What Are The Common Molding Processes For Prepreg? Analyze Their Respective Advantages And Disadvantages

 

Prepreg can be formed by different methods, and the more appropriate forming method can be selected according to different applications. The main prepreg forming processes include vacuum bag process, autoclave process, molding process, tube rolling process, pultrusion process, roll forming process, winding process, pressure bag process, etc.

Among them, the vacuum bag process is mainly used for interior decoration of the shipbuilding industry and railway system, the autoclave process is mainly used for high-quality composite materials, the molding process is mainly used for flat panels, sporting goods, sleds, and industrial products, the tube rolling process is mainly used for fishing rods, ski poles, golf clubs, and pipe fittings, and the pressure bag process is mainly used for masts, antenna poles, and various pipe fittings.

 

1. Vacuum Bag Process

 

Definition: The vacuum bag molding process is to seal the product between the mold and the vacuum bag, and pressurize the product by vacuuming to make the product more compact and have better mechanical properties.

 

Advantages:

 

1. High fiber content, better mechanical properties of the product;

 

2. Uniform pressurization, uniform performance of the product;

 

3. Effective control of product thickness and glue content;

 

4. Reduce bubbles in the product;

 

5. Can form complex and large parts;

 

6. Reduce the damage of volatile components to personnel.

 

Vacuum Bag Wet Process:

 

1. Mold preparation, apply release agent

 

2. Product layering (hand lay-up, spraying, prepreg)

 

3. Lay release cloth

 

4. Lay isolation film or perforated isolation film (optional)

 

5. Lay breathable felt

 

6. Paste sealing strips (can be done in advance)

 

7. Seal the vacuum bag film

 

8. Install vacuum valve, quick connector and vacuum tube

 

9. Connect the air source and check the vacuum degree

 

10. Vacuum, product curing

 

11. Product demoulding

 

2. Hot-pressing Tank Molding Technology

Features: uniform pressure inside the tank, fast, uniform air temperature inside the tank, high precision, high quality, wide range of applications, suitable for batches, reliable molding process; Disadvantages: high cost and large initial investment.

 

3. Roller Forming

Roller forming is mainly based on metal forming methods. The equipment consists of a series (one or more groups) of hot rollers and cold rollers. After the prepreg is heated, it first passes through a group of hot rollers to deform the prepreg, and then passes through a group of cold rollers with gradually decreasing spacing to form it.

 

Advantages:

(1) The blank is dense, strong and not easy to deform.

(2) Low moisture content, smooth and flat products, and consistent product specifications.

(3) High productivity and low labor intensity.

(4) Suitable for female and male mold forming, single machine connection.

(5) The operating technology does not need to be very high, which is convenient for forming an automated production line.

 

According to the forming process, it is divided into male mold forming and female mold forming.

(1) Male Mold Forming: Use a convex mold, the use surface of the blank faces the model, and the roller head determines the outer surface of the blank. It is suitable for flat disc products (shallow products).

 

Advantages: It is not easy to deform when drying with a mold (with support), the use surface can be engraved with patterns and curved edges along with the model, and it is relatively simple to take the blank and return to the mold in the production line. Disadvantages: The blank is easily deformed and cracked when it is taken out.

 

(2) Female Mold Forming: Use a concave mold, the non-use surface of the blank faces the mold, and the roller determines the inner surface of the blank. It is suitable for bowls, cups, plates and small-sized plate products (deep products).

 

Advantages: The blank is not touched when it is taken out, and the deformation is small. The spindle speed can be higher and it is not easy to fly mud. Disadvantages: It is more complicated to return the mold in the production line (turn 180°).

 

4. Winding Molding

 

Wrapping molding is a process method in which the prepreg is wound on the core mold in a certain way by controlling the tension and winding angle of the winding machine, and then cured to form a composite material part. The winding molding process is only applicable to rotating tubular products.

 

The reinforcing materials used for winding molding are mainly various fiber yarns: such as alkali-free glass fiber yarn, medium-alkali glass fiber yarn, carbon fiber yarn, high-strength glass fiber yarn, aramid fiber yarn and surface felt. For general civilian products such as pipes and tanks, unsaturated polyester resins are mostly used. For winding products with high requirements for mechanical properties of compressive strength and interlayer shear strength, epoxy resin can be used.

 

The inner mold of the molded hollow product is called the core mold. Under normal circumstances, after the winding product is cured, the core mold must be removed from the product. The winding machine is the main equipment for realizing the winding molding process. The requirements for the winding machine are:

 

①, it can realize the winding law and accurate yarn arrangement of the product design;

②, it is easy to operate;

③, it has high production efficiency;

④, it has low equipment cost.

 

Winding molding can be divided into dry winding, wet winding and semi-dry winding.

 

Dry winding is to use pre-impregnated yarn or tape, which is heated and softened to a viscous state on the winding machine and then wound onto the core mold.

 

Wet winding is to dip the fiber bundle (yarn tape) into glue and then directly wind it onto the core mold under tension control.

 

Semi-dry winding is to add a set of drying equipment on the way from fiber dipped into glue to winding onto the core mold to remove the solvent in the dipped yarn. Compared with the dry method, it saves the pre-gluing process and equipment; compared with the wet method, it can reduce the bubble content in the product.

 

Among the three winding methods, wet winding is the most commonly used; dry winding is only used in high-performance, high-precision cutting-edge technology fields.

 

Advantages Of Fiber Winding:

① The winding rules can be designed according to the stress conditions of the product, so that the strength of the fiber can be fully utilized;

② High specific strength: Generally speaking, the weight of fiber-wound pressure vessels can be reduced by 40-60% compared with steel vessels of the same volume and pressure;

③ High reliability: Fiber-wound products are easy to achieve mechanized and automated production. After the process conditions are determined, the quality of the wound products is stable and accurate;

④ High production efficiency: Mechanized or automated production requires fewer operators and fast winding speed (240m/min), so the labor production efficiency is high;

⑤ Low cost: On the same product, several materials (including resin, fiber and lining) can be reasonably selected and compounded to achieve the best technical and economic effect.

 

Disadvantages Of Winding Molding:

① Winding molding has low adaptability and cannot be used to wind products of any structural form, especially products with concave surfaces, because when winding, the fiber cannot be close to the surface of the core mold and is suspended;

 

② Winding molding requires winding machines, core molds, curing heating furnaces, demolding machines and skilled technical workers, which requires large investments and high technical requirements. Therefore, only mass production can reduce costs and obtain greater technical and economic benefits.

 

The fiber content of the winding molding process can reach 60%-80%, the product thickness is 2-25mm, the maximum product diameter is 4m, the length is 12m, the curing temperature is 80-130℃, the molding cycle is determined by the product size, the molding pressure is determined by the tension of the winding machine, the mold is a metal or gypsum core mold, and the production is continuous.

 

5. Compression Molding

 

Advantages of compression molding: High production efficiency, easy to realize specialized and automated production; high product size accuracy and good repeatability; smooth surface, no need for secondary modification; can form complex products in one step; mass production, low price.

 

However, the compression molding process is complex in mold manufacturing, high mold quality requirements, high molding pressure, large equipment investment, and limited by the press. It is most suitable for mass production of small and medium-sized composite materials. With the continuous improvement and development of metal processing technology, press manufacturing level and synthetic resin process performance, the tonnage and table size of the press are constantly increasing, and the molding temperature and pressure of the molded material are relatively reduced, making the size of the molded process products gradually develop towards large-scale. At present, it can produce large automotive parts, bathtubs, and integral bathroom parts.

 

The fiber content of the molded products is between 25% and 60%, the thickness of the products is 1-10mm, the curing temperature is 40-50℃ for cold pressing and 100-170℃ for hot pressing, the molding cycle is short, 5-60min, the molding pressure is 10-40MPa, the mold is generally a steel mold, and the cold mold can be made of fiberglass. The product size is limited by the mold size and the tonnage of the press. The production requires hydraulic presses, heating molds, cold molds and other equipment, which is suitable for medium-volume production of 100-20000T.

 

6. Pultrusion Process

 

Pultrusion process is to form and solidify the prepreg through an extrusion die under the action of traction, and continuously produce FRP profiles of unlimited length. This process is most suitable for producing FRP profiles of various cross-sectional shapes, such as rods, tubes, solid profiles (I-shaped, grooved, square profiles) and hollow profiles (door and window profiles, blades, etc.).

 

The pultrusion process consists of yarn feeding, dipping, preforming, curing and shaping, traction, cutting and other processes. Its advantages are:

① The production process is fully automated and efficient;

② The fiber content in the pultruded product can be as high as 80%, and the dipping is carried out under tension, which can give full play to the role of the reinforcing material and the product has high strength;

③ The longitudinal and transverse strength of the product can be adjusted arbitrarily to meet the use requirements of products with different mechanical properties;

④ There is no scrap in the production process, and the product does not need post-processing, so it saves labor, raw materials and energy consumption compared with other processes;

⑤ The product quality is stable, the repeatability is good, and the length can be cut arbitrarily. The disadvantages of the pultrusion process are large equipment investment, monotonous product shape, only line-shaped profiles can be produced, and the lateral strength is not high.

 

Among the raw materials of pultrusion process, the most widely used resin matrix is ​​unsaturated polyester resin, and the reinforcing material is mainly glass fiber and its products, such as untwisted roving, continuous fiber mat, etc. The mold is an important tool for pultrusion, generally composed of a preforming mold and a forming mold.

 

① Preforming Mold: In the pultrusion process, the prepreg must pass through a preforming mold composed of a group of yarn guide elements before entering the forming mold. The function of the preforming mold is to gradually form a preformed body with a shape and size similar to the mold control shape according to the cross-sectional configuration of the profile after the impregnation, and then enter the forming mold, so as to ensure that the yarn content of the product section is uniform.

 

② Forming Mold: The ratio of the cross-sectional area of ​​the forming mold to the cross-sectional area of ​​the product should generally be greater than or equal to 10 to ensure that the mold has sufficient strength and rigidity, and the heat distribution is uniform and stable after heating. The length of the pultrusion die is determined according to the traction speed and the curing speed of the resin gel during the molding process to ensure that the demoulding curing degree is reached when the product is pulled out. Generally, steel chrome plating is used, and the cavity surface is required to be smooth and wear-resistant to reduce friction resistance during pultrusion and increase the service life of the mold.

 

The fiber content of the pultrusion process can reach 60%-75%, the profile thickness can reach 12mm, the rod diameter is 40mm, the curing temperature is 100-160℃, continuous production is possible, the maximum traction force is 40 tons, the mold is the pultrusion unit mold, the product section size depends on the unit mold, and the length is not limited.