The Difference Between Vacuum Resin Infusion Process and Hand Lay-up Process

 

The resin vacuum infusion process is a low-cost manufacturing process developed in the past 20 years, which is particularly suitable for the manufacture of large products. The process of vacuum infusion of resin is to lay “dry” reinforcement materials (glass fiber, carbon fiber, sandwich materials, etc., which is different from the vacuum bag process) on the mold, then lay the vacuum bag, and extract the vacuum in the system to form a negative pressure in the mold cavity.

 

The pressure generated by the vacuum is used to press the unsaturated resin into the fiber layer through the pre-laid pipeline, so that the resin infiltrates the reinforcement material and finally fills the entire mold.

 

After the product is cured, the vacuum bag material is removed and the desired product is obtained from the mold. VIP uses a single-sided mold (like the usual hand lay-up and injection mold) to establish a closed system. The vacuum infusion process has been public for a long time.

 

This process appeared in the patent record in 1950. However, it has only been developed in recent years. Since this process was introduced from abroad, it has many names, such as vacuum infusion and vacuum infusion.

 

 

The hand lay-up process is a mold opening process, which currently accounts for 65% of glass fiber reinforced polyester composite materials. Its advantages are that it has a large degree of freedom in changing the shape of the mold, low mold price, strong adaptability, market recognition of product performance and low investment.

 

Therefore, it is particularly suitable for small companies, and also for the shipbuilding and aerospace industries, where large parts are usually disposable. However, this process also has a series of problems, such as excessive volatile organic compound (VOC) emissions, great impact on the health of operators, easy loss of personnel, many restrictions on permissible materials, low product performance, resin waste and large dosage, etc., especially unstable product quality, the ratio of glass fiber and resin, component thickness, layer material manufacturing rate, and layer material uniformity of the product are all affected by the operator, requiring the operator to have good technology, experience and quality.

 

The resin content of hand lay-up products is generally around 50% to 70%. The VOC emissions of the mold opening process exceed 500PPm, and the volatilization of styrene is as high as 35% to 45% of the usage. However, the regulations of various countries are all 50 to 100PPm.

 

At present, most foreign countries have switched to cyclopentadiene (DCPD) or other low styrene release resins, but there is no good substitute for styrene as a monomer.

 

The advantages of vacuum resin infusion process are introduced below.

 

Excellent Product Performance and High Yield Rate.

Under the same raw materials, compared with hand lay-up components, the strength, stiffness and other physical properties of components formed by vacuum resin infusion process can be improved by more than 30% to 50%. After the process is stabilized, the yield rate can be close to 100%. The increased glass fiber content can obtain higher strength and lighter products. VIP has many advantages over traditional processes.

Comparison of typical polyester fiberglass performance

 

典型聚酯玻璃钢性能比较

增强材料	无捻粗纱布	无捻粗纱布	双抽向织物	双抽向织物
成型工艺	手糊工艺	真空工艺	手糊工艺	真空工艺
玻纤含量	45	60	50	65
拉伸强度（MPa）	273.2	383.5	389	480
拉伸模量（GPa）	13.5	17.9	18.5	21.9
压缩强度（MPa）	200.4	215.2	247	258
压缩模量（GPa）	13.4	15.6	21.3	23.6
弯曲强度（MPa）	230.3	325.7	321	385
弯曲模量（GPa）	13.4	16.1	17	18.5
层间剪切强度（MPa）	20	35	30.7	37.8
纵横剪切强度（MPa）	48.88	52.17
纵横剪切模量（GPa）	1.62	1.84

 

Stable Product Quality and Good Repeatability.

Product quality is less affected by operators, and there is a high degree of consistency between the same component and between components. The fiber content of the product has been placed in the mold according to the specified amount before injecting the resin. The component has a relatively constant resin ratio, generally 30% to 45%.

 

Therefore, the uniformity and repeatability of the product performance are much better than those of hand-layup products, and there are much fewer defects. The resin impregnates the glass fiber under vacuum, and there are very few bubbles in the product compared with traditional manufacturing processes.

 

The Fatigue Resistance of Vacuum-Introduced Resin is Improved, Which can Reduce the Weight of the Structure.

Due to the high fiber content, low porosity, high product performance, and especially the improvement of interlayer strength, the fatigue resistance of the product is greatly improved. Under the same strength or stiffness requirements, products made by vacuum induction process can reduce the weight of the structure. It can minimize the loss of resin, and more importantly, it can save costs.

 

Environmentally Friendly.

The vacuum resin infusion process is a closed mold process, and volatile organic compounds and toxic air pollutants are confined in the vacuum bag. Only when the vacuum pump is exhausted (filterable) and the resin barrel is opened, there are trace amounts of volatiles. VOC emissions do not exceed the standard of 5PPm. This also greatly improves the working environment of operators, stabilizes the workforce, and expands the range of available materials.

 

Vacuum-Introduced Resin Products have Good Integrity.

The vacuum resin induction process can simultaneously form reinforcing ribs, sandwich structures and other inserts, improving the integrity of the product, so large products such as fan hoods, hulls and superstructures can be manufactured.

 

Reduce the Use Of Raw Materials and Labor.

When the same layer is laid, the amount of resin is reduced by 30%. There is less waste, and the resin loss rate is less than 5%. The labor productivity is high, and it can save more than 50% of labor compared to the hand lay-up process. Especially when forming large and complex geometric sandwich and reinforced structural parts, the savings in materials and labor are even more considerable.

 

Only one side of the mold can be used to obtain a product with smooth and flat surfaces on both sides, which can better control the thickness of the product. Save mold manufacturing costs and time. For example, in the manufacture of vertical rudders in the aviation industry, the number of fasteners is reduced by 365, the price is reduced by 75% compared with the traditional method, the product weight remains unchanged, and the performance is better.

 

Vacuum-Introduced Resin Products Have Good Precision.

The dimensional accuracy (thickness) of vacuum resin introducing process products is better than that of hand-laid products. Under the same layer, the thickness of vacuum resin diffusion technology products is generally 2/3 of that of hand-laid products. The product thickness deviation is about ±10%, while the hand-laid process is generally ±20%.

 

The surface flatness of the product is better than that of hand-laid products. The inner wall of the hood product of the vacuum resin introducing process is smooth, and a resin-rich layer is naturally formed on the surface, and no additional topcoat is required. The labor and materials of the grinding and painting processes are reduced.

 

Of course, the current vacuum resin introducing process also has certain disadvantages:

 

The Preparation Process is Long and Complicated.

It requires correct layering, laying of diversion media, diversion tubes, effective vacuum sealing, etc. Therefore, for small-sized products, its process time exceeds that of hand-laid process.

 

The Production Cost is High and More Waste is Generated.

Auxiliary materials such as vacuum bag film, flow medium, demoulding cloth and flow tube are all disposable, and currently a considerable number of them rely on imports, so the production cost is higher than the hand lay-up process. However, the larger the product, the smaller the difference. With the localization of auxiliary materials, this cost difference is getting smaller and smaller. At present, research on auxiliary materials that can be used multiple times is a development direction of this process.

 

Process Manufacturing has Certain Risks.

Especially for large and complex structural products, once the resin infusion fails, the product is easy to be scrapped. Therefore, there must be good preliminary research, strict process control and effective remedial measures to ensure the success of the process.

 

 

Introduction to FRP Vacuum Infusion Molding Process: Vacuum infusion process is divided into dry method and wet method.

 

The wet method is to cover the uncured product by hand lay-up or injection molding with a layer of vacuum bag film, the product is between the film and the mold, the periphery is sealed, and vacuum is drawn (0.07MPa) to remove bubbles and volatiles in the product.

 

The dry method is to lay the reinforced glass fiber on the mold, seal the vacuum bag film with the periphery of the mold, and introduce the resin from the other end of the mold into the mold through the pipeline while vacuuming to infiltrate the reinforced glass fiber. The difference between the two processes is that the glass fiber content of the dry method can be made higher.

 

Wet process: In the process of FRP vacuum infusion molding, no injection equipment is required, only a vacuum pump is needed; the mold only needs a single mold, and the negative pressure in the mold cavity is generally 0.1Mpa. The raw materials used in the vacuum infusion process include resin system and reinforcing materials, and generally no filler is added. The resin used in this process mainly requires a low viscosity of about 180 to 300 centipoise. The reinforcing material is mainly glass fiber, with a content of 50% to 70%; commonly used reinforcing materials include glass fiber chopped strand mat, composite mat and woven cloth.

 

Features of FRP Vacuum Infusion Process:

1. Products with single-sided finish can be manufactured;
2. The molding efficiency is average, suitable for medium-scale FRP product production (1,000 pieces/year);
3. It is a closed mold operation, which does not pollute the environment and does not harm the health of workers;
4. The reinforcement material can be laid in any direction, and it is easy to lay the reinforcement material according to the stress condition of the product;
5. There are many disposable consumables, garbage recycling is a problem, and the cost is relatively high.

 

Principle of Vacuum Infusion of FRP Production Process

The basic principle of vacuum infusion process refers to laying glass fiber, glass fiber fabric, various inserts, demolding cloth, resin penetration layer, laying resin pipeline and covering nylon (or rubber, silicone) flexible film (ie vacuum bag) on ​​the cured gel coat layer, and the film is tightly sealed around the edges of the cavity.

 

The cavity is vacuumed and resin is injected into the cavity. Under vacuum, the resin flows along the resin pipeline and the fiber surface to impregnate the fiber bundle, and the product is cured at room temperature or under heating conditions.

 

1. Advantages of Vacuum Infusion
3. High mechanical properties Compared with hand-layup components, the strength, stiffness and other physical properties of components formed by vacuum infusion process can be increased by 1.5 times.
4. Good repeatability The components have a relatively constant resin ratio, low porosity ≤1%, and hand lay-up ≥5%.
5. Light weight The fiber content is as high as 75-80%, and no additional material is required to connect the core material.Environmental protection The vacuum infusion process is almost a closed mold molding process, and volatile organic compounds and toxic air pollutants are confined in the vacuum bag.
6. Low cost and high efficiency The fiber content is high, the resin waste rate is less than 5%, and it can save more than 50% of labor compared with the open mold process. There is no need to wait for the resin to solidify before and after the core material is added. Especially when reinforcing the board, the savings in materials and labor are considerable.

 

 

2. Vacuum infusion process operation steps
3. Apply release agent (wax) on the mold surface
4. Lay dry fabric and sandwich
5. Lay isolation layer
6. Lay dispersion medium layer
7. Seal with vacuum bag
8. Inject resin and vacuum at the same time
9. Curing at room temperature or placing in oven

 

Specific Operation Steps of Vacuum Bagging Resin Injection Process

 

1. Apply Release Wax or Release Agent

After cleaning, the mold should be placed for more than 8 hours. Release wax (agent) can be applied only after the moisture on the mold surface is fully dried. When using polyvinyl alcohol release agent, the uniformity of its film formation should be carefully checked. There should be no omissions and sagging.

The gel coat can be applied only after it is fully dried. When using release wax, the new mold must be waxed 4-6 degrees, and the old mold must be waxed 2-3 degrees. The interval between each degree of wax must be more than 2 hours, and the release wax cannot be omitted or accumulated.

 

2. Spraying Gel Coat

The thickness of the sprayed gel coat should be controlled at 0.4-0.6mm, and the reinforcement material must be laid when it reaches “finger dry”. No omissions, accumulation or sagging are allowed. During the spraying process, it is generally applied twice, and the brushing direction between the two sides should be perpendicular to each other to avoid missing coating.

 

3. Laying Reinforcement Materials

The joint and overlapping position dimensions of glass fiber reinforcement materials should meet the process requirements. The overlap width is 50mm, the joints should be staggered by more than 100mm, and overlapping is allowed after five layers. In the deformed place, if it is inconvenient to lay, you can use scissors to cut the reinforcement material. The cut place should be locally reinforced.

If there is a gap between the layers, it should be compacted or glued together with positioning spray glue. When spraying positioning glue, the nozzle should be more than 30cm away from the spraying place, so that the glue can be sprayed evenly. If too much is sprayed, it will affect the infiltration of the resin into the reinforcement material, and even white spots will appear.

After the reinforcement material is laid, use scissors to cut off the excess fibers, and repeatedly roll the skirt with yellow sealing tape to remove the dust and fine fiber bundles on the skirt.

 

4. Laying Demoulding Cloth

Before laying the demoulding cloth, the large piece of demoulding cloth can be appropriately cut down, which is conducive to demoulding. The demoulding cloth should cover the entire reinforcement material, and generally there is about 1cm more at the edge of the reinforcement material. The overlap width between the stripping cloth and the stripping cloth can be 1cm. Too wide will waste materials and affect the impregnation speed. The stripping cloth is generally glued to the reinforcing material with a very small amount of black sealing tape instead of positioning glue, which will not affect the impregnation effect and save costs.

 

5. Laying the Guide Net

When laying the guide net, the edge of the guide net is about 3-5cm away from the edge of the reinforcing material, that is, the area of ​​the guide net is slightly smaller than the area of ​​the reinforcing material. When the resin is infiltrating the reinforcing material without the guide net, the speed is much slower than the place with the guide net.

This allows the resin to have sufficient time to infiltrate the reinforcing material and reduce the waste of resin. The overlap distance between the guide nets should be as small as possible, but there should be no place without the guide net. The guide net is generally also glued to the stripping cloth with a very small amount of black sealing tape instead of positioning glue.

 

6. Lay Out the Resin Flow Guide and Vacuum Tube

Place the hollow spiral tube as the resin flow channel and vacuum tube in the corresponding position of the mold cavity according to the pre-designed flow channel. When laying the hollow spiral tube, do not force the spiral tube to be too long, otherwise the spiral tube may collapse during vacuuming and fail to play the role of flow channel and vacuuming.

 

7. Place the Resin Injection Port and Exhaust Port

In the pre-design, the resin injection port and exhaust port are placed on the spiral tube. When making products with regular shapes, the resin injection port and exhaust port are generally placed at the position of the equal division point. For example, when two exhaust ports are placed on a spiral tube, the exhaust port is generally placed at one-third and two-thirds points.

When placing the resin injection port and exhaust port, i.e. the tee, a circle of black sealing tape can be wrapped around the end of the tee that connects the resin feed pipe and the exhaust pipe. In order to prevent the tee from moving, an appropriate amount of black sealing tape can be used to stick the tee to the flow guide net.

 

8. Connect the Vacuum Pipe

Connect the vacuum port tee and the resin collector interface with the vacuum pipe. Wrap a circle of black sealing tape around the interface.

 

9. Sealing of Vacuum Bag Film
10. Vacuuming, Bag Film Arrangement and Leak Detection
11. Connect the Resin Injection Pipe
12. Testing
13. Vacuuming and Leak Detection
14. Injecting Resin

 

When the required pressure holding effect is achieved, the resin can be injected. When injecting resin, generally open the injection port on the same resin flow channel first, and inject from the inside to the outside. When the resin flows through another row of flow channels, the injection port of this row can be opened and the resin can be injected. Fill the entire film cavity in this way.

Due to the irregular shape of the mold, it is difficult for the resin to reach the edge of the mold at the same time. When a part of the resin reaches the edge of the mold first and enters the vacuum pipe, the vacuum pipe can be clamped with pliers. This will not affect the filling effect of other parts, but also reduce the waste of resin.

 

Vacuum Curing

Before the resin is cured, you can use pliers to clamp most of the resin feed pipes and exhaust pipes, which can reduce the waste of resin. As for which resin feed pipes and exhaust pipes should be clamped, there is no specific requirement. Generally, they are clamped in turns.

 

Post-Processing

 

Demolding

When the FRP is cured to a certain extent, the product can be removed from the mold without damaging the product and the mold. First, loosen the surrounding of the product, and then remove the product. When demolding, pay attention to the following:

(1) Be careful and patient when demolding, and don’t rush;

(2) The knocking tool should be a wooden hammer or a rubber hammer;

(3) The knocking point should hit the key part, don’t hit randomly;

(4) Be careful that there must be no white spots or white spots on the surface of the product.

 

Surface Inspection

Mainly check whether there are wrinkles on the gel coat, poor gloss, sticky surface of the product, warping and deformation of the product, star-shaped cracks in the gel coat layer, delamination, bubbles and cavitations, pinholes and other defects, and mark them for later repair.

 

Trimming

First, draw a reference line on the rough product, use a cutting knife that is basically at right angles to the product, and try to cut it straight.

 

Selection of Vacuum Infusion Materials

Typical resins suitable for vacuum infusion process include low shrinkage polyester resin, vinyl resin, epoxy resin, etc. The viscosity of the resin system is generally 0.15~0.8Pa.S. The resin can completely impregnate the reinforcing material only under the action of vacuum force. Different processes have different requirements for gel time.

For example, some processes require injection within 35 minutes, while others require 4 hours to complete. Therefore, the gel time should be variable and easy to control, which is one of the keys to successful injection. The viscosity changes little during the impregnation process, and the curing exothermic peak should be moderate. High exothermic peaks will damage the mold and even the molded components.

 

Selection of Reinforcing Materials

The fiber reinforcement materials commonly used in hand lay-up processes can be used in vacuum infusion, and other forms of fiber fabrics, from chopped strands to thick knitted felts, can also be used. New knitted materials and plain unidirectional fibers are ideal choices.

 

Selection of Curing System

Since the vacuum bagging resin injection process generally uses resins with accelerators added, only the initiator needs to be added before use. The commonly used initiator is methyl ethyl ketone peroxide. The amount of initiator used has a great relationship with the required gel time and the temperature during mold filling. Because vacuum bagging resin injection is closed mold molding, humidity has little effect on the amount of initiator used.

 

Selection of Core Material

The core material is generally low-density foam and lightweight wood, and can also be thermoplastic materials, concrete materials, cured pultrusion materials, metal inserts, etc. Factors to be considered in specific use include differences in thermal expansion coefficients, surface treatment conditions, and compatibility with resins.

 

Analysis of Glass Fiber Reinforced Plastic Ship Resin Vacuum Introduction Technology

 

Glass fiber reinforced plastic is a new type of composite material for shipbuilding in the late 1960s. It has the characteristics of light weight, high strength, corrosion resistance and strong plasticity. After decades of development, glass fiber reinforced plastic 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 glass fiber reinforced plastic ship construction molding-resin vacuum introduction method.

 

Process Technology Introduction

The resin vacuum introduction 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 absorb 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 process section is shown in the figure below.

 

 

The vacuum infusion process uses a closed system in a single-sided rigid mold, and is a new type of large-size boat molding and construction technology. The process was patented in 1950, but was not introduced into China until recent years and has been developed. Since this process was introduced from abroad, it has many names, such as vacuum infusion, vacuum infusion, vacuum injection, etc.

 

 

 

Process Technology Principle

The vacuum infusion 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 infusion time, which is determined by 4 parameters; h is the resin viscosity, which refers to the viscosity of the injected resin; z is the infusion length, which refers to the distance between the resin feed port and the discharge port; 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 infusion time of the vacuum infusion process is proportional to the length and viscosity of the resin infusion, 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 a gel coat resin containing a catalyst/accelerator on the inner surface of the mold. Product gel coat or polished gel coat can be used. The types of selection include 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: Laying Vacuum Auxiliary Materials

On the reinforced materials 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 pump 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

 

1. Advantages of Process Technology

As a new type of FRP shipbuilding 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% to 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 used for 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% to 45%, while the resin content of the hand-pasted hull is generally 50% to 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.

 

1. 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. There are Certain Risks in the Process.

The characteristics of the vacuum infusion process determine the one-time molding of ship construction. It has high requirements for the work before resin infusion. 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 FRP 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, FRP ship construction will gradually transition to mechanical molding technology, and the resin vacuum infusion method will inevitably be widely used in more factories.

 

Detailed Explanation of the Vacuum Infusion Process!

Resin infusion is a refined process technology for producing high-performance, non-porous composite materials and even larger or more complex products. This process is very suitable for the manufacture of FRP parts, such as engine covers and ship shells. The same is true for small, intricate items, which are almost impossible to achieve perfectly using open mold build-up.

 

 

Resin Vacuum Infusion Process

This article describes the entire vacuum bag infusion process from the mold to the final bright product, as well as the equipment and materials used.

 

What is Resin Infusion?

First, the reinforcement, i.e., “dry” fabric (without any resin mixed in), is laid on the mold, then the vacuum auxiliary materials (such as mold release cloth, induction mesh, vacuum bag) are laid, and finally the vacuum is evacuated using a vacuum pump. Once all the air is extracted from the vacuum bag, the composite is completely flattened at atmospheric pressure, and the liquid resin (with mixed curing agent) can be guided into the reinforcement through a hose, flowing through the entire reinforcement under vacuum. When the resin is completely poured into the reinforcement, the resin supply is cut off (using a hose clamp), and the rest is the resin curing (still under vacuum).

 

If the resin infusion process is performed correctly, it can produce FRP parts with satisfactory strength and appearance quality. Compared with the traditional wet hand lay-up process, it avoids resin surplus and unstable performance. At the same time, it eliminates porosity (dead corners around the reinforcement when laying) and small bubbles in the part. The quality of resin infusion, i.e. the strength of the product, is close to that of using prepreg process.

 

Preparation – Materials and Equipment You Need

The key factors for successful resin infusion: the correct vacuum equipment, auxiliary materials, etc. need to be prepared in advance. Although this is not part of the process, in most cases, infusion failures or problems are caused by the use of incorrect materials or equipment.

 

Mold

The edge of the mold should have a large enough flange to fix the vacuum auxiliary material. The surface quality of the mold determines the surface quality of your product. If you use a composite mold, the surface material of the mold is preferably epoxy mold gel coat or vinyl mold gel coat; of course, other molds such as glass, metal steel or aluminum can also be used.

 

Equipment

Vacuum pump: can reach a vacuum degree of more than 99.99%.

Silicone nozzle: placed in the vacuum bag to connect the resin feed line and the vacuum line.

Resin collector: If you only use the infusion process once or twice, you can also use breathable felt instead of resin collector to collect a small amount of resin; but for long-term use, it is best to invest in a professional resin collector.

 

Vacuum Bag Auxiliary Materials

Vacuum Bag Film

Vacuum Sealing Tape (i.e. Sealing Strip or Putty Strip)

Introduction Net

Mold Release Cloth

Breathable Felt (if you don’t have a Resin Collector)

Spiral Tube

PVC Vacuum Tube

Paper Tape or Pressure-Sensitive Tape

 

Materials

Mold release agent (release wax, PVA or chemical mold release agent)

Reinforcement, such as introductory felt, chopped strand mat or fiberglass cloth

Vacuum infusion resin

 

Specific Steps (step by step guide to your operation)

 

Prepare the Mold Surface

The mold surface should be applied with mold release agent. Semi-permanent mold release agent is a very reliable high-gloss mold release agent. If used correctly, it will obtain a high-quality surface. Application: Apply chemical mold release agent to the mold surface with a lint-free cloth. Apply the mold release agent, wait a moment for it to evaporate (1 minute, depending on the temperature and the type of mold release agent), and use a second lint-free cloth to gently wipe off the excess mold release agent in circular motions. Apply 6 layers to the new, unused mold surface, with a minimum interval of 15 minutes between each layer. Allow at least 30 minutes after the last coat is applied before using the mold.

 

Cut and Lay Out Reinforcements

Cut the reinforcements to the appropriate size for molding. If possible, use one layer of reinforcement as a template to cut the remaining layers. Make sure you cut the fabric large enough to cover all edges of the part, and account for burrs that will need to be cut later. Lay out the reinforcements in the mold (you can also lay out multiple layers at a time). For complex molds, especially those with uneven or multiple contours, it is sometimes necessary to use a setting glue to adhere the fabric to the mold surface and to bond each layer to the other.

 

Add Release Cloth

The release cloth, in the resin infusion kit, is the first layer of process material that can be torn off the part after the product is completed, leaving a more consistent surface and also serving as an ideal bonding surface. Usually, only one layer of release cloth is laid, covering the entire surface of the reinforcement. It is not necessary to overlap the release cloth at the edge of the reinforcement, but it does need to cover the entire surface, so the cutting area is usually slightly larger than the reinforcement.

 

Adding a Flow Grid

The Flow Grid (also called a “Flow Media”) is used to ensure that the resin can flow freely from the resin flow tube (and spiral tube) into the laminate. The shape of the Flow Grid means that even under full vacuum, there are still gaps for the resin to flow. The Flow Grid also needs to be laid in one layer (you can overlap multiple layers if necessary without any problem).

 

Fixing the Spiral

The Spiral is a spirally wound plastic tube that is used to improve the flow of resin from the feed tube to the laminate. The resin will be dispersed along the entire spiral, so it is standard practice to fix the spiral at one end edge (the end where the resin feed tube is fixed), which means that the resin is quickly and evenly distributed from one side of the mold to the other. The spiral tube must be fixed directly on the Flow Grid. Ensure that the resin can flow more easily from the spiral tube into the Flow Grid.

Fix the Silicone Nozzle at the Resin Feed End

The Silicone Nozzle is a black silicone connector. It has channels at the bottom that run from one side to the other. The spiral tube can be passed through this channel, ensuring that resin can flow from the resin feed tube through the fitting and into the spiral tube. Fix the resin feed fitting in the middle of the spiral tube. Press the fitting onto the spiral tube so that the spiral tube can run at the bottom of the fitting.

 

Fix the Silicone Nipple On The Vacuum End

This is also the silicone nipple. Fix the silicone nipple to the guide mesh (sometimes, to reduce the risk of excessive resin absorption, the silicone nipple is fixed to the mold release cloth), on the opposite end of the resin feed spiral tube.

 

Sealing Tape

Sealing tape is like a very sticky rubber tape. This tape is widely used in all vacuum bagging processes. Its soft nature can effectively provide a seal environment.

 

Lay Out and Stick the Vacuum Bag

Now it’s time to vacuum bag everything. Good vacuum bagging film is not only strong (not easy to puncture) but also has the right amount of stretch. In the big picture, it is more wasteful to accidentally damage the product due to poor quality vacuum bags. When using vacuum bags, it is very important to ensure that the shape and contour of the mold are adequately covered by the vacuum bag.

 

If the bag is not laid correctly and any “bridging” occurs in the corners of the mold, it will result in excess resin in the final product, reducing strength and quality.

Cut a piece of bagging film that is large enough to be 50% larger than the mold area. Starting at the corner, peel off the paper backing of the sealing tape and press the bagging film onto the exposed sealing tape. Move along the edge of the mold, peel off the paper backing of the tape and then press the bagging film, peeling and sticking.

 

As you stick the film to the tape, be as careful as possible to avoid any wrinkles or creases in the bag. Press down firmly with your fingers, pressing and kneading the bagging film and sealing tape together to create a seal. If you have sealed the bag but there is a leak (we will do a leak test later), it may be caused by wrinkles or folds in the bag.

 

Connect and Seal the Vacuum Tube (also Called the Guide Tube) at the Resin Feed End

After the bag is bonded, you can make a hole in the bag to connect the resin guide tube. Use a pair of scissors or a knife to make a cut in the vacuum bag just above the silicone resin feed connector. Cut a length of PVC vacuum tubing from where the feed tank is to the feed connector on the vacuum bag. When you cut the PVC hose, make a bevel (about 45°) so that the hose does not accidentally seal at the bottom. Press the angled end of the hose through the small hole in the vacuum bag and into the hole in the feed connector. Make sure the hose is firmly connected to the feed connector. Seal the hose to the vacuum bag by wrapping a circle of sealing tape around the hose. Press firmly to make sure the sealing tape creates a seal.

 

Connect and Seal the Vacuum End of the Vacuum Hose

Repeat the previous step, this time using another piece of PVC tubing to connect the resin collector to the vacuum nozzle. Cut the hose at an angle, cut a small hole in the vacuum bag, push the hose into the vacuum nozzle in the vacuum bag, and seal with sealing tape.

 

Resin Feed Tank

The resin feed tank should be securely positioned near the mold. It is critical that the resin feed tank does not fall over during infusion. The infusion hose can be connected to the bottom of the feed tank all the way through, using a paper clip or something to keep the hose in place if needed.

 

Connect the Vacuum Pump and Resin Collector

The other end of the vacuum hose is connected to the resin collector, then cut a piece of PVC hose and connect the vacuum pump to the resin collector.

 

Clamp the Infusion Hose at the Resin Feed End

Place a hose clamp on the feed tube at one end of the resin feed tank to ensure a tight seal is created.

 

Turn On the Vacuum Pump

Everything is ready, now we turn on the vacuum pump.

Evacuate and Adjust the Vacuum Bag

When the air is removed from the vacuum bag, you will see that the vacuum bag is tightly attached to the mold surface. In this case, you should move and adjust the vacuum bag, and some complex areas on the mold require more vacuum bags. This process is essential for all vacuum bag processes (resin infusion process, vacuum bagging process and even prepreg process), so pay close attention to this step.

 

As the bag begins to tighten (of course, it is not completely evacuated and the bag will not be compressed), turn off the vacuum pump temporarily. You can now move and reposition it, crease it where necessary, and make sure that the bag is not bridging.

 

During your first few projects, you may underestimate the amount of vacuum bag film to use. If this happens, you will find that at this stage, the vacuum bag will bridge the mold at one point, but there will be no extra vacuum bag to adjust. Once this happens, unfortunately, you need to stop, tear off the bag (and the sealing tape), and start over with a larger vacuum bag. If you continue to use such a small vacuum bag, the quality of the product will be greatly compromised.

 

Vacuum Leak Test

Turn on the vacuum pump and apply vacuum. You will see the needle on the vacuum gauge begin to rise. Without turning off the vacuum pump, wait until the needle is close to full vacuum. Adjust the vacuum bag film as necessary to ensure that there is no bridging.

 

After a few minutes, the vacuum gauge will read close to 100%. Due to changing atmospheric pressure and calibration of the gauge, the needle may not accurately point to 100% vacuum even when full vacuum is reached. The most practical way to know when a perfect vacuum is achieved is to look at the vacuum pump and see if any vacuum pump oil is evaporating from the pump. When fully sealed, the sound of the pump will change slightly and the vapor of the vacuum pump oil will stop evaporating from the vacuum pump.

At this point, a vacuum “leak test” should be performed by closing the vacuum pump valve, turning off the vacuum pump, clamping the tube of the resin collector (on the side of the vacuum bag) with a pipe clamp, and waiting for at least 15 minutes. After 15 minutes, carefully observe the vacuum gauge and open the pipe clamp; if the reading drops, the vacuum bag has a leak and must be adjusted. If it is not completely sealed, press the vacuum bag and the sealing tape firmly until it is completely sealed.

 

Measure the Correct Amount of Resin

The amount of resin to use depends on the size and reinforcement of your project. The more layers, the more resin you use.

 

Stir the Resin and Add it to the Feed Tank

Once you have calculated how much resin you need, you will also need to calculate the correct hardener and stir it well.

All resins need to be in the correct proportions, accurately measured and thoroughly mixed.

Weigh and stir the resin and hardener well using an electronic scale. Stir the resin in a container and pour the resin into the resin feed tank.

 

Wait 10 Minutes for the Air Bubbles in the Resin to Come Out

Depending on the pot life of the resin you are using, you may have time to let the resin sit to let the air bubbles out. If you have chosen a resin with a very short pot life, skip this step.

 

Open the Clamp On the Resin Feed Tube

With the vacuum pump running, gently unscrew the clamp on the resin feed tube. As you unscrew the nut you will immediately see the resin being drawn into the tube and begin to flow. Loosen the nut to make sure there is no restriction on the resin flow.

 

Monitor the Flow

Depending on the size and shape of the part, the design of the spiral and the amount of reinforcement, the resin may take several minutes or an hour to fully fill the entire part.

Because the resin feed tank is at atmospheric pressure, only the viscosity of the resin is isolating the internal pressure around the feed port fitting, so the vacuum bag in this area will not be very tight and more resin will accumulate here. For this reason, for resin infusion on larger projects, the resin feed is often closed again to wait for the resin in that area to disperse into the part; then, the vacuum bag is pressed against the mold again before the feed port is opened again to continue infusion.

 

Infusion Cutoff

When the resin reaches the resin collector, close the infusion tube at the feed port end with a pipe clamp to stop infusion

 

Vacuum Cutoff

Depending on the size of the project, allow the resin to continue to be pumped out of the vacuum tube for 10 to 30 seconds. This helps to remove air from the resin and remove excess resin from the part. Then close the vacuum tube at the vacuum end with a pipe clamp.

 

Turn Off the Vacuum Pump and Wait for the Part to Cure

At this point, the part has been completely separated from the feed and vacuum, turn off the vacuum pump, and wait for the part to cure. Make sure to wait long enough for the resin to cure before attempting to demold. Epoxy infusion resins generally cure slowly, so it is usually best to wait more than 24 hours.

Because excess resin (mixed with curing agent) accumulates in one place and may release heat violently, you should pay attention to the resin in the feed tank and resin collector (if there is a lot). Note: The violent heat release of epoxy resin may cause spontaneous combustion!

 

Remove Auxiliary Materials and Demould the Product

When the product has cured, you can tear off auxiliary materials such as vacuum bags and spiral tubes, and then peel off the demoulding cloth and guide net (we recommend cutting some holes on one side of the guide net and demoulding cloth, so that you can easily peel them one by one). Except for the silicone nozzle, which can be reused, other auxiliary materials are no longer recycled. Remove the product from the mold!

 

Post-Processing

After demoulding, the product should have a perfect surface quality without any bubbles or pores; at the same time, the product is very strong and light, and the excess edges of the product can be cut or the product can be drilled and modified, etc. If it is a high-end carbon fiber product that needs to withstand more direct sunlight, friction or other wear and tear during future use, it is recommended to use 2K varnish to protect it.