Advantages and Disadvantages of Vacuum Infusion Molding of FRP Ships

 

 

FRP is a new type of composite material used in shipbuilding in the late 1960s. It has the characteristics of light weight, high strength, corrosion resistance and strong plasticity. After decades of development, FRP materials have been widely used in the construction of small and medium-sized boats, especially in yachts, high-speed boats and tourist passenger ships in recent years. This article focuses on the new process of FRP shipbuilding molding-resin vacuum infusion method.

 

Process Technology Introduction

 

The resin vacuum infusion method is to pre-lay the reinforcing fiber material on the rigid mold, then lay the vacuum bag, and remove the vacuum in the system to form a negative pressure in the mold cavity. The pressure generated by the vacuum is used to suck the unsaturated resin into the fiber layer through the pre-laid pipeline, so that the resin can infiltrate the fiber material and finally fill the entire mold.

 

After curing, the vacuum bag material is removed and the desired product is demolded from the mold. The vacuum infusion process uses a closed system in a single-sided rigid mold. It is a new type of large-size boat molding and construction technology. The process appeared in patent records in 1950, and it was not introduced into China until around 2000 and has been developed. Since this process was introduced from abroad, it has many names, such as vacuum introduction, vacuum infusion, vacuum injection, etc.

 

Process Technology Principle

 

The vacuum introduction process is based on the hydraulic theory founded by French hydraulician Darcy in 1855, namely the famous Darcy’s law: t=2hl／(2k(AP)); where: t is the resin introduction time, which is determined by 4 parameters; h is the resin viscosity, which refers to the viscosity of the introduced resin; z is the introduction length, which refers to the distance between the resin feed port and the outlet; AP is the pressure difference, which refers to the pressure difference inside and outside the vacuum bag; k is the permeability, which refers to the parameters of glass fiber, sandwich materials, etc. for resin infiltration.

 

It can be seen from Darcy’s law that the resin introduction time of the vacuum introduction process is proportional to the length and viscosity of the resin introduction, and inversely proportional to the pressure difference inside and outside the vacuum bag and the permeability of the fiber material.

 

Process Technology Flow

The specific process technology flow is as follows:

 

 

Step 1: Preparation for Work

First, make a steel or wooden mold according to the ship’s line and size. The inner surface treatment of the mold must ensure high hardness and high gloss, and the edge of the mold must be kept at least 15cm to facilitate the laying of sealing strips and pipelines. After cleaning the mold, apply the demoulding material, which can be waxed or watered.

 

Step 2: Applying the Hull Gel Coat

According to the requirements of ship production, apply the gel coat resin containing accelerator/initiator on the inner surface of the mold. Product gel coat or polished gel coat can be used. The types of selection are orthophthalic, isophthalic and vinyl. Construction can be carried out by hand brushing and spraying.

 

Step 3: Laying Reinforcement Materials

First, according to the hull line and basic structure, cut the reinforcement materials and skeleton core materials respectively, and then lay them in the mold according to the layup diagram and molding process. When laying, the influence of the material and connection method of the reinforcement material on the resin flow rate must be fully considered.

 

Step 4: Lay Vacuum Auxiliary Materials

On the reinforced material laid in the mold, first lay the demoulding cloth, then the guide cloth, and finally the vacuum bag, and use the sealing strip to compact and seal. Before closing the vacuum bag, carefully consider the direction of the resin and the vacuum pipeline.

 

Step 5: Remove the Vacuum in the Bag

After the above materials are laid in the mold, clamp the pipe system to introduce each resin into the pipe, use the vacuum pump to vacuum the entire system, try to evacuate the air in the system, check the overall air tightness, and make local repairs to the leaking areas.

 

Step 6: Mix the Resin Ratio

After the vacuum in the bag reaches a certain requirement, mix the resin, curing agent and other main machines in a certain proportion according to the environmental conditions, product thickness, laying area, etc. The prepared resin liquid should take into account the appropriate viscosity, suitable gel time and expected curing degree.

 

Step 7: Introduce Resin into the Mold

Introduce the prepared resin into the pressure pump, stir it thoroughly to remove the bubbles in the resin, then open the clamps in sequence according to the introduction order, and implement the resin introduction by continuously adjusting the pumping pressure to effectively control the thickness of the ship hull.

 

Step 8: Curing, Demoulding and Outfitting

After the resin introduction is completed, the hull should be placed in the mold for a period of time to allow the resin to solidify, generally not less than 24 hours, and it can be demoulded only after its Barcol hardness is greater than or equal to 40. After demoulding, necessary measures should be taken to support it to avoid deformation. After thorough curing, the hull is closed and outfitted.

 

Analysis of Advantages and Disadvantages of Process Technology

 

Advantages of Process Technology

As a new type of FRP ship building molding process, the vacuum infusion method has great advantages over the traditional manual paste process.

 

A1 The Strength of the Hull Structure is Effectively Improved

 

During the construction, the vacuum infusion process can simultaneously lay the hull, reinforcement, sandwich structure and other inserts of the formed ship, thereby greatly improving the integrity of the product and the overall structural strength of the ship.

 

Under the same raw material conditions, compared with the hand-pasted hull, the strength, stiffness and other physical properties of the hull formed by the resin vacuum infusion process can be increased by more than 30%-50%, which is more in line with the large-scale development trend of modern FRP ships

 

A2 The Weight of the Ship Hull is Effectively Controlled

 

The FRP ships produced by the vacuum infusion process have high fiber content, low porosity, high product performance, especially the improvement of interlayer strength, which greatly improves the fatigue resistance of the hull.

 

Under the same strength or stiffness requirements, ships built by the vacuum infusion method can effectively reduce the structural weight. When the same layer design is used, the amount of resin used is reduced by 30% compared with the manual paste process, with less waste and a resin loss rate of less than 5%.

 

A3 The Quality of Ship Products is Effectively Controlled

 

The vacuum infusion process is less affected by the operator than the manual paste process, and there is a high degree of consistency whether it is a single ship or a batch of ships.

 

The amount of reinforcing fiber of the ship has been placed in the mold according to the prescribed amount before injecting the resin, and the resin ratio is relatively constant, generally 30%-45%, while the resin content of the hand-pasted hull is generally 50%-70%, so the uniformity and repeatability of the ship are much better than the hand paste process.

 

At the same time, the accuracy of the ship produced by this process is also better than that of the hand-pasted ship, the flatness of the hull surface is good, and the labor and materials of the grinding and painting processes are reduced.

 

A4 The Factory Production Environment is Effectively Improved

The vacuum infusion process is a closed mold process, and the volatile organic compounds and toxic air pollutants generated during the entire construction process are confined to the vacuum bag.

 

There are only trace amounts of volatiles when the vacuum pump is exhausted (filterable) and the resin is mixed. Compared with the open working environment of traditional hand-pasting, the on-site construction environment has been greatly improved, effectively ensuring the physical and mental health of the relevant on-site construction personnel.

 

Disadvantages of Process Technology

 

B1 The construction process is relatively complicated

 

The vacuum introduction process is different from the traditional hand-pasting process. It is necessary to design the fiber material layer diagram, the guide pipe system layout diagram and the construction process in detail according to the drawings.

 

The laying of the reinforcement material and the laying of the guide medium, guide pipe and vacuum sealing material must be completed before the resin is introduced. Therefore, for small-sized ships, its construction time exceeds the hand-pasting process.

 

B2 The Production Cost is Relatively High

The vacuum introduction process has high requirements on the permeability of the fiber material, and most of them use continuous felt and unidirectional cloth, which has a high unit cost. At the same time, auxiliary materials such as vacuum pumps, vacuum bag films, guide media, demoulding cloths and guide pipes are needed in the construction process, and most of them are disposable, so the production cost is higher than the hand-pasting process. But the larger the product, the smaller the difference will be.

 

B3 Process has Certain Risks

The characteristics of the vacuum infusion process itself determine the one-time molding of ship construction, and the work requirements before resin infusion are very high. It must be implemented step by step in strict accordance with the construction process.

 

After the resin infusion begins, the process is irreversible. Once the resin infusion fails, the entire hull is easily scrapped. At present, in order to facilitate construction and reduce risks, general shipyards generally use two-stage vacuum molding of the ship hull and skeleton.

 

Conclusion

As a new type of fiberglass ship molding and construction technology, the vacuum infusion process has many advantages, especially in the construction of ships with larger main dimensions, higher speed and stronger strength. It has an irreplaceable role.

 

With the continuous improvement of the vacuum resin infusion construction process and the reduction of raw material costs, as well as the growing social demand, the fiberglass ship construction will gradually transition to mechanical molding technology, and the resin vacuum infusion method is bound to be widely used in more factories.