Introduction: How to Eliminate Z-axis Lines Caused by Peel/Separation Force in Bottom-up SLA/DLP/LCD 3D Printing?

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In the last article, we shared the effect of vacuum seal drawing in SLA/DLP/LCD 3D Printing. When we printed a bottle and cap, obvious uncured phenomenon appeared on the surface. In the gradual investigation, it was proved that the most influential factor was the vacuum seal drawing.

At the end of last article, we left two pictures as shown above. In the blue dotted box in pic 1, it could be seen that obvious Z -axis lines were caused by peel force/separation force, and even staggered layers occured. In pic 2, there were also two obvious Z -axis lines at the position of the blue dotted boxs which are still clearly visible after cleaning.

Step 1: The Separation Process in the Bottom-up 3D Printing

The culprit for those lines is the peel force/separation force which is unavoidable in the case of bottom-up SLA/DLP/LCD 3D printing. Why do we say so? First of all, let's understand the separation process in the bottom-up 3D printing again:

  • The build platform and parts are submerged into liquid resin,
  • Photosensitive resin is cured by the light source, and cured layer is stuck between the previous cured layer and the FEP film,
  • Build platform moves up, and the print is separated from the FEP film,
  • Then the build platform moves down, and this repeated actions continue to cure the new layers.

During the separation process of the prints and FEP film, the peel force/separation force is generated. So how to understand the peel force/separation force?

Step 2: Peel Force/Separation Force

There's a Stefan formula for that:

Peel force/Separation force =3π*resin viscosity*lifting speed*radius of new cured layer^4/2*height between new cured layer and FEP film^3

There is no need to understand this formula too deeply. As the name implies, we can know that resin viscosity, lifting speed, radius of new cured layer, and height between new cured layer and FEP film will affect the power of the peel force/separation force.

It's not hard to explain it. The resin viscosity can be thought of as pulling the object out of the water and out of the swamp. Because the swamp is more viscous, more force you have to exert to pull it out. While the lifting speed is also easy to understand. When placing suction cup on the glass, the more fierce the drawing, the stronger the adsorption. It should be noted that the doubling of the radius of the new cured layer causes a 16-fold increase in the separation force, not a doubling.

The height between new cured layer and FEP film can be imaged as if it’s large enough, the two objects will completely separate. So there will be no peel force/separation force at all. And the doubling of this distance, the resulting separation force is an 8-fold increase, not a doubling.

Tips: The quality of the FEP film will also affect the peel force/separation force.

Step 3: Eliminate the Z-axis Lines

It is easy to eliminate the Z-axis lines when the cause is clear. The first thing to know is that the peel force/separation force is impossible to disappear. The Z-axis lines can be eliminated in the following two ways:

  • Reduce the peel force/separation force
  • Stablize the print

Step 4: Reduce the Peel Force/Separation Force

The first method can affect the peel force/separation force by adjusting the parameters in the formula. It is more effective in changing the resin with lower viscosity, reducing the print or adjusting the orientation to reduce the radius, and decreasing the lifting speed. Of course, it is also necessary to regularly check the wear and tear of the FEP film and timely replace it with new one.

Step 5: Stablize the Print

If it is inconvenient to adjust these factors, we can stabilize the print. At the location of the cross section mutation (where the radius is relatively large), several more supports can be added to stabilize the print. As shown below, after stabilizing the print, the Z -axis lines are basically eliminated.

Further explanations:

The instability and inching of Z-axis will also cause Z-axis lines.

Model Source: https://www.thingiverse.com/thing:384578