Waxuum

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Waxuum

Release status: Experimental

WaxVacuum3.jpg
Description A subtractive toolhead for making patterns in wax.
License GPL 2.0
Author MattMoses
Contributors
Based-on [[]]
Categories Toolheads
CAD Models
External Link thing 1743


Contents

Introduction

The waxuum (waxuum = wax vacuum) is sort of a reverse-extruder, a reverse HotWaxSyringe. The idea is that a heated hollow needle is used to remove material from a block of wax in a controlled manner, analogous to conventional machining using an endmill. A vacuum pump pulls the molten wax through the needle, after which it is deposited in a reservoir where it can be recycled. The wax part can then be used directly, or more likely it can be used as a pattern for a mold. The process goes by several names; it was called the "hot wax vacuum method" in this article from Don Lancaster's classic Hardware Hacker column.

The waxuum is not intended to replace other methods of fabrication; rather it is meant to be used in special circumstances where it has unique advantages over, for example, extrusion and conventional machining.

Process Overview

1. A heated basin is filled with wax, which is then allowed to cool. This big block of wax forms the working material for our little setup.

2. A waxuum mounted where the extruder usually goes on your 3D printer carves a master pattern (including containment walls) out of the wax block.

3. Elastomer resin is poured directly into negative pattern in the wax block.

4. When the elastomer cures, it is peeled off the wax master and used as a mold to crank out large numbers of parts.

5. The wax master is remelted within its heated basin. The removed wax in the reservoir is added back. The basin is cooled, and the wax is ready to use again for a new master pattern.

Advantages

1. The wax is melted instead of cut, so high forces are not required at the tool-tip. The low force requirement means that the XYZ positioner can be much more flexible (and cheaper and easier-to-make) than is normally required for conventional machining. Unlike conventional machining, there is no need for chip removal.

2. In many cases the surface finish on parts is better than that on filament-extruded parts, due to the smoothing effect of the hot needle moving over the wax. There are also fewer warping, delamination, and anisotropy problems.

3. You get a mold instead of a part: casting from a pattern can produce parts at a much faster rate than printing. You also have a greater choice of materials (plastics, metals, ceramics, and wax can all be cast in silicone molds).

Disadvantages

1. You get a mold instead of a part: there are several additional steps and materials, including the manual work of pouring resins. Sometimes you really want to just print your parts and go.

2. Limited geometry: unless you make multi-part molds with cores and such, you are limited to relatively simple geometry. Hollow and concave parts are difficult.

3. The waxuum is relatively unproven compared to the many filament extruder designs currently in use.

Safety

In addition to the normal common-sense precautions one must take when working on an engineering project, the waxuum presents a few special safety issues that require attention. Molten wax is hot and sticky and can cause burns and fires. Be careful and use common sense when working with it. If wax is heated too quickly, pockets of super-heated vapor can form within a solid shell, possibly resulting in explosions*. Numerous tutorials are around that explain safe procedure.

The waxuum case normally operates under vacuum, but it can be lightly pressurized, for example to empty the molten wax into another container for recycling. If the needle becomes clogged with solid wax (which is easy to do) excess pressure can cause the case to explode, even with fairly low pressure. This actually happened with one of the test units, so be careful.

* Thanks to Sebastien for this warning.

Construction

The core of the waxuum is a heated needle. The “needle” here is just a thin-walled brass or copper tube (for example McMaster part number 8859K21). The heater is made from nichrome wire and JB weld, just like an early RepRap extruder as described on this page. The (old) standard RepRap thermistor circuit board and thermistor is used to measure needle temperature. A newer version of the board can be found here. The wax used is McMaster P/N 93955K73, which has a melting point at about 150 F (65 C)

The needle is placed into a plastic case that serves as a reservoir for the removed molten wax. The case has a port for a connection to the vacuum pump, and also ports for various vents and wires if needed. We used push-to-connect pneumatic fittings (e.g. McMaster 5779K386), but the type of fitting used is not especially important. The vacuum pump is a Thomas Industries 607CA22. This type of pump is probably overkill. A vacuum cleaner or maybe even a modified aquarium pump would probably also work.

Vacuum is needed primarily, but the pump is valved to the toolhead such that air can flow both directions - i.e. the toolhead case can be compressed or evacuated. Vacuum is used for normal material removal, while pressurization can be used to expel the molten wax from the case. If over-pressurized the container can explode (see above) so be careful.

Three different waxuum assemblies were built with different configurations of needle, vent, and heater. The purpose of the vent is to bring outside air into the “melt puddle” to facilitate removal of liquid wax. A vent is necessary when trying to drill a deep hole with straight sidewalls, but a vent is not really important for making a shallow cut on the top surface of the work. More details can be found by clicking on figures in the gallery below. The most successful waxuum was used in a simple test to build a master pattern for a mold.

Pictures

Video

This is a video of a first test with the wax vacuum toolhead. It was a trick to get the airflow and feedrate just right (feedrate is how fast the tool is pushed through the wax). If the airflow or feedrate is too high, the tool cools off and stops cutting. If the airflow is too low, the liquid wax is not removed from the work. An additional problem with too low airflow is that the molten wax inside the case overheats and there is a risk of case damage and fire.

Discussion

The test showed it was feasible to use a tool of this design to make wax patterns. The most important thing to consider seems to be optimizing heat transfer from the needle to the wax, while minimizing heat loss due to movement of cool air through the needle. The thin-walled brass tubing is just barely acceptable for this purpose. The next version will use a material with higher thermal conductivity, such as thick-walled copper tubing. The heater should probably be located outside of the case. Possibly two heaters should be used - one for the needle and one to keep the wax molten inside the case.

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