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Medical plastics: failure-free injection molding of medical devices.

Preventing failure of injection molded medical devices requires a thorough understanding of material properties, and a well-designed procedure for optimizing the processing parameters.

Some processors in the plastics industry have a mistaken notion that failures are "accidents" and little can be done to influence their occurrence. Detailed inquiries have revealed, however, that nearly all failures can be prevented, if causes and mechanisms are sufficiently understood and appropriate countermeasures taken.

Injection molded medical devices, while sharing many material and process requirements with other injection molding applications, differ in significant aspects. After molding, the component is often subjected to one or more washing or cleaning steps, dried at elevated temperatures, and, by means of any of a variety of techniques from solvent bonding to ultrasonic sealing, assembled into the final product. Before it leaves the plant, a terminal sterilization with either ethylene oxide or radiation is applied. Finally, shipping, warehousing, and shelf-life conditions at various temperatures and humidities are experienced by the device before it is ever touched by the ultimate user. To ensure failure-free operation, one must anticipate how all these conditions interact with the product.

Processing Parameter Optimization

To optimize injection molding processing parameters, all conceivable variables are taken into account:

* material type and mold design;

* mold and machine capacity match;

* material conditioning;

* flaw, knit-line, and notch reduction;

* degradation pathway identification; and

* processing parameter validation.

Often, the importance of the material type is overlooked, leading to excessive molded-in stress or improperly fused knit lines. Mold shrinkage, a property primarily governed by the degree of crystallinity, is of the utmost importance in mold and cycle design.

Economic necessity at times dictates mold rotation, i.e., mold transfer between machines within a facility or between facilities. A frequently neglected situation arises when the match between shot capacity and the molding machine barrel capacity gets out of balance. The resulting combination of excessive melt residence times with thermally sensitive materials can lead to serious thermo-oxidative and molecular-weight degradations that cause subsequent failures.

For example, an amorphous polyethylene terephthalate (PETG) was found to be extremely sensitive to thermal degradation when the melt temperature exceeded 230|degrees~C. Below 230|degrees~C, where thermal degradation was minimized, the resin was more sensitive to mechanical degradation. These degradation pathways alter the molecular weight as reflected in the melt viscosities.

In a similar manner, a hot runner mold designed to economize usage of a thermoplastic elastomer alloy was found to have oxidatively degraded the material, causing severe failures in a subsequent bonding step as measured by the fraction of samples that failed a leak test. The extent of the degradation was documented by a simple technique, the oxidative induction time (OIT) test, which quantifies the material stability in a differential scanning calorimeter (DSC) |L. Woo, M. Ling, and E. Chan, J. Vinyl Tech., 13, 199 (1991)~. Briefly, in the OIT test, a small, approximately 10-mg sample is placed in the DSC at an elevated temperature, and oxygen or air is introduced. The length of time necessary for all the stabilizer to be exhausted and the sample to start undergoing autocatalytic oxidation is designated as the OIT. Evidently, the excessive residence time in the hot runners led to oxidative degradation that significantly altered the chemical and surface properties of the material and interfered with the subsequent bonding step. The steepness of the failure curve vs. OIT is indicative of an underlying threshold phenomenon, that is, when the extent of degradation exceeds a certain limit, the failure rate grows nearly exponentially. This functional dependence potentially offers a simple method for process monitoring and control of this mode of degradation.

Another material requirement that needs particular attention is conditioning, especially drying. Many engineering thermoplastics are produced by condensation polymerization. In this type of reaction, one molecule of a low molecular weight species, frequently water or alcohol, is formed for each main chain linkage. However, these reactions are reversible. In the presence of moisture at the high melt temperatures in the injection molding press, each available water molecule can cleave a main chain bond, leading to a significant reduction in molecular weight. This is one of the most frequently encountered failure modes with engineering plastics. Figure 4 shows Izod impact strength measured against the time prior to molding that dry polycarbonate was exposed to ambient moisture conditions. In less than 10 hrs, sufficient moisture was absorbed to cause a drastic reduction in the impact properties of the molded components.

Knit lines are generated when polymer melt fronts converge from nearly opposite directions to form an integral part of the component. At the knit line, material orientation is normal to the direction of flow. If the time, temperature, and pressure are insufficient for proper chain entanglements and material relaxation to occur, a weak bond can form along the weld line. In addition, low molecular weight species such as oligomers, lubricants, and antiblock modifiers are often swept to the melt front during mold filling, further weakening the knit line. Knit lines frequently occur on surfaces where high stresses are present during assembly and end use. Therefore, weld lines are the sites of a high incidence of failures. To prevent such failures, a good understanding of the mold-filling sequence and a rigorous challenge or validation process is required.

Care must also be exercised to eliminate stress concentrations at corners. Because many of the "tough" plastic materials exhibit severe notch sensitivities, the presence of sharp notches results in brittle behavior.

In summary, before a product is deemed qualified, a full and well-designed evaluation of the injection molding process is needed. In a study we conducted on a low-density polyethylene (LDPE), a sharp dependence of molded part properties on melt temperature was observed. The high-temperature reduction in elongation was undoubtedly due to thermal degradation, while the low-temperature dropoff is believed to be due to a very high level of molded-in stresses in the semicrystalline LDPE, instead of shear degradation, as with PETG.

Designed Experiment Application

We have had significant success in using designed experiments to systematically quantify the dependence of product performance properties with molding conditions. The design process provides a maximum amount of data from a minimum number of experiments. The experimental design for the molding of a poly (4-methyl-1-pentene) component is shown in the Table. The melt viscosity of the molded component--a measure of molecular weight (MW)--is assigned as the dependent variable.
Table. Polymethylpentene Molding Experiment Design.
Process variables
Melt temperature: X1 +1 = 610|degrees~F
 0 = 580|degrees~F
 -1 = 510|degrees~F
Cycle time:X2 +1 = 42 sec
 0 = 32 sec
 -1 = 25 sec
Injection speed:X3 +1 = 80%
 0 = 50%
 -1 = 30%
Sample X1 X2 X3 |eta~*, |10.sup.4~ p
 1 +1 +1 0 1.77
 2 +1 -1 0 2.48
 3 -1 +1 0 4.43
 4 -1 -1 0 4.43
 5 +1 0 -1 1.93
 6 +1 0 -1 1.95
 7 -1 0 +1 3.90
 8 -1 0 -1 4.52
 9 0 +1 +1 3.30
 10 0 +1 -1 3.59
 11 0 -1 +1 3.77
 12 0 -1 -1 3.66
 13 0 0 0 3.52
 14 0 0 0 3.82
 15 0 0 0


It has been clearly established, both experimentally and theoretically, that at a given temperature for a given linear polymer, ||eta~.sub.O~, the zero-shear or rate-independent viscosity is related to the weight-average MW, |M.sub.W~, by:

||eta~.sub.O~ = K|(|M.sub.W~).sup.3.4~ (1)

where K is a constant.

In this study, the melt viscosities of components molded under various conditions were measured at shear rates between 0.1 and 100 radians/sec in the dynamic mode at 250|degrees~C on a Rheometrics fluid rheometer, and the low frequency (0.01 radians/sec) plateau limiting viscosity taken as the zero-shear value.

Figures 7a and 7b indicate the dependence of the component MW, as measured by viscosity, on melt temperature and cycle time, respectively. The melt temperature is easily identified as the most important factor. Figure 7c is a three-dimensional representation of the overall dependence for simple visualization of the variables.

The significance of the MW to product performance is shown in Fig. 8, again with MW represented by viscosity. In this case, failures were detected by cracking of the molded components after downstream processing. Evidently, molecular-weight degradation had shifted the material into a regime where the dependence of tensile strength on MW is nearly exponential. This is consistent with the widely accepted functional dependence of ultimate strength on MW:

||sigma~.sub.u~ = ||sigma~.sub.|infinity~~ - B/|M.sub.n~ (2)

where ||sigma~.sub.u~ is the ultimate strength; ||sigma~.sub.|infinity~~ is the strength at infinite MW; B is a material constant; and |M.sub.n~ is the number-average MW.

The dependence of failure on MW was found to be an even stronger function for polymethylpentene than for polymethylmethacrylate. This very strong dependence was corroborated in several other well-documented studies, making a compelling case for control of injection molding variables for the prevention of failures. Also, it highlighted the power of choosing key molecular properties as independent variables in the study.

From the functional dependence of failure occurrence with measurable properties, fundamental insights on component performance can be established. In addition, once the proper MW for product performance is identified by melt viscosity, it can be converted to a simple melt-flow-rate (MFR or MI) value, easily determined on a melt indexer, and thus become an easily implemented technique for quality control.

Conclusion

The critical nature of injection molding of medical plastics was examined and the proposal that all failures are preventable has been supported. However, to fully realize this goal, certain steps must be implemented. First, a thorough understanding of the material property performance and potential degradation paths must be established. Second, a carefully designed processing equipment selection and parameter optimization will ensure that the maximum properties are realized at all times. Finally, modern, designed experiments can frequently maximize understanding while reducing the number of experiments.
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Author:Woo, Lecon; Ling, Michael T.K.; Westphal, Stanley P.
Publication:Plastics Engineering
Date:Oct 1, 1992
Words:1670
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