Engineering/Medical Grades

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.

Written By:

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)