Pultrusion Process and Equipment

 

Principle of Pultrusion Process

 

Pultrusion refers to a method of continuously producing composite linear products by using glass fiber and other reinforcing materials under external traction, through a series of processes such as dipping, extrusion molding, heating and curing, and fixed-length cutting. After the reinforcing material is drawn out from the creel, it enters the dipping tank through the yarn dispensing device, and after being soaked in the resin glue, it enters the pre-forming mold to discharge the excess resin and bubbles, and finally enters the molding mold to gel and solidify. The cured product is continuously pulled out of the mold by the traction machine and cut to a fixed length by the cutting machine. It is different from other molding processes in that it requires external traction and extrusion molding, so it is called pultrusion molding process.

 

Figure 9-26 Pultrusion production workshop

 

The Pultrusion Molding Process is as Follows:

 

Reinforcement material (fiber and felt material, etc.) arrangement → impregnation → preforming → extrusion molding and curing → traction → cutting → finished product

There are six key factors in the pultrusion process:

①Reinforcement material delivery system, such as yarn rack, yarn passing device, felt rack and conveying device;

②Resin impregnation system;

③Preforming system;

④Mold;

⑤Traction device;

⑥Cutting device.

 

There are two ways to draw out the fiber: internal drawing and external drawing. When the fiber is drawn out from the inner wall of the yarn tube, the yarn tube is fixed but the fiber is twisted and twisted; when the fiber is drawn out from the outer wall of the yarn tube, the twisting phenomenon can be avoided, but a rotating mandrel should be used to reduce the drawing resistance and better control the fiber tension.

 

Pultruded Fiber Extraction

 

Figure 9-28 Pultrusion Product Traction

 

The impregnation of reinforcing materials such as fibers and felts is carried out in a glue tank filled with glue liquid. There are generally three forms: 1. Pressed yarn impregnation; 2. Straight groove impregnation; 3. Roller impregnation.

 

Among them, methods 1 and 2 are the most commonly used. The pressed yarn impregnation method is simple and easy to operate. It mainly uses tools such as yarn clamps, yarn holes, and yarn pressing rods to press the reinforcing materials into the glue tank for impregnation. Its disadvantage is that there is a certain amount of wear on the reinforcing materials, and at the same time affects the positioning and direction of the reinforcing materials.

 

With the increasing complexity of the structure of pultruded products, the application of straight groove impregnation method is becoming more and more common. Through the vacuum pump system, the continuous reflux of glue liquid is achieved, which not only ensures the good impregnation of the reinforcing materials, but also makes the arrangement of fibers and felts neat and smooth, making it easier to achieve the expected arrangement.

 

Figure 9-29 Application Picture Of Straight Slot Impregnation Method

 

The pre-impregnated reinforcement material passes through the preforming device. The preforming device is a guide device configured according to the cross-sectional shape required by the product, which has the following functions:

① Positioning of the reinforcement material;

② Arrange the reinforcement material in advance in an orderly manner according to the product shape;

③ Expel excess resin and bubbles, and then enter the mold for molding and curing, and pull it out of the mold.

 

Generally speaking, the mold is designed under the conditions determined by the system. According to the resin curing exothermic curve and the friction performance between the material and the mold, the mold is divided into three different heating zones, and its temperature is determined by the performance of the glue system. The mold is the most critical part of the pultrusion process. The length of the typical mold ranges from 0.6 to 1.2m.

The distance between the mold outlet and the traction device is generally about 3 to 5m. This distance must ensure that the composite product can complete the curing process and gradually cool down. The traction device is a crawler-type puller or two reciprocating clamping devices that can achieve continuous movement. Through the traction of the traction device, the product is continuously pulled out of the pultrusion die.

 

At the end of the pultrusion equipment, a cutting machine is set to cut the pultruded products into fixed lengths as required.

 

General Pultrusion Equipment

 

Figure 9-30 Typical Pultrusion Equipment

 

Pultrusion molding equipment has been developed since the 1950s and has been gradually improved and finalized. It can realize computer intelligent control to the greatest extent, and has shown a diversified trend with the development of pultrusion products and processes.

 

Although there are many types of pultrusion equipment, each device is composed of the following basic parts:

(1) Reinforcement material rack;

(2) Preform guide device;

(3) Glue impregnation device;

(4) Metal mold with heating control;

(5) Traction device;

(6) Cutting device.

 

In the pultrusion equipment, the inner tap glass fiber roving is arranged on the roving rack, and other reinforcement materials (such as continuous felt, surface felt, etc.) are usually placed in order on the multi-layer shelf in the tangential direction. When leading out, the parallel principle should be followed as much as possible to avoid crossing and confusion during walking.

 

The reinforcement material led out from the raw material rack is bundled and shaped by the yarn board and then enters the glue impregnation device. In the straight tank impregnation method, the glue impregnation device consists of the following parts: bracket, impregnation tank, glue receiving tray, glue storage barrel, and circulation pump. Guide plates are placed on both the front and rear sides of the impregnation tank, and yarn holes and felt grooves are set on the guide plates as needed to maintain the orderly arrangement of the reinforcing materials. The glue leaking from the yarn holes and felt grooves falls into the glue receiving tray, is collected in the storage barrel, and is pumped into the impregnation tank through the circulation pump for use. If necessary, a circulating water jacket and a heater to control the resin temperature can also be set to optimize the process.

 

Figure 9-31 Fiber and Felt Extraction

 

After preforming, the impregnated reinforcement material enters the mold. Depending on the product structure, the preforming system of the pultrusion process is diverse and rich. Its main purpose is to make the reinforcement material naturally and smoothly transition from a divergent state to a cross-section similar to the product according to the pre-designed layer structure, complete the final positioning, and smoothly enter the mold cavity.

 

For the production of preforming, plastic plates with low friction resistance are often used, and holes are punched on them for guidance. For the fiber positioning of products with complex structures, positioning tubes can also be used to directly introduce the fibers into the mold entrance. Compared with the preforming of fibers, the guidance of felt materials requires the maker to have more advanced skills. It is a challenging and skillful job to transform the felt material from a single plane state to a three-dimensional form that fits the mold cavity.

 

Round Tube Preforming Mold

 

 

The preform can be fixed together by the frame, mold and mold support, or it can be set above or below the impregnation area according to the layer structure of the reinforcement material. The preform of hollow products is generally designed as an integral part of the core rod support, which is convenient for adjusting the concentricity and saving the operation space.

 

After the impregnation reinforcement material is preformed, it will smoothly enter the heated forming mold. The mold is usually heated by electric heating and segmented heating. The temperature of each zone of the mold can be preliminarily determined by gel test and adjusted according to the product quality during the production process.

 

In the production process, setting the matching mold temperature and traction speed, controlling the appropriate gel time and curing degree are the key to ensuring the quality of the product. When producing large hollow products, due to the thick core rod and slow heat transfer speed, there will be a large deviation in the temperature of the inner and outer walls when the material moves in the mold cavity, which affects the curing of the product. A heater can be installed inside the core rod for temperature compensation to make the product heated evenly inside and outside.

 

The curing degree of the product just out of the mold cannot reach the ideal state, and it needs to be further post-cured during continuous operation and gradually cooled before reaching the traction device. Profiles in high-speed traction sometimes need to be cooled by fans to reduce the wear of the traction rubber pads caused by high temperatures.

 

There are two commonly used traction systems: upper and lower crawler traction systems and alternating reciprocating traction systems. The crawler traction system consists of two opposing, continuously rotating transmission belts, which tightly clamp the profile and drag it forward. This traction system is inexpensive, but its versatility is slightly poor. For products with complex shapes, the corresponding clamping rubber blocks need to be reprocessed and wrapped on the upper and lower crawlers.

 

The alternating reciprocating traction system overcomes the shortcomings of the crawler type, adopts a pneumatic or hydraulic design, and uses two pairs of clamping rubber blocks to cyclically reciprocate to achieve continuous production. When one pair of rubber blocks clamps the product and moves forward, the other pair of rubber blocks releases the product and retreats to reset at the same time, waiting for the next clamping. This system is easy to replace the traction clamps and is easy to operate. It has a wide range of applicability when there are many types of products.

 

At the end of the pultruder is the cutting system, which usually uses a diamond saw blade and is equipped with cooling and lubrication. When cutting, the system moves synchronously with the traction device and is controlled by a fixed-length cutting switch. The use of hydraulic servo drive and program control has further improved the automation of pultrusion equipment.

 

Figure 9-34 Crawler Traction

 

2. New Pultrusion Process and Equipment

 

(1) Preheating Process and Equipment

The use of radio frequency (RF) transmitters to preheat the reinforcing material is a leap forward in the development of pultrusion technology. This innovation combines RF energy preheating with heat conduction of the metal mold to shorten the curing time of the glue and greatly increase the pultrusion speed. The RF transmitter is placed between the impregnation section and the mold section. Before the wet fiber bundle enters the mold, it passes through the transmitter and generates instantaneous molecular friction heat under the action of RF, which reduces the viscosity of the glue, enhances the wettability, and provides the energy required for the glue reaction. In this case, even if the mold length is shortened, the glue can still gel and solidify, reducing the time for the material to pass through the mold and reducing the pulling force. This major invention significantly increases the pultrusion speed, especially in the production of large products.

 

(2) Winding Pultrusion and Braiding Pultrusion

In order to improve the circumferential performance of pultruded products, winding and braiding technology elements are introduced into the traditional process. The resulting winding process and braiding pultrusion process have made a breakthrough in the performance of pultruded products and have better overall performance.

 

Winding Pultrusion Process

 

Braiding Pultrusion Process

 

(3) Pultrusion and injection coating process Some pultruded products with special requirements need to be coated with a layer of coating on the surface. A small injection molding machine can be set up, and the plasticized plastic liquid is placed in a tank. The profile pulled out of the mold then passes through the plastic liquid tank, so that a layer of coating is applied to the surface of the profile, achieving higher requirements in appearance, performance, corrosion resistance, etc.

 

(4) Curved pultrusion equipment The principle of curved pultrusion is to artificially control the curing and post-curing process of the pultruded product so that the product can achieve the designed curved shape. This breaks through the traditional thinking mode that pultrusion can only produce constant-section, linear products, and can meet the needs of variable-section product research and development.

 

3. Pultrusion Die

In the pultrusion molding process of FRP profiles, the die is the intersection of various process parameters and is one of the cores of the pultrusion process. Compared with plastic extrusion molding, pultrusion molding has similarities with it, but the former is only a physical change process, while the latter is also accompanied by dynamic chemical reactions. In contrast, the working conditions of pultrusion dies are much more complicated than those of the former, so the design and manufacture of pultrusion dies are of great significance. It is not only related to the success or failure of the pultrusion process, but also determines the quality and output of pultruded products, and also affects the service life of the pultrusion die.

 

From a process perspective, pultrusion dies generally consist of two parts: a preforming die and a forming die.

(1) Preforming die During the pultrusion process, after the reinforcing material is impregnated with resin (or while being impregnated), before entering the forming die, it must pass through a preforming die composed of a group of yarn guide elements. The function of the preforming die is to further remove excess resin from the impregnated reinforcing material, eliminate bubbles, and gradually form an approximate forming cavity shape and size before entering the die. Through preforming, the reinforcing material gradually reaches the required shape, and the distribution of the reinforcing material in the cross section of the product meets the design requirements.

 

Figure 9-39 Pre-molding Picture

 

(2) Molding mold The ratio of the cross-sectional area of ​​the molding mold to the cross-sectional area of ​​the product should generally be greater than 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 pulling speed and the curing speed of the resin gel during the molding process to ensure that the product reaches the demoulding curing degree when it is pulled out.

 

Figure 9-40 Pultrusion Die Picture

 

The cavity surface of the pultrusion die is required to be smooth and wear-resistant to reduce the friction resistance during pultrusion and increase the service life of the die. The choice of die material directly affects the performance of the pultrusion die. The die material is required to have the following properties:

1) High strength, corrosion resistance, fatigue resistance and wear resistance;

2) High heat resistance and small thermal deformation;

3) Good cutting and surface polishing performance;

4) Low friction coefficient, small resistance and good dimensional stability;

 

The surface of alloy die steel is smooth and dense, with high hardness, easy to demold, not easy to damage when cleaning the mold, easy to nitriding treatment and hard chrome plating on the cavity surface, so pultrusion die generally uses alloy die steel. After rough machining and fine machining, the surface is plated with hard chrome or nitrided and carburized, so that the hardness of the inner surface of the cavity reaches 50-70HRC (Rockwell hardness), and finally polished with a polishing tool to make the surface reach a very high degree of finish, and the surface roughness reaches 0.2um, which can meet the above requirements very well. This will not only reduce the friction coefficient and extend the service life of the mold, but also improve the anti-stick properties of the resin.

 

After decades of development, the most widely used mold steels in the US pultrusion industry are mainly a few grades such as 4140, P20 and A2. In the production of domestic pultrusion molds, quenched and tempered steels such as 40Cr, 38CrMoAl, 42CrMo, and 5CrNiMo are more commonly used, and the use effect is better, but there is still a big gap compared with the processing level abroad.

 

Table 9-1 lists the chemical composition and HRC hardness of the above three American steels after heat treatment for our reference in pultrusion mold design and material selection.

 

Table 9-1 Chemical composition of American pultrusion die steel

 

Item	Alloy chemical composition (%)						Hardness after heat treatment (HRC)
	C	Si	Mn	V	Cr	Mo
P20	0.35	0.50	0.80	–	1.70	0.45	54
4140	0.40	0.25	0.90	–	0.90	0.21	52
A2	1.00	–	0.60	0.25	5.25	1.10	62

 

From the perspective of the entire pultrusion industry, electroplating pultrusion dies still dominate, and non-electroplating dies are still in the development stage. my country’s pultrusion manufacturers use pultrusion dies with surface nitriding treatment, and the thickness of the nitriding layer is 0.2-0.3mm, HRC ≥ 60. From the limited use experience, the corrosion problem still exists, and the friction resistance is slightly greater than that of the chrome-plated die, and there is still a long way to go.

 

(3) Design of Pultrusion Dies

Pultrusion dies are usually assembled from several separately manufactured die components. The number of components and the selection of parting surfaces depend on the cross-sectional structure of the pultruded product, the mold processing technology and the use requirements. In order to ensure that the pultruded product corresponding to the mold parting surface or the mold seam has good appearance quality and does not form burrs, the parting surface is minimized as much as possible to ensure that the joint is tight while meeting the mold manufacturing requirements.

 

When the glass fiber enters the molding die after being dipped in glue, the fiber bundle enters the mold under the traction of the molding machine. Because the fiber bundle at the mold inlet is very loose, it often accumulates and entangles at the entrance, causing fiber breakage. Furthermore, during long-term use of the mold, due to the influence of accumulation and entanglement, the inlet is often seriously worn, affecting the product quality. To solve this problem, an elliptical cross-section fillet should be inverted around the inlet of the mold, and the inlet should be tapered with an angle of 5 to 8 degrees and a length of 50 to 100 mm, which can greatly reduce the occurrence of fiber breakage and improve the quality of pultruded products. As shown in the following figure:

 

 

Figure 9-41 Pultrusion Die Inlet Design

 

When designing the die, the length of the die should be determined by considering the raw materials used and the cross-sectional shape of the product. At present, the domestic die length is generally designed to be 900mm-1200mm. The size of the die cavity is determined by the size of the product and the shrinkage rate of the selected resin. Under normal circumstances, the shrinkage rate of unsaturated polyester products is 2%-4%, and that of epoxy resin is 0.5%-2%. For hollow products, special attention should be paid to the design of the core rod. Generally, the effective length of the core rod is 2/3 to 3/4 of the die length. In the pultrusion process, the convenience of fixing and adjusting the core rod should be considered. In addition, the larger core rod should also consider the problem of counterweight and heating to maintain horizontal balance and uniform heating. Comprehensively considered, for a die with a length of about 900mm, the length of the core rod can be designed to be about 1500mm.

 

(4) Maintenance and Repair of Pultrusion Dies

Unused molds usually need to be cleaned and protected to avoid corrosion from water and dust. The core rod should be hung when idle to prevent deformation caused by gravity.

 

After the electroplating pultrusion die is used for a period of time, the local chromium layer may fall off. If the area is not large, it can be used again by grinding. The grinding method is as follows: First, use 600 mesh water sandpaper for grinding. When it is polished to a certain degree, use finer sandpaper. The grinding order is as follows: 600 mesh → 800 mesh → 1000 mesh → 1200 mesh → 1500 mesh. During the grinding process, the mold must be continuously rinsed with aviation kerosene to wash away the particles ground by the sandpaper to avoid scratching the mold.

 

After the water sandpaper is polished to 1500 mesh, use a special electric polisher and wool polishing disc for polishing. At the beginning of polishing, use a polishing agent with slightly coarse abrasive particles, and use a slightly harder wool disc, and polish it 2~3 times. Rinse the mold with kerosene to rinse the thrown particles, and then use a slightly softer wool polishing disc to polish the mold. During the polishing process, the polisher moves in one direction and cannot stop in one place to avoid heating the mold surface and burning the mold. This process is carried out 2 to 3 times. After polishing, the mold cavity is very bright and has a mirror effect. It can continue to be used, as shown in the figure below.