Injection molding of medical plastics: A review

SPECIAL REPORT

Injection Molding of Medical Plastics: A Review

KARIM AMELLAL, COSTAS TZOGANAKIS", ALEXANDER PENLIDIS, and GARRY L. REMPEL Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

ABSTRACT Given the tight competition in the area of plastics for medical applications, the requirements for consistent quality, standardized, safe, and low-cost medical components have become a big challenge for molders. The survival of injection molding companies for medical applications depends strongly on the success of design, manufacturing, and marketing strategies that different companies have established to meet these requirements. Very often, the design and manufacturing operations to produce medical plastics for the health-care market have to be adjusted on a daily basis in order to satisfy increasingly strict, government-regulated, biomedical requirements in addition to all the other pressures expected in this market. As a result, molders of medical plastics are always seeking more systematic ways to improve product quality and produce more at a minimum cost. This brings an advantage to those processors that can combine high-tech molding with low-cost production. In this article, based on the open literature, we present a review of the latest activities (last decade) in the field of injection molding of medical plastics together with some comments on modern injection molding processes that can be used. 0 1994 John Wiley & Sons, Inc.

I Introduction

ecause the medical/health care industry is one of the fastest growing markets for plas-

tics, it is becoming increasingly demanding and challenging for injection molders to adjust to the product and process developments. In addition, there is an intensive pressure on molders to cut

* To whom all correspondence should be addressed.

their costs. At the same time, customer expectations with respect to the cleanliness and efficiency of the molding environment and the precision of the parts are getting stricter. Therefore, injection molders must invest time and effort in finding ways to bring about a more financially viable product quality/production volume compromise than their competitors.

In addition, the daily design and manufacturing operations are affected by cost-consciousness, improved performance, solid-waste management

Advances in Polymer Technology, Vol. 13, No. 4, 315-322 (1994) 0 1994 by John Wiley & Sons, Inc. CCC 0730-6679/94/040315-08

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through source reduction, recyclability, waste-to- energy conversion, growing demands for single- use products, and the ability to withstand various sterilization processes.

Finally, with respect to the increasing need to unify the specifications of the Food and Drug Ad- ministration (FDA), the International Standards Organization (ISO), and the North American, Eu- ropean, and Japanese regulations, there is a tendency toward the creation of global standards.

In this article, we present a review of the latest activities (last decade or so) in the field of injection molding of medical plastics. In addition, modern injection molding processes that can be used in medical applications are discussed.

Medical-Grade Materials

THE INJECTION MOLDING PROCESS The injection molding process starts by filling

the cavity with molten polymer (filling stage). Once the cavity is filled, extra polymer melt is in- jected into the cavity at a higher pressure to com- pensate for possible shrinkage during polymer so- lidification (packing stage). Then, cooling takes place until the molded part is sufficiently rigid to be ejected (cooling stage). The post-filling stage is a combination of the packing and cooling stages. Af- ter the cooling stage, the mold opens (mold-open stage), the part is ejected, and the mold closes again to start a new cycle. Therefore, a typical injection molding cycle consists of the filling, post- filling, and mold-open stages. Simultaneous heat transfer and fluid flow take place within the polymer melt during the filling and post-filling stages, and heat removal by conduction is very important in controlling final product specifications.

MATERIALS Almost all plastic medical devices are injection

molded. In addition, plastics used in medical devices should be transparent, bondable, and readily sterilizable. Depending on the ultimate application of the medical device, chemical resistance, water absorption, oxygen transmission, elasticity, impact strength, and deflection temperature may be among the important properties to be considered

when selecting the appropriate plastic material with respect to its relative cost.

From a comparative analysis between cost and performance of plastics used in medical device^,^ polyvinyl chloride (PVC), styrene acrylonitrile co- polymer (SAN), polycarbonate (PC), and polyester seemed to be the materials with the best compromise. However, according to Luke,3 modified acrylics, which have experienced tremendous growth in the medical device field, offer excellent clarity, chemical resistance, toughness, and rigid- ity. Although transparent, modified acrylics have less clarity than PVC, SAN, PC, and polyester, and their cost is slightly higher than PVC and SAN.

Rigid thermoplastic materials are used in various disposable medical devices. In order to determine the appropriate plastic for a medical device, one must consider the physical and optical properties of the finished product. It is a well-known fact that medical devices which are in contact with the body or bodily fluids must be, without exception, sterilized before use. The sterilization method depends on the type of thermoplastic ~ s e d . ~ , ~

STERILIZATION TECHNIQUES The two commonly used high energy steriliza-

tion techniques for disposable medical devices are ionizing radiation (gamma and electron beam) and ethylene oxide gas. Because of environmental regulations, the use of ethylene oxide is restricted and needs special safety procedures. Among the high energy ionizing radiation sterilization methods, gamma radiation is the most widely used technique.

However, a recent study,6 reviewed and reported elsewhere in the literature,’ investigated the effects of the electron beam sterilization on thermoplastics in comparison with the gamma radiation sterilization. Results from this study indi- cated that thermoplastic resins exposed to both sterilization techniques (electron beam and gamma radiation) exhibited similar physical and visual property changes. Based on the test results, it was concluded that the manufacturers can consider the electron beam technique as an alternative to gamma radiation for disposable device sterilization. However, more considerations should be taken into account in terms of the size and flexibility of the packaging, the complexity of the device, and the sensitivity of various plastics to degrada- tion.

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APPLICATIONS Plastic materials are used in all medical sectors

including the development of blood-handling devices.* The spectrum of the plastics used is as di- verse as their application^.^'^ Increasing demands on hygiene have increased the popularity of disposable and reusable articles with high functional- ism and reasonable cost/performance ratio.I4 For example, polymeric materials have led to great advances in the development of implants.

In medical engineering, degradable polymers represent an attractive alternative for conventional materials to be used as implants in the body for certain applications. Degradable polymeric materials used in drug delivery systems and as "plates" and "screws" for broken bone restoration can help eliminate the second operation which is generally required for removal of these parts. The reason is that they are completely broken down by the hu- man body. The main problem is the processing of these polymers, because no stabilizers are allowed. This causes a partially relevant loss in molecular weight, which influences directly the mechanical properties. As reported re~ently, '~ tests have been carried out on an injection molding machine and on a cone-and-plate rheometer to study the influ- ence of temperature, shear, residence time, and moisture on molecular weight reduction and mechanical properties. It has been found that an interaction exists among these factors. Based on the results obtained, it has been concluded that it is possible to minimize molecular weight reduction.

I Design and Manufacturing

Molders are meeting the challenge of competing in the market with various manufacturing and marketing strategies. 16,17 The tendency of large medical molders is to seek partnerships or prefer- red-supplier relationships in which product development responsibilities and costs are shared with the end-users. The other tendency is to develop value-added services such as on-line packaging, since, due to the critical cleanliness requirement, the part does not Ieave the clean-room molding environment unless it is packaged.

Some companies compete by stressing their part design and tool-making capabilities. Regardless of their size, these companies rarely bid on blueprints

to fully design parts, but in fact most of their work derives from product development using 3-D computer-aided design (CAD) units and tool shops. However, as already mentioned,16 not all medical molders are in agreement with the philosophy of in-house tooling, but many molders maintain that toolmaking is an art distinct from molding. Hence, instead of investing in toolmaking, they would rather deal with mold shops.

Other companies com.pete by building new fa- cilities that would allow for expansion or production changes. A company producing a proprietary line of medical products, faced with a crowded, old, and inefficient facility, made a major move to ensure long-term productivity growth.'* The old plant was abandoned for a brand-new plant designed for maximum productivity. This risky but successful transformation has improved product flow and housekeeping and helped cut scrap to about zero. This business adventure, according to Hassell,I8 may serve as an example for other companies trying to ensure their survival in our trou- bled economic times.

USE OF HOT-RUNNER SYSTEMS In some cases, health care companies require

uniform, thin-walled products that can be molded in large volume to the tightest tolerances, but at a reasonable cost. To satisfy these requirements, hot-runner systems have been added to the molds. Their cost-effectiveness has become more evident in recent years, during which an increasing number of companies have specified them because of the promise of cycle time reduction, scrap material elimination, and overall energy cost savings. I9f2O

Economical hot-runner systems that satisfy even the most difficult medicallhealth care applications are now available. On the other hand, a poorly designed hot-runner system may cause several problems, especially when it does not allow for temperature uniformity, gate control, simplicity, and flexibility. In the case of small-part moIds with a high number of cavities, an advanced hot- runner design must be considered in order to meet additional design specifications: leakproof design, individual drop control, balanced fill for dimen- sional control of parts, and minimum center-line distance between drops. Therefore, by providing high reliability, simplicity, and reduced mainte- nance, an effective hot-runner design allows the molder to benefit from the economic advantages of runnerless molding.

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MOLDS AND MOLD MAKING The mold-making business ranges from very

small shops to very large organizations. The methods and capabilities of each shop may vary widely since some shops build tooling for hardware items using standard mold-making techniques and materials, while other shops build tooling for mi- crominiature molded products using advanced processes and high-grade materials. Therefore, as reported in the literature,21 medical product manufacturers should choose the appropriate mold- making shop based on its ability to meet their particular mold requirements rather than on cost alone.

Mold-making process phases, such as mold classifications, mold design, project planning, fabrication, and final assembly and testing, must be considered to help product designers, engineers, and procurement personnel make informed choices among suppliers when new molds are required. These methods and procedures, detailed elsewhere,21 reflect sound manufacturing practices and are considered standard procedures for the fabrication of high quality injection molds for high- technology products. These mold-making process procedures have been outlined to help the medical device industries understand the complexities involved in designing and building injection molds and decide on the type of mold-making source to use, based on the particular product requirements.

PREMATURE PART FAILURE In order for the performance of the injection

molded parts to be successful over the expected life, one must consider a number of factors to avoid premature part failure. A recent studyU has described how premature failure can be successfully designed out of parts and assemblies by giv- ing proper consideration to service conditions, material selection, structural design, residual stresses, molecular orientation, moldability, geometric features, mold design, environmental comparability, and finally, assembly methods and related stresses. Other design aspects, such as design for assembly, disassembly, and recyclability, have lately gained a lot of attention and need to be considered. One can get overwhelmed by all these design aspects if they are not kept in perspective and considered in order of importance. In addition, great caution is required during their application since overemphasis on some may result in oversight on others. As a general caution, paying

proper attention to the engineering design of plastic parts and assemblies can reduce unexpected part failures. A properly performed failure analysis can be used as an important tool for failure preven- tion.

I dean-Area Environment

The manufacture of injection moldings under clean-room conditions has been a big concern in recent years.16,23-26 Medical molders state that cleanliness standards are getting tighter, and customer (and regulatory authorities) expectations regarding documentation and process validation are getting higher.I6 As also reported, not all applications require a clean-room environment. The general standard is a Class 100,000 rating (i.e., no more than 100,000 particles of 0.5 microns in each cubic foot of air. On the other hand, molders who are planning to expand into medical applications should expect to spend considerable resources im- plementing cleanliness (a process that will add at least 5% to their overhead).

Injection molding machines should be installed in an environment with special air filtration units removing particulate matter and operators wear- ing caps and gowns. Hydraulic machines should have greaseless bushings and must be equipped with advanced microprocessor controls. Material- handling systems should be tailored so that boxes, pallets, and auxiliary equipment are not on the shop floor. Some companies include dedicated pneumatic conveying systems to reduce resin con- tamination, as well as constant monitoring of temperature and humidity in the molding environment. Other medical molders have installed their auxiliaries in underground tunnels (under the molding machines). In particular, there are injection molding machines which are built on slopes, placing the molding shop on the second floor and auxiliaries on the first floor. Both floors are accessi- ble to trucks and other necessary vehicles.

Choice of Injection Molding Machines

A consensus seems to exist among medical molders regarding the use of smaller-tonnage in-

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jection molding machines equipped with molds having a lower number of cavities as long as large production runs are not a concern. Companies have 90- to 500-ton machines using 32 cavity- molds are investigating the possibility of establish- ing a facility with small-tonnage machines which will use 1- to 4-cavity molds. With today’s market state, the philosophy of using low-number cavity molds is due to the fact that they are repeatable, make the final properties less likely to change from part to part, and are simple to handle (set-up/ change-over). ’This new tendency helps industries to be flexible and use this flexibility as a key to competitiveness.

The quality of molded plastic parts used in medical devices depends on the choice of material, accuracy of mold and part design, and control of processing variables as well as the quality of the injection molding machine. Since there are so many suppliers of injection molding machines, each one offering a variety of different models, selecting the most appropriate machine to meet the particular molding product requirements can be a difficult task.

Therefore, design features,27 such as the clamping unit, injection unit, programmable electronic controls, energy efficient design, and safety features should be considered before making the final choice. In medical device manufacturing, where molded plastic components must meet stringent quality requirements, the choice of an injection molding machine should be based on a thorough knowledge of machine design and its capabilities.

I Software Packages for Process Simulation

CHOOSING THE RIGHT COMPUTER PROGRAM Short molding cycles may be obtained by opti-

mizing the molded parts and molds on paper during the design stage, rather than on steel in the workshop. Thus, what is needed is a device for achieving optimum designs of parts and molds. Computer software packages for process simulation provide essential support for this procedure. The availability and performance of relevant programs have greatly improved lately. Companies, willing to incorporate in-plant simulation of the injection molding process, are facing the problem

of choosing the right program. Therefore, it is important to know how a computer program system can be designed optimally for the products of a particular company.

A summary of currently available software packages, their areas of application, and their performance characteristics has been reported recently.28 As pointed out, simulation programs can be clearly differentiated, partly in terms of their user-friendliness and computational accuracy, and partly in terms of their theoretical basis. Therefore, molding can evolve as a planar, 2-D model, or as a spatial layer or shell model. The current tendency is heading towards 3-D programs and 2.5-D mod- elling, where surface elements of constant wall thickness are involved.

The procedure for choosing the dedicated simulation software is outlined in terms of setting-up a requirements profile; provisional choice; definition of reference products; experimental determination of current data; simulations on reference products; comparison of results; final choice; creation of plans for stepwise introduction of system; and, finally, the introduction of the chosen system.28 Once the choice is made for the best suitable software based on this procedure, one still has to worry about how to use it successfully.

As reported recently,29 in order for any manu- facturer to successfully use a computer-aided engineering (CAE) simulation software product for plastic part, mold, or process design, one has to rely on customer-driven engineering, integration, and third generation analysis tools. The concept of customer-driven engineering consists of three pri- mary issues associated with the development of any new plastic product: (a) feasibility; (b) manu- facturability; and (c) performance.

Integration provides ”electronic dataways” that promote communication between various types of hardware and software across all types of organi- zational boundaries. In addition, with the current ”information superhighways” in North America, there is a possibility of sufficient capacity to allow engineers at different locations to simultaneously manipulate computer simulations and CAD models, which will make customer-driven engineering more accepted.

The third generation analysis consists of interac- tions between different analyses and inputs. For example, residual stresses calculated using this ap- proach are influenced by the material properties, processing conditions, and part geometry as well as process-induced anisotropy of thermo-mechani-

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cal properties and crystallization. The values of residual stresses are computed as a function of time and position and are used in structural analysis to predict shrinkage and warpage.

FINITE ELEMENT MESHING Mold filling analysis (MFA) using computer

software has attracted interest from a number of plastics industries. Regardless of software algo- rithms, the finite element mesh has always been the basis for MFA. The quality of the simulation results is directly affected by the quality of the mesh.

In a recent publ i~at ion,~~ subtleties of mid-plane models, evaluation of critical areas, selection of mesh density, and adaptive meshing techniques are factors that were discussed in the context of linear elements. Several rules of thumb were developed: processing time increases with the fourth power of node density; weld line accuracy is pro- portional to node density; contour accuracy is loosely coupled to node density; small curves (i.e., fillets) should have at least three nodes per 90" bend; rib height should mean height above the floor, not true mid-plane height; and, finally, extra elements should be added to tapered areas on the main flow path. This means that, by paying attention to the design of a finite element mesh for MFA, the accuracy of the results will increase and the processing time will be reduced.

I Statistical Methods

STATISTICAL PROCESS CONTROL Fine-tuning of tooling coupled with advanced

statistical process control (SPC) methods has en- abled top medical molders to achieve impressive accuracy levels (less than four defective parts per million). Some medical molders seek greater precision by developing SPC software packages that monitor each shot on the machine. The parameters are set for an acceptable part and are programmed into the controller. Then, the program monitors each shot to the preset profile. The machine is in- structed by the program to reject any part that does not meet molding specifications. l6

demonstrated how quality can be planned and optimized with the use of off-line technique^.^^,^ A practical application of the off-

A recent

line strategy was done on the injection molding process. The strategy for off-line process analysis consisted of two phases. The first phase consisted of determining the material, mold, machine, and quality characteristics, selecting the influencing factors and determining the variation range. The second phase included the statistical planning of experiments, the proof of significance, the determination of the optimum machine settings and, finally, the confirmation of the experiments. The connections established during the experiments were confirmed by the process, which means that the procedure can be applied in principle. Based on the data, it was possible to obtain target-value and robustness oriented machine settings [robustness oriented machine settings are "optimal" settings that make the operation less sensitive (and hence more tolerant) to process perturbations]. The study revealed that, by increasing robustness, the process reacted less sensitively to material fluc- tuations. In addition, the trials arrangement, con- sisting of external and internal experimentation, combines low expenditure with high value results.

In the more recent literature, a design of experiments was combined with inductive reasoning to determine the optimum molding conditions and the most influential variables governing the range of part weights of a stabilized process.34 This combination was demonstrated to be a dependable way to predict and improve an injection molding cycle and, regardless of its prediction ability, it can be considered as a valid method for quality im- provement.

Another recent implemented an algorithm that runs in real-time and provides immedi- ate feedback to a process operator about the current operating point. The algorithm was applied to to the on-line tuning of process inputs for the plas- ticating and injection phases of an injection molding process. Quality criteria were defined based on the injection cycle time, and the presence of both flashing and underfill. The quality control problem was defined as a multiobjective optimization problem. This formulation allowed the simultaneous incorporation of many different quality factors and provided for decision-making on the evaluation of unmeasurable quality aspects.

SPC techniques have also been used to identify experimental procedures to systematically adjust the processing conditions so that injectable parts can be produced with a quality closest to desirable nominal values.36 The second-order response surface model developed was used to control the

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thickness deviation of the injectable parts and proved to be appropriate and effective. By follow- ing the procedure to run the machine at the best settings, the quality of injectable parts can be sig- nificantly and economically improved, before spending money to change or add any expensive machine equipment.

DESIGN OF EXPERIMENTS Designed experiments can also be used to ob-

tain the maximum amount of information from a minimum number of experiments. The design process was recently used in preventing failure of injection molded medical devices.37 Injection molded medical products, while sharing many material and process requirements with other injection molding applications, differ in significant aspects. After molding, the article is often subjected to washing or cleaning steps, dried at elevated temperatures and assembled into a final product by any of a variety of techniques from solvent bond- ing to ultrasonic sealing. A terminal sterilization with either ethylene oxide or radiation is applied to the final product before it leaves the plant. Finally, shipping, warehousing, and shelf-life conditions at various temperatures and humidity levels are experienced by the product before it is used. In order to ensure failure-free operation, one has to anticipate how all these conditions interact with the product itself.

The use of designed experiments had significant success for systematically quantifying the depen- dence of product performance properties on molding condition^.^^ Specifically, the proposal that all failures of injection molded medical plastics are preventable was supported. The conclusion was that carefully designed experiments using statistical methodology (e.g., see Reference 38) can maxi- mize process understanding while reducing the to- tal number of experiments.

Modern Injection Molding Processes

In addition to thermoplastics and thermosets, the injection molding process is being extended to rubbers, polymerkeramics, and polymer/pow- dered metals systems. Lately, innovative injection molding processes have been developed to pro-

duce parts with special features which cannot be achieved using the conventional methods. An overview of the latest innovations in injection molding was recently reported and discussed with an emphasis on computer simulations of these processes.39

CO-INJECTION MOLDING The co-injection molding process, which pro-

duces parts with a sandwich configuration, offers the inherent flexibility of exploiting the optimal properties of each polymer to reduce the material cost, injection pressure, clamping tonnage, residual stresses, and/or to modify the properties of the molded part. This process has the advantage of using low-cost or recycled plastics40 as the core material (invisibly) sandwiched within thin, decora- tive, and more expensive skin virgin plastic material. Therefore, in view of the increasing ecological and economic pressures for recycling plastic materials, it seems like a promising process. Research on the co-injection process has been intensive in the automotive, electronics, packaging, and furni- ture i n d ~ s t r i e s . ~ ~ , ~ ~

GAS-ASSISTED INJECTION MOLDlNG The gas-assisted injection molding process con-

sists of a partial or nearly full injection of polymer melt followed by an injection of compressed gas into the core of the melt to assist the filling and packing of the cavity. The gas would normally hol- low out a network of predesigned, thick-section gas channels. This allows an effective transmission of pressure by the gas and therefore a fairly uniform pressure distribution as the gas penetrates the part extremeties. Therefore, the gas pressure required to fill the mold cavity is lower than the required entrance pressure for conventional injection molding. This process has drawn considerable attention recently due to its potential to produce stronger, yet more complex, parts with less stress, better surface finish, and smaller sprue

APPLICATIONS For specific medical applications, co-injection

and gas-assisted injection molding would be promising processes for minimizing raw material cost and operating energy and for respecting environmental regulations. The gas-assisted injection

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molding process has the potential to produce rigid parts incorporating both thick and thin sections with fewer residual stresses (i.e., less warpage) and better surface finish than conventional injection molding. In addition, it offers more design freedom and cost savings with respect to weight reduction, lower tooling, and press capacity requirements. However, since the innovative gas-assisted injection molding process involves a dy- namic interaction between polymer melt and gas flowing in complex cavities, and because of the lack of engineering know-how, the product, tool, and process designs for this process are quite com- plicated and less well-understood.

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Injection molding of medical plastics: A review