2010
09.04

After my previous post on alternative ABS use on the Stratasys, I wound up with a pile of informative comments. Jeff directed me towards US patent application 2009/0295032, which contained this gem:

The extrusion runs of Examples 1-12 were performed with a modified ABS material commercially available under the trade designation “CYCOLAC” MG94-NA1000 ABS from General Electrics Co., Pittsfield, Mass. The extrusion runs of Comparative examples A-D were performed with a standard ABS copolymer commercially available under the trade designation “AG700 ABS” from The Dow Chemical Company, Midland, Mich.

We surmised that Dow AG700 ABS may likely be the standard Stratasys ABS, and given the date of filing and material properties, the GE MG94-1000 ABS was likely the newer ABS-M30 material.  I started looking into the AG700 resin first – while I was able to find something of a datasheet for the product, the fact that Dow now only sells to the automotive market (in North America, at any rate) pretty much quashes any possibility of acquiring the resin. Still, we now have a set of material properties to use for comparison, and I’m wondering which properties specifically make for an ideal Stratasys/RepRap/FDM feedstock. The “high flow” and “low gloss” aspects jump out at me in particular – obviously high flow is needed given the small orifice sizes, but I wonder if the glossiness of the extruded filament is indicative of its surface energy, and hence the amount of attraction that two layers will have for each other (thus potentially causing warp).

I had much more luck with the MG94-1000NA resin – GE sold their plastics division in 2007 to SABIC, so I dug around on their site (after needing to create an account, grrr) and pulled up the datasheet for the material. It turns out that the ‘-1000NA’ is simply the color code – they have dozens of colors available, and -1000NA is the plain old uncolored ‘NA’tural one. After a call to sales, I found that the smallest quantity I could order was 55lbs. at a whopping $30.61 per pound. Yowza. However, the price decreases drastically with quantity, and 330lbs. would only be $7.69 per pound. I don’t exactly have $2500 burning a hole in my pocket, but the quantity and pricing certainly isn’t out of reach for a few dedicated hobbyists to try. There’s a few other possibilities to research before falling back to that, however.

Recently there’s been a bit of buzz on the RepRap forums about a low-cost, very RepRap-ish 3D printer from China. What caught my eye the most was that this printer is actually using Stratasys sized 0.070″ feedstock rather than the 3mm RepRap standard. They note an ABS price of $50/kg, which comes out to $22.69/lb. Not as cheap as from NIP or other sources, but if the plastic acts just like Stratasys ABS, it would be worth it.

Speaking of NIP, I called up Donna to see what other possibilities there were for ABS. She and Jim were kind enough to provide me with the datasheet for the specific ABS they use, which is Chi Mei Polylac PA-747. They’re able to get other Chi Mei resins as well, so I took a look at their high flow offerings. The highest flow formulation, PA-756H, looks promising – the low impact strength is one of the properties that stuck out on the AG700 resin. I’ve emailed Donna to see if they can acquire this material.

So much for material musings – I have 5 pounds of HIPS that I need to wind onto spools! I’m not about to wind a half mile of filament all by hand again, so I needed to figure out a good automated system. I originally figured I’d just chuck the empty spool on the lathe and wind it that way, but then I decided to use the mill instead, as the head is variable speed on-the-fly. How to hold the empty spool, though? An expanding collet would be great, but I don’t have any as big as the 2″ bore in the Stratasys spools. A little bit of digging through scrap bins, and I found a Delrin cylinder that would work perfectly for making a pair of bullnose centers mounted on a bolt. Here’s what the assembly with spool looks like on the mill:

I clamped a live center into the mill vise to support the spindle from underneath

What to do on the unwind side was a bit trickier.  Based on my winding by hand of the ABS, I knew that keeping the coil of filament in a rather static shape was very important, so I opted to make a simple spool that could be assembled around the coil itself.  I used Masonite for the sides and drilled holes  through each piece to allow for joining screws.  Multiple sets of holes were drilled so that I could adjust the screw locations to perfectly fill out the center of the coil.

The coil of HIPS filament as it arrived from NIP, still bound with shrink-wrap, is placed onto one of the spool sides. The screws that will attach the other side are placed as close to the interior diameter of the coil as possible. 2" pieces of pneumatic tubing are slipped over each screw to serve as spacers.

The fully assembled spool, mounted and ready to unwind. The radial lines on the top piece of Masonite are a result of using an angle grinder to deburr the holes. It may sound crazy, but it worked far better than the countersink I tried first.

I clamped a piece of 3/4″ rod in the bench vise and slipped the spool over it (a collar on the rod kept the spool at an appropriate height).  It rotated pretty freely, so I took the loose end of filament and tied it to the inside of the empty spool.  I set the mill spindle to the lowest speed, and hit the power.  60 rpm is perfectly fast for winding, though I cranked the speed up to around twice that once I had a good start on the winding.  I held the filament in one hand (with a rag so that I’d actually have skin left) to provide continuous tension, and the material unwound from my adjustable spool just as nicely as I had hoped, with no snarls.  About a half hour later I had two spools fully refilled.

This beats the heck out of doing the winding by hand.

I gave the NIP HIPS a try in the Stratasys, noting that the Stratasys support material is slightly more brittle than the NIP HIPS.  I ran a small test part with NIP ABS at 250° C, and the HIPS at 265° C.  The HIPS performed admirably, and had good adhesion to the NIP ABS.

The part separated from the base cleanly. I'm at a loss as to why the top crosshatch fill turned out nice and tight. The only reason I can think of is that perhaps the drive rollers were beginning to slip, resulting in a diminished flow rate.

The Stratasys is currently running a large plate of parts, and we’ll see if I have less warping this time.

4 comments so far

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  1. 1- Awesome. You gotta love it when you just have all of the stuff you need on hand to put build the required tools, right? Very, very cool.

    2- What’s up? No update on that plate you mentioned in the last paragraph? Or do we have to wait for a new post?

    • Yeah, that plate of parts… On Monday I went in to work to see the results. It wasn’t pretty, though I took a picture for posterity. In my desire for optimum efficiency, I had wound the spools of HIPS right to the edge of the flange. In retrospect, this was a poor decision, as the filament easily slips off the edge of the reel, and wraps itself around the shaft on which the spools are placed in the Stratasys. Eventually the drive rollers can’t overcome the friction, and material feeding stops. In short, the support nozzle was starved for material and the ABS could not be deposited cleanly as a result. Pretty messy.

      I fixed the issue and started another plate that would finish around noon on Wednesday. Tuesday morning I checked on it, only to find another mess inside the build chamber – the HIPS filament had made a birdsnest inside the head, and I had to do a fair bit of work to clear out all the filament. When trying to feed the HIPS into the extruder, I had noticed that at some point the filament tends to kink between the drive wheels and the extruder entrance. I’m guessing that the HIPS from New Image is perhaps a little less stiff than the HIPS support material from Stratasys. A call to Donna @ NIP will be in order tomorrow to see if I can get material specs for the HIPS. I’ve also opted to remove the nozzles from the head and give them a good cleaning – I’ve accumulated a fair bit of charred material on the bottom of the head as a result of my experimentation, and it’s started to gum up the up/down operation of the support nozzle.

  2. Yikes… So, i’m picturing what happens when you let go of the end of a spool of thin steel (as for jewelry) wire… it tries to straighten out and basically “loosens” a bunch of the coils around the spool? Would adding some tension to the spool help prevent this?

    • No, the plastic filament doesn’t try to straighten – it actually does take on a ’set’, and wants to stay coiled. I just have to make sure to wind to within perhaps 1/4″ of the edge of the rim next time. The much bigger issue is that the HIPS from NIP seems to be just soft enough to bend more than the Stratasys material, and I’m not yet sure of what the best solution for that is. I could try another HIPS formulation, but I’m wondering if I should just rig up some sort of guide block that will sit between the feed rollers and the liquifier tube entrance.