MJP Best Practice When Dealing with Warping

 

P/N 31-D117_ Rev A

 

MJP Best Practice When Dealing with Warping

The images in this document depict the ProJet® 2500 platform however, these instructions can also be used on the ProJet ® 3600 & 5600 printers.

 

What causes warp?

  • Warp is primarily caused by a buildup of internal stress in MJP parts.

- This stress is then released during post processing as the parts are heated to remove the support wax.

- Post processing in a non-optimal fashion can also cause additional warp to occur in some parts.

  • This buildup of stress in acrylate materials is a byproduct of the process that gives them their excellent material properties.

 

  • For most parts, this stress can be contained by the part geometry itself.

- However, if the part has a large flat section and is printed in a non-optimal orientation, the part may warp during the build, or warp during post processing.

  • To stop warp in it’s tracks, we’ll use two main optimizations.

- Orientation

- Post Processing

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Orientation

  • To minimize warp, parts should be oriented so they have the smallest possible flat X/Y cross-section.

– This will most often increase print time, but the part quality gains are typically worth the additional time.

  • This action minimizes the produced internal stress, as the greatest stress is produced from polymer chains in the X/Y plane.
  • If the part is oriented vertically, the internal stress will be built up in a plane that is more easily contained by natural part shape. This, in-turn, makes the part less susceptible to movement during post processing.

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Example STL

  • This bracket needs to be printed as true to CAD as possible and have the 90 degree angle preserved.
  • If printed in the proper orientation, this is easily achievable.

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Non-Optimal

  • This orientation places a large, flat section of the part parallel to the X/Y plane (plate).
  • This will likely result in the downward face warping up slightly.

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Optimal

  • For the best possible result, this part should be oriented so that the X/Y cross section is as small as possible.
  • This orientation will allow the natural geometry of the part to contain the now minimized internal stress during printing and post processing.

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Post Processing

  • Once the internal stresses are controlled with orientation changes, if the post processing is controlled, then parts will come out with minimal warp.
  • Optimizing post processing for warp control focuses on two main areas:

– Thermal Curves

– Thermal Cycling

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Thermal Curves

Controlling the flow

  • Keeping the flow of heat into and out of a part is important for controlling warp.
  • Applying heat and cooling slowly allows for the part to come to temperature evenly.
  • Uneven heating can cause, or exacerbate warping problems.

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Thermal Curves

What to do

  • When putting parts in the oven, don’t let the part touch bare metal.

- Metal transfers heat into the part more quickly and can cause the area touching the heat to expand more quickly than the surrounding part, causing upward warp.

  • When cooling the parts, allow them to cool slowly.

- If a programmable oven is available, use one to ramp the temperature from the 65 degrees needed to process the wax down to room temperature over ~2 hours for larger parts.
- At a bare minimum, when parts are removed from heat, cover them with a thermal insulator to stop convective cooling from pulling away heat unevenly. This can be as simple as using paper towels to wrap the part up while it cools.
- Once the part reaches room temperature, it can be used as normal.

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Thermal Cycling

Minimizing Thermal Shocks

  • Since cleaning MJP parts is a multi stage process, each stage needs to be thermally controlled. Every stage of cleaning should be done as close to 65 degrees C as possible.
  • The parts being cleaned should go directly from 65C oven to 65C oil to 65C water to a slow cooling environment.
  • If the part cycles from hot to cold to hot multiple times, the warp will be more pronounced and unpredictable.

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Summary

Putting it all together

  • Print parts with minimal X/Y cross sections parallel to the plate.
  • Post process the part at 65C without letting it cool between stages.
  • Allow the part to heat up and cool slowly to control for thermal warp.

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