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

Bioplastics, aside from being derived from renewable resources, have the advantage of not releasing harmful toxins during their production, processing or degradation. Many conventional plastics can release known or suspected carcinogens such as formaldehyde or benzene during production, processing or destruction.

Growing the sources for bioplastics also reduces carbon dioxide in our atmosphere. Since the production of conventional plastics produces so much CO2 the use of bioplastics in place of a conventional plastic has a cumulative effect, with the substitution of just one ton of bio for conventional plastic having the net effect of reducing multiple tons of CO2 in the atmosphere. This not only takes into account the production methods for each type of plastic, but also the photosynthesis process in growing biomass or raw material for bioplastics. Bioplastics show great promise in reducing both our industry’s carbon footprint and impact on rising global warming.

What can plastics processors do until bioplastics are perfected in properties and reduced enough in costs to truly compete on a large scale with conventional thermoplastics? This is where the 3 R’s apply in injection molding. The 3 R’s in molding don’t stand for “reading, riting and ‘rithmetic,” but rather: reduce, reuse, and recycle. At Matrix Tooling / Matrix Plastic Products, we have been molding with bioresins, including bioabsorbables for a number of years, but as responsible members of the environmental community, we also have been practicing the 3 R’s.

Reduce: Scrap (and resin usage) is reduced through cold runner and sprue size reduction where possible without affecting moldability. In many cases we have reduced sprues and runners to the prescribed percentage of regrind allowed in the product specification. Hot runners and hot sprue bushings also have been used wherever possible. We have also thinned out wall stocks on parts where the product integrity wouldn’t suffer.

Reuse: We reuse what regrind we can and have come up with applications to use up to 100% in-house regrind. We utilize returnable/reusable packaging where possible and where allowed by the customer. We also have a closed circuit water system to reduce consumption and also filter, monitor and analyze hydraulic oil to avoid indiscriminate unneeded oil changes.

Recycle: Where we can’t reuse in-house regrind, we try to find it a good home. We sell the regrind where possible or even give it away for free if it can be used but there isn’t a paying market. Packaging is recycled also. We even collect our soda pop cans!

Matrix is serious about being environmentally responsible, using bioresins, and abiding by the 3 R’s. It not only makes environmental sense, but favorably affects the bottom line.

Written By:

Brent Borgerson

Senior Process Engineer (Older Molder)