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08.05.2021, admin
Until recently, producing a single, functional part from composite materials was impractical. Much of the problem was due to the rough and wavy surface that usually resulted when these materials were not used inside a mold.
Also, non-artistic builders find it intimidating to sculpt the shapes freehand. Finally, many resin materials are formulated to be used inside a mold and do not work properly when exposed to air.
Consequently, someone wanting to build a single part or prototype, even the plug for a mold, would often be discouraged and turn away from composites before even getting started! New materials and techniques enable significantly improved results! Sculpting polyurethane foam around simple templates, reinforcing with fiberglass and finishing with an advanced primer system such as Duratec permits a single, functional and cosmetically attractive part to be made, even by someone new to fiberglass! Moldless composite construction is the process through which a functional composite part is built in the first production cycle without the time and expense needed to generate a female or cavity type mold.
It could more accurately be named internally molded construction because a form does need to be made, but this form is covered by reinforcement and often stays with the finished part.
The internal form differs from a classic male mold because it must be made from materials durable enough to be put into service when the part is complete. The plasters, styling clays, body fillers, and wooden frames of plug or male mold construction cannot withstand the rigors of regular use. They are also very heavy and usually fill the entire volume within the part leaving no hollow room for occupants or other internal requirements.
All quality composite parts must be designed to optimize the type of construction being employed. Moldless composite construction requires slightly different design features than classic open molding. Molded parts receive their smooth surface from the mold itself and require little post finishing.
Moldless parts must be built from the bottom up with compatible materials throughout. The final surface will only be as good as the work done on the substrate below it. What effort is expended to build parts this way is mostly done to create the shape and to achieve a smooth outer surface. Moldless construction is similar to plug construction in that a high quality reproduction of a desired shape is the final goal.
It differs from plug construction because it must still have the advantages of a finished composite part. Namely, it must remain lightweight, strong, resistant to vibration and corrosion as well as any other factors necessary for the application. Foam and wood are the most widely accepted materials for constructing the internal formers.
They are easily shaped, have relatively high compression strengths, and are cost effective. A moldless composite part can be designed in two ways. The first way is to create a form which is skinned with fiberglass on the outside. The form is then removed from the inside leaving just the reinforced outer shape. This will yield a thin lightweight fiberglass shell of the desired shape that often remains quite flexible. This is a good method to use when the part will be fully supported and the inside cavity must remain as large as possible.
The second way is to leave the form in place and reinforce it from both the inside and the outside. This takes advantage of the stiffness associated with a sandwich core structure. Obviously, access must be available on both sides of the form to do this. When planning a moldless project, consider all the objectives the finished part needs to meet.
Weight, stiffness, internal dimensions, and the final external shape all need to be considered before beginning. Material selection is important to achieve the balance between all these properties. Since the intended shape is always known, it is common to begin designing from the outside and work in.
Choose a material which will shape easily to the external contours. Polyurethane foam sands and cuts well so it is used very often for this type of construction. If the curves are very broad and the cavities quite deep, select a thick foam so the sanding will not go all the way through it.
Individual sheets of foam can be stacked and glued together to make a panel of any thickness. Experience also teaches to use thicker foam across any broad unsupported areas. The longer the unsupported span, the thicker the foam should be. For instance, 1" thick foam can be used to fill any cavity up to about 36" wide.
However, all cavities broader than 36" should use 2" thick foam so the shape does not distort while sanding. Next, consider how much room must remain on the inside of the part. A legitimate concern with Moldless Fiberglass Boat Construction Words this technique is that you can lose a little space on the inside due to the thickness of the foam.
If the first method of moldless construction is followed where the internal form is torn out after molding, this is not a problem. However, if some of the foam is to remain to help stiffen the part make sure adequate clearance is maintained around all moving or hot parts which might be housed within the cavity. If a thick foam was necessary for the external shaping but more space is needed internally, the foam can be sanded thinner on the inside after the outside is reinforced with fiberglass.
Weight, corrosion resistance, and stiffness must also be addressed. Moldless parts can be as light as their molded Moldless Fiberglass Boat Construction Project counterparts, and both epoxy and polyester resins will provide good protection in most settings.
However, stiffness does need more forethought because material selection can effect it dramatically. If the part needs to be extremely light and does not need to be very stiff, the thin shell can be made and the form removed. Just enough fiberglass can be used to have the part hold its shape, usually two layers of a medium weight woven cloth. If the part must be moderately stiff and light, more layers of fiberglass can be added with carbon fiber used in spots as a selective stiffener. If the part needs to be 'rigid', leave as much of the foam core in place as possible to form a sandwich.
Skin this on the inside with fiberglass or carbon fiber depending on the ultimate strength needed. The last thing to consider is the ultimate size of a composite part. Moldless parts actually 'grow' as they are reinforced.
If the application is dimensionally critical, one must subtract the thickness of the outer fiberglass skin from the desired finished shape so the internal form is made small enough that everything will equal out when the reinforcement is added.
Following the designed plans, construction begins on the internal form or mold. Artistic individuals may be talented enough to sculpt the entire shape freehand, but a more reliable approach is to create dimensionally accurate templates which represent the final shape of the part.
The templates are two dimensional silhouettes of the shape from the side or front of the structure. These are usually placed at the edges of the part or anywhere the shape changes dramatically. A good example to help visualize the silhouettes are the ribs in an airplane wing. Each rib is located in a precise spot in the wing and is responsible for creating the shape of the wing in that area.
The templates provide support and a stopping point for the sanding blocks used to shape the foam. The templates can be made of wood, metal, or plexiglass. They need to be harder to sand and cut than the foam. This permits the foam to be sanded without affecting the overall shape of the structure. Draw the shape onto cardboard, cut it out, and trace it onto the chosen template material.
Use a jig saw to cut out the form. Be sure to cut it slightly on the large side so it can be hand sanded to the exact final shape. Material cannot be added if the template is cut too small, it will have to be remade.
The templates must be perfectly sized. It takes less time to fix an incorrect shape at this stage than to correct a problem later. A loose framework is usually constructed to hold the templates rigidly in place. If the framework can be incorporated into the finished part such as the floor in a car, this step is quite easy.
If the framework cannot exist in the final part, it will be necessary to design the support so it can be removed after the reinforced shape has been created. It is impossible to offer more specific advise on this procedure as the applications are virtually unlimited. Just remember to plan for this measure before proceeding. The structure should now have the appearance of a skeleton.
The next step is to fill in all the open space along the surface with foam. Select the foam that is stout enough to hold its shape across the span. Again, the broader the span the thicker the foam must be to remain rigid.
Begin filling in the largest and flattest portions first. Measure the space in the frame and mark the foam accordingly. A long adjustable razor knife and a metal straight edge are used to cut the foam sheet.
Try to cut all the way through the foam in one pass, as multiple cuts usually result in an uneven cut. If the cut is uneven or does not fit closely enough, use a sanding block with 40 grit paper to correct the fit. Glue the foam to the framework using a quality wood glue or a glue gun. A hot glue gun will speed the procedure significantly. It is important to note two lessons in foam placement which must be followed for satisfactory results. First, the foam must be fastened to the templates so its surface remains higher than the templates in all areas.
The excess foam will be sanded off in the shaping procedure to follow. Second, keep the glue below the outer edge of the template so that it does not have to be sanded. It is easy to gouge or sand away too much good foam while trying to level off a stubborn drop of glue. The hole can be filled with fillers , but cleanliness and forethought will eliminate the problem.
When the flat spans are filled with foam , cut beveled pieces to fit into the contoured areas. Thicker foam sections work well for this because they tend to be easier to shape.
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