Hydroforming revolution

C4 Carbon has been the first to professionally propose carbon fiber blades for freediving and spearfishing fins and has led for many years the international development of such material applied to diving. Today another Revolution is made by the Italian company, with the technical lead of its founder, Marco Bonfanti. The moulding process is done utilizing the hydroforming technique. Using a liquid instead of air stabilizes the process obtaining an even more precise result in each single part of the blade. Performance, resistance, durability and perceived quality all improve. This is also visible with one’s bare eyes, where the extremely high perceived quality of the matt surface of the blades of the new C4 MB 001 and C4 MB 002 is clearly visible.

C4 MB 001 & MB 002 fins

Why hydroforming – Marco Bonfanti’s genius

We have wanted to interview Marco Bonfanti about this new revolution he has thought, developed and made possible, to understand how it works, what it means and the evident and expected effects.

Traditional processes

“To join the layers of carbon it is necessary to apply a uniform compacting force over the entire surface concerned. Air is normally used as a means of transmitting force, both for operations in an autoclave and for those performed with vacuum alone.

The autoclave allows the compacting force to be controlled, so several atmospheres of pressure can be used. In the case of a vacuum the pressure is limited to the atmospheric one, therefore less than 1 BAR. This applies both in the case of using a vacuum bag and in the case of infusion or resin injection.

Alternatively, a mechanical means such as a rubber pad can be used, therefore always an elastic material, which, like for gas, adapts to the surface to be compacted by copying it.

It is necessary to consider how the polymerization of the resins, both liquid and/or prepreg, is not instantaneous. Thanks to the temperature variation, the viscosity of the resin varies from liquid, to very liquid, and then congeals until complete solidification.

By squeezing and/or compacting with an elastic force system, the pressure should necessarily change following the variation of liquidity of the resin. Differently, it will be difficult to control the quantity of resin in the final product. Something can be varied with the autoclave, while it is impossible to do so with a vacuum or with an elastic rubber buffer.

The adaptation of the autoclave regulation is not perfect because it is in linear variation, while the density variation of the polymerization resin is progressive and not linear.

I therefore thought of modifying these compaction systems, based on the elasticity of the compaction medium, replacing them with an inelastic system. I did this in order to obtain perfect compaction, that does not depend on the fluidity of the resin and consequently does not affect the percentage of resin in the product with the resulting quality variations.”

Simply remove the problem at the source

“I did the first tests years ago with bicycle frames. It was very complex to laminate the frame and difficult, almost impossible, to completely eliminate the liquid from the sac inside the frame after molding, so I abandoned it.

I subsequently tried it with the fins, and after numerous tests of various hydraulic bags and various modifications to the system and to the moulds, tests that required several years of work, I was able to apply it to the production of the MB fins.

There was also to consider how with a single mould it was necessary, purely for production reasons, to make several models of fins with different stratifications. The 3D shape to be compacted is complex and the variations are minimal given that a layer of carbon is on average 22/100 mm. C4 has always used particular shapes for the internal layers and this amplifies the problem. For a company like C4, production is not a secondary topic.

So this C4 application in hydroforming instead of using an elastic medium, as the name itself suggests, uses a liquid, in our case mineral oil since the temperature of use is 120°C and plain water would boil.

The oil is contained within a shapeable bag which forms part of the forming mould, the pressure of the oil compacts the carbon layers in an inelastic and homogeneous way, therefore theoretically perfect. I don’t know of other known examples of carbon fiber molding with a hydrobag system.

To get an even rough idea of what it entails and how it works, think of the difference between a hydraulic cylinder and a pneumatic one. At equal pressure, the pneumatic one springs, the hydraulic one is immovable.”

Perceived quality of the blade

  • “The resulting quality is clearly perceptible in the carbon moulding as homogeneity of the surface without an abundance of resin. It is usually an abundance of resin that makes the surface of the blades “nice and shiny”, basically a covering and leveling layer. In our case the percentage of resin is minimal, the fibers are evident on the surface and the homogeneity is such that it is easily perceptible.”


  • “Given that performance depends on the whole of the fin, shoe and blade, and not on just one technical aspect, the perfect compaction with a minimum amount of resin makes the MB extremely reactive, the water test will prove it.”

Resistance and durability over time

  • “A composite so controlled in its construction and with minimal resin can only improve its resistance to fatigue. It wasn’t a necessity for C4 to improve reliability, but if you can, why not?!”

Design of the C4 MB 001 & C4 MB 002

The C4 200 series, with the S-990, are somehow the revolutionary top of the line offer by the Italian Company, and the Umberto Pelizzari polymer blade fins are the amazing offer in the entry level professional segment. The best interpretation of the concept of traditional carbon fiber fins by C4 Carbon are definitely the MB 001 and MB 002. With a classic shape of the blade, the two fins C4 MB001 and C4 MB002 differ for the length, while both utilize the 250 foot-pocket. Let’s analyze them better.


T700 carbon fiber

100% carbon fiber of course, the C4 MB 001 and C4 MB 002 utilize the T700 texture, where the bigger squares (measured equal to 8 mm per side), compared to the traditional twill material, improve the reactivity. This is a solution utilized by C4 since a few years.

The length of the blades, as anticipated, differs between the MB 001 and the MB 002, with the first equal to 806 x 195 mm and the second to 874 x 195 mm. The total length of the fins is 920 mm for the MB 001 and 980 mm for the MB 002.

Reverse Parabolic lamination

But the other real innovation, in addition to the “revolution” of the hydroforming, is based on the “Reverse Parabolic” lamination. This is a parabolic curvature which, instead of having the part with greater flexion at the end of the blade, has it near the foot. A complete inversion of concepts and an evolution of what the C4 DPC (Double Parabolic Curve) idea is. The great improvement of performance comes from the fact that now most of the fin blade area generates thrust, while only a minor part remains in an almost straight position. As we know, in fact, the area in front of the foot-pocket generally produces little thrust, but still moves water and so requires effort.

C4 MB 001 and MB 002 fins

According to C4 the improvement in performance is so evident that it is possible to use stiffer blades with the same effort, but evidently better thrust. For such reason the indication of the stiffness of the blade has a “+”, as a 25+ for example has actually the stiffness of a 30 but the feel and requested effort of a 25. Restarts from the bottom, accelerations and speeds of the 25+ are thus higher and comparable with a 30, but require the effort of a traditional 25. The available stiffnesses are: 20+ ExtraSoft /Soft, 25+ Soft/Medium, 30+ Medium/Medium Hard

Profile of the tail

C4 MB 001 and MB 002 fins

The profile of the “tail” of the C4 MB fins blade is straight on the sides to utilize all the length of the blade, with a central V helps to additionally stabilize the fins. In such way, together with the long water rails, the kick is extremely stable in every condition.

Water rails

The blades of the C4 MB fins feature new rounded section water rails which improve water containment along the blade and reduce water vortexes on the outer sides. The measured total height is constant and equal to 15 mm. The action of the water rails determines an increased and correct amount of water sliding along the blade. This water is the one that generates the thrust, so the correct design of the water rails improves the performance of the blade. As, thanks to the Reverse Parabolic lamination, the C4 MB feature a greater thrust surface of the blade, the water rails have been designed to cover the entire length, from the foot pocket to the tip of the blade. Such solution evidently also helps to protect the tip of the blade on the sides, especially during the spearfishing action among the rocks.

250 foot-pocket

We have already presented the new 250 foot-pocket with the Umberto Pelizzari fins x-rays. The 250 puts together and improves the qualities of the 300, extreme lightness and compactness united to great force transmission, with the ones of the 400, meaning great comfort and ease in putting them on and taking them off.

Rigid sole

The best way to transmit the force from the foot to the blade is to utilize a rigid sole, already present on the 300 and also on the 200 foot-pockets. The C4 250 has a rigid sole in polypropylene co-moulded in the foot-pocket, a solution that requires an advanced design and top-level manufacturing systems. The preformed angle is the traditional by C4.

Soft thermo-rubber

To guarantee the best comfort, the C4 250 is produced with a special thermo-rubber compound. The use of soft but resistant thermo-plastic material is possible thanks to the rigid sole. The softness of this material gives extremely high comfort, permitting the use also without socks. Excellent ease of donning and doffing of these foot-pockets completes the top comfort features.

Extremely reduced weight

With the weight of only 247 grams (255 grams measured by AP) in the size 41/42, the third goal has been evidently well achieved! Believe it or not, less weight, even in the water, definitely determines less fatigue, especially during long sessions. Maybe something not perceivable by the less experienced freedivers and spearos, but indeed very important. Whether it is a freediving session or a long spearfishing day, the limited effort to move the fins determines reduced tiredness at the end of the session or the spearfishing day.

Available sizes

An important aspect of the C4 250 foot-pocket offer is the amazing number of sizes. EU options are 36/37 (223 x 87 mm), 38/39 (242 x 94 mm), 40/41 (261 x 101 mm), 41/42 (270 x 104 mm), 42/43 (280 x 108 mm), 43/44 (288 x 111 mm), 44/45 (299 x 115 mm), while for US/Canada size references are 4/4.5, 5.5/6, 7/8, 8/9, 8.5/9.5, 9.5/10.5, 11/12.

Additional pluses

The lateral rails reduced to the minimum (less than 3 cm) are something that C4 “invented” in 2006 with the launch of the Mustang foot-pockets. The reduced presence of lateral horns of the foot-pocket produces the best performance of the blade, as they do not interfere with its flexibility. The result is also an extremely easy and fast substitution of the blade when needed, even during the same diving session. To ensure the best connection between the C4 250 foot-pocket and the carbon fiber (or polymer) blade, C4 has designed an extremely refined locking system. A single large-diameter stainless steel screw connects to a big brass nut embedded in a generous size plastic element. The screw has an Allen head size 4 and is complete with a threadlocker.