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[Andy M]

I was heading for my shed with a bit of light ply and the sunshine showed what the inside of it is like, bit knotty in there! The outer plys are perfect, you would never know. Seems to do the job fine though, knots and all.

[derekwarner]

Wow..............you wouldn't believe 


I understood all 'ply wood' timber was manufactured to certain Grades [not sure how this is defined is US only, or European or ISO Standards?]


On face evidence, it would appear that there is no issue or fault, simply that the ply didn't conform to whatever Standard was assumed


It is also reasonably clear to accept, that a 3 ply structure with such [edit] 'gross directional, material and structural inclusions' .....could be of a Standard which did not allow or preclude such ??? inclusions


Amazing 


Derek

[Colin Bishop]

There has been a bit of discussion about the make up of lite play on the Model Boats forum and I came across this interesting topic on the Model Flying forum:

https://www.modelflying.co.uk/forums/index.php?/topic/46582-liteply-compositionpoplar-87/

Colin

[derekwarner]

Not altogether surprising      

I intentionally stayed away from European, British or ISO Standards, as these Publications are usually only available to view at great $ cost, but secondly being of a technical nature in content, do not necessarily provide any simple understanding    

The following Finnish publication is rather informative

https://pdf.archiexpo.com/pdf/wisa-plywood/handbook-finnish-plywood/65783-199305.html

Page 10 of the document offers ..........

"GRADES OF FINNISH BIRCH FACE VENEERS IN ACCORDANCE WITH SFS 2413 B (I) Pin knots permitted. Other knots and holes permitted up to 6 mm diameter, limited to a cumulative diameter of 12 mm per m2. Closed splits and checks permitted up to an individual length of 100 mm and one per metre of panel width. Slight discoloration and streaks permitted. Other defects strictly limited. Pin knots permitted. Sound intergrown knots permitted up to an individual diameter of 20 mm, limited to a cumulative diameter of 50 mm per m2. Other knots and repaired holes permitted up to 10 mm diameter, limited to a"...

Page 11 is evening more frightening

"and checks, repaired, permitted up to 2 mm wide and 200 mm long not exceeding 1 per metre of panel width. Discoloration, roughness and sanding through permitted if all slight. Wooden patches permitted up to 3 % of area. Glue penetration limited to 5 % of panel surface. WG (IV) Pin knots and sound knots permitted up to 65 mm diameter, limited to a cumulative diameter of 600 mm per m2. Other knots and holes up to 15 mm diameter limited to a cumulative diameter of 100 mm per m2. Open splits and checks up to 4 mm wide and 2 per metre width of panel. Discoloration, streaks, roughness"

So from this, looks like I have to eat my words again  .......as clearly those pin knots Andy has shown.......[hidden in the middle of the sandwich] are OK in accordance with SFS 2413 B

'Every day in life is a learning day' ................Derek...

[kinmel]

I have always understood that plywood has low quality sheets in the centre and good quality surfaces, otherwise an awful lot of timber will be going to waste.

If you watch the following video it shows sheets being sorted whilst being stacked pre-glueing and although most sheets have knots and splits, the finished sheets have blemish free surfaces.
https://www.youtube.com/watch?v=SFomXXXYMXw

[Colin Bishop]

Yes, as long as the outer sheets are good the inner one(s) don't matter. It is voids in the cheap stuff you have to beware of.

Colin

[kinmel]

There used to be 3 grades, now only Type A is generally available.

 A = Marine Ply, waterproof glues, high quality surfaces
B = High quality surfaces , not water proof.
C = Shuttering etc,  low quality surfaces and glue. 

[Andy M]

I have been thinking about the knot bits... I suppose they are ok on a flat panel but I am lucky I didnt manage to get this bit of window frame on a big knot, as the outer ply's grain is horizontal, a knot here could have been bad.
 If I am doing anything similarly fragile, I might just check out the sheet before marking and cutting.