Proper Care and Handling of Your Print Head

Print Head Versions

Some Do’s and Don’ts

 Some Background

   In general, your model maker print head is an electro-mechanical device that delivers small droplets of phase change hot melt materials.  The heart of the device is a small potted assembly of a piezo tube, an aluminum housing, a high temperature plastic nozzle, a couple of wires and some potting compound.  The remainder of the device deals with maintaining temperature, providing a consistent filtered supply of material to the nozzle, and a cable for delivering power and drive signals.

    All in all it sounds quite simple.  In fact, the precision that is required throughout the manufacturing process is so sensitive that a few percent deviation from the process renders a non functioning component.  The orifice diameter for example is held to within .0002 in.  If it were not, the droplet size could not be controlled and the stream of droplets and the associated dynamics could not be controlled sufficiently to produce accurate models. Beyond this, the device is increasingly sensitive. 

   The major vulnerable areas are: the delicate orifice, the sensitivity to over heating, and the fragility of the piezo.  The nozzle is made of a high temperature plastic that is very soft and delicate when it is at operating temperature.  Touching it at this time can easily damage it.  Even in light of this apparent drawback, it will provide service for years if not physically damaged.  It is extremely resistant to fluid abrasion.  It is strongly advised not to touch it directly or risk permanent damage doing so. 

    Then there is the sensitivity to thermal damage.  The potting material has a glass transition temperature slightly above the normal recommended operating temperature of the build jet.  When the print head temperature is raised above this TG, the associated increase in the co-efficient of thermal expansion risks cracking the piezo.  Therefore, Ballistic Fluid Technologies exclusively installs a small thermal fuse that acts to protect the print head against overheating as a result of a thermal run away  from a controller malfunction.  This DOES NOT protect against a thermal run away of the heated line controller.  Such a failure can still damage the print head with or without the fuse. 

    There is an additional risk to overheating the piezo; the loss of poling.  Poling is the phenomenon that gives the piezo its ability to molecularly shift and apply energy to the fluid.  When the temperature gets too high, the piezo can lose its poling and lose its ability to shift.  When this happens, the jet will no longer function. 

    All of the above problems will render a print head useless.  The only one that is correctable is the last one, poling.  As a matter of course, all piezos are re-poled when returned for service.  Print head owners can be reassured that long term storage of spare print heads has not effect on the piezo poling.  Print heads can be stored at room temperature for years without suffering any ill effects.  Storing print heads at temperatures lower than room temperature will do absolutely nothing in preserving print head functionality.  The cost of re-poling is built into the type of service requested, or can be provided separately.

 Print Head Versions

    There are some noticeable differences between the versions of print heads.  The oldest version is the 6 Pro print head.  There are basically three types of 6 pro heads representing all of the developed stages of the print head technology.   The first is the type with the removable hexagonal shaped cap.  This the oldest technology and the most difficult to repair and maintain.  It is also the least reliable and was only used in the 6 Pro line of model makers.  The next design that is used in the  6 Pro head is basically the MMII/Patternmaster/T6-6/T6-12 internal works mounted on a 6 Pro shoe.  This is an improvement in reliability in that it no longer has the hexagonal cap.  It has a dip tube on the inside of the print head that allows for longer duration printing without re-establishing a bubble (a bubble is required inside the print head to provide inertial dampening to counteract the movement of the print head). The major faults with this design are that the bubble has to be injected manually and that there is an occasional freeze problem at the top of the build jet where the heated line is attached.  Also, this jet is typically not repairable.  Since the parts that go together to make this jet are interference press fit together, they don’t come apart readily.  Special tooling is required to repair these print heads.  These jets are also limited in their frequency response and are not capable of running reliably at resonance. 

     The third version available for the 6 Pro is the SDI version used in their RTM and 20/20 and BigFoot series model makers.  This version possesses the following advantages.  First, there is no need to inject an air bubble.  Although one is still required, it gets established automatically each time the jet is purged with the proper procedure.  Secondly, at the top of the print head there is an extension where the heated line mounts.  This extension supplies heat from the print head further up inside the heated line.  This prevents freeze offs at that location.  Thirdly, the general design of the print head is one of a modular concept.  This modularity allows the print head to be easily repaired.  Although it still requires significant technical skill, repair is performed with relative ease.  Fourthly, there is the increased frequency response.  The SDI version print head is capable of operating reliably at resonant frequencies.  The fifth difference has the orifice plane slightly recessed to offer more protection from lateral damage and better heat delivery to the orifice.  Finally, the piezo is better protected internally against thermal damage.   The latest SDI print heads can tolerate temperatures in excess of 160 degrees C without cracking the piezo.  Unfortunately, the potting compound cannot.  So if the jet is exposed to elevated temperatures, it is the potting compound that suffers permanent damage.  Potting compounds are developed specific to the application and any requiring this technology to operate at higher or lower temperatures can be accomplished by developing alternative potting materials.

    The only cross over applications between 6 Pro, MMII and RTM comes from Sanders Design International, Inc.  For several years, SDI has been providing RTM print head technology for 6 Pro and MMII applications.  With the relatively recent introduction of the T6-6 and the T6-12 into the market, SDI’s RTM  jet technology has not yet found its way into these applications.  Close review of the “T” model makers print heads indicate that this would be an easy transition.

    Ballistic fluid technologies is currently positioned to service all of these technologies with the latest technology plus new advancements to come.

1. Regardless of the advice or the source, there is never a valid reason to raise the print head temperature above 125 degrees C.  Depending on the calibration of the temperature controllers, doing so may actually expose the print head to higher temperatures and cause permanent damage.
    
2. Never touch the tip of the print head especially when it is hot as previously mentioned.  The plastic that the nozzle is made of becomes very soft at operating temperature and is vulnerable to tears and scratches.  Attempting to remove a clog or debris from the nozzle will almost assuredly damage the jet beyond recovery and cause extensive repair costs.
    
3. Do not change the print head material from build to support, or support to build.  If a support jet changes to build, the residual support material left behind inside the jet combines with the newly introduced build material and forms another compound at these locations.  The new compound has a different melt temperature and other characteristics which spell trouble.  We understand the frustration of needing a spare build jet when all you have is a spare support jet, but this generally only provides more business for BFT.  Since the support material is a wax based formula, it is more difficult to clean up than the build material.  Converting a build jet to a support jet is much more successful in that the build material is easily soluble in warm alcohol.  It is less likely therefore to leave sufficient residue of the build material behind to combine with the new support material and form a third compound.  It is not recommended to convert print heads in either direction, but what one does with ones own property is ones business.  If you have decided to do a conversion out of desperation, we recommend that you only go from build to support.  Therefore, it makes more sense to have additional spare build jets at hand rather than spare support jets.  Ballistic Fluid Technologies basically recommends that print heads are sent back to perform a material change, however, if the owner is willing to assume all responsibilities for the ultimate results, then something similar to this could be used.  First, remove the heated line and use a syringe filled with air to vacate all of the current material from the jet while it is at operating temperature.  Recharge the syringe again with air and set the jet temperature to 70 degrees C.  Slowly pass air through the jet as it goes lower in temperature.  This should maintain a clear orifice.  After the temperature of 70 degrees C has been achieved, introduce a syringe of alcohol (90% Isopropyl) slowly through the print head.  DO NOT apply much pressure.  It is very easy at this point to over pressure the jet with the syringe.  This could cause the purge cap to blow off.  Also, be aware of the increased pressure caused by the hot alcohol inside the jet.  Placing a thumb or finger on the end of the purge cap to prevent it from launching is recommended.  Be extremely careful when handling alcohol not to expose it to any flame or ignition source.  Also, eye protection should be worn to prevent splashing alcohol in an eye.  This is very painful but not significantly damaging.  Return the jet temperature to the set point for the material to be used and reconnect the heated line.  Re-purge the jet and attempt to continue printing.  This procedure introduces air in the jetting chamber which may take some time to get out.  Print heads will not work if there is air in the jetting chamber.  If all attempts fail to re-establish printing, allowing the print head to set at temperature for several hours will dissolve the air an re-establish printing functionality.
    
4. Auto bubble restoration in the SDI version print head is simply a matter of purging the print head through the side spout.  It is never necessary to inject air into an SDI version print head provided this procedure is followed.  After removing the side purge cap from the spout, allow the jet to set idle for a minute before purging.  This is ample time for all material inside the jet to siphon back to the storage tank.  Begin purging and when the flow of material is clean a steady, stop the purge and replace the purge cap.  The cap should be replaced within 5 seconds, although allowing as much time as 10 seconds will probably still be okay.  The risk is allowing too much time for the material to siphon back to the tanks and allowing air to go up the heated line.  If you allow 10 seconds or more and find that your jet is operating sporadically then re-purge and allow less time before replacing the purge cap. 
    
5. It is strongly recommended that whenever print heads are being purged that purge tubing is used on the purge spout.  The build material is electrically conductive when molten.  Allowing it to flow indiscriminately all over the top of the print head is only asking for problems.  Also, when molten, it has very low viscosity and will flow into very small crevices.  This especially spells problems when it comes time to perform service on the jet.  Not only are the insides of the jet caked with material but the shear mass of it can cause temperature consistency problems.  Then there is always the risk of shorting something out on the inside such as the RTD or the heater thereby causing even more problems.  Ballistic Fluid Technologies generally supplies, with returned print heads, an ample quantity of short Teflon tubes to be used for purging.  Please use them and if more are required, please request them.  They are available at a very modest cost.  Also, in most cases they can be reused by applying a little heat or by dissolving the material from them.
    
6. Ballistic Fluid Technologies has designed a new purge cap for use on most versions of print heads covered in this primer.  Up to this date, purge caps have been of various designs but have always employed Viton as the inner tube material.  Viton has tremendous chemical resistance, elasticity and high heat stability.  However, it has been observed that it is not terribly abrasion resistant.  The problem that has been discovered is that over time and after many insertions and removals, small particles of Viton have become involved in the filter and orifice.  The new cap employs a two position sliding technique and uses Teflon tubing as the inner tube.  Teflon in much more resistant to abrasion but is not as elastomeric as Viton.  When the cap is first positioned on the spout, the Teflon tube protrudes some from the end of the cap.  As the cap is pushed onto the spout, the Teflon tube slides up and over the barb on the spout.  Continued pushing of the cap further positions it over the spout and allows the outer sleeve of the cap to slide over the Teflon tubing already in place.  This action applies some support to the Teflon tubing and seals the cap from leaking.  Upon removal, the outer sleeve is the first to move and frees the Teflon tubing so that it can expand slightly as it slides over the barb on the purge spout.  When fully removed, the Teflon is then in the correct position to be reapplied.