Detailed Explanation Of The Design Principles, Methods And Production Technology of RTM Fiberglass Molds

 

In the closed-mold molding process where products pursue two-sided gloss and high precision, the reasonable design of lightweight RTM FRP molds is particularly important. If there is a problem with the mold design from the beginning, the mold may not be used, resulting in capital loss.

 

The design of FRP molds should fully consider factors such as polyester shrinkage, and focus on product precision requirements to ensure that tolerances are controlled within the allowable range. This requires that RTM closed-mold FRP molds should fully consider and follow the following design principles.

 

 

1. The surface of the FRP mold must be flat, smooth, free of defects such as cracks and pinholes, with a glossiness of more than 95 and a surface roughness of less than 10μm to ensure the excellent surface quality of the product.

 

2. The mold is composed of a positive and negative mold, which is matched by a positioning pin and requires high precision to avoid misalignment and affect the dimensional accuracy of the product.

 

3. The mold should have sufficient strength, rigidity and surface hardness. The mold should be able to withstand the combined effects of its own weight and product weight, as well as vibration and live load during the production process. For large FRP products, in addition to meeting the strength requirements, rigidity is also important to prevent mold deformation, affect product quality and the service life of the FRP mold. The mold surface hardness should reach a Barcol hardness of more than 40 to reduce the force damage to the mold during demolding.

 

4. The key factor in mold design: how to demold. Demolding should be convenient and easy, and the FRP mold should not be damaged during product demolding. Therefore, the radius of curvature of the mold corners should be as large as possible, the radius of curvature of the inner corners should not be less than 2mm, and the overall mold should be designed with a demoulding slope, and demoulding should be performed with the help of water pressure and air pressure when necessary. For the hand-laid FRP mold forming process, the demoulding method is generally combined with ejection and air release. For FRP products with complex shapes, auxiliary structures such as local assembly molds and side core pulling should sometimes be considered.

 

5. The mold manufacturing material should have sufficient low shrinkage and thermal stability to prevent mold deformation caused by resin curing exothermicity and external heating curing, which will affect product quality and FRP mold life.

 

In short, the construction of any FRP mold is a trade-off between mold physical properties, material cost and construction time. Usually, more time, energy and money are invested in the design stage of FRP molds, but potential production problems can be avoided or reduced later.

 

 

How to release the mold from the original mold and the molding process of subsequent parts will affect the overall design and structure of the lightweight RTM FRP mold. Some common FRP mold design methods are as follows:

 

1. Reasonably select the parting surface, which should be conducive to demolding, not produce demolding sheath, and be conducive to the regularity and integrity of the product surface. In order to facilitate product demolding, a certain demolding slope should be considered when designing the overall mold. For FRP molds, the general demolding slope is at least 1°. For parts that are difficult to demold, air release and ejection devices must be set at appropriate locations.

 

2. Adopt the three-point positioning principle. One point is center positioning, so that the upper and lower molds are on the same center line; the other two points are to use positioning pins to ensure the matching accuracy of the mold.

 

3. Determine the appropriate FRP mold thickness and structural type according to the requirements of specific products. Polyester molds should have sufficient strength, rigidity and service life. The mold thickness should be considered comprehensively based on the thickness, geometry and size of the product. Generally, the mold thickness is 3-4 times the thickness of the product.

 

3. According to the use conditions of the mold, the structure of the FRP mold is generally designed as a composite structure of gel coat layer/transition layer/structural layer/reinforcement layer, and the layer type depends on the specific product.

 

Usually, the specific FRP mold design needs to consider more factors, such as mold shrinkage, material cost and production time, etc., which are not listed here one by one. Careful consideration of these factors will help you spend less money and get more ideal results.

 

 

Detailed Explanation of FRP Mold Making Technology

 

Although the production process of FRP molds has undergone some new changes and improvements in recent years, this article is still a good article. It describes the entire mold-making process very clearly, especially the details.

Zero, main mold production

1. Main mold production: There are many materials for making the main mold. Generally, the materials used for the main mold are required to be easy to form, easy to trim, and have good stability. Such as wood, plaster, wax, etc. We usually use wood. According to the product drawings or mold drawings, the carpenter makes a wooden main mold.
2. Main mold trimming: The main mold must be repaired before the production mold can be remade. Trimming includes puttying, shaping, size correction, and reinforcement. This process mainly involves basic treatment of the surface and overall wooden mold to ensure that the size and form of the wooden mold match the drawings.
3. Main mold surface treatment: This process includes spraying gel coat, gel coat curing, grinding, polishing, and mold release wax. Spray the gel coat on the master mold processed in the previous process, and then wait for the gel coat to solidify. After the gel coat solidifies, use sandpaper to polish the gel coat surface. Generally, it starts from coarse sandpaper of dozens of numbers to fine sandpaper of about one thousand numbers. After sanding, start polishing the mold and finally apply the demoulding product. Until this time, the production of the master mold is over. Next, the master mold can be used to make the production mold.

Many materials and tools are used in this process. Wood and corresponding woodworking tools are necessary. In addition, there are: putty (also known as atomic putty), sandpaper, from coarse sandpaper of dozens of numbers to fine sandpaper of more than one thousand numbers, gel coat, mold cleaner, sealer, polishing paste, demoulding wax, etc. In addition, some small tools are also used, such as grinders, polishing wheels, gel coat spray guns, air pumps (or other air sources).

4. Reproduction of production mold: After the main mold is waxed, the production mold can be remade. The production mold remaking process is as follows:

 

1) Spraying mold gel coat: Since the production mold is being made, a mold gel coat with better performance must be used to ensure the final effect of the mold. And the spraying is required to reach a certain thickness (0.7-0.8mm).

2) Mold layering: After the mold gel coat is initially solidified, the layering can begin. The layering process cannot be too fast. Generally, 2-3 layers of fiberglass cloth or fiberglass mat are laid in one day. A certain amount of mold resin is required for layering. The performance of this resin is better than that of ordinary resin. During the layering process, workers need to prepare glue, that is, add an accelerator and a curing agent to the resin, and then use a glue-spreading tool to apply glue, lay a layer of fiberglass fabric and apply a layer of glue. At the same time, use an iron roller to flatten the fabric, remove bubbles, and make the glue uniform. After reaching the specified thickness, the layering is completed. In general, the thickness of the mold should reach 3-5 times the thickness of the product. Therefore, the layering time is generally longer and can last for 6-7 days.

3) Mold curing and reinforcement: The mold can be cured naturally or by heating, but it is generally best to have a natural curing period. After the natural curing period, the mold should be reinforced to prevent the mold from being damaged during the production process.

4) Surface treatment of the production mold: After the production mold has cured for the required time, it can be removed from the main mold. The demolding method can be manual demolding by workers or high-pressure gas demolding. The production mold after demolding also needs to be surface treated again, including sanding, polishing, marking process lines, and demolding products. After the demolding products are finished, they can be used to produce products.

The materials used in this stage are: mold gel coat, mold resin, ordinary resin; curing agent, accelerator; glass fiber surface mat, glass fiber mat, glass fiber cloth; fine sandpaper, mold cleaner, sealer, polishing paste, demolding products (mold release wax, semi-permanent mold release agent, etc.). In addition to mold processing tools, the tools used also include tools for laying layers: such as rubber rollers, rubber brushes, iron rollers, etc. Mould making is a delicate and lengthy process. Generally, the production cycle of a mould takes about one month.

 

1. Conditions for Manufacturing Molds

To manufacture top-quality molds, we should start from the three aspects of workshop hardware, management level, and personnel training and achieve excellent standards. We must do solid and effective work in the selection of mold materials, reasonable arrangement of process procedures, and quality testing.

 

1. Environmental Conditions

Tempurature: The ideal temperature range is 21-28°C. This ambient temperature should always be maintained during the manufacturing of the original mold and mold. The base material used in mold manufacturing should have the best chemical reaction process. If the environmental factors are not suitable, it will prolong or accelerate this process and have an adverse effect on the performance of the mold.

 

Humidity: Humidity has an important influence on the curing of gel coats and resins. If the relative humidity is greater than or equal to 70% when manufacturing the mold, it will have an adverse effect on the curing of the base material. We should be cautious and wait until the appropriate production conditions or take dehumidification measures.

 

2. 2. Perfect Workshop Management

The workshop for mold manufacturing should have chemical product storage conditions, equipment for controlling temperature and humidity, and a clean workplace. In terms of management, it is necessary to divide the area according to the process flow, and the control documents such as process specification cards, operation instructions, and quality tracking cards are complete and detailed.

 

3. Compressed Air

Pure and dry air supply is another factor in manufacturing high-quality molds. A small amount of water or oil in the air pipe will have an important impact on the mold gel coat. It is necessary to match an air compressor placed in a good environment, and equip it with an effective air dryer and oil-water separator, and a voltage stabilization and pressure regulation system.

 

4. Raw Materials

Using the best quality raw materials as much as possible is another important condition for manufacturing high-quality fiberglass molds. The cost of raw materials used to manufacture molds accounts for only a small proportion of the total cost, so don’t try to save.

 

5. Arrange a Reasonable Process Flow

Whether it is a traditional layering method or a low shrinkage system, the required time is determined by the curing characteristics of these materials. Attempting to take shortcuts or speed up the process will have an adverse effect on the quality of the mold.

 

6. Well-Trained Technical Workers

The so-called well-trained means having common sense basic knowledge and professional skills, good behavioral habits, and being able to consciously regulate personal behavior.

 

Material Selection

 

2. Original Mold Material

The original mold is the most important prerequisite for manufacturing high-quality FRP molds. The selection of the base material of the original mold is mainly determined by the mold type and size, as well as the experience gained when making different types of molds. Based on the principle of layer thermodynamics, in order to eliminate defects such as different densities, seams, and screw holes on the surface of the original mold, there must be a layer of material with consistent heat transfer speed and certain rigidity as the base material of the surface treatment material. On this substrate, easy-to-grind materials are used to correct the accuracy and smoothness of the contour dimensions of the original mold surface, and air-drying gel coats with high-gloss surfaces are used to obtain high-brightness original mold surfaces. The accuracy, smoothness, and brightness of the contour dimensions of the original mold surface directly determine the quality of the FRP mold.

 

To ensure the accuracy and smoothness of the contour dimensions of the original mold surface, materials with the following characteristics are required:

1) Air-dried spray type, which can quickly accumulate a certain thickness

2) Low porosity, easy to grind and shape

3) Good temperature resistance

 

To ensure the high brightness of the original mold surface, materials with the following characteristics are required:

1) Air-dried spray type

2) Low porosity, easy to water grind

3) Good temperature resistance

4) Smooth and delicate surface

 

2. Requirements for demoulding materials on the original mold surface

1) Ensure demoulding

2) Do not affect the brightness of the original mold surface

3) The demoulding material does not transfer to the mold

 

3. Mold Material

The most important factor in measuring the production level of molds is the brightness, smoothness, gloss, surface porosity, etc. of the mold gel coat surface. This is determined by the process level. The length of time the mold surface quality is maintained, which is usually called the mold life, is determined by the performance of the mold material.

 

1. Mold Gel Coat

Mold gel coats currently mainly include vinyl mold gel coats and isophthalic unsaturated polyester mold gel coats.

The resin structure of the mold gel coat gives the final mold surface mechanical strength, brightness, weather resistance, chemical resistance and flexibility. The operation process of mold gel coat is a crucial step in mold manufacturing. The mold gel coat is the entire mold, and all the back layers and structural skeletons serve the mold surface gel coat layer. Mold gel coats require higher use and curing conditions than general product gel coats. High-quality mold surfaces require very precise mold gel coat operation and mixing processes.

Therefore, the performance of mold gel coat materials, environmental control equipment, and process technology level are the key to determining mold production.

 

1. Mold gel Coat Construction Process Equipment

The use of spray equipment to apply mold gel coat is an important link in the mold manufacturing process. There are two types of spraying equipment available, one is the equipment typically used for production gel coat spraying, and the other is an air atomizing pressure spray can. Both types of equipment can produce high-quality gel coat surfaces. However, you must consider the following points when operating:

For typical gel coat spraying equipment, the curing agent is injected into the raw materials by internal or external mixing. These spray guns are easy to use and are generally more efficient than spray cans. However, it should be noted that gel coat defects may occur if the proportion and distribution of the curing agent are distorted, the delivery system is faulty, or the fan-shaped mixture is not distributed properly.

Many mold makers choose to use an air atomizing spray can to eliminate deviations in the production process. In the spray can gun, the curing agent is manually mixed to ensure the correct proportion. In addition, the distribution of the fan-shaped mixture produces a very fine atomization effect, but the spraying speed is slower than the pumping system, and its working time is limited by the gel time of the raw materials.

 

1. Mixing

Thoroughly stir each bucket of mold gel coat with an air-powered agitator before use, and ensure that all materials in the bucket are thoroughly mixed. Do not use manual mixing, and do not reduce the degree of mixing because of the fear of air mixing into the mold gel coat. Make sure that the gel coat at the bottom and side of the barrel is also fully mixed. Then allow the gel coat to stay for a few minutes before use to restore its own viscosity and defoam.

 

1. Temperature

Verify whether the temperature of the gel coat is within the range of 23-28°C. If it is not within the range, measures should be taken.

1. Test and select the performance of the curing agent in different types of materials and different processes.
2. Gel Time

 

The next step in gel coat quality inspection is to verify the gel time, which will prove whether the mold gel coat is within the specified range. Verifying the gel time is a simple process and does not require any equipment.

1) The test sample should reach the standard test temperature of 25°C;

2) Add the specified amount of curing agent and mix well;

3) Record the time from the addition of the curing agent to the gel.

The gel coat supplier will recommend equipment and provide detailed instructions to guide users to conduct gel coat quality inspection in their own factories. It is important to conduct quality inspection of mold materials in the factory and can be regarded as part of the molding process. After applying the release agent on the original mold, it is time to start spraying the gel coat. The gel coat is sprayed in two layers. The thickness of the mold gel coat needs to be strictly controlled. This requires that after each spray of the gel coat, the thickness of the gel coat layer must be accurately measured at many places on the mold surface.

 

1. The correct gel coat operation method is as follows:

1) The first spray thickness is about 0.15-0.2mm.

2) Spray the second coat of gel coat in a perpendicular direction to the first coat.

3) Spray the last coat of gel coat in the direction of the first coat. Now the thickness of the whole layer of gel coat is 0.5-0.6mm.

4) After the first layer of gel coat gels, another layer of gel coat can be sprayed. The thickness of this layer of gel coat cannot exceed two-thirds of the thickness of the first layer of gel coat.

5) Spray the second layer of gel coat of 0.3-0.4mm in the same way as the first layer of gel coat (steps 1 and 2 above).

The spraying of gel coat follows the principle of difficult first then easy, groove first then plane, try to avoid local excessive thickness. In the case of unsatisfactory spraying angle, you can use the method of hand brushing to assist.

 

3. Mold Laying

The mold gel coat takes two to three and a half hours, that is, the initial setting time of the mold gel coat under ideal conditions is fifty minutes and the curing time is one hundred minutes. After the second layer of gel coat gels, the base layer can be laid. There are two common laying methods for making FRP molds. One is the traditional multi-step laying method, and the other is the fast low shrinkage system laying method.

 

1. Traditional Mold Laying Method
2. Raw Materials:

1) Mold Resin

Mold resin should have a high heat deformation temperature and low shrinkage. Isophthalic resin, vinyl resin (such as FUCHEM854, 854M, 881, 881T) and DCPD resin can be used as mold resin.

Vinyl polyester resin has high solid content, thixotropy, and has promoted excellent process performance, low shrinkage performance; excellent mechanical properties, heat degradation resistance, and chemical corrosion resistance. It has excellent strength retention at high temperatures and high heat deformation temperature. It can be used as the first choice of mold material when manufacturing high-end molds. The mold gel coat is used to ensure the surface quality of the product, and the performance of the mold resin is used to ensure the stability and consistency of the product quality.

 

2) Glass Fiber Reinforced materials

Surface mat is often used as the first layer of reinforcement material because of its fine fiber, convenient infiltration and easy defoaming. Chopped strand mat is an ideal mold reinforcement material because it can minimize the printing of fiber texture.

3) Quality Assurance

In order to ensure that the mold resin meets the quality requirements, it is also necessary to conduct quality inspection like the mold gel coat. Including: material storage period, test gel time.

 

1. Operation Process

First layer (the first layer after spraying the gel coat)

The mold layering starts with the paste of the first layer. According to the complexity of the mold shape, the fiber can be selected from surface mat, 300 grams or 450 grams of chopped strand mat when paste-making the first layer. The quality requirements of the first layer are very strict, and it is necessary to avoid repairs after the mold is completed. In order to ensure the quality of the mold, the layer should be rolled very carefully to ensure that all bubbles are driven out, and a reasonable resin-to-fiber ratio should be maintained. Special tools should be used to roll the resin. After the first layer is cured, the layer should be thoroughly checked, including all corners and curved surfaces, as well as all possible problems. If bubbles are found, they must be carefully removed and the damaged surface repaired.

 

C、 Thickening Layer

One method of laying a thickening layer is to use all glass fiber to make it. This is the most traditional method and will not be explained in detail here.

Here, I would like to introduce some new methods to you.

When manufacturing the mold, you can use strong core mat as a fast thickening layer. It can effectively improve the surface quality of the mold and prevent the texture of the glass fiber or sandwich material in the mold reinforcement layer from being printed out. You can lay a layer of 450G mat after the first layer, lay a strong core mat, and then a layer of 450G mat. The three layers are laid at the same time. Such a layer can effectively improve the surface quality of the mold. Because the structure of the strong core mat is organic fiber and microbeads, such a layer can also appropriately improve the rigidity of the mold.

Structurally, strong core mat is a composite structure of hollow microspheres and organic fibers. It is a thin core material often used in the FRP industry. Using this material can increase the stiffness of FRP appropriately and effectively improve the surface quality of the mold or product.

 

Another mold ply is to consider whether to use a sandwich structure when making the mold. The sandwich structure refers to the use of balsa wood, PVC and other materials instead of glass fiber in the FRP ply.

1) Balsa balsa: The amount accounts for more than 50% of all sandwich materials. It is a core material with the longest history of use, which has been used for more than 50 years. It has the best processability compared to other core materials, the highest compressive strength and compressive modulus, the most resin-saving, and the best adhesion to the panel. It can be used as the main structural sandwich.

 

2) PVC: Linear and Cross-Linked.

Sandwich materials have been widely used in composite material structures. For molds, the use of sandwich structures can obtain high-rigidity and light-weight plies, while also reducing the requirements for external structural skeletons.

If you are making a thin product, the resin releases low heat or the product cycle is long, then using a sandwich mold will not cause any problems. However, if the product is very thick, a high heat release resin is used or the production cycle is short, it is best not to use a sandwich mold.

 

1. Determine the Layer Plan Based On The Barcol Hardness

A good way to make a layer plan is to observe the changes in the Barcol hardness of the layer. When the Barcol hardness of the layer reaches 80%-90% of the fully cured hardness, you can continue to lay the next layer. When using a Barcol hardness tester to measure the hardness of the layer, please note: you need to record multiple readings and calculate the average value, and use this value as the hardness value. This method eliminates data errors caused by time differences, and this method is usually faster than laying layers based on a layer time schedule.

 

1. Rapid Molding Process

The key to this mold molding method is to use a fast low shrinkage mold resin system, which uses a special resin and filler that shrinks very little or even does not shrink during curing. Therefore, the speed of lamination can be accelerated, and glass fiber injection can be used to obtain higher efficiency. This process can be combined with traditional processes, or it can be combined with the use of strong core mat, balsa wood sandwich and other materials in the layer design.

This high-quality resin system can produce high-quality, non-deformed mold surfaces, but it also has strict requirements on temperature and operation. Therefore, the production conditions and methods must strictly follow the requirements of the resin system supplier.

 

Rapid mold making resin, which can be applied by ordinary hand lay-up process or spray process. The operation steps are as follows:

 

• Hand Lay-up Process:

1) After the mold gel coat is cured, the first layer of reinforcement material uses vinyl resin. After the operation is completed and cured, the rapid mold making resin is used.

2) Continuously lay 6 layers of 450 g/m2 or 300 g/m2 alkali-free chopped strands to obtain a thickness of 3 to 4 mm. When laying each layer, it is best to use the roller coating method to make the resin evenly distributed. At the same time, use a defoaming roller to eliminate bubbles.

3) When the resin is cured, the layer turns white, and the next 4 to 6 layers can be laid one hour later. The operation method is the same as step 2.

4) Repeat the above steps until the required thickness is reached.

 

Spraying Process:

1) After the mold gel coat is cured, the first layer of reinforcement material uses vinyl resin. After the operation is completed and cured, start using rapid molding resin.

2) Spray 3-4 mm thick resin and chopped fibers, use a defoaming roller to eliminate bubbles and compact.

3) When the resin is cured, the layer turns white. One hour later, you can start spraying the next 3-4 mm resin and chopped fibers, use a defoaming roller to eliminate bubbles and compact.

4) Repeat the above steps to lay the layers to achieve the required thickness.

If combined with the use of strong core mat, balsa wood sandwich and other materials, the resin used on the upper and lower sandwich materials is vinyl resin. Using rapid molding resin, the mold reinforcement layer can be completed in one day. Due to the use of zero shrinkage curing system, the mold has better dimensional stability and better surface effect.

 

4. Mold External Structure

When the mold is laid, the next step is to make the external reinforcement structure of the mold. The purpose of the reinforcement structure is to make the external force evenly transferred to the mold surface to prevent the surface deformation of the mold.

The most basic reason for the cracks in the mold gel coat is that the mold is deformed by external force. Because of this, improving the rigidity of the mold in the design will greatly prevent the cracking of the gel coat. The use of strong core mat and light wood sandwich materials in the mold laying can greatly improve the rigidity of the mold and reduce the damage to the mold by external forces during the demolding process.

Whether it is a steel frame structure, a plywood box structure or a skeleton structure, if the structure is directly connected to the mold surface, different heat conduction will cause marks on the mold surface. Therefore, at the junction of the structure and the mold, insulation materials are used as the basic layer of the reinforcement structure or low thermal conductivity adhesive materials.

 

5. Demolding

When the frame structure is completely cured, the mold can be ready for demolding after 3 to 5 days of maintenance on the mold. Start by cutting the edges neatly, then use multiple demolding wedges to distribute and insert between the mold and the prototype, apply force evenly, do not apply force alone to prevent damage to the mold, and finally demold completely.

 

6. Processing The Mold

After demolding, carefully check the mold surface. Any surface defects should be marked and repaired. Ideally, the mold does not need to be polished, only the mold can be polished. However, in some cases, the mold surface needs to be properly processed and modified.

Pay attention to the following during the polishing process of the mold:

1. During the rough polishing process, eliminate the marks caused by repairs, water grinding, mold release agents or other reasons.
2. During the use of fine polishing materials, eliminate the marks caused by rough polishing.
3. When using polishing materials, pay attention to whether the material contains brightening ingredients and whether the polishing is the true brightness of the mold gel coat.

Mold manufacturing is an organic and continuous process. The key to successful mold making is: using the best raw materials, choosing the appropriate mold making working environment and conditions, well-trained employees, and simple and effective management.