Not all HIPS is created equal, either

I managed to get a decent pin vise at last and drilled out the support nozzle with a 0.011″ drill.  The other day I got around to testing the cleared nozzle to determine if buildup/blockage might be the root cause of my difficulties in running the high-impact polystyrene from New Image Plastics.  I powered up the Stratasys, let the support nozzle come up to temperature, and started feeding it the NIP HIPS.  I noted that the extruded plastic was still curling right around and contacting the exterior of the nozzle – re-boring a 0.012″ nozzle with a 0.011″ drill bit apparently does zilch in this regard (makes sense, as I wasn’t actually removing any metal), but at least I could be fairly sure that any possible blockage was gone.

After perhaps 2 inches of feedstock, the HIPS filament buckled once again between the feed rollers and the liquifier entrance.  I struggled with this for some time, and found that increasing the support temperature to 275° C (10 degrees higher than the normal setpoint) and reducing the feed to 60% (normally at 100%) improved things greatly.  Improved, but not fixed – I could still expect the HIPS to kink very roughly once in every 18 inches or so.  Additionally, I noted very faint wisps of smoke emerging from the nozzle area – I’m pretty sure I was witnessing outgassing of the HIPS as it emerged hot out of the nozzle.  I had never seen this with the OEM material.

I had a little bit of OEM support material left and figured I should try running it to see if it would also buckle – perhaps I still had some sort of buildup inside the liquifier, and maybe the OEM material would act the same way.  I dialed the temperature back down to the OEM setting and bumped the feed back up to 100%.  It ran just as happily as it had done prior to my mad experimentation with alternate materials.  With that in mind, I started to do some light research on what exactly high-impact polystyrene consists of.  Styrene, as Wikipedia points out, is an oily liquid – many styrene molecules need to be linked together in order to create polystyrene, which is the clear plastic used for car windshields and airplane canopies in plastic model kits (modelers hardly ever say ‘polystyrene’, referring to such material as simply ‘styrene’) .  Polystyrene (sometimes abbreviated as GPPS, for General Purpose Polystrene) is itself rather brittle, so to toughen it up, a bit of polybutadiene rubber can be added to improve the impact resistance and we get High Impact Polystyrene (HIPS).  The polybutadiene ruins the transparency of the polystyrene, so uncolored HIPS is a milky white color.  Perhaps this particular HIPS had enough polybutadiene in it that made it buckle more easily than the OEM material?  A brief side note – there are apparently a LOT of ways to mix styrene and butadiene.  I discovered SBS (styrene-butadiene-styrene), which Chris DeArmitt did an excellent job of contrasting with HIPS.  There is also SBR rubber which shares the same CAS # (9003-55-8) as HIPS, even though the two materials seem quite dissimilar (I am not a chemist, so this may all make perfect sense if one is versed in the ways of polymer science). And if that isn’t confusing enough, have a dose of CAS # 91261-65-3, styrene-butadiene block copolymer. Yeah, my eyes glazed over long ago as well, and now my brain hurts.

In looking through various MSDS sheets for different grades of HIPS, I noticed that many formulations contain a few percent of mineral oil.  I wondered if perhaps vaporization of the oil might account for the faint smoke I was seeing – the 265° C nozzle is hot enough to boil some grades of mineral oil.  Once again, NIP was kind enough to provide me with material specs for their HIPS, which turns out to be Styron 487.  The MSDS sheet indicates that less than 5% of the composition is not in fact mineral oil, but is a ‘copolymer mixture’ marked as being a trade secret. Furthermore, the sheet only cautions against temperatures over 300° C as causing decomposition.

I went back to playing with running filament through the Stratasys, remembering that the machine has a torque monitor setting.  If the filament jams in the extruder, the machine will detect that the drive rollers are pushing too hard and pause the system.  I’m not sure that I’ve ever actually seen this torque limit tripped, but when feeding filament through, you can hit the torque button for a realtime display of how hard the feed motor is working.  Comparing the torque when feeding the OEM support material versus the HIPS should tell me…  well, something.  Setting the support extruder for 265° C and 100% feed, I ran the OEM support material through and the torque was around 65 (I’m pretty sure this is a unitless number) – interestingly, I couldn’t see any sign of smoke or vapor with the OEM material.  I then ran the NIP HIPS through with the same settings (as expected, faint wisps of smoke/vapor were visible once more), and it was a whopping…  What?  Only around 60?  What the heck is going on?  I noticed that both materials were extruding much more nicely this time – they would still curve a bit after exiting the nozzle, but not as much as before.  I ran about 6 feet of HIPS through without a hitch before getting tired of waiting for it to kink, which it never did.  As best I can guess, there must have been a little bit of buildup or blockage that the OEM material flushed out.  Could there be some magic cleaning additive in the OEM support material?  Possible, but unlikely – the P400 support MSDS sheet notes the only ingredient as being approximately 99% styrene-butadiene copolymer.  Eh, maybe I was just lucky.

As long as I was looking at torque values, I thought I’d see how the OEM ABS compares to the NIP ABS.  At 275° C and 100% feed, the OEM ABS showed a torque of about 70.  The NIP ABS at the same settings was much higher – about 110.  I also noted that again, faint smoke/vapor was present with the NIP material, but not with the OEM. The MSDS sheet for the Polylac PA-747 that is the NIP ABS notes that additives comprise less than 2% of the material, but it doesn’t say what those additives might be. One possibility is that NIP may be adding some sort of chemical to their batches in order to improve processing, but this is only a wild guess – I don’t know much about the extrusion process and would have to ask Jim @ NIP if they add any sort of special sauce.

6 thoughts on “Not all HIPS is created equal, either”

  1. Thanks for the mention in your excellent blog. It was a pleasure to help. Let me know if you have any other questions on plastics. As you said, for a non-chemist it can be bewildering.

    The 5% additives in HIPS are mineral oil (as you stated) and antioxidants to protect the plastic during extrusion and later during use. The polybutadiene rubber in HIPS is easily attacked by heat and oxygen so you need to stabilize it with antioxidants or the HIPS goes brittle.

    In ABS the 2% additives are antioxidants again to protect the polybutadiene rubber. Possibly some silicone oil to boost the impact resistance and also surfactant left over from the manufacture of the rubber particles (made by emulsion polymerization).

    I can explain the difference between HIPS, SBR and SBS block copolymer when I have a few more minutes spare.

  2. Thanks for the info, Chris – you are truly a fountain of knowledge! If you have a few minutes, I’d love for you to elaborate further on how the materials differ – your explanation on the eng-tips forum was excellent, but I assume there may be even more details in the mix.

  3. Many of the Stratasys manuals I have read stress ‘drying’ the ABS, as plastic is hygrosopic. Could excessive moisture be giving you the fits with the HIPS material?

  4. Possible, but unlikely – I left the HIPS in the dry box with freshly dried desiccant tins for several weeks. Furthermore, HIPS appears to have a notably lower absorption than ABS. My Stratasys manual also stresses keeping the filament dry, but the only effect that it seems to mention is increased nozzle ooze as moisture content increases.

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