post curing large laminates

[Buckle]

Just wondering who in the industry post cure very large laminates whilst still in the mould?

By large, I thinking between 40-100ft structures (i.e. hull topside or deck).

In addition, what sort of heating do you use. Water, electrical, oil etc.

Am in need for compairing ideas to people in similar positions.

[Jimbo1490]

How hot do you need to go?

Jimbo

[jonsailor]

We wall off a portion of our boat building shed and post cure at about 70 C
fully assembled out of the mould.
Your resin supplier should have all the curing times and process.
If you go over 85 degrees, keep it in the mould as it will move.
With a very good system, you may get up to 120 degrees but this is a very delicate operation due to out gassing and movement.
We use gas operated heaters with good air circulation from fans etc.

[Buckle]

Hi,

Before I get this topic going, this is the basis for dicussion.

Current production is based around poylester of types used on hulls and decks. Typically with DCPD or vinylester skin coats (depending upon gelcoat colour).

After a month of curing out of the mould, hull topsides typically have a HDT of around 55 deg.C.

Sadly we have a very high turn around of hulls. 4 men laminating can knock a 42 footer out in 6 days. Likewise, 6 men, for a 100footer in 4 weeks. Current production schedule doesn't allow for a good stable cure in the mould.

With infusion on its ways soon, we can see another smallish hull being knocked out every 3 days. Post curing of hull topsides can help eliminate potential warantee claims of hull surface distoration. I would love to get a hull topside to 80 deg.C.

Sadly I need a very rebust and reliable moulding. Moulds are often moved during their lifetime.

Mould are typically made from Rapid tooling systems and are good for post curing applications. Moulds are typically 1" balsa cored structures to help reduce weight. Laminate skins typically 4-6mm.

If we place water pipes or electrical heating elements directly behind the skin coat, are can achieve an even surface temp of 80 deg.C

This is all very well, however if theres a leak or a electrical circuit break, we are stuffed. THe practical solution is to place the heating system directly behind the balsa core, and lots of lagging on the backside to keep the heat in. Sadly 80.deg C is impossible to each.

Any ideas or advice???

[Jimbo1490]

How about a bottomless box that you can drop down from above the mold. This creates a sort of removable 'oven' over the mold. This could be made of lightweight paper cored honecomb with aluminum skins to retain heat. Such panels are available off the shelf for use in low-cost housing and are already certified as flame retardant.

When working the mold, the box, which could be either whole or divided into 'U' shaped sections, is hoisted clear. When laminating is complete, the box is lowered over the entire mold. The heat available from the current setup may be sufficient if enough is retained. If not, heating the air under the box will get the temp in range. Did this on two mold setups. Much smaller overall but the idea is scalable. My setup was about 2' by 2' by 42'. The oven box hung from the ceiling and lowered onto the layup and heated air circulated within. A process controller kept temps in range. I could acheive 90C with this setup. The cost was very low, a few hundred dollars.

Jimbo

[Buckle]

Firstly, I forgot to mention, we also lag the inside of the hull with an ultra light weight, but highly thermo insulting jacket. Heat loss it minimal only a couple degrees.

Secondly, some large hulls have a 4m + topside. The height on the hull from rubbing band down toward the keel is easily 6m on some large moulds. Hull mouldings just release by only a couple inches when craned out of the mould upon release. A large collapsible box isn't practical. A box would also take too long to erect and dismantle. Production schedules are very tight (despite the fact the product is some cases is worth millions).

A disadvantage of using box type ovens is uniform heat distribution. We require a maximum ramp rate of approximately 10 deg.C/hour (build-up and build-down). Air boxes are prone to hot pots etc.

Thanks for the good idea but sadly it isn’t practical for a number of reasons.
Any other ideas?

[mobjack68]

Hey Buckle, this is gonna sound a little wierd, but the portable "box" heater may be feasible after all. My wife owns greenhouses (I did boat repair/construction, f'glass polyester fabrication/moldmaking for years) and the idea of a movable tent may truly work for you. Our greenhouses are metal frame, plastic covered arrangements, but what if you could use a pvc pipe frame instead of metal ribs? The real tidbit that I wish to share is this, our greenhouses are covered with 2 sheets of plastic and the plastic is inflated, not a high pressure fill, just enough to keep the air layer about 4-6 inches thick. Therma-pane!! Makes the heat loss/gain a whole lot more manageable, I have no idea what your latitude is, but there has to be a way to capture solar heat (passive or active) to heat your "tent". I continue to look at our greenhouses and wish I could capture just some of the heat that is being pulled out by the cooling fans. It's a shame you aren't building boats in our backyard, we could just install some 48" tubes to your buildings...you could have ALL the heat, I'm trying to get rid of....
mobjack68

[fiberglass jack]

i use lights to heat the moulds, i do some carbon work , the company is in the film busness so i have access to 18 k lights i set up 4 of them aimed at a corner of the shop the walls are covered with foil she goes to 250 c if i want more heat i use more lights

[fiberglass jack]

why dont u get a heating contrator to help u out, they place that floor heating system for marble flooring, u could have this runing through your mould it can be controled also , the same for the green house set a tent over the mould cover with foil sheets this will keep in the heat and place alot of heaters,

[Richard Hillsid]

The floor heating system by fiberglass jack, reminded me that we once routed in grooves to a 90’ plug and installed electrical heating wiring into it before glassing over; it had 6 separate circuits as heat tends to rise and les was needed at the top, the bottom in this case.

[Buckle]

Hi,

Having an electrical or water system imbedded close to the mould gelcoat surface is a great idea. However we need a failsafe system. What if the heating system fails in some way?

Approximately 5 years ago, one of the first large structures I infused was a 25m wind turbine blade. The system worked really well, until one day we received a phone call reference a partial electrical heating element failure.

The post cure was vital for the epoxy curing process. Resultantly the mould cut up and the section repaired from the backside. The cost was tremendous with loss of production. A lesson learned the hard way.
From my knowledge, this company still uses this system today often suffering from the same failure owing to regular movement of moulds.

[Jimbo1490]

With the electric heating elements imedded diretly in the mold. it's almost impossible to have a fully failsafe system. But with water pipes imbedded instead, a failsafe system can be realized. Using an electric water heater with an automatic backup generator could protect against power failure, while thermomechanical valves can prevent over temp conditions in case a process controller or primary thermostat on the water heater fails leaving the elements on. Water heaters and boilers always have a 'T&P' fail safe valve additionally. And the maximium temps are those acheivable with water at or just above atmospheric pressure anyway, perhaps 120C.

Jimbo