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SUPERYACHT #11
Winter 2007

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news


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Article by
Stefano Beltrando
Q.I. Composites


MALE AND FEMALE MOLDS

 

Building a long carbon fiber mast

 

The first revolution in mast manufacturing occurred in the '30s when for the first time aluminum masts, instead of the traditional wooden masts, were used on the large J-class boats participating in the America's Cup. Then in the mid '90s the second radical change occurred: the first boats with carbon-fiber masts appeared on racecourses. Since then, carbon-fiber-manufacturing techniques have been significantly refined and today single-piece masts exhibit the intrinsic characteristics of these fibers. Let us see, then, how these highly performing and resistant carbon fiber masts are manufactured nowadays.

Two main methods are used for building a laminated structure: one in a mold (female mold) and the other one around a mandrel (male mold). In the first case the first layer of fabric is the one exposed to sight and to the environment; in the second case the opposite occurs, the first layer is hidden while the last one is exposed.

Thus, carbon fiber masts can be manufactured in two ways: by laminating the fabrics inside two bigger halves of a tube (the molds), then by detaching the two obtained mast halves from the molds and finally by uniting them (generally the forward half with the rear half, rarely the right half with the left half); by wrapping (laminating) the fabrics around a mandrel and by pulling out the mandrel to have the finished mast.

At a more superficial analysis the male mold method appears easier because the difficult operation of attaching the two halves is avoided (even more so in the case of larger masts) and because it offers other advantages that we will try to describe below.

Nevertheless, this method hides several traps, the first one being the esthetical finish. As a matter of fact one should always remember that an expensive item such as a carbon fiber mast should also appear beautiful, especially if it is to be fitted on a superyacht worth several million Euros! Even though the appearance seems to be needless for an object used to increase the boat performance, it is undeniable that the carbon fiber appeal does not lie in its mechanical characteristics but, also (or above all?), in the high-tech image that it conveys to the whole boat. A mast built in a female mold will definitely be smoother and more uniform than the one manufactured in a male mold. In the first case the fibers are in contact with a perfectly smooth surface, the mold, while in the second case - working with a mandrel - the final product will have a rugged surface with wrinkles (due to the inevitable application of reinforcements and not the manufacturer's fault).

Solution: after pulling out the mast from the mandrel, it will have to be ground, perfectly smoothed and painted either with a transparent varnish revealing the fiber or with a colored one.

This article deals with the necessary steps for manufacturing a carbon fiber mast around a mandrel.

The mandrel

This element may be in composite materials but most often it is in aluminum and in some cases it is a mast itself. First, a primer is applied, generally a Teflon film, to ensure the easy pulling off of the carbon-fiber mast; second, the mandrel is stiffened to prevent it from deforming when the fabric is applied during handling or curing time.

Lamination

The application of "prepregs" follows a specific plan called lamination schedule. Such schedule is the result of the structural analysis carried out by the designer according to the definition of the loads on the mast and on the determination of the number of layers needed to support such loads.

Prepregs, in carbon fiber with epoxy resin, are transported and preserved in a freezer to guarantee their workability and duration. From there, they are pulled off and cut just before application.

Below there is an example of a lamination plan of a mast about 30 meter (98 ft) long which may have up to 60 layers of carbon fiber.

At this point the ability of the laminators is essential for the realization of a good mast because the tension with which the fabric is laid determines the quantity of voids, that is to say, its stiffness and duration. It is intuitive that, when applying fabrics on convex surfaces, the risk of having voids is lower than on concave surfaces, for in the first case fabrics can be stretched more easily to make them adhere while in the second case fabrics must be pressed.

In general, during lamination, the application of layers is periodically interrupted to carry out the intermediate consolidation with vacuum bagging.

It is obvious that greater the number of interruptions and consolidations better will be the final result. Unfortunately such operation requires hours of work often with disposable material (vacuum bags and mastic), this is why manufacturers tend to reduce to the minimum the number of intermediate vacuum bagging cycles. From our point of view, as product quality supervisors, consolidations every 1-3 layers lead to a final perfection while less frequent cycles increase the risk of "voids" that cannot be eliminated.

The application of reinforcements

The mast needs local reinforcements capable of supporting loads or special friction in the following points: mast step; halyard exit; vang and boom attachments; spreaders attachments; shroud and stay attachments; masthead.

In these points additional layers are to be added to the lamination for reinforcement. In some cases, such reinforcements may even double the initial thickness creating sorts of "bumps".

In the case of a female mold, they are easily applied on the inside of the mast, on the two open halves before the final gluing operation; in this way they will not appear on the outer surface.

"Heat Curing"

Heat curing is the term used today for the hot polymerization process of the resin. Before curing the mast, apply a last vacuum bag and introduce it, if available, in the autoclave that, as a hyperbaric chamber, increases on request the environmental pressure inside it. This autoclave bagging cycle is used to press the fibers so as to reduce to the minimum, or better eliminate, all voids.

Usually, approximately nine hours are needed for heat curing at a temperature between 80 and 100°C. Nevertheless, for safety reasons and in order to avoid thermal shocks the preferred heating cycle is: initial heating of the oven at a rate of 1° per minute up to 90°, then heat curing for approximately nine hours and, finally, slow cooling for about three hours. Result: approx. 13 hours in the oven.

The pulling off and finishing phase

The mandrel, which for easier extraction, for practical and space reasons may be modular is pulled off by means of hydraulic pistons - if the mast were 30m long the single-piece mandrel should be at least as long thus a 70m long or bigger shed would be necessary.

Once the mandrel is off, the mast can be chemically or ultrasound tested. Lastly, the mast shall be completed with: drilled holes (halyards, spreaders, etc.) spreaders tangs boom and vang goosenecks an electricians wire fish for passing halyards electrical cables wind and radio station stays and shrouds.

This last finishing phase is the slowest.

There is one more difference between the female and male mold manufacturing methods. In the first case, the two halves have to be inevitably joined and, usually, this operation is done with structural adhesives, supported by a large quantity of rivets or screws during heat curing that join the flanges of the two halves to allow for a good hold of the adhesive.

In this case, two aspects clearly emerge: The high number of holes (in some cases even a thousand) requires extra fabric to prevent them from weakening the structure, thus resulting in extra weight, If the person in charge of the gluing phase is not experienced the damage may be enormous.

In the table below, the two manufacturing methods of the custom production of a 30m long mast are compared:

DetailMale moldFemale mold
Esthetical finish In most cases it needs much fillerNo need of fillers
Presence of voids Normally a volume of less than 2% If vacuum bagged less than 2% otherwise higher
Repair workSimpleSimple on the tube, difficult on the reinforcements
Mechanical properties Good on the averageGood on the average - much depends on the operators
Recurring defects Cracks in the fillings, uneven surfaces Voids at the edgesCracks may occur along the gluing line