HighTemperatureMaterials

Extruder Heater Materials

PTFE Breaks down at 230C, so replacement marterials are required in order to safely process plastics with high melting points such as PLA and HDPE.

Several high-heat areas are present in the nozzle and its insulator, with different requirements:

• The heater barrel - Low thermal resistance needed.
• The heater cladding - High thermal resistance, some "give".
• Heater insulator - High thermal resistance, rigid.

Promising materials are:

• BBQ Paint, or other "high heat" paint.
• Plaster of Paris
• Fibreglass insulation
• Perlite/vermiculite (possibly pumice)
• Cement or concrete

The proposed approach is to use ceramic-filled BBQ paint to insulate the heater barrel from the nichrome heating coil. BBQ paint is good for up to 600C+ but does outgass on its first heating.

To protect and enclose the heater element, I am proposing a fibreglass wrap saturated in plaster of paris, much in the same way a plaster cast is made. The fibreglass can be arranged to make sure the thermistor and all protruding wires do not rip through the plaster jacket if tugged. It should be resilient enough to cope with thermal expansion and contaction of the heating element.

To attach the barrel to the extrusion mechanism, a rigid block of insulating concrete is proposed. This need be no more than 50mm in diameter and 60mm high. Any narrower in diameter, and it will be hard to get enough concrete around the threaded rod that will attach it to the extruder mechanism.

A standard cement mix with added perlite and vermiculite should suffice. Due to the small size of the insulator, a fairly damp mix may be needed to ensure the concrete cures before it dries out. This, of course, weakens the concrete so a happy medium needs to be found. Absolute minimum use of water and precautions against evaporation should be tried.

A 3mm knitting needle through the centre of the block will maintain the central cavity, and hold the heater barrel in allignment. The needle will be removed before the concrete fully hardens and the hole tidied manually. Hardener or ceramic paint may be necessary to prevent wear at the edges.

One end of the block will have threaded rods embedded in it to facilitate attachment to the extrusion mechanism; washers or nuts may be embedded to improve adhesion. The other end will hold the heater barrel by means of a cluster of nuts and washers. This cluster of nuts and washers will be embedded, allowing the barrel to be unscrewed for maintenance or replacement. Some smoothing of the finish and washing will be necessary for the concrete component, to keep debris to a minimum.

A removable nozzle will be installed to ensure that the barrel and central cavity can be properly cleaned of debris before extrusion commences. Chance of successful alignment on first pass is slim, but there is some lattitude for error. A jig will simplify the alignment process.

Design Aims

The design has to hold a 200C+ degree nozzle on one end, and maintain a temperature of <60C on the other. This will allow Polymorph/polycaprolactone extrusion. Other plastics require a higher nozzle temperature, but in turn can take more heat at the other end.

While at full temperature, the 4 threaded rods inserted in the block need to withstand a force of at least 10N each, and the nozzle itself 20N.

Test effect of using black BBQ paint on concrete portion to increase heat radiation and protection of concrete exterior.

Fairly obviously, we don't want to use any materials that degrade at <300C.

Purchasing notes

For small quantities of premixed cement, ask your DIY store for a cement "patch pack", used for small repairs.

Check out several brands of black "high heat" paints. Prices vary by a factor of 2. Automotive stores also sell this paint for spraying engines and exhaust systems.

White cement varies from ordinary grey cement only in terms of colour and additional cost.

Pelican Wire (and presumably other suppliers) do 0.2mm (32-33AWG) nichrome coated with silicone and fibreglass insulation that is good up to 400C+. Nice if you can get it as the heater barrel does not then need the BBQ spray for electrical insulation: http://www.ec-securehost.com/PelicanWireCo.Inc./Nichrome_80.html

Test Article

A test cylinder was made from 5 parts cement premix and 3 parts damp greenhouse vermiculite/perlite mix together with just enough water (quantity not measured). A woodscrew and 3mm knitting needle were placed in the concrete.

The concrete was kept in place by a thick cardboard tube 37mm I/D and blocked at one end with gaffer (duct) tape The tube was dampened before use to avoid drying out the concrete. The components were taped in place to stop them sinking, and the whole assemblage left in shady surroundings. All cats were dissuaded from entering the experimental zone.

Time started, 4:30pm. Ambient temp 21.5C, gentle breeze, some cat hair.

After 1 hour the needle was removed by pulling upwards. This resulted in some damage to the upper surface of the block and the coating on the tip of the needle. Pushing downward may have been a better idea, or perhaps a more comprehensive mould is required.

10am next day, the concrete is just starting to show small grey patches on the upper surface. Initial curing is expected to be complete within an hour or so, but the test article will be left alone for at least 24 hours.

It is already becoming apparent that the exit hole left by the needle is substantially larger than 3mm. A sneak peek reveals that the hole at the other end is much closer to 3mm.

Test Results

After 48hrs, the cylinder had cured to the point where it could be handled and worked. It shed a lot of sand, but the overall integrity was good. It is mechanically not dissimilar to pummice, and a firm rub can easily dislodge surface material.

The concrete had a great many voids in it, and some of this may need removing. Perhaps a sander or similar vibration system could settle the concrete more thoroughly?

The screw embedded in the concrete comfortably supports a 12N load. A firm, steady pull just using fingers failed to dislodge the screw.

The central 3mm hole had rough sides, but was well-formed with little debris.

First Attempt

A 52mm diameter plastic cylinder hacked off a plastic flowerpot was used as the former, 65mm high. A tin lid with a 6mm hole in the middle was secured to the bottom with gaffer tape. It then occurred to me that a disposable cup of the appropriate dimensions would have sufficed. Oh well.

A nut, washer, nut, washer and final nut were put on one end of the 6mm brass heater barrel. Thin glavanised steel wire was twisted about the top two nuts and splayed out in loops to give the concrete something to gain purchase on. This was then inserted into the former, and the free end of the heater barrel was passed through the hole. A final nut under the lid secured it in place and made the join leak-proof.

A zig-zagged cylindrical form of wire was then added on top, to strengthen the concrete. All wires terminated at least 10mm below the edge of the former and in loops where possible.

A 3mm knitting needle with a sipke on both ends was then inserted into the former, and placed firmly into the hole at the top of the heater barrel.

A variety of long M3 bolts and threaded rods were attached to the bottom of the Mk2 extrusion mechanism, so that they would engade with the concrete. This included 2 x 65mm lengths of M3 with the last 15mm bent into a J-shape. Two 45 degree bends are required, closely spaced, or you'll break it.

5 Heaped desert spoons of cement mix were mixed with 3 heaped spoons of vermiculite/pearlite and a small quantity of water. Underdo the water if anything - the plasticisers in ready-mix need remarkably little water.

This was packed into the form within 5mm of the top, and vibration applied with the pad of a small orbital sander. This causes bubbles to rise to the surface by turning the concrete into quicksand. It also causes concrete to dribble out of every gap in the bottom of the form.

The extrusion mechanisim with protruding bolts was then inserted into the concrete, sliding down the knitting needle. The concrete was vibrated again, topped up a bit, vibrated once more and left to cure. A large dollop of leftover concrete was kept to act as a guide to the set of the concrete in the form.

Results of First Concrete Form

After 3 1/2hrs, the stuff still isn't set. This batch is much slower for no obvious reason. After 12 hrs the needle was removed. After 48hrs the mould was cut away with a Dremel tool, revealing some very soild concrete. I'm letting this dry after some rudimentary shaping to remove the leaked bits of concrete. Images of the form are shown below.

• The first concrete insulator just out of its mould (10-Feb-2006):
  

Note that the concrete is quite dense, though some bubbles are visible. The whiter patches are mostly vermiculite/perlite.

-- Main.VikOlliver - 04 Feb 2006