Printing a hollow semi sphere with DAP SiliconePlus

We at Structur3D Printing realize that silicone is one of the most popular materials our customers want to print. In fact, we have been printing with silicone a lot as well. For this case study we selected a silicone material and a geometry to provide all our case study data. The off-the-shelf silicone material we have chosen is the one we have had the most success with. A picture can be seen below.

GCODE model generated for Hollow Sphere

We already know that paste-like materials are most suitable for printing flat parts. However, in this case study we wanted to explore the boundaries of silicone regarding 3 dimensional parts with overhang features. For this reason, we have chosen a hollow semi-sphere, because the spherical surface should indicate limitations of the achievable surface finish. As indicated in the picture to the left, we printed the part with the ‘cut surface’ in contact with the building platform. This way, the material’s overhang capabilities should be obvious. 

The printer we used was the DIY version of the FELIX 3.0 from Netherlands based FELIXprinters.

The Slicing software we used was Slic3r and the software used to control the printer was Repetier-Host.


DAP SiliconePlus from Hardware Store

Material

The FELIX printing conditions used for testing can be found here.

The factors that have proven to be most influential on the print’s quality are:

  • Speed
  • Nozzle diameter
  • Offset
  • Flow rate

Additional factors are:

  • The parts size
  • Steps per mm of the motor
  • Silicone’s freshness

In the following, I will discuss each factor more in detail.

Speed

The speed can be set in the prints speed settings and 100% speed is considered to be:

  • Perimeters 10 mm/s
  • Small perimeters 10 mm/s
  • External perimeters 70% (%-value constant)
  • Infill 15 mm/s
  • Solid infill 15 mm/s
  • Top solid infill 15 mm/s
  • Support material 15 mm/s
  • Support material interface 100% (%-value constant)
  • Bridges 20 mm/s
  • Gap fill 15 mm/s
  • Travel 130 mm/s (%-value constant)
  • First layer speed 70% (%-value constant)

Compared to thermoplastics (like ABS and PLA), silicone takes longer to cure. This is a result of the different solidification mechanisms. Thermoplastics solidify due to the phase change from liquid to solid as soon as the temperature drops below the melting range. Silicone on the other hand cures due to a chemical reaction cause by atmospheric moisture. As a result, the silicone’s initial adhesion to the building platform is strongly dependent on the printing speed. If the speed is too high, the extruded silicone line will be stretched before it’s being deposited which causes the formation of holes in the layers. By setting a lower speed, this effect can be avoided. 

Nozzle diameter

The internal nozzle diameter affects not only the amount of extruded material but also how the material is being extruded. Considering different nozzle diameters but a constant pressure in the syringe and tube respectively, the velocity of the extruded silicone will vary. For a smaller diameter, the velocity will be higher than for a bigger diameter. The higher the velocity, the higher the curling effect of the extruded silicone at the nozzle. This curling effect causes the silicone to stick and accumulate at the nozzle. 

Velocity and flow rate too high:

Velocity and flow rate too low:

Velocity and flow rate adequate:

Offset

The offset (distance between nozzle and building platform/layer) for printing with silicone needs to be smaller than the actual layer height. If the offset is bigger, the extruded silicone is more likely to stick to the nozzle instead of the layer, which causes holes in the layer’s geometry. 

In order to avoid this problem the offset needs to be smaller than the layer height. For the first lines being printed onto the building platform this means there will be a certain spread of the silicone around the nozzle. For all consecutive layers this means the nozzle is actually slightly immersed in the previously extruded material. 

Offset little bit smaller than layer height during first line:

Nozzle slightly immersed in layer:

One problem that is caused by this procedure is a material accumulation at certain points of the geometry. This is caused by cured silicone sticking to the nozzle, which drags material from the top layer across the surface. During this process, the amount of material sticking to the nozzle will increase until it sticks to the surface eventually.

This problem in combination with the problem of a decreasing flow rate causes the pillar effect.

Flow Rate

Air bubbles in cartridge tubing

The flow rate refers to the amount of material extruded over time and strongly depends on the pressure in the syringe and the tube. Since we are printing with a paste like material (silicone), its compression characteristics are dynamic and thus the flow rates varies over time. Initially, after a new syringe has been filled, there will most likely remain air bubbles in between the material, which cannot be removed. The air bubbles are being compressed first before any mayor compression of the silicone is achieved. For one thing, these air bubbles cause variation in the compression of the silicone and thus in the overall pressure. Another aspect to consider is that these bubbles are being extruded as well. This means, eventually, the compressed air will leave through the nozzle and cause an overall pressure drop. These bubbles might even be big enough that it makes sense to pause the print shortly before they are being extruded through the nozzle, extrude them and the adjacent material before and after the bubble at an overspill area and continue the print afterwards.

Hollow Sphere with too low flow rate

Overall, the flow rate tends to drop over time. If the print is continued unassisted this will cause holes in the geometry. If the nozzle diameter is too small, it can even cause the extrusion to cease completely.

This problem can be avoided by the described procedure of pausing the print and manually extruding a certain amount of material until a sufficient flow rate is achieved again. 

A video of insufficient and sufficient flow rate can be viewed above.

In the beginning, the flow rate will be at its peak, which generally causes excess material extrusion. 

Some excess material extrusion at the beginning of the Hollow Sphere print

Silicone’s freshness

All the problems I encountered came up regardless whether I was using fresh or old silicone. However, the silicone I was using was never older than 2 days. Inside the syringe, the silicone seems to be sealed off enough to prevent almost any curing. The silicone inside the tube, especially at the nozzle on the other hand, cures faster. I recommend replacing the tube when you are using silicone that is not fresh in order to lower the pressure difference between the syringe and the tube.  

The part’s size

Apart from demanding more time and material, a bigger print can also have an influence on the print quality. This is especially true for prints with complex geometries. The more complex a layer is the more inconsistent the motion of the nozzle will be. Characteristic for the silicone is its ‘inertia’, which means it lags behind the nozzles motion and takes a little time to be actually deposited to the point where it sticks to the layer. Especially if the flow rate is low, a complex geometry will not be realized if the nozzle changes its motion direction fast and frequent.

If the print’s size is increased, which means the geometry is scaled up by a certain factor, these directional changes became less frequent and slower. This gives the silicone more time to be deposited and form the desired geometry and thus increases the print’s quality. 

Steps per mm of the motor

Using the FELIX printer and Repetier-Host, this setting can be adjusted live, during the print. An increased value will increase the revolutions of the motor shaft per time and this increases the flow rate. However, in all prints in which I used a higher value (increase from 1200 to 5000) I could not detect a significant influence. After I increased this value for the flow rate in print 13 it showed no beneficial effect. In any case, the effect of pausing the print and extruding material until the flow rate increases visibly is much more effective. 


Practices

Previous manual extrusion

After a syringe has been filled, a certain pressure needs to be built up inside the syringe and tube respectively, before the print can be started. First of all, the material needs to be extruded through the tube until it reaches the end connected to the nozzle. This can be done with the manual control in Repetier-Host or by hand. However, compressing the silicone in the syringe by hand requires a certain amount of physical strength and takes more time.

As soon as the material is close enough to the nozzle, pressure needs to be built up to establish an initial flow rate. More information on the characteristics of the flow rate of silicone can be found in the section about flow rate. For the first layers it is sensible to establish a flow rate that extrudes a little bit of excess material in order to realize a thick base layer. Another reason for this practice is to prepare the print for the later drop of the flow rate. This means, by the time the print reached the first layers above the base layers the flow rate will have reached a more appropriate level. A video of a sufficient and insufficient flow rate can be seen in the flow rate section.

The drop of the flow rate to a level where there is no excess material extrusion can occur during the print of the first layer. This can cause the layer height of the infill to vary visibly if the first and last lines are compared.

An ideal flow rate for the first couple layers of the Hollow Sphere print

Pausing print, manual extrusion

Temporary oversplill area

In order to avoid the problems caused by the dropping of the flow rate it is useful to pause the print and establish a sufficient flow rate before continuing the print. Prior to performing this procedure, an overspill area needs to be identified. I used the home position of the x-axis and mounted a custom cut cup with tape as an overspill reservoir.

After pausing the print, the z height needs to be increased first. Otherwise, the nozzle will interfere with the printed geometry if it’s being relocated. After the print was paused and the z height was increased homing the nozzle on the x-axis is the most sensible procedure. This should be done with the x-axis home button. Moving the nozzle incrementally through the manual control is also possible but this can cause problems. If through incremental manual moves you somehow exceed the physical range of the axis, the software will remember the incorrect ‘total movement’ rather than the actual total physical movement. If the print is continued afterwards, the repositioning of the nozzle will be off.

A video of sufficient and insufficient flow rate can be seen in the flow rate section. After the flow rate is at a sufficient level, the z height should be reduced to its initial value first before continuing the print. The reason for this procedure is the fact that the printing will reposition the nozzle first after ‘continue print’ has been clicked and the system will adjust the z height after. Considering the time it takes to adjust the z height, this will cause excess material extrusion at the point where the part is being continued. 


Problems

Pillar effect and line failures at the periphery

The pillar effect occurs while printing a circular geometry and is caused by a combination of an insufficient flow rate and material accumulation. The reason for the material accumulation is explained in the offset section. As a result, material accumulates at several points of the geometry across the circular path of the geometry. In addition, if the flow rate drops below a certain threshold, there will be not enough flow and time to deposit the silicone onto the layer in between those points. Instead, the extruded material sticks to the nozzle until it is wiped off onto one of the positions where material has already accumulated. 

This causes further material accumulation on the same points repeatedly. For a part with a circular cross sections, like a hollow semi sphere, this results in the formation of pillars on the outside and the inside.   

Examples of the Hollow Sphere with pillaring effect.

Even though the pillar is a geometrical failure, it is consistent. This however, is not always the case. As mentioned in the offset section, ideally, the nozzle is immersed into the material during printing which can cause the dragging of excess material. This dragged excess material can also be wiped off on random places and stick to nozzle again later only to be dragged to another place. As a result, the finished geometry will consist of medium and large sized material accumulations with no material in between. 

Hollow Sphere with pillaring due to material on the nozzle tip

On a smaller scale, an insufficient flow rate can also cause holes in the cross sections of layers. A small hole in the cross section of a layer can cause a repeated lack of material deposition over that hole to fill it up. As a result, this hole is further expanded in the z height.


Conditions and results of other projects

In addition to all settings mentioned above, the overall conditions of the room have most likely played a role as well. The majority of all prints were performed on a hot summer day at temperatures in between 25 °C – 30 °C. Since our machine shop is on the top floor, the atmospheric temperature of the room was probably in the same range, maybe even higher.

Moreover, these days were also very humid. The resulting higher amount of moisture in the air was probably even more influential than the temperature because it favored the curing the procedure. We know from a previous process involving a similar geometry that a circular hollow geometry can be printed with better results.

Examples of circular bridging effect on previous semi-sucessfull prints

The pillar effect was apparent in this print as well but not to the extend it was in the print of this project. Further, the overall layer resolution was still good.

Another factor was probably not the material freshness but the material itself. Several, different silicone materials exists that are suitable for printing. Since the materials have differences, different outcomes can be expected regarding the prints quality. One example is one of our print from another previous project.

Examples of previous successful prints using DAP Silicone 100% Transparent with no bridging effects


Tips and Tricks

Z height adjustment

The z height of the FELIX 3.0 DIY printer can be adjusted by turning the physical z-axis of the printer by hand. This can be useful if the offset required a minimal adjustment.

Moreover, there should be a difference between the z height of the silicone nozzle and the thermoplastic nozzle. If they are on the same level, sooner or later the idle nozzle will likely interfere with the printed geometry. Thus, the idle nozzle should always be at a higher position. 

Printing onto a substrate

Instead of printing directly onto the building platform, printing onto another removable substrate, like standard wax paper used for baking is sensible. For one thing, this eliminates the need to clean up the building platform.  Another advantage is that simply picking up the removable substrate can relocate the printed part. During the print, the substrate needs to be fixed to the platform with tape and flattened out. Flattening the side at which the nozzle enters the substrate is especially important. If this is not properly done, the nozzle will rip off the substrate due to the small offset. 

The FELIX 3.0 ready for printing Silicone with a layer of wax paper on the bed

One practice that helps to increase the adhesiveness of the substrate to the building platform is to introduce a little bit of water between the two.

Once the print is cured, it may be stuck to the wax paper. Simply prepare a bowl of hot water, and place the print in the bowl. The hot water will melt the wax and disintegrate the paper after 5 or so minutes of soaking. Any remaining paper can be easily scratched off without damaging the print.

Filling syringe

Filling up a syringe with silicone can be very annoying and messy. As explained earlier, we want to have as little air trapped in the material as possible to realize a continuous flow. Further, this reduces the required time to build up a sufficient level of pressure before starting the print. The following tips can make the procedure a little bit more pleasant.

First of all, use gloves.

Second, before filling the syringe, mount the tube on the extruding end. This way, the air trapped in the sections between the filled material and the extrusion end can escape. At the same time, the material can already enter the tube without causing any trouble.

Adding holes to the plunger to remove air from the cartridge

Thirdly, use anything similar to the plunger to compress the material inside the syringe. For example, the rubber seal of a plunger can be removed easily by hand. If this plunger without the seal is used to compress the material inside the syringe by hand, residual air can escape to the big opening through the sides. Additionally, Four holes can be drilled inside the plunger.

By using a long stick-like object with small diameter (e.g. wire), trapped air inside the material can be released as well. While compressing the material with a plunger, this object can be inserted along the sidewall. This enables the trapped air to escape when the material is further compressed.

Fourth, after inserting the final plunger used for printing (with seal) use a wire inserted along the sidewall to remove the trapped air between the plunger and the material inside the syringe.

Fixing the tube to the syringe

What happens if the tubing is incorrectly connected

Fixing the tube to the syringe properly is important. The compression of the silicone can create enough pressure to cause the tube to come off the adapter. 

To prevent this from happening, make sure not to screw the adapter for the tube too tight onto the syringe.

Moreover, fix the tube with tape to gain additional stability. 

Cleaning

Before you clean up the silicone wait until it cures. For most surfaces, the silicone can be peeled off by hand easily without leaving any visible residual material. 

Code

Don’t use the g91 command in Repetier-Host during the manual operation. The motor of the Discov3ry system tends to operate not as well after this command is used. The motor shaft tends to stop turning after multiple motion commands have been put in. This behavior is similar to the malfunction if the setting for the steps per mm is too high or too low.  

To enable the retraction function, set the setting for the steps per mm lower than the value for the extrusion. The motor tends to operate better under these conditions in the retraction mode.