Application of Closed Mold Technology in the Shipbuilding Industry

 

1. Content Summary

The traditional manufacturing process of composite shipbuilding mainly adopts hand lay-up and injection molding, both of which belong to open mold process

The closed mold process shipbuilding discussed in this exchange is:

Vacuum induction Wet vacuum bag pressing

RTM LRTM

1. Why use closed mold process for shipbuilding?
2. Case interpretation How to use vacuum infusion for shipbuilding?
3. Introduction to other applications of closed mold process shipbuilding
4. Discussion of common problems FAQ5. Vision of closed mold shipbuilding development

 

2 Why use closed mold technology to build ships?

 

1. Reduce exposure to harmful chemicals and improve the working environment

Main VOC (volatile organic compounds):

Styrene

LD50: 5000 mg/kg (rat oral); LC50: 34.5 g/m3, 2 h (mice inhalation) LC50: 26.0 g/m3, 4 h (rat inhalation)

Irritate the respiratory tract, affect the skin, nerves, liver and lungs, etc.

NIOSH REL: 50mg/m3 (maximum exposure of 10 hours per day, maximum exposure of 40 hours per week)

Domestic emission standards: GB14554-93

U.S. Navy test data at a yacht factory in Washington in July 2006

Testing equipment:

Pac III Drager portable gas monitor

XS EC organic vapor sensor (specially for testing styrene)

Substances that need to be reduced in contact

 

Epoxy diluents

The toxicity of active diluents such as glycidyl ether compounds is mainly manifested in skin irritation, which is easy to cause allergies. Taking n-butyl glycidyl ether (501) as an example, oral-rat LD50: 2050 mg/kg;

Epoxy curing agent

Anhydride curing agents are prone to cause dermatitis, but the oral toxicity is relatively small. Amine curing agents are relatively toxic, and aromatic amines are more toxic than fatty amines. They are irritating to the skin and mucous membranes and can cause allergies. They are not easily metabolized by the liver. Taking m-phenylenediamine as an example, the rat oral LD50: 930mg/kg

 

2. Improve product quality and performance

Common defects in hand lay-up boats:

1. The interface bonding during the secondary resin lamination is not strong enough, which will affect fatigue resistance
2. Bubbles and bridges are likely to exist in the lamination, and the waterproofness is poor
3. Partially rich in resin, heavy
4. Products from different batches are unstable

Compared with traditional hand lay-up, the closed mold process can make the hull layer structure denser. In the equal strength design, it plays a huge advantage and reduces the weight of the product. Stronger, lighter, faster, smaller driving force (engine horsepower), more fuel-efficient,

 

3. Change the labor-intensive status quo

What managers have been worrying about

3 Case Study: How to use vacuum infusion to build a ship?

 

1. Using vacuum infusion to build a yacht

Case: 24-meter racing sailboat (epoxy carbon fiber layer, with foam sandwich structure in some parts)

Location: Slovenia

Build a simple mold and complete the surface lamination process to meet the requirements of the vacuum infusion process

Why use a simple mold for vacuum induction?

 

.For such a large racing boat, there are generally few orders. The cost of opening a composite female mold is very high and the cycle is very long. Using a simple mold is a more economical and efficient method.

 

.The surface of the product is post-painted, and the flatness and brightness of the mold surface are not very strict. The disadvantage of not being easy to find the level and smoothness during the female mold treatment will not have much impact.

 

.Compared with hand lay-up, what mold factors should be paid attention to when making a boat with vacuum induction?

 

.(Mold closing) Sealing. The bottom of the boat often uses a split mold, and catamarans also use a three-piece mold. Flanging to prevent deformation and make good sealing strips.

 

.Heat shock resistance. Vacuum induction concentrates heat release and vacuum pressure, which easily forms cloth patterns and core material marks on the mold surface.

 

Sample test before making a boat

Vacuum Inlet for Cabin Board

Interpretation

 

The importance of flat plate test before shipbuilding

 

.Different layers, different resins and construction temperatures will affect the flow rate. Collecting the resin flow rate data of the vacuum induction process under different conditions can play a great reference role in the design of the diversion pipeline.

 

.For thicker layers, there is often a wetting gap between the upper and lower surfaces during the vacuum induction process. Through sufficient tests on glass plates, the auxiliary material laying and process operation can be adjusted to avoid the problem of surface resin penetration but local dry areas in the bottom layer during shipbuilding.

 

.Product samples can test mechanical properties and verify the layer design

 

.The cabin plate can be combined with the test

 

Complete the laying of layer materials and auxiliary materials

Interpretation

 

What are the requirements for material laying?

 

.Avoid bridging. Both the ply materials and auxiliary materials should avoid bridging, otherwise, on the one hand, resin will be wasted, and local resin-rich accumulation will release heat, which may affect the mold and vacuum bag film; on the other hand, resin channels will be formed, affecting the flow of resin and bringing uncertainties.

 

.Pay attention to overlap and docking. The overlap width of fiber overlap is generally specified in the ply design, and the overlap position is staggered as much as possible. The docking of core materials should generally minimize the gap. Foam and light wood can be nailed with a plastic nail gun, and strong core mat can be sewn.

 

.If a hand-laid transition layer is made, it is best to build a construction ladder to prevent pre-demolding

 

.When the ply structure lacks a diversion medium and the resin flow rate is slow, a more dense pipeline is often used for laying and a large area of ​​diversion net, which will require more resin. If there is a core material in the ply to provide a diversion effect, the diversion net and diversion pipe can be greatly simplified.

 

.Splicing and folding of vacuum bag film. If the vacuum bag film is not long or wide enough, or the local folds are not sufficient, it may be over-stretched during vacuuming and easily rupture.

 

Interpretation

 

What are the requirements for material laying?

 

• Pipeline laying is diverse and should not be limited to a single method. In terms of form, it can be in the shape of fish bones (divergent type) or multiple parallel diversion pipelines in the picture, and combined with each other (multiple decks). It mainly changes according to the shape and layer of the product.

Vacuum Boat Inlet

Interpretation

Under what circumstances can we start to introduce resin?

Prerequisites: 1. Are the resin raw materials calculated and prepared?

2. The gel time is within the controllable range, too short or too long is not acceptable (considering the factors of heat release from accumulation and ambient temperature)

Check the sealing in sections, do the pressure holding test, and use a vacuum leak detector to check the leak point

Section 1, vacuum pump → resin collector

Section 2, resin collector → mold exhaust port

Section 3, mold. The larger the product, the longer the pressure holding time is required

Section 4, mold injection port → resin barrel

 

Interpretation

 

Under what circumstances can we start to introduce resin?

 

Prerequisites: 1. Are the resin raw materials calculated and prepared?

2. The gel time is within the controllable range, too short or too long is not acceptable (considering the factors of heat release from accumulation and ambient temperature)

Check the sealing in sections, do the pressure holding test, and use a vacuum leak detector to check the leak point

Section 1, vacuum pump → resin collector

Section 2, resin collector → mold exhaust port

Section 3, mold. The larger the product, the longer the pressure holding time is required

Section 4, mold injection port → resin barrel

 

Deck complete vacuum infusion

Interpretation

 

What are the main differences between vacuum induction on deck and vacuum induction on bottom?

1. There will be more changes in the layers, and the flow rate of the resin in different layers will be quite different.
2. There are more corners, which are easy to form bridges, which become uncontrollable factors in the diversion process.
3. When designing the diversion network, it is often necessary to make some spare diversion pipes or even exhaust pipes.

4 Introduction to other applications of closed mold process shipbuilding

 

1. Vacuum infusion to make molds

80-foot racing sailb

Material: Epoxy-glass fiber/carbon fiber

Surface: Mold-free gel coat

2. Wet Vacuum Bagging to Make Rowing Boats

Material: Epoxy carbon fiber/aramid, linear PVC foam

Surface: Post-painted

Material:

Epoxy carbon fiber/aramid, NOMEX honeycomb

Surface:

Post-painted

Sailing Competition Sailing

Material:

Epoxy-carbon fiber prepreg, NOMEX honeycomb

Surface:

Post-painting

Material:

Unsaturated resin/glass fiber, polyurethane rigid foam

Surface:

ISO-NPG gel coat

Typical application 3: LRTM process for inflatable boats

 

Material: Unsaturated resin/fiberglass, Balsa wood

Surface: ISO-NPG gel coat

5 FAQ

Q1. What changes are needed when we switch from hand lay-up to vacuum infusion? How to change?

The suggestion is to consult a professional design company. The lay-up must meet the process construction requirements, as well as the structural strength and rigidity requirements, while taking into account the cost.

In the shipbuilding industry, there are many units that can provide composite material structure design. Through finite element analysis and simulation software, many detours can be avoided. At the same time, they have a better understanding of material parameters and properties and can provide the most optimized solution.

The general changes are:

The single-board structure of traditional hand lay-up will be transformed from mat cloth combined lay-up to axial fabric lay-up, reducing the strength loss of the fiber and improving the processability of resin introduction.

The main keel often uses foam, which is vacuum-infused with the hull as a whole.

The hull structure with single-board reinforcement will be replaced by a sandwich structure for overall molding.

Q2. How to choose a suitable process to make a boat?

It has something to do with the material and structure. For example, prepreg/honeycomb structure can only be made with wet vacuum process.

(Take 5M fiberglass speedboat as an example)

It depends on the specific project situation. Different processes have some advantages and disadvantages. Which requirement is the most important is the determining factor.

Q3. What should I do if the fabric marks and core material marks on the surface of the product gel coat are obvious after the hull made by vacuum infusion is demolded?

On the one hand, it may be a problem with the resin and curing agent, with too large a shrinkage rate, too fast reaction, and high heat release. The main problem is that there are many problems with unsaturated resins, and you need to contact the resin manufacturer or curing agent manufacturer to find a more suitable raw material.

On the other hand, you can reduce the marks by hand-laying the transition layer and spraying the barrier gel coat.

You can also adjust the vacuum infusion ply structure and core material, use special strong core mat TF on the first layer, put ±45° axial fabric before 0/90° fabric, and ask the core material manufacturer to adjust the slotting and punching method and aperture, slot width, etc.

Finally, if the mold has poor heat shock resistance or is still in the aging process, and the mold surface has formed marks, you need to process the mold surface.

 

Q4. Why do you watch the resin vacuum infusion process very perfectly, without any flaws before curing. But after the hull is cured and demolded, there are some problems with the inner surface being discolored and the fibers being whitened?

One reason is that the local lack of glue causes whitening. Although the resin is fully infiltrated, the resin is stopped in advance, and the vacuum pipeline extracts a lot of resin in the ply during the final curing process. The resin content is too low, and the fibers are exposed and white and mottled.

Another reason is that the vacuum is turned off too early, and the fiber rebound and delamination occurs between the plies, resulting in local whitening between the layers.

In addition, too much spray glue is used, which cannot be seen during the introduction process, but white mist and white spots of the spray glue can be seen after demolding.

Finally, in order to make it easier to tear, the resin curing degree is not complete when removing the demolding cloth and the guide net, which damages the surface glass fiber and causes it to delaminate and whiten.

 

Q5. When vacuuming the boat, the resin guide tube presses the inner cabin with obvious indentations, affecting the interior decoration. How to avoid it? It is recommended to use Omega tubes or guide strips ENKA CHANNEL. Another method is to wrap the guide tube with a guide net, leaving some overlapping distance for the guide net. When vacuuming, use a vacuum bag film to “lift” the guide tube in the guide net.

 

Q6. Why are ships made with the LRTM process often overweight? Since the upper mold of the LRTM process is a semi-rigid mold, when the layer material is not laid in place, the upper mold cannot be completely flat under 1 atmosphere of pressure, which often affects the mold clearance between the upper and lower molds, causing the extra gap thickness to be filled with resin, resulting in overweight. At the same time, as the rubber seal ring ages, it will also affect the mold clearance between the upper and lower molds, resulting in overweight.

 

6 Closed-mold shipbuilding development vision

 

1. Development towards more ship types, replacing wooden and steel structures

Fishing boats. Japan’s fishing boat technology is far ahead, especially ocean-going fishing boats. China’s fishing boat reform can drive the development of the composite ship industry, and it is necessary to use closed-mold technology to catch up. Reduce the weight of the ship, reduce fuel consumption, and form a sandwich structure with foam for refrigeration and insulation. Military ships. More and more structural components and accessories will be made using closed-mold technology, and the entire ship will be made using closed-mold technology. It is also very necessary to replace steel. Give full play to the performance of light weight, high strength, stealth performance and high wave transmittance. Foreign large minesweepers and large landing hovercraft have mature technology.

Warship cannon turret

Better stealth, while meeting the requirements of bulletproof and waveproof

2. Modular Application

Use closed-mold modular parts to add and reduce resource waste.

For example, the DUFLEX sheet of ATLCOMPOSITE in Australia provides a way of thinking, which has a better application effect in some special projects.

At the same time, modular parts should be able to improve the plasticity and processability of materials in the future. Through the semi-cured state performance or thermal deformation performance of special materials, closed-mold modular parts are expected to be directly formed on the 3D printed mold base