Place of Origin : Xiamen,China
Post Date : 29-04-2020
Expiry Date : 08-03-2131

Post-curing Is A Stage Of SLA Printing That Is Often Overlooked But Which Can Greatly Influence The Final Properties Of A Printed Part. Post-curing Isn’t A One-size-fits-all Process. There Is Skill And Experience Involved In Deciding On The Right Length Of Post-curing For Different Materials And Part Sizes. For Some Parts Post-curing Is Essential, While For Others It Is Not Important At All. Let’s Take A Closer Look At The Science Behind SLA Post-curing And Discuss Why It Important To Get It Right.
What Is Post-curing?
During SLA Printing, Specific Areas Of Resin Are Exposed To A Laser Which Causes Them To Cure. Once This Process Is Complete The Entire Part Will Be Cured, So It May Not Be Completely Clear Why There Is A Need To Post-cure. To Answer This Question, We Have To Understand The Properties Of Hotosensitive Resin And How It Transforms From A Liquid To A Solid. While In A Liquid State, The Resin Is Made Up Of Lots Of Unbonded Monomers Mixed With Photoinitiators. Exposing This Mix To The Strong UV Light From The Laser Causes The Photoinitiators To React And For The Monomers To Bond Together And Form A Polymer. This Is The Primary Sla Curing Stage. The Cured Resin Is A Crosslinked Macromolecule, Which Means That Every Part Of It Is Directly Connected To Every Other Part Of It. However, After Printing There Will Still Be Many Sections Which Are Not Crosslinked To The Extent They Could Be, Which Affects Tensile Strength And Other Properties. UV Light Can Be Used To Complete The Molecular Bonding And Make Sure Every Crosslink That Could Be Made Is Made.
For The Purposes Of Post-curing, The Sun Is Actually Not A Bad Source UV Light. Many Hobbyists Using Consumer SLA Printers Will Simply Place Printed Parts Outside In The Sun For A While And That Gets The Job Done. However, In More Professional Settings It Is Generally Standard Practice To Use A UV Oven, Which Is Much More Controllable.
Why Post-cure?
Post-curing Not Only Makes Parts Stronger, It Also Makes Them More Stable. Having Unreacted Photoinitiators And Unbonded Polymers In The Part Will Mean Its State Could Continue To Alter Long After It Is Printed, Particularly When Exposed To Sunlight. For Parts That Need To Be Precise, Such As Dental Inserts, This Is Completely Unacceptable. Unpainted Parts May Also Change Color When Exposed To Sunlight If Not Properly Post-cured. This Is An Issue With SLA Resins In General But Will Be More Pronounced If The Part Is Under-cured.
The Primary Reason For Post-curing Though Is Usually To Increase Tensile Strength And Remove Any Surface Tackiness. Some Printed Materials May Be Slightly Soft Prior To Post-curing And Weaker Sections Of The Build Could Sag Or Droop, Affecting Accuracy.
There Is Also A Question Of Safety As Uncured Resin Is Toxic To Humans. If Parts That Are Not Fully Cured Are Handled With Bare Skin There Is The Potential For Irritation Or Other Issues, Particularly If The Handler’s Fingers Later Make Contact With Their Mouth. Post-curing A Part Has The Effect Of Making The Resin Completely Stable And Safe To Touch.
The Chemistry Of Curing
Curing Is Affected By Both Heat And Light. Heat Increases The Energy In The Crosslinked Polymer Network And Causes More Movement At A Molecular Level. This Increase In Energy And Mobility Makes It Much Easier For Reactive Groups To Come Into Contact With Each Other And For Further Bonds To Be Made.
However, UV Light Is The Primary Ingredient Needed In Post-curing. Although The Equipment Is Referred To As An Oven Or A Furnace, In Reality The Temperatures Inside Are Not High At All, Not Much Hotter Than A Tanning Bed. The Resin Used In SLA Printing Is Photosensitive And Exposing It To Light Causes A Reaction To Take Place. As Discussed Above, During The Original Curing Phase Of Printing Not All Bonds That Could Be Made Are Made, And So Further Reactions Need To Be Stimulated. When Exposed To UV Light During Post-curing, Free Radicals Are Formed Which Bond With Nearby Groups And Create Polymerization.
The Right Kind Of Light
However, It Isn’t The Case That The More Light You Provide The Faster The Process Will Be Or The Stronger The Material Will Become. What Is More Important Is The Wavelength Of The Light Used In Relation To The Resin The Part Is Formed From. Photosensitive Resins Are Designed To React To Certain Wavelengths Of Light And Using A Different Wavelength To This In Post-curing Will Give Suboptimal Results. This Is Why One Of The Reasons Why Professional 3d Printing Curing Teams Use Specialist Equipment For Post-curing. In The Case Of ProtoFab, Our Resins Cure Best Using Light With A Wavelength Of 355 Nm. If Resins From Another Supplier Are Used Then This Value Will Be Different. For Example, Formlabs’ Resins Cure Best Using 405 Nm UV Light. This Is A Very Good Reason To Use ProtoFab’s Own UV Oven If You Are Already Using Proto Fab Inc Resins.