RTM Molding Process Analysis and Production Precautions

 

RTM Molding Process and Classification

 

1.RTM

The so-called closed mold molding process is a process method for molding composite material components when the female and male molds are closed. SMC, BMC molding, injection molding, RTM, and VEC technologies are all closed mold molding processes. Due to the formulation of environmental laws and the improvement of product requirements, the open mold molding of composite materials has been increasingly restricted, which has prompted the application of closed mold molding technology, and has especially promoted the innovation and development of RTM technology in recent years.

 

2. Types of RTM

RTM process, namely resin transfer molding process, is a new type of compression molding method. It has the advantages of low mold cost, short production cycle, low labor cost, less environmental pollution, precise manufacturing dimensions, smooth appearance, and the ability to manufacture complex products. Since the 1940s, this process has been developed to adapt to the molding of aircraft radomes. At present, RTM molding process has been widely used in construction, transportation, telecommunications, health, aerospace and other fields. Here are several RTM technologies.

 

1) RTM, resin transfer molding. This technology is derived from polyurethane technology. During molding, the mold is closed and resin is injected into the preform. The glass fiber content is low, about 20-45%.

2) VARIT, vacuum-assisted resin transfer injection molding. This technology uses vacuum to suck the resin into the preform, and can also press the resin in. The vacuum degree is about 10-28 inches of mercury.

3) VARTM, vacuum-assisted resin transfer injection molding. The product generally has fewer pores and the glass fiber content can be increased.

4) VRTM, vacuum resin transfer molding.

5) VIP, vacuum infusion method.

6) VIMP, variable infusion molding method. The resin moves with the help of vacuum or deadweight, and the impregnation is compacted.

7) TERTM, thermal expansion RTM. Insert the core material into the preform, let the resin impregnate, and heat the mold and the molded product. The core material expands due to heat, compacting the layer. Use this compaction effect and combine it with surface pressure molding.

8) RARTM, rubber-assisted RTM. In the TERTM method, rubber is used instead of core material. The rubber mold presses the molded product to greatly reduce the porosity, and the glass fiber content can be as high as 60-70%.

9) RIRM, resin injection cycle molding. Combining vacuum and pressure, resin is alternately injected into multiple molds to circulate the resin until the preform is fully impregnated.

10) CIRTM, Co-Injection RTM. Co-injection RTM can inject several different resins, and can also use several preforms. Vacuum bags and flexible surface molds can be used.

11) RLI, resin liquid impregnation (infiltration). Resin is injected into the lower mold, and after the preform is placed, the upper mold is covered, heated and molded with the molding pressure of the autoclave. Heating reduces the viscosity of the resin, improves fluidity, and is easy to impregnate.

12) SCRIMPTM Seaman Composites Company’s resin impregnation molding method. Patent application. The resin is pressurized and impregnated using a vacuum bag, with a fast impregnation speed and a wide area. The resin can also be fully impregnated in the thickness direction of the preform, and a vacuum bag and a soft surface mold must be used.

13) UVRTM, ultraviolet (curing) RTM. Similar to the SCRIMP method, it cures quickly and requires the use of ultraviolet light sources, vacuum bags that can transmit ultraviolet light, and soft molds.

14) VECTM Virtual Design The core technology of VEC is the patented “floating mold” concept. The composite mold is installed between two liquid-filled steel pressure vessels, and the mold forms a seal along the entire length of the container. The container is filled with a compressible heat-conducting liquid, which is usually water.

 

RTM Molding Process Mold Technology

 

1) RTM overall process route

RTM has three important components:

*1 Raw material system

*2 Injection equipment

*3 Mold system

 

2) RTM molding mold technology

All RTM products require a mold suitable for the process, and RTM is no exception. RTM molds can be made of aluminum steel and FRP. Since aluminum steel molds are not easy to deform but are expensive, they are not introduced here. The following mainly introduces FRP molds.

 

*1 RTM Mold Type and Material

FRP molds are used for RTM. According to general regulations, a 7-10mm thick mold layer is made, and then a heating core is installed under the surface of the base mold to form a sandwich structure. The total thickness of the mold layer is 20mm. Since this thickness is not enough for the strength required by the RTM molding process, it needs to be further enhanced. Box-shaped steel profiles are much cheaper than composite materials and are generally reinforced with box-shaped steel profiles.

 

FRP mold practice has proved that molds made with inferior resins have a very short service life and have a direct impact on product quality, so the mold surface is required to be made of heat-resistant and chemical-resistant materials. Most of the mold manufacturing costs are labor, and the material selection costs are almost irrelevant to the total mold cost. At present, vinyl ester mold resin systems and gel coats are generally selected, and they have been proven to have better service life and temperature resistance than traditional epoxy materials. According to foreign data, molds made with vinyl ester mold resins have been molded more than 18,000 times and are still in use.

 

*2 Mold Heating

Most resins used in the FRP industry have a curing curve that is directly related to temperature, so it is of great practical significance to find a way to control the temperature of the production mold, which helps to optimize production efficiency. In fact, for every 10°C increase in mold temperature, the gel time will be halved. Therefore, at ambient temperature (20°C), it may take 60 minutes to cure and demold a part, while at 50°C, the same resin system can demold the same part within 7.5 minutes.

 

Heating with Electric Heating Cloth

The electric heating method of applying heating cloth to the mold memory in RTM mold has been used for many years. The heating is relatively uniform and the mold can easily be heated to more than 100℃, but the standard application maximum mold temperature is 75℃.

 

Liquid Heating

Liquid heating is to install the hot and cold circulation system into the appropriate mold structure, which can replace electric heating. This also provides a cooling system for the mold. The temperature can be controlled by the circulating medium in the pre-buried pipe.

 

*3 Mold Sealing

All closed molds used for RTM and vacuum molding (VM) require a sealing device to control resin overflow at the edge of the mold cavity. In the case of VM molding, an additional external mold flange vacuum sealing device is required.

There are many different forms of seals, but the seal material is required to have high temperature resistance and consistent recovery rate. At present, organic silicone materials are the most successful for basic mold resin sealing. If used correctly, it is enough to provide a service life of more than 1,000 times.

 

Passive Seal

The almost exclusive solid silicone rubber seal section is designed to compress 1.0-1.5mm at the “Z” end closing position when the mold is closed. To achieve effective sealing without increasing the mold closing force, the hardness of the seal and the choice of compression size are critical. However, because the required pressure is too large, it is easy to cause mold deformation. The mold contact surface is generally 3-5mm enough to prevent resin seepage and achieve effective sealing.

 

Dynamic Seal

The dynamic form is better than the passive seal form. It can permanently control the cross-sectional changes. The cross-sectional size change of the illustrated form can be up to 4mm. This allows the vertical sealing track to be embedded in the mold flange. On the contrary, the passive seal will only “plastically deform” and wear extremely. Once the inner section of the dynamic seal is pressurized, it can provide a good adjustment of the sealing effect. When the mold is closed or opened, the seal elastically deforms with the vacuum gap, which prevents wear caused by “plastic deformation” in the vertical direction.

 

*4 Injection Port

Technicians engaged in RTM molding process pay special attention to the inlet position of the injection molding resin. Practice has proved that it is most reliable to set the RTM injection port in the center (depending on the shape of the mold cavity).

 

*5 Mold Accuracy

RTM molds are often under stress, so mold accuracy is one of the key factors for a successful closed mold. The mold cavity accuracy is controlled at ±0.2mm, which is the target accuracy of the closed mold. Failure to achieve this accuracy will inevitably lead to glue shortage and unpredictable resin filling, and the size of the molded part is out of tolerance. The most common is overthickness, and the material shrinkage parameter must be taken into account.

 

 

RTM Molding Process Operation and Material Selection

 

*1. RTM Process Operation

The RTM process injection operation is generally required to be completed within 1/4-1/2 of the gel time, the transfer time is 2-15 minutes, and the transfer pressure is 0.3-07Mpa.

The resin transfer pressure is the main parameter that should be controlled in the RTM process. This pressure is used to overcome the resistance encountered when injecting into the mold cavity and soaking the reinforcement material. The time for the resin to complete the transfer is related to the system pressure and temperature. A short time can improve production efficiency. However, if the resin flow rate is too large, the glue will not have time to penetrate the reinforcement material, and it may cause accidents due to the increase in system pressure. Therefore, it is generally required that the rising speed of the resin liquid level entering the mold during the transfer process is not more than 25mm/min. Monitor the resin transfer process by observing the discharge port. It is generally believed that when all the observation ports on the mold have glue overflowing and no bubbles are discharged, and the actual amount of resin added is basically the same as the expected amount of resin added, the transfer process is completed. Therefore, the setting of the discharge port should be carefully considered.

 

*2. Resin Selection

The selection of the resin system is the key to the RTM process. To get the resin into the mold cavity and quickly soak the fiber, the viscosity of the resin should be 0.025-0.03Pa•s. Polyester resin has a low viscosity and can be completed by cold injection at room temperature. However, due to different performance requirements of the product, different types of resins will be selected, and their viscosities are not the same, so the size of the pipeline and injection head must be designed to meet the fluidity requirements of the special components.

Resins suitable for RTM process include polyester resin, epoxy resin, phenolic resin, polyimide resin, etc.

 

*3. Selection of Reinforcement Materials

In the RTM process, glass fiber, graphite fiber, carbon fiber, silicon carbide and aramid fiber can be selected as reinforcement materials. The varieties can be selected according to the design needs. Short-cut fiber, unidirectional fabric, multi-axial fabric, weaving, knitting, core material or preforms.

 

Common Defects, Causes and Solutions of RTM Products

 

1. The surface of the product is partially rough and dull

The main reason for this phenomenon of RTM products is the production of mild mucous membranes. Touch the mold with your hand, and when you touch these parts, you will feel extremely rough. Usually, this problem will occur after the product is produced for a period of time, and the mold needs to be cleaned in time. First, use water sand to polish the rough parts of the mold, then use cotton wool dipped in acetone to wipe the entire mold, and finally apply a release agent to the mold.

 

2. Wrinkling

This is one of the common drawbacks of gel coat products. The main reason for the wrinkling of the gel coat is that the gel coat resin is not completely cured before the resin is injected, and the monomer (styrene) in the injected resin partially melts the gel coat resin, causing expansion and wrinkles. Therefore, before injecting the resin, check whether the gel coat is cured.

 

3. Leakage

The main reason for leakage is that the mold is not tight after closing or the sealing gasket is not tight. Before closing the mold, check whether the sealing gasket is intact and whether there are cracks. If found, it should be replaced in time. Check the sealing condition when closing the mold.

 

 

4. Bubbling

The main reasons for this phenomenon are: 1) The heat release of the resin in the mold cavity is too high, and the curing time is too short, so the gas in the mold cavity is not completely discharged. 2) When the resin enters the mold cavity, too much air is brought in, and the bubbles cannot be completely discharged within the injection time. 3) The viscosity of the resin is too high, and the bubbles cannot all overflow from the product during injection. 4) The pressure of the resin injected into the mold cavity is too high, causing the bubbles to be contained in the resin and difficult to be discharged.

 

5. Dry Spots Inside The Product

The main reason for dry spots inside RTM products is insufficient glass fiber infiltration. If the dry spots in the products of the same period are a certain part of a certain product, it should also be considered whether it is caused by contamination of the glass fiber cloth quilt. Usually, the appearance of dry spots inside the product is also related to the viscosity of the resin, so the viscosity of the resin should be analyzed and adjusted first. Check whether the mold runner is too long or too narrow, and modify the mold in time. Check the feed pipe, improve the feed pipe, improve the feed pipe, increase the feed point

 

6. Core Material Movement

The movement of the core material during injection is caused by the instability of the flow, which can be solved by opening a hole in the core material or ensuring that the closing pressure applied to the core material is much greater than the pressure of the resin at that location. The movement of the reinforcing material, such as the flushing of the fiber, is also due to the insufficient mold closing pressure relative to the injection pressure.

 

 

Phenomenon	Causes	Countermeasures
Cracks	Too much resin	Add felt and cloth Add putty to corners (increase R) Uniform distribution of preformed glass fiber
Uneven thickness	Too much deformation during demoulding	Increase stiffness and curing degree Appropriate demoulding treatment
	Unskilled spraying; Too much heat	Use low exothermic resin and thin wall
Bubble	Lack of glass fiber at corners	Scrape putty on corners
	Too fast resin injection speed	Reduce injection speed and increase resin viscosity
Poor impregnation	Too much glass fiber in some areas	Uniform distribution of glass fiber
	Poor resin fluidity	Set air holes and change injection port position
White spots	Poor curing	Increase the curing dosage of gel coat and resin Extend mold filling time
	Insufficient gel coat thickness	The thickness should be above 0.3mm
	Too much glass fiber	Appropriate amount of glass fiber
	Resin shrinkage during curing	Add filler and use low shrinkage resin
	Thick yarn, high hardness	Select brand
Bends	Misalignment of glass fiber flow	Use binder that is effective for preform binder and slow down injection speed
	Poor quality of glass fiber type	Select good quality glass fiber
Flexion	Incomplete curing during demoulding	Promote resin curing and use reinforcing materials to increase stiffness Use correction fixtures
	Resin shrinkage during curing	Use low shrinkage agent and filler

 

Comparison of Production Costs Between RTM Process and Hand Lay-up Process

 

 RTM Molding Process

The Specific Process Flow of RTM Process is as Follows:

 

Features of RTM Molding Process:

1. Short production cycle
2. Low labor cost
3. Less environmental pollution
4. Accurate manufacturing dimensions
5. Smooth appearance and can manufacture complex products

 

Hand Lay-up Molding Process

Hand Lay-up Molding Process is the earliest molding method of composite materials and also the simplest method. Its specific process flow is as follows:

 

 

Characteristics of Hand Lay-up Molding Process:

1. Not limited by size and shape;
2. Simple equipment and low investment;
3. Simple process;
4. Reinforcement materials can be added at any position to easily meet product design requirements;
5. The product has high resin content and good corrosion resistance.

 

Comparison of Production Costs Between Hand Lay-up Process and RTM Process

1. Comparison of mold material consumption between RTM process mold and hand lay-up process mold
2. Comparison of RTM and hand lay-up product costs