How to Make FRP L-RTM Mold

 

 

The content of the first part is the same as: “L-RTM Technology Exchange and Sharing”

 

The content of the second part:

 

1. Types of molds RTM is formed under low pressure, and the mold rigidity requirement is relatively low. A variety of materials can be used to make molds. Common mold types include fiberglass molds, electroformed nickel molds, aluminum molds, cast iron molds and steel molds. Table 6-8 lists the comparison of various types of molds.

 

Table 6-8 Comparison Of Different Types of Molds

 

	FRP mold	Electroformed nickel mold	Aluminum mold	Cast iron mold	Steel mold
Strength/MPa	150～400	300	50～500	100～200	>300
Modulus/GPa	7～20	200	71	～150	210
Toughness	Damage caused by impact	Low impact resistance	Easy to cause scratches	Not easy to damage	Not easy to damage
Density g/cm3	1.5～2	8.9	2.7	7.2	7.8
Coefficient of thermal expansion ×10－6℃	15～20	13	23	11	15
Thermal conductivity/W·m-1·K-1	～1	～50	200	70	60
Specific heat capacity/J·kg-1·℃-1	～1000	460	913	500	420
Maximum operating temperature/℃	80	>Resin curing temperature	>Resin curing temperature	>Resin curing temperature	>Resin curing temperature
Tolerance	Deformation due to shrinkage during curing	Depends on the original mold	High precision	High precision	High precision
Surface finish	Can be polished	Polishing improves finish	Polishing improves finish	Not as good as aluminum and steel molds	Polishing to improve finish
Repair	Easy to repair gel coat damage, difficult to repair reinforcement layer damage	Not easy to repair	Easy	Easy	Easy
Manufacturing cycle	2～4 weeks	30～40 days	30～40 days	30～40 days	30～50 days

 

 

2. Mold Structure Design

 

The structural design of RTM molds includes product structure parting; mold structure forms such as embedded molds, combined molds, embedded structures, sandwich structures, etc.; special locking mechanisms, demoulding mechanisms, special sealing structures; vacuum structures; mold lamination structures, rigidity structures, mold heating forms, etc. The mold structure design mainly follows the following principles:

(1) Simplify demoulding parts as much as possible: When manufacturing the mold, the product demoulding should be considered. A certain demoulding angle should be left on both edges of the cover, and the joints of each part should have a smooth transition.

(2) Make the layout of the pouring system as convenient as possible.

(3) Facilitate gas discharge: In order to facilitate gas discharge, the parting surface should coincide with the end of the resin flow as much as possible.

(4) Mold sealing and vacuum-assisted molding: The mold adopts a double sealing structure and uses vacuum to evacuate the mold cavity before impregnation, which is conducive to reducing mold deformation, reducing porosity, improving production efficiency, and reducing finishing processes.

(5) Convenient placement of the live block: When the parting surface is opened, there must be a certain amount of space to facilitate the placement of the live block, and ensure that the live block is placed firmly. The live block of the cover mold adopts vacuum adsorption, which is conducive to positioning and stability.

(6) Difficulty of mold manufacturing: The overall structure of the mold is simplified, the number of parting surfaces is minimized, and a straight parting surface is used.

 

 

3. Design of Injection Port and Exhaust Port

 

There are three types of injection ports on the mold:

 

(1) Center position: The injection port is selected at the geometric centroid of the product to ensure that the resin flows in the mold cavity with the shortest distance.

 

(2) Edge position: The injection port is designed at one end of the mold, and a distribution channel is provided on the mold. The resin is injected from the edge channel, and the exhaust port is symmetrically designed at the other end of the mold.

 

(3) Peripheral: The resin is injected through the peripheral distribution channel, and the exhaust port is selected at the center or near the center. Although the flow channel for peripheral injection is also at the edge, it is closed, and the exhaust port is at the center of the mold.

 

Regardless of how the injection port position is selected, the purpose is to ensure that the resin can flow evenly and penetrate the fiber. Having multiple injection ports on the mold can improve the injection efficiency, but it is necessary to ensure that when the flow edge reaches the next injection port, the injection port can be opened in time and the previous injection port can be closed in time to avoid dead corners caused by interruption or turbulence.

 

The exhaust port is usually designed at the highest point of the mold and the end of the filling flow to facilitate the discharge of air and the full infiltration of the fiber. The ideal injection and exhaust ports can be better determined with the help of flow simulation software.

 

4. Mold Sealing Structure

The mold is usually sealed by sealing strips of different structures. The sealing strips used include O-type, rectangular and special-shaped sealing strips. Silicone rubber is the best material. In order to ensure the effective sealing of the mold and the need for vacuuming in the mold cavity, a double sealing structure is often used.

 

5. RTM Mold Heating Method

There are two major methods for heating the resin in the mold in the RTM process: one is the direct heating method. For example, the American RP/CMachinery company directly passes radio frequency power to the resin in the mold cavity to solidify the resin. This method is more advanced and has the highest thermal efficiency, but it is difficult and is not yet mature; the other is the indirect heating method: the heat energy is carried by the medium (gas, water, oil, steam), and is conducted to the resin through the mold backing, shell, and mold surface to solidify the resin. There are three types of indirect heating methods.

 

(1) Backing pipeline method. The heat medium pipeline (circulation pipeline) is laid in the mold back lining. Since the pipeline is a certain distance away from the mold surface, and the composite material is a poor conductor of heat, heat conduction is difficult, so the heating speed is slow and the heating cycle is long. The application of this method requires improving the thermal conductivity of the mold material and selecting a relatively flat product type so that the pipeline can be laid easily.

 

(2) Shell pipeline (or electric heating wire cloth) method. The heat medium circulation pipeline (or electric heating wire cloth) is laid in the shell close to the mold surface, and the control of the heat medium flow is coordinated with the operation of the press and the clamping of the mold periphery. One heating system can supply multiple molds at the same time, with the fastest heating and cooling speed, but the cost of each mold is the highest.

 

(3) Whole mold heating method. After the material is filled into the mold, the entire mold is placed in a curing furnace (or autoclave) for heating. The heat energy is conducted through the mold to the resin in the cavity, causing the resin to solidify. The mold material must have good thermal conductivity and the curing furnace must be able to accommodate the entire mold. If the curing furnace is large in size, multiple molds can be heated at the same time. This method has low heating efficiency and a long curing cycle.

 

In addition, Sterling Heights in the United States has developed a high-speed RTM process, which uses a steel mold with pipes inside the mold, steam at about 140°C in the pipe, and resin curing in 45 seconds, combined with other “high-speed technologies” such as pre-mixing, pre-catalysis, special single resin, fast feeding, fast injection, etc., with a molding cycle of about 1 minute

 

6. Manufacturing of FRP RTM Molds

 

The basic production process of FRP RTM mold is shown in Figure 6-16

 

 

Figure 6-16 Flow Chart of FRP RTM Mold Production

 

The most important item in the preparation of the original mold is inspection. The molding of composite materials is irreversible, so each process needs to be inspected and verified to be correct and qualified before the next process can be carried out. This is also the most effective way to ensure timely adjustment when problems occur in mold production and minimize losses.

 

Internal accessories include injection port inserts, exhaust port inserts, sealing strips, auxiliary mold ejectors, mold guide positioning parts, and product embedded parts positioning mechanisms, etc.

 

The gel coat layer is the basis for ensuring the mold surface. The thickness should be 0.5-1.0mm. It can be sprayed in 2-3 times. The curing time is judged by hand touch. It is better to feel sticky and the fingertips are not covered with gel coat.

 

The transition layer can be made of 30-50g/m2 surface mat paste. During the paste process, it is ensured that there are no bubbles. After curing, the reinforcement layer begins to be pasted. During the paste process, the most important thing is to control the uniformity of the layer and the uniformity of the glue content.

 

For flat-plate molds with simple shapes and large areas, the alternating mat-cloth plying design can be used in the reinforcement layer. This method can further improve the strength of the plate shell, while molds with more curved surfaces and complex shapes can be directly made of chopped mat plying. However, cloth reinforcement should be carefully selected according to different mold quality requirements, because its strength and content are directional, which can easily cause the mold to deform due to uneven shrinkage.

 

In addition, according to different resin systems and different mold requirements and paste systems, in general, the fewer the number of paste layers per curing, the higher the mold size accuracy, the lower the mold strength, and the lower the probability of mold deformation; on the contrary, the more the number of paste layers per curing, the greater the overall strength and the greater the probability of shrinkage deformation.

 

For molds with heating layer design, the thickness of the reinforcement layer should be 6-8mm, which can ensure the surface strength and the insulation effect will not be too prominent. The heating layer is composed of heating pipes and heat-conducting medium. The heating pipes are made of copper pipes and attached to the surface of the reinforcement layer. They are first partially fixed and then cast with heat-conducting materials.

 

The mold reinforcement should use rigid materials such as metal. According to the design results of plate-shell theory, the size of the flat unit area should be determined and grid reinforcement should be performed. After the frame is welded, stress relief treatment must be performed and the paste should be applied to the back of the mold to make it bonded to the mold as a whole. The mold with a heating layer should also be made of an insulation layer at the end. Depending on the insulation material, the thickness of the insulation layer is generally 50 to 100 mm and cast.

 

Mold Making Steps

 

1. Data or drawings
2. 2. Model processing
3. 3. Model (wood processing)
4. 4. Model (easy to polish primer treatment)
5. 5. Model (high gloss gel coat treatment)
6. 6. Demolding treatment (mold release agent application)
7. 7. Mold gel coat spraying
8. 8. (Fatigue resistant layer) surface mat paste
9. 9. (Fatigue resistant layer) chopped mat paste
10. 10. (Reinforcement layer) chopped mat paste
11. 11. (Sandwich layer) production
12. 12. Wrapping layer production
13. 13. Steel skeleton production
14. 14. Positive mold processing
15. 15. Cavity processing
16. 16. Air chamber production
17. 17. Injection and exhaust port design
18. 18. Back mold production
19. 19. Back mold processing

 

• Drawings or Photos
• 
• First of all, whether the drawing is suitable for L-RTM process, whether some parts need to be modified, demoulding angle, embedded parts, local thickening, block division, etc. The skirt and width of the L-RTM mold need to be added on the basis of the original product drawing. The processing model needs to determine which side is smooth and how the sealing strip turns at the skirt of the mold, which will not affect the sealing effect of the mold in the future. Whether the exhaust port affects the appearance of the product, etc.

  Model Processing

• There are many processes and methods for model making, including block substitutes, high-density polyurethane foam, polystyrene foam and paste substitutes, wood, etc. Considering the comprehensive cost and effect, paste substitutes are generally selected. When designing the three-dimensional model, it is necessary to leave enough space or thickness for the substitute layer. The model processing factory uses manual wood coating, spray gun spraying, and machine coating of substitutes. The bonding strength between the substitute and the foam below must also be considered.Model (Substitute Wood Processing)

 

After the model is processed, it needs to be measured against the drawings, such as pallet, scanning, three-coordinate, photography and other means of detection. After the model is processed, it is necessary to measure the surface effect to determine the treatment plan, such as whether it is necessary to seal the holes, scrape the putty and then spray the easy-to-grind primer.

Model (easy-to-grind Primer Treatment)

 

 

The recommended amount of easy-grinding primer is 300g/square meter. Clean the surface before spraying. The spraying distance is about 400 mm. The spray gun should be adjusted to achieve the best effect, otherwise it will cause fluffing (dry overspray) when spraying. When spraying, start with the difficult part and then the easy part, and finally spray the large area of ​​the model. When sanding, you must first determine the mesh of the sandpaper. The most important thing is to understand the structure of the model. If you do not pay attention to the structural modeling, the lines, arcs, flatness, etc. will be polished out of shape during sanding, so you can’t achieve the ideal effect, and in serious cases, it will cause unqualified products in the future.

Model (high-gloss Gel Coat Treatment)

 

 

The recommended dosage of high-gloss gel coat is 200g/square meter. Before spraying high-gloss gel coat, the surface should be cleaned, the spraying distance is 400 mm, and the spray gun should be adjusted to achieve the best effect. When spraying (the surface of the model needs a thin layer), start with the difficult and then the easy, and finally spray the large area of ​​the model. For the polishing of high-gloss gel coat, first choose the mesh of sandpaper. The most important thing is to understand the structure of the product model. If you don’t pay attention to the structural modeling, the lines and arcs will be distorted during polishing. Products with higher surface requirements can find out whether they are satisfactory after polishing the high-gloss gel coat.

Mold Gel Coat Spraying

 

 

The recommended dosage of mold gel coat is 800g/m2 (1000g/m2 for small mold). Mold gel coat is very important for molds. Sufficient preparation work needs to be done before spraying mold gel coat. It is best to make a sample block to test whether this gel coat can meet your high requirements. When spraying mold gel coat, adjust the distance and gas volume according to the type of spray gun. When spraying, thin edges and multiple layers should be used to avoid local thickening too quickly. When spraying, first difficult and then easy.

(Fatigue-resistant layer) Surface Mat Paste

 

 

The surface layer of the mold should be pasted very carefully. Make sure the surface mat is dry. Wet surface mat will cause many problems. The resin should be allowed to gel for a long time. The surface mat should be pasted first when the gel coat is sticky but not glued. This will increase the bonding strength and reduce the amount of resin. The surface mat should be butted when laid. The amount of resin should be moderate. When using a brush locally, apply force at 90 degrees, otherwise the surface mat will be pushed away or accumulated by the brush. Visually, the resin is shiny, which means there is too much resin. Ensure the resin content.

(Fatigue-resistant layer) Chopped Mat Paste

 

 

Before making the chopped strand mat paste, make sure it is dry, master the gel time of the resin, and try to arrange the paste time for the gel coat layer during the day. Before making the chopped strand mat paste, make sure that the surface mat is completely cured and remove bubbles, control the amount of resin, and try to butt the chopped strand mat when laying it (the mold has a certain thickness and there is no need to consider it will break). The amount of resin should be moderate. When using a brush locally, use 90 degrees to apply force, otherwise the chopped strand mat will be pushed away or accumulated by the brush. For complex mold surfaces, choose 150 to 300g of mat for the first two layers. Try to use a bristle roller to remove bubbles.

(Reinforcement layer) Chopped Strand Mat Paste

 

 

Most mold reinforcement layers use zero shrinkage resin, and it is recommended that the thickness of the zero shrinkage resin layer should be more than 3 mm. When applying several layers of zero shrinkage resin together, the most important thing is to get the first layer right. Don’t wait until several layers are applied before rolling bubbles.

Demolding

 

 

Demolding is even more critical for molds and products. You need to use high-quality release agents, have a clean environment and temperature, and know the risk of mold sticking in advance through some methods.

(Sandwich layer) Production

 

 

The mold sandwich layer needs to be firmly attached. There is also a special putty for the sandwich layer. The putty under the sandwich layer can also be mixed with low shrinkage resin and filler. The putty under the sandwich layer should not be too thick to stick to it.

Wrapping Layer Production

 

 

Before pasting the wrapping layer, check whether the sandwich is firmly adhered, and make the corners smooth and polished before pasting the wrapping layer.

Steel Frame Production

 

 

The lightweight RTM mold does not require a strong strength for the steel frame, but in order to consider the stress of the steel frame, the stress must be removed after the steel frame is welded. If there is no condition to remove the stress, silicone can be used to bond the steel frame, and silicone will have a buffering effect.

Positive Mold Completed

 

 

When processing the positive mold, a clean environment is required. The mold has a good effect after demolding. Use high-grit water sandpaper for processing. If there is a grain of sand at this time, it will grind many scratches on the gel coat surface. Even the sandpaper and pad for grinding must be cleaned before use. When grinding, you cannot directly use the faucet to flush. It is best to use a bucket of water for standby. When cleaning the mold, use pure cotton cloth to wipe the mold in one direction. After wiping, the cloth should be swung in the bucket and washed before use.

Mold Cavity Processing (Injection Port and Exhaust Port Design)

 

 

Before pasting wax sheets in the mold cavity, the ambient temperature must be ensured. If the temperature is too high, the wax sheets will become soft and deformed. If the temperature is too low, the wax sheets will become hard and the bonding strength will be very poor. Considering the effect of the wax sheet joints, you must have a plan for the mold before pasting the wax sheets, where to start and where to hand over, and apply force evenly when pasting each wax sheet. The key is to ensure that there is no air under each layer of wax sheets. The shape of the product structure is easy to leave the mold, and vacuum assistance must be taken into consideration before pasting the wax sheets. Especially when pasting wax sheets in the concave mold, the wax sheets are more likely to leave or have gaps, and using vacuum assistance will help.

Air Chamber Production

 

The key to making the air chamber is to ensure the sealing effect of the two subsequent layers. The design should be reasonable before paving, and the pre-buried internal and external sealing channels should be ensured to be non-deformed and firmly adhered when paving.

Design of Injection Port and Exhaust Port

 

 

The setting of mold injection port and exhaust port should be considered from many aspects. First of all, the success rate should be considered, and then the reduction of workload and material saving should be considered.

Back Mold Production

 

 

The premise of the back mold is to consider the shrinkage effect. The paste making of the sealing channel and the air chamber should be arranged in a reasonable manner to ensure that they will not shrink or deform. If they deform, it will directly affect the sealing effect of the mold.

The Back Mold is Completed

 

 

The back mold has low requirements for finish, but the sealing strength of the injection port and the exhaust port is very important. As long as there is a slight air leak in the back mold, it will have a great impact on the product.

 

 Causes and Analysis of Dry Areas and Vortexes in RTM Products

L-RTM product has problems	Analysis of problems with L-RTM products
Product vortex in the same position	Irrational design of injection port and exhaust port, move exhaust port position
Product vortex in different positions	Uneven thickness of mold cavity, poor fiber infiltration, increased resin viscosity
The product had no problems before, but recently there were dry areas and vortex	Whether to change materials, temperature drops, equipment bubbles are not completely exhausted, filler sedimentation
The product has heavy fiber texture on the surface	Add surface felt, increase curing temperature, select resin with low shrinkage, increase filler
The product has orange texture on the gel coat	Select high-quality gel coat and curing agent, increase gel coat atomization, maintain mold surface
The product has resin on the gel coat after demoulding	Gel coat release, select high-quality release agent, gel coat thickness is uneven, pay attention to fiber placement