Drying is an important part of the process for any product made of hygroscopic (meaning affinity for moisture) thermoplastic.   For medical implants made of bioabsorbable polymers, dryness is particularly critical.  Inadequate drying can produce a variety of problematic results.  These include:  lack of tensile properties and impact resistance, as well as varying flow characteristics.

Bioresins, much like other hygroscopic thermoplastic resins, can suffer three types (or a combination of these three types) of degradation:  thermal, mechanical or hydrolytic. In most thermoplastics these types of degradation occur chiefly during the molding process. With bioresins such as the PLA, PLG , and PGA families, hydrolytic degradation also occurs before and after the molding process.

An implantable device must decay or degrade in the body as part of the absorption process. Different materials and part designs have different rates of degradation in the body (where it is in a moist environment).  The rate of degradation and retention of mechanical properties is affected in no small degree by the way the resin was dried and how the dried resin and finished part were handled.

If a bioresin grocery bag degrades quicker than it was designed to, the results can be the bottom falling out and groceries on the ground. If an implantable device degrades quicker (or slower) than designed to, the results can be harmful to the patient. The degradation process of the implant is key to resorption in the body.

Run of the mill dryers are generally not sufficient to control the moisture as well or reach the super-low moisture levels desired for absorbable implants. Many implant molders opt for vacuum dryers or compressed air with membrane dryers.  Since most implants are small, vacuum ovens designed for lab use is another option for resin drying.  In any case, the drying schedule and temperatures provided by the resin manufacturer must be strictly followed.

In many cases, the resins must be dried to less than 0.02% (200 ppm) and the resin and finished product must be maintained dry. This requirement mandates an inert gas such as nitrogen atmosphere in any non-vacuum dryer hopper, humidity controlled atmosphere in the cleanroom, vacuum packing with a desiccant and nitrogen, and refrigerated storage of the resin prior to drying.

It is not enough to strictly follow the drying and handling procedure, the resin dryness must be well tested, documented, and controlled.  The dryness data is so important because it must be correlated with part degradation data to be able to predict implant device performance and absorption in the body.  Lost weight or halogen type moisture analyzers are relatively economical devices but should be equipped with data acquisition and logging technology.

Drying bioabsorbable resins requires specialized knowledge, methods, and equipment, but is key to successful bioresin implant molding.

Drying engineering resins is crucial to obtaining desirable end products with these high–performance and often expensive resins. Thermoplastic resins are being called on to be as strong as metal and to survive in harsh environments. To achieve these end properties, a resin must be processed correctly, and one area of proper processing is to ensure that the resin is molded at or under the manufacturer’s specified maximum moisture content (%).

At Matrix Plastics Products, we are very careful (almost to the point of being neurotic) about our resin drying and dryness assurance procedures. We take a multi-pronged approach to these issues including some of the techniques and procedures as follows:

  • •  Drying Time: We follow the manufacturers’ recommendations as a minimum for drying time before beginning molding as well as residence time in the dryer. These steps are carefully documented for accountability.
  • •  Drying Temperature: Again, resin makers’ guidelines are strictly followed.
  • •  Dew Point Monitoring of the Dryer: Our dryers feature dew point monitors and alarms which are consistently observed. The dew point on a dryer is the best indication of the proper function of the dryer, which allows us to foresee many impending problems.
  • •  Moisture Analyzer: Our Quality Inspection lab features an OMNIMARK Mark IV moisture analyzer which can be used to test and verify results. This is the last line of defense and is used whenever there is any doubt about the dryness of a resin. In the case of sensitive jobs, moisture analyzing test are routinely used and documented.

A part molded with wet resin (moisture content above the manufacturer’s suggested max percentage) may not be a cosmetically unappealing part, but it is almost always a structurally weak part. Hydrolysis – the result of heating moist resin – produces an action in the resin that is essentially akin to thermal degradation. The molecular structure and integrity are affected, and a weak and/or brittle part is the result. Some of these problems are not always readily detectable, especially during the early life of the product, but premature and unexpected failures can result from molding with “less-than-dry” resin. We try our best to avoid this situation.

Written By:

Brent Borgerson
Senior Process Engineer (Older Molder)

Pat Collins
Molding Operations Manager

 

The thought of processing PEEK (polyetheretherketone) or other high-temp resins can send nervous tremors through many a molder’s body. I know, as a molder who learned the craft on a steady diet of PP and PE closures with their low melt temperatures and cold molds, my first PEEK experience made me edgy to say the least. But I’ve since come to realize that PEEK is just another thermoplastic resin and, like the others, can be molded safely and efficiently with just a few precautions.

PEEK is widely believed to be one of the highest performing thermoplastics on the market and its end properties more than justify any trials and tribulations you may encounter processing it. PEEK is a linear aromatic, semi-crystalline thermoplastic having excellent wear, chemical and hydrolysis resistance. It has very low flame/smoke toxicity and excellent electrical properties that preclude the need for additives in many cases.

PEEK processes at a high melt temperature nearing 720°F, and both the press barrel and controls must be capable of this. On many molding machines the high heat software is an option and I recommend ceramic high-temp heat bands whenever possible. A special screw and barrel are generally not needed, but consider hard units if running filled PEEK resins. We typically use sliding ring non-return valves, GP or Eliminator™ tips and don’t recommend ball checks or shutoff nozzles.

A hot mold is the key to achieving crystallinity in PEEK parts. Purging PEEK allows you to see the color change from a translucent to a solid colored crystalline state. If the mold is too “cold” (i.e. not hot enough) the parts will have that discoloration or partial translucency, and the quality of the end product will be compromised. The mold, in most cases, must be between 350°F and 450°F. This is steel temperature and requires oil or cartridge heat to maintain this level. Complex parts may require better temperature control so oil would be the preferred option. We also recommend the use of thermocouples to verify and monitor the steel temperature.

These molds must be specifically designed to run high-temp materials with draft, finish, undercuts and steel types all factored in from the beginning. Insulator plates between press platen and mold clamp plates are a must. The preferred steel type would depend on whether or not the resin uses any abrasive fillers but should have a minimum hardness of 52-54 Rc.

The resin also must be very dry to process well and achieve the desired end properties. This means that the resin must be at 0.02% moisture or below. We typically recommend drying the resin at 300°F for at least 3 hours. We also suggest the use of a moisture analyzer to assure dryness.

PEEK can be quite costly, but you should be able to use 30% dry first-pass regrind with unfilled PEEK and 10% with filled PEEK.

Safety should be a primary consideration when molding PEEK, both for purging and while working with the mold. Wear safety glasses and/or a face shield, Kevlar or Kevlar/stainless steel sleeves, and heavy cotton cloves when purging and reaching into the mold.

When preparing for your PEEK experience, research it well with your resin supplier. The above information is based on our experience, but it should used as a reference only. Also, make sure you don’t neglect recognized scientific principles when working with any thermoplastic material. With a bit of common molding sense, your PEEK experience can and should be a rewarding one.

Matrix Tooling, Inc. & Matrix Plastics Products has a great deal of mold design, building, as well as processing experience with PEEK resin, much of it in the medical field.

Posted by:

Brent G. Borgerson
Senior Process Engineer (Older Molder)

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