Cracking - Medial Axis

12min

Cracking - Medial Axis
Cracks/fissures form along the medial axis flow lines.
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How to Identify
Medial axis cracking can be diagnosed as follows:
Cracks appear as thin, dark lines running along medial axis flow lines.﻿
Medial axis flow lines occur on surfaces parallel to the platform, facing the window.
Lines appear after baking. Parts look fine off of the printer and after washing.
Medial axis cracking is differentiated by other types of cracking by where it occurs on the part. This is an orientation-based form of cracking that occurs solely along medial axis lines on flat surfaces facing the window. Reference Cracking - Bulk﻿ and Cracking - Excessive Solvent﻿ for other types of cracking.
Cause - Inconclusive
Cause - Inconclusive
Cause - Inconclusive
Troubleshooting
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Process, Material, and Hardware Checks
If any of these conditions apply to your print, take the corrective action and reprint to see if the problem is resolved.
Follow solvent exposure times closely in the resin post-processing protocol.
Do not exceed maximum solvent exposure times and reduce exposure time when possible if parts are fully clean.
Use swabs to remove excess solvent along medial axis lines.
After washing, dry parts on a blue towel or raised mesh (with air flow around the whole part) until all solvent is evaporated prior to baking.
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Check that the correct bake schedule was used per resin protocols. Resins most susceptible to cracking use a bake schedule that ramps up temperature gradually.
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Print Preparation or Part Design Adjustments
The following solutions will address print or part-related causes of the defects. Choose among the suggested actions below and select the best approach for your application. Usually only one type of adjustment is needed.
Orient parts to keep window facing surfaces at an angle (recommended).
Keep surfaces parallel to the platform smaller than 10 mm across.
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Reference Print Plan Adjustments﻿ (login required) for guidance on the following parameters:
1 Pump Up Stage
Pump Height Parameters
Minimum Pump Height
Minimum Distance from Peak
Distance from Peak Multiplier
Force Peak Motor Stop Percentage
If problems persist, please reach out to Carbon Support.
Explanation & Causes
Explanation
Medial axis cracking occurs along the medial axis flow lines on surfaces parallel to and facing the window. Resin flows in from all sides of a part, and where those varying flow directions meet is the medial axis flow lines. These lines act as weak points in the material where stress during the chemical changes of baking can cause cracks. 
The most susceptible conditions of medial axis cracking are: 
Large surfaces parallel to and facing the window
Excessive solvent exposure
Brittle resins: CE and EPX
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Causes
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Root Issue
Problem
Why Defect Presents
Washing
Solvent exposure
Too much solvent exposure weakens the material, making stress points weaker.
Baking
Wrong bake schedule
The most common resins for medial axis cracking are the brittle resins (EPX) that use a ramping program to raise the temperature slowly and prevent thermal shock.
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Root Issue
Problem
Why Defect Presents
Orientation
Surfaces parallel to and facing the window
Medial axis lines only occur on surfaces parallel to and facing the window. These lines are weak points that can crack under stress.
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Printing Stage
Parameter Type
Type of Adjustment
Why Defect Presents
1 Pump Up
Pump Height
﻿Decrease﻿
A shorter pump height may not provide enough time for resin to relax into the medial axis flow lines, leaving them weaker as a result.
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Root Issue	Problem	Why Defect Presents
Washing	Solvent exposure	Too much solvent exposure weakens the material, making stress points weaker.
Baking	Wrong bake schedule	The most common resins for medial axis cracking are the brittle resins (EPX) that use a ramping program to raise the temperature slowly and prevent thermal shock.

Root Issue	Problem	Why Defect Presents
Orientation	Surfaces parallel to and facing the window	Medial axis lines only occur on surfaces parallel to and facing the window. These lines are weak points that can crack under stress.

Printing Stage	Parameter Type	Type of Adjustment	Why Defect Presents
1 Pump Up	Pump Height	Decrease	A shorter pump height may not provide enough time for resin to relax into the medial axis flow lines, leaving them weaker as a result.

Cracking - Bulk

Cracking - Excessive Solvent