Vacuum System in Vacuum Infusion Process of Composite Molding

 

Composite materials molded products made by the resin vacuum infusion process have many advantages such as high surface quality, few defects such as pores, and high strength. They are increasingly widely used in the mold molding industry.

 

Basic Principle Of Resin Vacuum Infusion Process

 

As shown in the figure below, the reinforcing material is laid on the mold, and then the vacuum bag is laid, and the air in the system is extracted 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 reinforcing material and finally fills the entire mold. After the product is cured, the vacuum bag material is removed to obtain the desired product from the mold.

 

 

Why Use Vacuum Infusion Process?

 

The main function of vacuum infusion process is to avoid defects such as pores and looseness in the molded products. These defects are mainly caused by air mixed in the mold or fiber layer, or steam generated during the molding process.

 

Steam refers to gas that is gas or solid at normal temperature and pressure and volatilizes under vacuum or high temperature. In the composite molding process, steam is an “invisible enemy”. These steams will increase the pumping time and produce leakage-like phenomena; they will produce pores or serious strength defects in the finished product. Water vapor is the most common steam. Other common steams include gases volatilized from resins or resin infusion systems, such as styrene, butanone peroxide (MEKP), etc.

 

Water Vapor

Due to the certain humidity of the air, some water will be adsorbed on the surface of the mold and in the gaps of porous materials such as fibers. This adsorbed water cannot be seen and will not have any effect at normal temperature and pressure, but when vacuuming, water can boil at room temperature.

 

We all know that the boiling point of water will decrease as the air pressure decreases. For example, at high altitudes, water will boil at 80 degrees. If the pressure continues to decrease, then at a certain pressure value, water will boil at about 20 degrees at room temperature.

 

The relationship between the boiling point of water and pressure (vacuum degree) is as follows:

 

 

If you have a vacuum pump that can produce 99% vacuum, and connect it to a transparent container of water, you can see the water boiling at room temperature.

 

Volume Expansion Under Vacuum

 

Boiling under vacuum can help us remove water, which is a good thing. However, when water boils, a large amount of water vapor will expand dramatically, and the higher the vacuum, the greater the volume expansion. The following table shows the volume of 1 kg of water at different vacuum levels:

 

The volume of 1 kg of water in liquid state is 1L, but at 20mbar (98% vacuum), it will produce 67,000 L of water vapor after boiling and vaporization, which is 67,000 times larger! Since the vacuum pump has a very low pumping speed when it reaches the limit pressure, taking our most commonly used 20 m3/h vacuum pump as an example, it may take up to several hours to remove this water vapor.

 

What does this Mean in Actual Process?

If the target pressure (absolute pressure) of your molding process is below the vaporization pressure at the corresponding temperature, the vacuum pump must be kept working until all the water vapor is removed, which may take several hours. For large parts, it may even take a whole night.

 

Some materials, such as medium-density fiberboard (MDF), which is sometimes used to make molds for composite molding, can contain up to 7% of their weight in water. Such a high moisture content will greatly increase the pumping time.

 

 

From the above description, you may think that as long as the pumping pressure is above the vaporization pressure of water, it will be fine. However, there is another point to consider, which is the temperature rise caused by heating or heat release of resin. If the temperature rises above the boiling point corresponding to the corresponding pressure, then the vaporization of water will inevitably occur, which will cause the stratification of the final product, especially when using materials that are not stored in a dry environment.

 

In addition to prolonging the pumping time, water vapor will condense into water after being pumped into the vacuum pump for compression, which will cause the vacuum pump oil to emulsify, thereby causing the pump oil to lose its sealing and lubricity, resulting in a decrease in vacuum pumping capacity and abnormal wear of the pump. Agilent’s vacuum pump can avoid water vapor condensation in the pump as much as possible through the following two methods:

 

• The vacuum pump itself is designed to have a high operating temperature (70℃-110℃), and water vapor is not easy to condense at relatively high temperatures.

 

• The pump itself is designed with a gas ballast valve to prevent water vapor condensation. By mixing clean air into the pump to reduce the partial pressure (concentration) of water vapor in the pumped gas, the partial pressure of water vapor in the pumped gas is lower than the condensation pressure at the corresponding temperature when the vacuum pump is exhausted.

 

Similar to water vapor, the solvent in the resin will also vaporize into steam under vacuum, and it is even easier to vaporize than water. You can ask the supplier to provide relevant data, or determine its vaporization pressure by placing the resin in a transparent container and evacuating it. Make sure that the process pressure is above the vaporization pressure of the solvent. Otherwise, if the pressure is too low, the solvent will vaporize, and the final product will be porous and loose. The vaporized solvent will enter the vacuum pump and dissolve in the pump oil, affecting the lubricity and even causing damage to the vacuum pump.

 

The Composition of a Typical Vacuum Pumping System is as Follows:

 

 

Selection of Vacuum Pump Type

 

Generally choose an economical and practical single-stage oil-type rotary vane vacuum pump. Note: Be sure not to choose the vacuum pump used for air conditioning and refrigeration system vacuuming. This type of vacuum pump is used to work at very low pressure (very good vacuum).

 

When the working pressure is high, the vacuum pump oil is very easy to emulsify, and a large amount of oil smoke will be sprayed out of the exhaust port. These oil smoke will not only pollute the working environment, but also pollute the mold and fiber materials.

 

The model of the vacuum pump generally contains letters and numbers. Taking Agilent EM20/B ​​as an example, 20 represents the pumping speed, that is, the pumping speed of the vacuum pump is 20m3/h; EM represents a product series. Generally, a series of products have similar designs and the same limit pressure

 

 

From the table above, we can see that the ultimate pressure of the EM series is 2mbar, and the ultimate pressure of the EM/B series is 20mbar. However, the lower the ultimate pressure, the better:

 

• Lower ultimate pressure will increase the risk of water vapor and solvent vaporization;

 

• If the vacuum pump with lower ultimate pressure is used for too long when the vacuum is poor (when making larger parts), smoke will often appear at the exhaust port.

 

The vacuum introduction process of composite molding generally requires the ultimate vacuum of the vacuum pump to be 10-20mbar.

 

It can be seen that the ultimate pressure of the /B vacuum pump is not the best, but it is more suitable for the vacuum introduction process.

 

Selection of Pumping Speed

Large or complex parts require a higher pumping speed of the vacuum pump during pre-vacuuming. In addition, the pumping speed required for the vacuum introduction process is generally not large. If the pre-pumping time is not high, you can refer to the following suggestions to select a vacuum pump:

• Fiber material area <1 square meter-vacuum pump pumping speed 4 m3/h
• Fiber material area <20 square meters-vacuum pump pumping speed 8 m3/h
• Fiber material area 20-50 square meters-vacuum pump pumping speed 20-28 m3/h
• Fiber material area 50-100 square meters-vacuum pump pumping speed 40-60 m3/h
• Fiber material area >100 square meters-vacuum pump pumping speed 60 m3/h*fiber material area/100

 

According to our experience, the connection between the vacuum pump and the buffer tank is very prone to leakage. Using the vacuum hose connection and joints shown in the figure below will reduce the probability of leakage.

 

 

Vacuum Pump Layout

There are many layouts for vacuum pumps and buffer tanks. The following are for reference:

 

 

The buffer tanks used in the above two arrangements are open, and can be placed inside the barrel for collecting resin. The buffer tanks used in the following arrangements are closed, generally used for larger parts, and require a liquid barrier before the gas enters the buffer tank to prevent the resin from entering.