Engineering/Medical Grades (3)
Engineering and medical grade resins can pose unique challenges in the injection molding process. These entries highlight the trials and successes we've experienced while producing parts from these demanding materials. Contributed by various employees of Matrix Tooling, Inc. & Matrix Plastic Products.
In addition to building injection molds for medical applications, Matrix specializes in injection molding with a wide range of medical grade polymers. Among these materials are bioabsorbable resins for surgical implants which we process in our ISO -8 / Class 100,000 cleanroom. Examples include PLA, PLG, PLC, PDO, PDLG, and PG. These dissolving resins have very unique (and sometimes challenging) requirements, and several years of experience have taught us how to process them successfully. These bioabsorbable materials represent the majority of our cleanroom production at Matrix.
Recently we were awarded a medical program that involved processing an implantable-grade PEEK polymer . Due to its high-strength, biocompatibility, and high temperature & chemical resistance, PEEK is being specified for an increasing variety of long-term implantable medical device applications. While Matrix has been building PEEK tooling and molding PEEK in production for many years, this was the first time we would be processing PEEK in our cleanroom.
Over the years, we have designed our PEEK tooling to utilize either hot oil circulation or electric cartridge heat. In our experience we have found that hot oil provides more consistent heating than electric heaters; and while some PEEK parts can be processed using either heat source, many critical parts definitely require hot oil. In the past, we always had the flexibility to pick and choose where to run these jobs: on our main floor or in the cleanroom. Now, however, we had an implant that, due to customer requirements, had to be processed in the cleanroom. At the same time, it required hot oil heating. We had a dilemma: “how do we use hot oil in our cleanroom?”
Those responsible for overseeing our cleanroom operations would not consider using oil in the room, so we began our search for temperature control alternatives. The solution was the purchase of a Single Temperature Controls high-pressure hot water thermolator capable of reaching 430° F. We're in the process of making a few design tweaks to strengthen the final part which is still being qualified, but the thermolator has been tested several times in samplings and we are prepared for the production phase of this project.
Medical grades of biodegradable/bioabsorbable PLA, PLGA, and PLG resins can sell for upwards of $1,500 USD per pound. This and the delicate nature of biopolymers can send shivers down the spine of any injection molder. Really though, the standard precautions and best practices of any high level molding operation familiar with running engineering-grade resins similarly apply to the processing of these bioabsorbable resins. Every aspect of the manufacturing process must be closely monitored, and proper procedures strictly adhered to.
Mold construction must take into account the rheology or flow characteristics of the resin , shrinkage , and venting requirements. Due to the relative infancy of the bio-materials there is little data available from the resin makers, so all mold construction should be steel safe. Selection of mold material should keep corrosion resistance in mind.
Cleanliness, of course, is paramount. There can be no contamination present in an implantable device. Purging an injection unit after a run with a different resin, no matter how thorough, is not enough. The screw must be pulled and all residue removed from the screw and barrel physically. Don’t use copper gauze (here we differ slightly from the standard molding procedures) for fear of fragmenting and heavy metal contamination. A plastic abrasive cloth would be better, and the unit should be wiped, dusted, and blown or vacuumed free of any minute debris after cleaning. Having a dedicated injection unit is a better option, with a dedicated molding machine, dryer, and loading system being the best case scenario.
The molding machine really is not specialized. Usually a general -purpose screw is fine with about a 3:1 compression ratio and 20:1 length over diameter ratio (again, a general – purpose unit). Clearances should be tight in the injection unit, and the check ring non-return valve should be in good shape. Bioresins are not very heat or shear stable, so sizing the barrel capacity of the machine to the intended shot size is very critical, and high compression screws are not advisable. Since the resins are so costly, any scrap will immediately affect the bottom line. Savvy molders start and stop the run with a “commodity” bioresin with similar flow characteristics.
PLA/PLGA/PLG for implantables, like most bioresins, are slow to give up their heat and also can be slow to obtain optimum crystallization, so cooling times and pack/hold times can be quite long which lead to longer cycle times. Here, corners can’t be cut by running a colder mold in quest of faster cycles because you risk endangering the end characteristics of the implant.
In addition to shear and heat degradation, bioabsorbable resins are susceptible to hydrolytic degradation. Careful drying is important, with drying in the cleanroom next to the molding machine being advisable. Minimum out-of-dryer/in-molding-machine hopper time is also essential. Like many engineering resins, bioabsorbable resins must be dried and maintained at or below 250ppm (0.025%) moisture. A moisture analyzer is mandatory.
It is important to fast-track the process building and optimization portion of any injection project for PLA implants, if only due to the resin costs. One tool for this is DOE, or Design of Experiments. Here again, process building with a commodity PLA resin and final tuning with the PLA implantable resin is advised.
The implantable device must be made, handled, and packaged under the strictest of cleanroom conditions. Packaging must protect the implantable and be hermetic to keep it from picking up ambient moisture. Vacuum packing with laminated foil pouches is an option, and inclusion of a desiccant in the master carton is also popular. Sealing options include vacuum and heat sealing. Finally, sterilization methods and their impact on product end characteristics must be cautiously considered.
Care must be taken with these expensive and fragile resins; but the discipline is good for a molder, and the results for following good methods and procedures can be quite rewarding.
Brent G. Borgerson
Senior Process Engineer (Older Molder)
We recently had been asked by a potential customer why a polycarbonate would crack post-molding. They had been having this issue on a specific part from one of their current suppliers.
Our first step was to ask if we could get a sample of the part and the process sheet. After looking at the part and reviewing the process sheets we noticed the following: First, key set points like the dryer settings were not included in the process sheets. We saw this as a potential red flag. With polycarbonate it is very important that the material be dried correctly with the proper equipment. Polycarbonate requires a dryer setting around 240 degrees for four hours (following the material recommendations of course, some may vary around 250 degrees for four hours) but doing this requires a high-heat dryer. It is always good to verify that the moisture is 0.020% or less prior to molding.
Further looking into the setup sheets we noticed that the injection pressures were all on the high side of the recommended range. This can be a sign that the gate size or nozzle orifice may be a potential suspect. Running the incorrect gate size or nozzle size can induce molded-in stress.
We also noticed a lack of process monitoring; the set limits would allow the press to continue to run outside the manufacturer’s recommendations. If uncontrolled, incorrect barrel temps, pressures or screw cushion can all be reasons for in-molded stress.
In looking at the part, the molded stresses were obvious, particularly when looking through a polarized lens under strong lighting. The stresses create a rainbow effect in the translucent material. Our next step was to measure the gate size and we found it to be much smaller than what we would recommend for PC.
So we had plenty to consider from the start, and these are just a few possible reasons for PC cracking. We’ve also been told by the material manufacturer that some mold release sprays can attack polycarbonate. They even had a story about an operator whose hand lotion was found to be the culprit for cracking parts. This is one reason we do not allow silicone mold release in the plant and insure our operators use gloves on polycarbonate jobs.
After ruling out all of the above possibilities, it’s possible that some part designs may require annealing for stress relief. Annealing of the plastic part is the process of heating the post molded part to just below its softening point, then keeping it at the high temperature for a period of time before cooling it slowly back to room temperature. This can relieve some molded-in stresses but isn’t a desirable solution in most cases.
Processing polycarbonate at the manufacturer’s recommendations is the key to stress-free and crack-resistant parts. If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots.
Molding Operations Manager