Review of the Development of Composite Materials Technology and Equipment at Home and Abroad Part V – Pultrusion (Part II)

 

 

In the early 1990s, there were reports of variable cross-section braided pultrusion in the United States. As shown in Figure 32, unidirectional fibers, ±45° braiding, 90° fiber winding, uniform cross-section, and tubular cross-sections can be pultruded through a die to change the circular cross-section into the required shape [2].

 

 

ITV Denkendorf has developed a thermoplastic braided pultrusion technology to commercialize thermoplastic pultrusion technology. Four braiding machines are used in one pultrusion line to obtain a certain wall thickness and produce a hollow profile for the automotive industry. Because the polyamide (PA) fibers on the market shrink too much, special low-shrinkage PA fibers are specially produced and mixed with glass fibers. Each braiding machine uses energy-saving infrared heating technology to ensure that the fiber bundle is well preheated, so the fibers are well impregnated. The selection and heating of the matrix or matrix fibers are the key to thermoplastic pultrusion. Figure 33 shows the ITV thermoplastic braided pultrusion production line.

 

 

In 2015, the Institute of Technology (ITA) of Aachen University in Germany developed a braided pultrusion process, which uses induction heating or frictionless heating for preheating and heating before post-forming. Figure 34 shows the ITA thermoplastic braided pultrusion production line.

 

I think the ones with practical value in thermoplastic pultrusion are mixed yarn (reinforced fiber thermoplastic matrix fiber weaving) pultrusion (see Figure 35) and thermoplastic resin melt injection pultrusion (see Figure 36).

 

The latter is the development direction. In 1998, the author saw mixed yarn for thermoplastic pultrusion at the Institute of Composite Materials (IVW) of Kaiserslautern University in Germany and in recent years in China.

 

Brunei Forever Energy Technology’s thermoplastic matrix (can contain short fibers) pultrusion + injection molding process (its process diagram is shown in Figure 37) is different from the process of fiber reinforcement and matrix thermoplastic fiber combined pultrusion, and its technical and economic efficiency has been improved. In fact, the French Saint Gobain company took the lead in developing in 1988 to produce glass fiber thermoplastic composite yarn (its trade name is Twintex) by direct drawing method. Because its technical and economic efficiency was challenged, the market shrank and its growth was limited. There are precedents in China for direct impregnation of thermoplastic resin for fiber winding and outer covering of pultruded parts.

 

 

Fiber-Reinforced Two-Component Polyurethane Resin Pultrusion

Two-component polyurethane resin pultrusion is a reaction injection pultrusion process. In the early 21st century, Europe and the United States began industrial production of fiber-reinforced polyurethane resin pultrusion. In 2008, Bayer (now Covestro) of Germany held a polyurethane resin seminar in Shanghai. In 2009, Suzhou Grete Grille Co., Ltd. (now Zhejiang Deyilong Technology Co., Ltd.) took the lead in developing polyurethane fiberglass doors and windows. In 2013, Hangzhou Tianyun Technology Co., Ltd. successfully developed cable trays. There are now more than 200 two-component polyurethane resin pultrusion lines in China.

 

Two-Component Polyurethane Resin Pultrusion Equipment Requirements:

(1) Two-component injection machine.

Because polyurethane is a two-component system, a two-component injection machine is required for mixing. It is required to have accurate metering, uniform mixing, continuous material discharge, and easy adjustment of injection volume.

 

(2) Injection box.

Because the polyurethane system reacts quickly, gelation occurs quickly, the viscosity increases, and the wettability decreases. In order to improve the impregnation and avoid the solidification of polyurethane in the impregnation device, a special injection box is required for each product.

 

(3) Pultrusion machine.

Traditional pultrusion equipment (hydraulic reciprocating or crawler type) can be used. Crawler type is recommended within the rated traction to provide continuous and uniform pultrusion force. Nowadays, the use of servo motors and closed-loop controlled hydraulic reciprocating types can also meet the requirements.

 

(4) There is a water cooling device between the injection box and the mold of the European and American two-component polyurethane pultrusion production line.

The water-free cooling solution pioneered by Nantong Matech New Materials Co., Ltd. has been successfully promoted.

 

(5) Although the temperature of the product is very high after demolding, in my opinion, no additional cooling is required.

The distance between the mold outlet and the cutting machine can be increased as appropriate. A company in Zhejiang uses a distance of 6 m.

 

Figure 38 shows the polyurethane resin pultrusion process. Figure 39 shows the polyurethane resin pultrusion production line. Figure 40 shows a two-component injection machine. Figure 41 shows the PU reaction injection pultrusion mold and injection box.

 

 

 

FFU Composite Sleepers

Composite sleepers are made of fiber reinforced urethane (FFU) foam by pultrusion, with a glass fiber content of 50% to 80%, and the sleepers are covered with chequered cloth. In 2005, the author saw the production line of this product at the Shiga Ritto Plant of Sekisui Chemical in Japan.

 

In 2007, the company set up a factory in Shanghai to produce this product. After 2011, Xiamen Guanyu Equipment Manufacturing Co., Ltd. and others have launched the production line of this product to the market. CSSC 725 Institute introduced the Xiamen production line many years ago, and the sleepers produced by it have been widely used. The polyurethane/phenolic foam composite sandwich panel production line is shown in Figure 42. Figure 43 is the FFU composite sleeper pultrusion machine.

 

 

PU Pultrusion + RTM (Resin Transfer Molding) Process

 

In recent years, FFT of Germany has combined the two core processes of PU pultrusion and RTM to produce various types of profiles. Its production line is shown in Figure 44.

 

 

 

Curved Pultrusion

 

In 1972, Goldsworthy Engineering of the United States developed the first pultrusion machine for producing arc springs in response to the requirements of NASA (National Aeronautics and Space Administration) (see Figure 45). The entire production line consists of a yarn rack, a dip tank, a radio frequency heater, a guide device, a fixed die, a rotary die, a turntable and other components [9]. In 1978, the first automotive arc spring pultrusion machine was born, as shown in Figure 46. Figure 47 shows the dipped fiber bundle entering between the rotary die and the fixed die. Figure 48 shows the rotary die components and the cutting saw.

 

 

 

Reverse Pultrusion

In 2009, Thomas Technology Innovations of Germany developed the “reverse pultrusion process”. The technological breakthrough known as the “curve pultrusion” process made it possible to produce constant radius curved profiles. Continuous constant radius stacked objects, such as spiral springs and other types of coils, can be continuously produced with this process. This process solution is the reverse process of traditional pultrusion. The profile is no longer pulled through the die, but the die is moved along the profile in a step-back method to produce it.

 

According to this principle, not only constant radius, coils and similar structures can be manufactured. “Reverse Pultrusion” can also develop a lot of fiber reinforced profile solutions. Reverse pultrusion can produce not only constant radius curved profiles, but also variable radius profiles. The above-mentioned “step-back process” can be achieved by using elastic or segmented dies.

 

Applying the reverse pultrusion process to the production of straight profiles can minimize the tension, which in the traditional pultrusion process will deform the composite fabric preform. Therefore, this makes it possible to integrate ±45° prepreg or laminated materials on the profile without any distortion. Reverse pultrusion can produce continuous rigid or flexible tubes of almost any diameter, which are currently only manufactured by filament winding.

 

For large diameter products, no mandrel is required, so there are no restrictions on diameter and/or length, which is determined by the process. Reverse pultrusion can process hollow textile preforms, which is not possible with standard pultrusion processes. This is because in standard pultrusion processes, the core of the hollow profile must be fixed at the entrance of the die and the textile reinforcement must have at least one gap.

 

The reverse pultrusion process only requires a much shorter machine, and the core can be fixed on one side of the produced profile. With this setting, hollow textiles (such as tubes with multiple braids) can be processed like hollow textile material tubes.

 

Figure 49 shows the reverse pultrusion process principle. Figure 50 shows a German Thomas pultrusion machine and a drawn curved thermoset composite profile.

 

Figure 51 shows a cylindrical helical spring produced by reverse pultrusion. Figure 52 shows a thermoset composite profile pultrusion line.

 

 

 

 

Pultruded Thermoplastic Curved Profiles[8]

 

ITV uses a special pultruder (see Figure 53) to pultrude glass fiber yarn and polypropylene yarn to produce thermoplastic curved profiles.

 

Its Pultruder Includes:

① a weaving machine that weaves two fibers into a belt, which is covered with unidirectional fibers;

② a special preheating device;

③ a bending mold;

④ a bending cooling device;

⑤ a curved track, which is converted from a normal track to a special curved track with high traction. The curved track is shown in Figure 54.

 

 

 

Variable Cross-Section Pultrusion

 

Figure 55 is a schematic diagram of the principle of a pultrusion technology for producing variable cross-section shapes, and its typical product is a reed. This technology can form products with constant volume and variable shape, and can also achieve the production of products with variable volume and variable cross-section shape by adding or reducing reinforcing materials.

 

The main molding mold of this process is a series of mother molds, which have their own heating system and are installed on a ring or belt device that can move continuously. The mold ring rotates and pulls out the resin-impregnated fiber into the mother mold for heating and curing.

 

At the same time, there is a heated flexible steel belt covering the mother mold and contacting the surface of the pultruded product to ensure the outer surface quality of the pultruded product [2,11]. The equipment used (see Figure 56) is similar to the 1970 American Goldwasser bending pultrusion prototype.

 

 

Microwave Heating of Pultrusion Dies

 

Microwave heating is mostly used for preheating the reinforcement after it is impregnated with resin and before entering the steel mold. It can also be used as a molding mold. The preheating and molding molds must be insulated and are usually made of ceramic materials. Energy saving is a major advantage of this method. Microwaves promote the molecular movement of the molding material, drive away low molecular weight substances, and the matrix quickly gels and solidifies. The molding material has a high temperature, while the mold is usually 40 ℃. Microwaves of different frequencies are used for different matrices.

 

Many years ago, Xi’an, Rugao and other places have adopted microwave heating on pultrusion lines, but the evaluation is different. Figure 57 shows that the glass fiber roving enters the microwave oven after being impregnated with glue. Figure 58 shows the inner cavity of the microwave oven. Figure 59 shows the ITV microwave pultrusion process production line.

 

 

New Pultrusion System

 

In 2017, KraussMaffei of Germany demonstrated its iPul new pultrusion system (see Figure 60), which made two revolutionary improvements to the long-standing production process.

 

First, iPul changed the previous completely open dipping tank setting and used a closed injection box to dip the fiber, successfully introducing epoxy, polyurethane, nylon 6 and other resin systems with fast response characteristics into the pultrusion system.

 

Second, iPul significantly increased the pultrusion speed from the original 0.5 m/min~1.5 m/min to 3 m/min, and the production efficiency was close to the extrusion speed of PVC.

 

Figure 61 shows KraussMaffei’s fiber-reinforced epoxy resin wind turbine blade pultrusion machine.

 

 

 

The World’s Smallest Pultrusion Machine

 

In February 2020, German TTI (Thomas Technic & Innovation) announced that it had successfully developed a pultrusion machine of only 3.5 m (see Figure 62), which is only 1/4 of the traditional pultrusion equipment, and can be quickly and conveniently transported and installed, eliminating the trouble of building/expanding a new plant. The equipment uses TTI’s patented technology and can produce straight/curved profile products without length restrictions.

 

 

Pipe Drawing Die

 

The general pipe drawing die is shown in Figure 63. The core die is placed in the support outside the die. The alignment between the core die and the die needs to be adjusted repeatedly. In the die shown in Figure 64, the flange where the core die is located is connected to the flange of the die body, which saves adjustment time.

 

The flange should have a “stop”, that is, one of the two flanges has a round flange and the other has a corresponding round groove to ensure concentricity. Figure 65 shows a pultrusion tube die. For composite tubes with a wall thickness greater than 4 mm in pultrusion, the core die used should be heated to prevent the inner wall of the tube from sticking and causing the inner wall of the finished tube to be rough or even break during the pultrusion process.

 

 

 

Prepreg Pultrusion

 

One of the characteristics of the equipment used for prepreg pultrusion is the preformed roller. Nanjing University of Aeronautics and Astronautics in China took the lead in developing it successfully in 2014. Figure 66 shows the prepreg pultrusion process flow chart of Nanjing University of Aeronautics and Astronautics. Figure 67 shows the prepreg pultrusion machine developed by Nanjing University of Aeronautics and Astronautics. Figure 68 shows the prepreg pultrusion CFRP channel steel of Nanjing University of Aeronautics and Astronautics.

 

 

 

Thermoplastic Coating Of Pultruded Parts

 

The pultruded products are coated with thermoplastic plastics to protect the pultruded parts, improve the feel and increase the aesthetics. Since 2014, thermoplastic coating of pultruded parts has been gradually promoted in the industry. Xingtai Hongbang Company was the first to adopt this method in the industry. It used ABS to coat pultruded profiles and was well received by customers. The required equipment consists of an extruder, a crosshead (coating mold), and a crawler tractor. These equipment are produced in Qingdao, Zhangjiagang, Guangdong and other places. Figure 69 shows the thermoplastic coating equipment.

 

 

Photocuring Pultrusion

 

Photocuring pultruded FRP can greatly increase the speed of pultrusion production line. Carbon fiber and aramid are not suitable for photocuring, and mixing them with glass fiber is a solution. The pultrusion mold must be able to transmit ultraviolet light, and quartz glass is the most practical mold.

 

Depending on the specific requirements of the product, photocuring can also be completed outside the mold. Photocuring was used for composite materials in Taiwan Province of my country earlier, and it was introduced to the mainland in the 1990s. Its products include ultraviolet lamps for photocuring. Tianjin Jiuri New Materials Company now ranks first in the world in terms of photocuring agent production/capacity, but it is mainly for export. It is recommended to promote photocuring pultrusion in the industry. Figure 70 is a schematic diagram of the structure of a photocuring pultrusion mold.