow analysis predicted a cycle with injection pressure of 23,000 psi and melt temperatures of 323°C. Previous work indicated that molding polycarbonate tubes at these conditions results in degradation of the molecular weight of the polymer. Although in that case the molecular-weight damage was not enough to significantly reduce the polymer's mechanical properties, it was demonstrated that processing polycarbonate at 323°C significantly limits allowable residence time of the polymer in the barrel and narrows the permissible processing condition window.

Molding thin-wall polycarbonate tubes on conventional injection molding machines with maximum injection pressures of 138­158 MPa results in molecular-weight degradation of the polymer because of the requirement for excessive melt temperatures. In light of this fact, we began to investigate the effects of high injection speed and pressure on our ability to fill a 100-mm, 0.68-mm wall cannula while protecting the polycarbonate construction material from loss of molecular weight.

EXPERIMENTAL CONDITIONS, RESULTS, AND DISCUSSION

Procedural details and results are discussed below for both preliminary experiments and follow-up studies. Analysis of these findings led us to formulate final conclusions and thus determine a successful manufacturing strategy.

Injection molding trials were conducted on various injection molding presses with accumulator-assisted injection control. Each machine is described more fully within the discussion of the experiment in which it was used. The melt temperatures listed were actually measured with a pyrometer on polymer melt extruded from the nozzle under low-shear conditions. In all the following experimental work the polycarbonate material used was Dow 2081-15 (melt index =15 g/10 min).

Preliminary Experiment. First, to study the effects of injection speed on the ability to fill a 100-mm cannula with a thicker wall (0.76 mm), injection speeds were gradually increased while a constant melt temperature of 296°C was maintained on a Sumitomo SG 125, a 125-tn, 122-g machine. At speeds between 200 and 300 mm/sec, a completely filled, stress-free tube could not be manufactured. It was found that in the 300­375-mm/sec range, an acceptable tube could be molded. At a speed of 375 mm/sec, injection pressure was 83 MPa. Measurement of molecular weight of the material in the molded tubes showed that no degradation during molding had occurred.

Based on these results, it was theorized that high-speed shear is more effective in reversibly decreasing the viscosity of polycarbonate than is elevated melt temperature.

The results corroborated information reported by polycarbonate suppliers claiming that polycarbonate viscosity is dramatically affected by shear rates.^1,2 As stated in one report, "The combination of injection speed and gate size allowed us to achieve the desired span of shear rates."¹

Initial Thin-Wall Cannula Molding. A new cannula design mandated that the wall thickness of the tube be reduced to 0.63 mm. The most challenging design feature for the production of this cannula was the requirement for ribs on the outer diameter. The rib design featured a height of 0.25 mm, with a 0.05-mm radius at the tip.

In designing the molding tool and selecting the injection molding equipment for this project, two items were of prime importance. The injection molding equipment needed to be capable of achieving injection speeds up to 500 mm/sec, and venting of the tool was perceived to be critical.

The Nestal Series N 150-tn clamp injection molding press was at first selected for this job. This machine features a 225-g barrel with injection speeds up to 500 mm/sec and injection pressures up to 214 MPa. Tooling was designed with inserts in the sleeve area to provide for facile modifications. Because the optimum gate size was an unknown, an intermediate-size, 1.27-mm gate was chosen.

With an H-13 steel insert, a melt temperature of 315°C, and the machine operating at its maximum pressure, the cannulae molded were clear, the cobalt blue tint characteristic of the resin having disappeared. The rib section of the part was not filled or packed out completely. Attempts using vacuum assist did not improve the process, and it was concluded that any further work with H-13 inserts would be futile.

The H-13 inserts were then replaced with Porcerax II porous steel inserts in the sleeve area of the cavity. Satisfactory cannulae could then be produced at melt temperatures of 304°C, with an injection speed of 500 mm/sec and a resultant injection pressure of 158 MPa. The porous steel supplemented the venting, and both part quality and rib definition were significantly improved.

At this point, it was decided that the Nestal N series injection molding press was not capable of achieving the injection rates and pressures necessary to optimize the processing conditions, and a Nestal Synergy 150-tn injection molding press was subsequently selected for further work in this program. Fitted with a 85-g barrel, this press was capable of speeds up to 1000 mm/sec and pressures up to 207 MPa.

High-Speed Injection Experiment. In the next experiment a different five-stage injection profile was devised by trial and error for each polymer melt temperature at 5.5°C intervals between 287 and 315°C, inclusive. Each stage of each profile was set for a 0.1-second duration. Minimum injection speeds at each stage that were capable of producing visually acceptable parts were selected for the profiles. The stages of the profiles were scaled down from high speed to low speed to maximize the rheological effects of the high-speed injection while minimizing jetting of the melt in the cavity. The injection profiles that resulted from this exercise are shown in Table I.

Table I. Melt temperatures and injection-velocity profiles used in a high-speed injection experiment.

Under the conditions of the experiment, the injection pressure peaked at 220 MPa for 0.05 seconds and then subsequently dropped to approximately 138 MPa or slightly less for the higher temperatures tested, 304° to 315°C.

When molecular-weight studies were conducted on the molded cannulae, no molecular-weight degradation could be found for product molded at any of the melt temperatures. This result was especially significant in light of earlier studies on cannulae with 0.89-mm. walls and no ribs, molded at conventional injection speeds of 100 to 150 mm/sec through a 2.3-mm gate. These parts had displayed significant molecular-weight degradation probably due to the use of excessively high melt temperatures--approaching 337°C--to achieve fill.

Gate Effects. Subsequently the effect of gate size on the molding process was examined. When the 1.27-mm gate, used in the work described above, was increased to 1.52 mm, the required level of shear thinning of the polymer did not occur. With the larger gate, higher melt temperatures were required to fill the cannulae at each injection speed than with the 1.27-mm gate. When gate size was reduced to 1.02 mm, gate freeze-off occurred and prevented assessment of the effect of this orifice size on the polymer flow characteristics.

It has been stated that "thermoplastics are non-Newtonian in nature, which means that their viscosity will change dependent on their velocity; i.e., the amount of shear experienced. This non-Newtonian nature is key in thin-wall molding."² So is the short-lived temperature increase in the polymer, which is believed to occur due to shear heating during the high-speed injection step. Together these effects provide the low polymer viscosity needed for full, stress-free parts to be produced. The results of the current study support the concept that polycarbonate can be injection molded at high speeds and shear rates if properly controlled. Material flow characteristics, gate position and size, and venting must all be balanced to obtain a structurally sound part.

For polycarbonates, the standard assumption that injection speeds should be below 250 mm/sec has been discredited. Rather, an intelligent application of shear effects can open up the possibility of molding long, thin walls without very high melt temperatures and the accompanying polymer degradation. To achieve success in such processing, the type of injection molding equipment is critical. The injection molding machine should be capable of minimum injection speeds of 500 mm/sec, and injection pressure must be greater than 207 MPa. Tooling should be designed for maximum venting, and gating should be in the direction of flow. Gates should be small enough to promote shear thinning and transitory heating of the polymer but not so small as to cause premature freeze-off.

CONCLUSION

The results of the numerous experiments described above support the belief that injection molding of thin-wall polycarbonate parts under high-shear-rate cavity filling and moderate (287°­ 315°C) melt temperature conditions provide superior parts with less molded-in stresses and polymer degradation than parts formed under conditions of lower shear and higher melt temperature. The shear regimen during filling of the cavity is conveniently controlled by injection speed and gate diameter. The ability to fill the part well under these conditions results from extreme thinning of the polymer, probably due to both its pseudoplastic nature and the short-lived temperature rise it experiences because of the shear energy imparted to it. While the short-lived elevated temperature of the polymer produced by the shear energy as the polymer passes rapidly through the restricted flow channels is effective in helping to thin the material, it is much less damaging to it than a high temperature (315°­343°C) experienced by a resin throughout its residence interval in the barrel. High-speed, high-shear, moderate-melt-temperature injection regimes are recommended for the production of high-quality polycarbonate medical device parts.

REFERENCES

1. Serrano M, Little J, and Chilcoat T, "Critical Shear Rate for the Injection Molding of Polycarbonate, Polystyrene, and Styrene Acrylonitrile," in Society of Plastics Engineers, Inc., Technical Papers, vol XLI (ANTEC 95), Brookfield, CT, SPE, pp 357­365, 1995.

2. Fassett J, "Thin-Wall Molding: Differences in Processing over Standard Injection Molding," in Society of Plastics Engineers, Inc., Technical Papers, vol XLI (ANTEC 95), Brookfield, CT, SPE, pp 430­433, 1995.

Robert T. Alvarez is a plastics consulting engineer for Ethicon Endo-Surgery, a Johnson & Johnson Co. (Cincinnati). He holds a degree in plastics engineering from the University of Massachusetts Lowell, and has more than 30 years' experience in polymer processing and design. Jorge Gutierrez, also at Ethicon Endo-Surgery, is a staff development engineer with 16 years' experience in the biomedical field. He holds a degree in mechanical engineering from Georgia Tech University, and has worked in various roles in manufacturing and product development. Mac Russelburg is project manager at Tech Group Tempe (Tempe, AZ). He has been involved in injection mold making for plastics for approximately 18 years and also has extensive experience in processing technology.

Copyright ©1997 Medical Device & Diagnostic Industry