Running radius mouldings, expansion issue inner edge curling off the glass bench

Samson2

Member
Hi , I am running some window mouldings on a glass bench approx. 4 inch wide using Fine casting plus , the straight runs are only 1.5m long and are spot on but the radius are warping from the inner upwards on a radius of 2feet/ dia 4 feet tried making them in half but still they curl up from the inner thinner edge.. I have tried Keramicast as advised by the reps but even that's doing the same ...Doing my head in I can get a great running mould completed then it curls any advice greatly appreciated Lads.
 

Samson2

Member
I use some plastic spacers sat on silicon thing stays put, its on about 2 inch at each end enough to run against then just cut off the ends same applies to straight runs and the curved ones
 

Samson2

Member
31882
 

owls

Private Member
Because they should be run on a timber bench coated in shellac, using a gig stick attached to your profile for your radius, that way they can’t move or warp, that’s how we used to do it with no issues.
 

Samson2

Member
I like to clean up Hobo! is that so bad? Owls thank you, So I guess if timber is used the plaster dries differently on glass? but if its shellacked its still not as absorbent but can see it being better at moisture control over glass but I thought its more of the fact its expansion on the outer to the inner material as no where to go unlike on a straight section that is equal in both length and width …
 

Samson2

Member
I thought that glass would be better to use, slide on etc and keep clean, the straight bits are a doddle to be honest and pop off a treat. It was trial and error at first I use hot glue gun to stick down "stops" but the water/moisture got under and they slipped then used silicon and not looked back. Sadly the arched units I make are lifting as stated still being anchored to the table at the ends via my "stops" I can make a cracking run but after several minutes or even a tad longer they lift on the inner edge. I have placed weights and clamps as soon as its hard enough and this keeps the movement down until fully dried and once moved to a wooden desk to dry I have to add weight again to stop movement...I thought its a combination of expansion along the length and a issue to the drying as well..
 

The Hobo

Well-Known Member
some times get a lift up when making thin section stuff but that's down to the lath inside it /but just weight it as u are doing
 

Samson2

Member
I used lathes I thought it might be the difference in the wood effecting drying and expansion , then I did a few runs with no hessian or laths still the b*****d moved! I see until they are fully dried/ white they have stopped moving the time factor is a pain .. I also did a rough profile some 2/3 mm less than the finish let it dry minimise movement pva it and run a finish coat over that , that works but time intensive and still have to weigh it down , granted the movement is less... The section is 4 inch wide at the outer its 1 1/4 inner edge 1/4 with profile getting proportional smaller from the outer edge , this is the drying issue s well as expansion , thought that they might be a trick I'm missing hence the forum contact .. Karamicast as a slightly less expansion factor but not much from fine casting plaster plus , its certainly harder but finish seems not a great a FCP+..
 

The Hobo

Well-Known Member
I used lathes I thought it might be the difference in the wood effecting drying and expansion , then I did a few runs with no hessian or laths still the b*d moved! I see until they are fully dried/ white they have stopped moving the time factor is a pain .. I also did a rough profile some 2/3 mm less than the finish let it dry minimise movement pva it and run a finish coat over that , that works but time intensive and still have to weigh it down , granted the movement is less... The section is 4 inch wide at the outer its 1 1/4 inner edge 1/4 with profile getting proportional smaller from the outer edge , this is the drying issue s well as expansion , thought that they might be a trick I'm missing hence the forum contact .. Karamicast as a slightly less expansion factor but not much from fine casting plaster plus , its certainly harder but finish seems not a great a FCP+..
just a thought could it be when plaster is setting the heat being made cant get through the glass and is causing this
 

Samson2

Member
I thought that also , my brain was hurting as to think through reasons but to be fair its glass it should act as a eat sink and dissipate heat created but after all my thoughts expansion was on my hit list, but trying karamicast after advice thought it might cure it, it hasn't so its a combination of expansion and drying rates and possibly surface of glass keeping moisture on the back but thought with a shellac surface it would be similar .. trick is to try it on a wood surface and see … must say I like glass it cleans easy slides well and see it used a lot... folk do use granite etc but the sliding was an issue till I used the silicon .. as you stated its clean on a pass I clean and cleans easy.. I suspect a lot easier than a wood bench .. perhaps I do the straight bits on glass and if so the arched bits on wood once proven a wood base bench solves the issue.. I need to get off here and try … again :aburrido:
 

The Hobo

Well-Known Member
that's a vent rose no doubt a church or chapel???
no far from it they are in a stable block of a country house/they are big vents going in to a wooden ducting to take the smell out a someone used to live above them even cornice moulds in there
 

The Hobo

Well-Known Member
I thought that also , my brain was hurting as to think through reasons but to be fair its glass it should act as a eat sink and dissipate heat created but after all my thoughts expansion was on my hit list, but trying karamicast after advice thought it might cure it, it hasn't so its a combination of expansion and drying rates and possibly surface of glass keeping moisture on the back but thought with a shellac surface it would be similar .. trick is to try it on a wood surface and see … must say I like glass it cleans easy slides well and see it used a lot... folk do use granite etc but the sliding was an issue till I used the silicon .. as you stated its clean on a pass I clean and cleans easy.. I suspect a lot easier than a wood bench .. perhaps I do the straight bits on glass and if so the arched bits on wood once proven a wood base bench solves the issue.. I need to get off here and try … again :aburrido:
if you use wood bench shellact u will still need a releasing agent or it wont let go of shellac
 

Samson2

Member
and in the full length one half done yesterday I have cut in a saw cut and within seconds it closed and re cut it weighed it down best I can let it set better and dry better and pva it then top coat it in one go with finish profile then silicon it for casting its one half of a 68 inch arch moulding
31911
 

Samson2

Member
Hi Hobo I thought that too in the beginning so I only used one at the direction end I was running to so it had something to push on and stop slipping so it could expand backwards but still did it rise in the middle the highest was near 1/2 an inch!!! that's a lot.. typical movement is about 1/4 inch if left alone and even off the glass! we are on the same wavelength me thinks ..
 

Samson2

Member
Like I stated earlier I decided to run a base profile and leave it to dry with minimal movement and weighed down, then after a few days changed my profile plate to finish and it did work . where it lifted it was compensated for the difference in my profile plates so some plaster made its way underneath but no problem as it completed the final profile perfectly and when dry on the glass weighed down and after a day removed and left to dry fully still weighed down.. that was my only perfect unit time intensive and dropped the bast**d !!! well banged it moving it.. typical that's on the picture with a short straight bit shellacked.
 

Samson2

Member
I have 24 arches to make, insitu makes for a much messier job, i'm capable but thought because 12 are the same size, run the straights on the table easy enough for all sizes, then cast the arches in 2 halves from a master for repetitive ones and then the one offs run on the table and repeat.. failing that if anyone interested in helping please get in touch!
 

Samson2

Member
the final finish profile seems to work better than all in one go, minimises the problem but takes much longer, still need to weigh it down.
31913
 

Samson2

Member
just had a good idea why don't you run your curved one insitue its easy enough
Must say keep pondering it but gravity is a bitch! done straight repair in-situ on some renovations I did years on a period house, but a rail across the spring point and apply material .. never done a arch in situ b4 maybe now's the time.. and stop farting about eh???
 

The Hobo

Well-Known Member
Must say keep pondering it but gravity is a bitch! done straight repair in-situ on some renovations I did years on a period house, but a rail across the spring point and apply material .. never done a arch in situ b4 maybe now's the time.. and stop farting about eh???
worth a try /but failing that if you have a lot to make I would make a reverse mould then pour them
 
Top