Composites

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The weekly assignment focuses on designing/machining a 3D fabricatable CAD model for moldmaking, basic knowledge of mold releases and demolding procedures.

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My project related to composites involves the design of a boat hull. The hull is very accurate, I’ve studied naval architecture, so the proper commands and tools have been used in order to optimize the design. The software package used for design is McNeel Rhinoceros (trial version for 90 days). Once the design has been completed, the shape has been exported in a STL file.

The exported STL file has been inported into MESHCAM, the software used to shape-to-code generation. A lot of parameters need to be set, i.e. : side to machine, material, size, retract height and so on. Of course the size of the TOOL need to be entered. ENDMILL (diameter 8 mm flat-head) has been used for roughing and BALLMILL (diameter 3.2 round-head) for finishing.The mentioned settings have been resumed by means of the following figure.

Endmill and ballmill setting. (FEEDRATE, PLUNGE RATE, STEPOVER, DEPTH PER PASS)

Then we moved to CNC machine. First step is the most important: clean the machine ! Every square centimeter under the block of material need to be cleaned in order to avoid verticality errors.

As usual, the security measure need to be set, including: headphones and glasses, aspiration and compression system. The block of material has been secured with screws on the four edge touching the base support. At the end of starting settings, the transition from CAD to CAM to MACHINE is completed. The machine starts working.

After a couple of hours both works end. The shape has been correctly digged but,maybe for a stepover problem, small lines appear. Sandpaper solve manually this problem. The shape is now ready for tissues positioning.

WET LAY UP. Wet lay-up is a molding process that combines layers of reinforced fiber with liquid resin to create a high quality laminate. This process involves the positioning of reinforcement material into or against a mold in layers. These layers are then impregnated with a liquid resin system, either with a brush or roller, to ensure a good wet-out of the reinforcement material. The step is repeated until reinforcement thickness is achieved.Curing can be performed at room temperature or under heat, depending on the selection of the resin system. This can be accomplished with or without the use of a vacuum bagging process,recommended for a higher quality parts(see also this page for further details).Used components are showed above and summarized in the following: Peel Ply-Plastic Laminate , Holes-Breather, Linen Fabric and Air-Vacuum Bag (usage detailed in the following), bleeder and of course epoxic resin.

The first layer (used to harden the mould) is the GELCOAT(mix: 100% GELCOAT – 2% REAGENT)

After a few hours, when the gelcoat is hardened, the other layers have been placed. Thus the order is: gelcoat, release agent in the mold, peel ply, three layer of linen fabric and resin (alternated), peel ply, bleeder , breather (allow uniform application of vacuum pressure over laminate and removal of entrapped air or volatiles during cure)and vacuum bag (closed with sealer tape).

FIBER ROTATION. The intermediate layer of linen fabric has been roated of 45 degrees. The orientation of fibers at +45 degree or -45 degree increase the stiffness the global resistance of the structure. This is a well known procedure in composite design.

RESIN TO LINEN RATIO CALCULATION. The ideal ratio between resin and fabric it's 30% resin and 70% fabric. In this specific case, considering the textile material (linen) the ratio considered is 50% resin and 50% linen. The specific weight of linen is 200 grams/square meter. The three layers total area is approximately 0.3 square meter. So, the linen weight is approximately 60 grams. 60 grams of resin are required and considering the ratio 2:1 (resin:hardener) the hardener weight is 60/2 grams equal to 30 grams.

The procedure is very complicated, because layers move and they can not be fixed.

At the end of the first part of the procedure, the entire object is placed under vacuum.

Demoulding.

And finishing.

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Download area

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The following files are downloadable:

Type of file	Description	Link (Type of file)
Fabrication files: vectors(s) / 3D model(s)	Rhinoceros file	(3DM)
Fabrication files: vectors(s) / 3D model(s)	MeshCAM file	(MCF)
Fabrication files: vectors(s) / 3D model(s)	Exported mesh file	(STL)

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