Micro Molding Using Bio-Resorbable Polymers
8/10/12: Introduction
Micro Molding in bio-resorbable PLA and PGA materials creates challenges with small gate sizes for these highly shear sensitive and highly moisture sensitive materials. This article dives into some of these challenges such as shear stress through small gates, humidity control for extremely small shot sizes, and integrating macro to micro technologies to produce near micron level geometry in precision, micro molded bio-resorbable components. (https://www.microengineeringsolutions.com/process/micro-molding/ )
As implantable and bio-resorbable molded parts approach micro or nano in size, even a toothpick-sized runner is still too large and too costly. Bio-resorbable raw material costs are between $3,000-$22,000/lb. Within this article is a case study for micro molding with bio-resorbable polymers creating an instant return on investment for molding with net zero material losses in runner and sprue scrap.
Definitions
MICRO MOLDING: Although there is no standard definition of micromolded components in the industry, most micro manufactured components have one or more of the following attributes:
- Fractions of a plastic pellet or weighing fractions of a gram
- Having wall thickness of less than .005”(0.127mm)
- Having tolerances of .0001” to .0002”(0.0025 to .0050mm)
- Having geometry seen only by use of a microscope
BIO- RESORBABLE MATERIAL: Bi-resorbable polymers have been on the market for over 20 years. These polymers are typically PLA based (Poly Lactic Acid) or milk based. They are commonly compounded with PGA called PLA/PGA compounds (Lactide/glycolide). These materials are used in implantable applications when the device is only needed in vitro short term.
INTRINSIC VISCOSITY: Most polymer processing uses melt flow index as an indicator for processability. With BIO-resorbable polymers, an IV (Intrinsic Viscosity) test is used to determine the characterization of the polymer as it relates to Molecular Weight, processability, and in vitro stability. IV is a measure of a polymer’s capability to enhance the viscosity of the solution it lives in. It is important to find the viscosity at different concentrations and extrapolate to zero concentration.
Bio-Resorbable Applications
Figure 1.1 shows common applications for Bio-Resorbable micro molded components. In this life cycle curve, most of the work being done is in the new product area, R&D developments, drug-eluding products, and implants being used as pharmaceutical “carriers”. Growth products in the hundreds of thousands of parts annually are bone screws, anchors, and swallowable pills. Bone screws typically made of Titanium can be replaced with bio-resorbable materials so that patients are not “stuck” with that bone screw for the rest of their lives. The resorbable bone screw, unlike the titanium one, after an approved amount of time, the bone fuses together and no longer required the screw, and the resorbable material then gets absorbed by the body and turns into carbon dioxide and water and is flushed from the human system naturally.
Mature products is also a very busy segment due to what is called “a work around method” whereby conventional molders may be putting micro components into large mold frames and conventionally-sized molding machines. These programs are almost always an immediate return on investment to re-tool due to the runners and sprues being so very costly in material scrap they generate.
Figure 1.1 MES Bio-Resorbable Market Life Cycle Chart
High volume (Growth) bio-resorbable products are facial implants from reconstructive surgery for aesthetic reasons or accident/disfiguring reasons. Additional high volume products (100k+) are seen in bone screws and sutureless devices such as resorbable sutures, anchors, and staples. (see Figure 1.2)
Figure 1.2 Bio-Resorbable Polymer Applications (provided by MES)
Processing Challenges of Bio-Resorbables
There are many different compounds of PLA/PGA. Most common is the 82/18 version (82% lactide, 18% glycolide). A very high concentration of glycolide creates material handling and feeding difficulties due to “gooey” nature of the glycolide.
An abundance of information can be found using several of the bio-resorbable polymer suppliers (Purac, Boehringer Ingelheim, Lakeshore, DSM to name a few). When it comes to micro molding, however little or no information is available on the market due to proprietary processing techniques and lack of industry-specific testing for custom compounds.
The first challenge encountered by processors of bio-resorbables is material handling, which is the single largest area for error. PLA/PGA materials are highly susceptible to moisture and heat. They must be stored properly (usually in a freezer) at a specified temperature in nitrogen-sealed foil pouches. They must then be used according to the processing run quantities needed and material drying cycle. The material useage must be matched with the injection screw and shot size in an injection molding machine so that the material is not sitting in an improperly sized machine where over-drying and over-heating can occur due to the prolonged temperature and drying exposure.
Mold Design
A mold design with a properly sized gate and a very small runner and sprue (if any) is critical to product quality and cost. Bio-resorbable raw material costs are between $3,000-$22,000/lb and even a toothpick-sized runner and sprue adds up to hundreds of thousands of dollars of scrap annually.
Mold venting is also important as clogged vents will also degrade the polymer prematurely and cause burning of the implant during processing.
If gates and runners are used, a micro molded part is typically easier to degate using an edge gate vs. a sub gate. Degating methods such as ultrasonic degating, tiny knives in a fixture, or in-mold degating are used to avoid this effect. If the bio-resorbable implant is in direct contact with skin or arteries, even a very small gate vestige is detrimental to the implantable application as that picky piece can pierce an artery or vein. If the wall thickness allows, cutting into the part vs. leaving a small vestige is better than chancing the vestige. (+0.00/-0.05mm is typical)
PROCESSING
Micro molding machines are also a key component to processing bio-resorbable polymers. Because they are highly shear and heat sensitive, proper fit of the shot size for a micro molded part to the screw and barrel is critical. The residence time (time the polymer sits in the barrel) can affect the IV (Intrinsic Viscosity) of the material. Small shot sized machines are typical in the design of micro molding machines. Some machines used reciprocating screws and some use screw over plunger technology. Some other machines are being developed by processors of bio-resorbable materials because even the smallest shot sizes available on the market are too large to properly process small amounts of bio-resorbable polymers. These machines are typically proprietary and primarily uses internally or through licensing agreements.
Many challenges exist in micromolding but there are ways to minimize these challenges and corresponding risk of failure to component manufacturers. These challenges include:
Modeling of Micro Components – There remains a limited understanding of the fundamental physics at the micro scale, which are necessary to develop reliable models. Perfecting the mesh is critical to obtaining the correct result in any analysis. Because micro parts have such small features (and therefore very large solid model sizes), painstaking processes in meshing a very high resolution model is key to an accurate analysis.
Environment – As a fraction of one single degree of temperature change can affect precision when machining (or measuring) at the submicron level, many micromolders and micro machining experts enclose the entire machine and/or inspection area in order to create a controlled working environment.
Metrology/Inspection Techniques – Inspection techniques in measuring very small micromolded parts requires customized vises, tweezers, and fixturing (not to mention extreme patience). Inspecting steel measurements usually provides a flat, robust surface that can be measured with non-contact means or in some cases contact measurement. These same surfaces that make the molded components can be used to “certify” the dimensions much closer in repeatability and reproducibility than attempting the same corresponding measurement in the micromolded components. It’s not uncommon for the first article inspection to consume as much time if not more time than the entire micro moldmaking and micromolding project combined. One of the latest time saving techniques in this area, however is laser scanning of the micro part which scans the part, turns it into a point cloud data, and that data can then be directly file compared to the nominal solid model to see visually in color where problem dimensions exist.
Gage R&R from client to vendor requires duplicate fixtures and exact methods of inspection technique to repeat the results to near micron tolerances. Only a select few sources of inspection equipment exist that are capable of measuring to sub-micron tolerances and extremely clean and hepa-filtered, air controlled rooms are necessary to the environment needed for repeatable measurements.
It’s also common in macro components and specifically with medical devices to insist on 1.33 Cpk or better with respect to performance to drawing dimensions or tolerance. 1.33 Cpk on .0001” tolerances requires nearly a mathematical impossibility in some cases when the gage R&R and operator R&R are taken into account. Component manufacturers and micromolders require similar inspection machines with identical fixtures to validate tolerances in micro components.
Properly sized machines – It’s very common to see micromolded components that have sprue and runner systems amount to 75% or more of the total shot. For many molders trying to enter this market, micromolding parts in larger machines is commonly attempted. Molding parts in this manner is not recommended on machines larger than 0.5 ounce because it is hard to control such small shot sizes. Also, long residence times and material degradation would occur with oversize screw and barrel combinations. Tabletop machines are not considered good candidates for micromolding, as they are not usually designed for high-volume production and process control capability.
Part Handling/Static – Part handling can be challenging given the sizes of micromolded components. Many micromolders use edge-gated runners to carry their parts from one location to another and many are used as part of the automation process. If parts cannot be edge-gated, customized end of arm tooling, vacuum systems, reel-to-reel take-up equipment and blister packs are utilized accordingly.
Static electricity is a micromolders nightmare. Parts as small as dust can easily be lost if proper grounding of part collection systems, robotics, packaging, and inspection systems are not performed. Static guns, wands, air curtains, and grounding mats are commonplace in micromolding facilities.
MATERIALS TESTING
In order to determine if the bio-resorbable implant is robust enough, it is important to characterize the material during many different phases in the injection molding process. For example, PLA/PGA pellets in their raw form are stored in nitrogen sealed pouches. Opening this pouch and exposing the polymer to a small degree of temperature and humidity starts to degrade the polymer immediately. (as if it were in the body already and starting to do its job).
Consequently PLA/PGA compounds must be dried in nitrogen sealed hoppers in most cases and IV’s must be validated throughout the injection molding process. Temperature from shear in the injection molding screw and barrel temperatures also decrease the materials IV. Additional shear from small mold gates are also a source for decreasing the IV. Once the PLA/PGA component is molded, however it has a protective skin around the molded part and can be left for a small period of time outside of a nitrogen-sealed environment.
VALIDATION:
Due to the nature of bio-resorbable polymers and their use in implantable devices, they are often processed in a classified cleanroom and be validated using ISO 9001 and/or under ISO 13485 quality systems.
Throughout the molding process, Intrinsic Viscosity values should be validated. Samples should be taken from the bag, after the drying process, after the molding cycle by testing both the runner and the part to compare shear effects through the gates, and after a period of time in the package, and through different temperatures for shelf life tests.
This testing will insure the validity of the implant throughout its living cycle in vitro for properly form, fit and function of the implantable device.
DESIGN OF EXPERIMENT TESTING
By the time a 4.0 IV material is processed, rapid deterioration of polymer properties can take place. If improperly processed, the material will “act” improperly in vitro and cause an implant to resorb prematurely. There are tools available to test the impact of processing conditions on PLA/PGA materials. One of these tools is a gate shear test and tensile bar test shown in Figure 1.4. The gate shear test is 8 cavities with varying wall thickness from .002” to .009” (0.05-0.23mm) and the gate is always 75% of the wall thickness. The varying gates will simulate varying shear on the PLA/PGA materials. These “coupons” are then tested for IV loss and simulate what happens to a particular compound before an expensive shaped mold is built using a similar gate size. The tensile bar, an ASTM standard for micro molding, can be used to test tensile properties of a given wall thickness.
Figure 1.4 Spiral Flow Test and Micro Tensile Test Bars
CASE STUDY
Mature products, although not truly mature in time years, but mature in cost and depreciation are seen when conventional injection molding methods are used to create the molded parts. This is typical when using 40Ton-80Ton presses which require larger surface area molds to fit in them. The material loss due to large paths in sprues and runners are extremely costly for $3,000/lb materials. These mature products always return on their initial investment to transfer to a micro molding sized mold and return even quicker with a runnerless mold.
The following example in Figure 1.5 shows a typical ROI for what is referred to as a “work around” conventional method to a micro molding method. $4,000,000.00 in cost savings is realized when changing from a toothpick-sized runner and sprue to a runnerless bio-resorbable mold. It would take just one molding run to pay for the capital of micro runnerless tooling/molding machine. This is a Controller’s dream and what is commonly referred to in the industry as a “no-brainer”.
Figure 1.5-Typical Bio-Resorbable Product ROI- Conventional molding vs. Micro molding approach
Conclusion
In conclusion, there are critical steps required to handling, testing, processing, and validating bio-resorbable molded components. Material characterization throughout the molding process is critical to understand what happens to the Intrinsic Viscosity of bio-resorbable polymers such that when they are in the body, they will not prematurely resorb OR stay too long for the implant to properly function.
As is usually the case in micro molding, this type of processing requires specialized equipment, design, and validation expertise. Choosing the right supplier, and one that is experienced in bio-resorbable processing and micro feature generation can create a faster path to success.
Donna Bibber is a Plastics Engineer with 25 years of experience in micro manufacturing. Ms. Bibber has written and lectured hundreds of technical papers on micro and ultra-precision manufacturing associated topics worldwide and was voted onto the List of 100 Notable People in Medical Devices.
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