Import 2 objects, position them overlapping each other (e.g. Head and Helmet here)
On Halmet, Shift+ R click → Smooth
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Alt+Shift +D to delete history
→
Modify → Freeze Transformation
→ Hide the Helemt
Delete the polygon below neck part in Head layer (face mode → L click 1 face, then Shift+double click) → Use default material → select the head
delete history
→
Modify → Free Transformation
In Head layer, open Node Editor → remove all nodes inside from the graph (icon below Options tab in editor) → Drag Head layer into Node graph
Select the head → Face mode → select the whole area of the head →
Shift + R click → Extract Face (2 nodes will create in node editor)
Select polyChipOff1 in node graph → set the values as follow (translate Y should be 0)→ Untick Keep Faces Together
Set Key for the adjusted values above at 30F; set all values to 0 at 10F
Create new node in node editor → Drag Helemt this time to the graph → repeat set procedure to add Extracted Face Nodes → set same Poly Chop Off History as the Head layer
→
Go to 20F to set Key for the values above, while 40F for all values to 0 (opposite to the Head)
Select the Head layer, Go to FX (F5) → MASH (top bar) → Utilities → Create MASH Blend Deformer (a new node created in Head node graph) → Select the defromer node attribute
→ Open Connections tab → drag Helmet layer to Blend Mesh box
Set key of Blend value 0 at 10F, value 1 at 30F
Remark: The Helemt layer is suppose to be hidden all the time, ust let the Head layer does the work.