Fiberglass Reinforced Plastics (FRP) is a new type of composite material manufactured by ships in the late 1960s,with light mass,high strength,corrosion resistance,the characteristics of the plasticity.After decades of development,FRP materials have been widely used in the construction of small and medium-sized boats,Especially in recent years,it has been widely used on yachts,high-speed boats and tourist passenger boats.This paper focuses on the construction and molding process of FRP ships — resin vacuum introduction method.

1 Technology introduction

Resin vacuum import method is on the rigid mold layup reinforced fiber materials in advance,and then spread vacuum bag,the vacuum pumping system,form a negative pressure in the mold cavity,using the vacuum pressure lay unsaturated resin through the pipe into a fiber layer,wetting behavior of unsaturated polyester resin for fiber material,Finally,the whole mold is filled,the vacuum bag material is removed after curing,and the desired product is obtained from the mold demoulding.Its craft profile shown below.

Vacuum lead-in process is a new technology for forming and building large size boats by establishing a closed system in a single rigid die.As this process is introduced from abroad,there are also a variety of names in the naming,such as vacuum import,vacuum perfusion,vacuum injection,etc.

2.Process principle

The special technique of vacuum import is based on the theory of hydraulics created by The French hydraulics Darcy in 1855,namely the famous Darcy’s Law: t=2hl／(2k(AP)),Where,t is the resin introduction time,which is determined by four parameters; h is the viscosity of resin,guiding the viscosity of resin,z is the import length,refers to the distance between the resin inlet and outlet,AP is the pressure difference,refers to the pressure difference between inside and outside the vacuum bag,k is permeability,refers to the parameters of resin infiltration by glass fiber and sandwich materials.According to Darcy’s law,the resin import time is proportional to the resin import length and viscosity,and inversely proportional to the pressure difference between inside and outside the vacuum bag and the permeability of fiber material.

3.Technological process

The specific processing process of the special agent is as follows.

first,Start the preparatory work

First of all,steel or wood molds are made according to the shape line and size of the ship.The internal surface treatment of the molds must ensure high hardness and high gloss,and the edge of the molds must be kept at least 15cm to facilitate the laying of sealing strips and pipelines.After cleaning the mold,apply demoulding material,you can play demoulding wax or wipe demoulding water.

second,Apply hull gelcoat

According to the requirements of ship production,the inner surface of the mold is coated with gelcoat resin containing catalyst promoter,which can be used as product gelcoat or polished gelcoat.The type of choice is phthalate,m-benzene and vinyl.Hand brush and spray can be used for construction.

Thirdly,Layup reinforced material

First,according to the hull line and basic structure,the reinforcement material and skeleton core material are cut respectively,and then laid in the mold according to the lay-up drawing and forming process.The effect of reinforcement material and connection mode on resin flow rate must be taken into account.

Fourthly,Layup vacuum auxiliary material

On the reinforced material laid in the mold,the stripping cloth is first laid,followed by the diversion cloth,and finally the vacuum bag,which is compacted and closed by the sealing strip.Before closing the vacuum bag,carefully consider the direction of the resin and vacuum line.

Fifth,Vacuum the bag

After the laying of the above materials is completed in the mold,the resin is imported into the clamping tube system,and the vacuum pump is used to vacuum the whole system,and the air in the system is evacuated as far as possible,and the overall air tightness is checked,and the leakage place is repaired locally.

Sixth,Blending resin ratio

After the vacuum in the bag reaches a certain requirement,according to the environmental conditions,product thickness,spread area,etc.,the resin,curing agent and other materials are allocated in a certain proportion.The prepared resin should have appropriate viscosity,appropriate gel time and expected curing degree.

Seventh,Mold lead-in resin

The prepared resin is introduced into the pressure pump,and the bubbles in the resin are eliminated by full stirring.Then the clamps are opened in turn according to the order of introduction,and the resin guide is implemented by constantly adjusting the pump pressure,so as to effectively control the thickness of the ship body.

Eighth,Curing stripping outfitting

After resin introduction is completed,the hull should be used in the mold for a period of time to allow resin curing,generally not less than 24 hours,in its Bacor hardness is greater than or equal to 40 before demoulding. After demoulding,necessary measures should be taken to support to avoid deformation.After complete solidification,hull closure and outfitting began.

4 Analysis of advantages and disadvantages of process technology

 A.advantages of process technology

As a new kind of molding technology in the construction of FRP vessels,the vacuum insertion method has great advantages over the traditional manual paste process.

A1 Hull structural strength has been effectively improved

During the construction process,the hull,stiffeners,sandwich structures and other inserts of the ship can be laid at the same time,thus greatly improving the integrity of the product and the overall structural strength of the ship.In the case of the same raw material,compared with the hand-pasted hull,the strength,stiffness and other physical characteristics of the hull formed by resin vacuum introduction process can be increased by more than 30%-50%,which is in line with the large-scale development trend of modern FRP ships.

A2 Boat to effectively control the weight of the ship

The FRP ship produced by vacuum introduction process has high fiber content,low porosity and high product performance,especially the improvement of interlaminar strength,which greatly improves the anti-fatigue performance of the ship.In the case of the same strength or stiffness requirements,the ship constructed by vacuum lead-in method can effectively reduce the weight of the structure.When the same layer design is used,the resin consumption can be reduced by 30%,the waste is less,and the resin loss rate is less than 5%.

A3 The quality of ship products has been effectively controlled

Compared with manual pasting,the quality of the ship is less affected by the operator,and there is a high degree of consistency whether it is a ship or a batch of ships.The amount of reinforcement fiber of the ship has been put into the mold according to the specified amount before the injection of 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 is much better than the hand-pasted craft.At the same time,the precision of the ship produced by this process is better than that of the hand-pasted ship,the flatness of the hull surface is better,and the manual and material of the grinding and painting process are reduced.

A4 The factory’s production environment has been effectively improved

Vacuum lead-in process is a closed mold process,the volatile organic compounds and toxic air pollutants generated during the whole construction process are confined to the vacuum bag.Only in the vacuum pump exhaust (filter) and resin mixing when there is a small amount of volatile,compared with the traditional manual paste open working environment,the site construction environment has been greatly improved,effectively protect the relevant site construction personnel physical and mental health.

B,Shortcomings of process technology

B1 The construction technology is complex

The vacuum lead-in process is different from the traditional hand-pasting process,It is necessary to design the lay-out diagram of fiber materials,the layout diagram of diversion tube system and the construction process in detail according to the drawings.The paving of reinforcement materials and the laying of diversion medium,diversion tube and vacuum sealing material must be completed before the resin lead-in.Therefore,for small size ships,the construction time is longer than hand paste technology.

B2  Production costs are relatively high

The special vacuum importing technique has high requirements on the permeability of fiber materials,which can use continuous felt and unidirectional cloth with high unit cost.At the same time,vacuum pump,vacuum bag film,diversion medium,demoulding cloth and diversion tube and other auxiliary materials need to be used in the construction process,and most of them are disposable,so the production cost is higher than the hand paste process.But the bigger the product,the smaller the difference.

B3  There are certain risks in the process

The characteristics of the vacuum filling process determine the one-time molding of ship construction,which has high requirements for the work before the resin filling.The process must be carried out in strict accordance with the process of resin filling.The process will be irreversible after the resin filling begins,and the whole hull will be easily scrapped if the resin filling fails.At present,in order to facilitate construction and reduce risks,general shipyards adopt two-stage vacuum forming of ship body and skeleton.

5  Conclusion

As a new forming and construction technology of FRP ships,vacuum import technique has many advantages,especially in the construction of ships with large master scale,high speed and strong strength,which can not be replaced.With the continuous improvement of the construction technique of vacuum resin import,the reduction of raw material cost and the increasing social demand,the construction of FRP ships will gradually transition to mechanical molding,and the resin vacuum import method will be widely used in more factories.Source: Composite Applied Technology.

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