Pultrusion Process Analysis and Production Precautions

 

Pultrusion is a method for continuous production of composite material profiles. It is an automated production process that impregnates the untwisted glass fiber roving and other continuous reinforcement materials, polyester surface felt, etc. on the creel with resin, and then passes through a molding mold that maintains a certain cross-sectional shape, and continuously ejects the mold after curing in the mold, thereby forming a pultruded product.

 

The tensile strength of the products produced by the pultrusion process is higher than that of ordinary steel. The resin-rich layer on the surface also makes it have good corrosion resistance, so it is the best product to replace steel in projects with corrosive environments. It is widely used in transportation, electrical, electrical, electrical insulation, chemical, mining, ocean, boats, corrosive environments and various fields of life and civil use.

 

Pultrusion Process Flow

There are many forms of pultrusion processes and many classification methods. Such as intermittent and continuous, vertical and horizontal, wet and dry, crawler traction and clamping traction, in-mold curing and in-mold gel and out-mold curing, heating methods include electric heating, infrared heating, high-frequency heating, microwave heating or combined heating, etc.

 

The Typical Process Flow of Pultrusion is:

Glass fiber roving arrangement – impregnation – preforming – extrusion molding and curing – traction – cutting – finished product

 

 

Pultrusion Equipment Composition:

1. Reinforcement material delivery system: such as yarn rack, felt spreading device, yarn hole, etc.
2. Resin impregnation: The straight groove impregnation method is most commonly used. During the entire impregnation process, the fibers and felts should be arranged very neatly.
3. Preforming: The impregnated reinforcement material passes through the preforming device and is carefully transferred in a continuous manner to ensure their relative position, gradually approaching the final shape of the product, and extrude excess resin, and then enters the mold for molding and curing.
4. Mold: 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 resin system. The mold is the most critical part of the pultrusion process, and the length of the typical mold ranges from 0.6 to 1.2m.
5. Traction device: The traction device itself can be a crawler-type puller or two reciprocating clamping devices to ensure continuous movement.
6. Cutting device: The profile is cut to the required length by an automatically synchronously moving cutting saw.

The function of the forming mold is to achieve the compaction, forming and curing of the blank. The cross-sectional size of the mold should take into account the molding shrinkage of the resin. The mold length is related to the curing speed, mold temperature, product size, pultrusion speed, and the properties of the reinforcement material, and is generally 600-1200mm. The mold cavity should have a high finish to reduce friction, extend service life, and facilitate demolding. Electric heating is usually used, and microwave heating is used for high-performance composite materials. A cooling device is required at the mold inlet to prevent the glue from curing prematurely. The dipping process mainly controls the relative density (viscosity) and impregnation time of the glue. Its requirements and influencing factors are the same as those of prepreg.

 

The curing molding process mainly controls the molding temperature, mold temperature distribution, and the time (pultrusion speed) of the material passing through the mold. This is the key process of the pultrusion molding process. During the pultrusion molding process, a series of physical, chemical, and physicochemical complex changes occur when the prepreg passes through the mold, which is still not very clear. Generally speaking, the mold can be divided into three areas according to the state of the prepreg when it passes through the mold. The reinforcement passes through the mold at a constant speed, while the resin is different. At the mold entrance, the behavior of the resin is similar to that of a Newtonian fluid. The viscous resistance between the resin and the inner surface of the mold slows down the forward speed of the resin, and as the distance from the inner surface of the mold increases, it gradually returns to a level equivalent to that of the fiber.

 

 

During the process of prepreg moving forward, the resin undergoes cross-linking reaction when heated, the viscosity decreases, the viscous resistance increases, and it begins to gel and enter the gel zone. It gradually hardens, shrinks and detaches from the mold. The resin and the fiber move forward evenly at the same speed. It continues to cure when heated in the curing zone, and ensures that the specified degree of curing is reached when it is ejected from the mold. The curing temperature is usually greater than the peak value of the exothermic peak of the glue, and the temperature, gel time and traction speed are matched. The temperature in the preheating zone should be low, and the temperature distribution should be controlled so that the curing exothermic peak appears later in the middle of the mold, and the detachment point is controlled in the middle of the mold. The temperature difference between the three sections is controlled at 20-30°C, and the temperature gradient should not be too large. The effect of the exothermic reaction of the curing reaction should also be considered. Usually, three pairs of heating systems are used to control the temperature in the three zones.

 

Traction force is the key to ensure smooth ejection of the product. The magnitude of the traction force depends on the interface shear stress between the product and the mold. The shear stress decreases with the increase of the traction speed, and three peaks appear at the entrance, middle and exit of the mold. The peak at the inlet is caused by the viscous resistance of the resin at that location. Its size depends on the properties of the resin viscous fluid, the temperature at the inlet and the filler content. In the mold, the viscosity of the resin decreases with the increase of temperature, and the shear stress decreases. As the curing reaction proceeds, the viscosity and shear stress increase. The second peak corresponds to the detachment point and decreases significantly with the increase of the traction speed. The third peak is at the outlet, which is caused by the friction between the product and the inner wall of the mold after curing, and its value is smaller. Traction is very important in process control. To make the surface of the product smooth, the shear stress (second peak) at the detachment point is required to be small and to detach from the mold as soon as possible. The change in traction reflects the reaction state of the product in the mold and is related to the fiber content, product shape and size, release agent, temperature, traction speed, etc.

 

Main Raw Materials for Pultruded FRP

 

Resin Matrix

Pultruded FRP mainly uses unsaturated polyester resin and vinyl ester resin, and other resins also use phenolic resin, epoxy resin, methacrylic acid and other resins. In recent years, due to the advantages of phenolic resin such as fire resistance, foreign countries have developed phenolic resin suitable for pultruded FRP, called second-generation phenolic resin, which has been promoted. In addition to thermosetting resins, thermoplastic resins can also be used as needed.

 

Fiber-Reinforced Materials

The fiber-reinforced materials used in pultruded FRP are mainly E-glass fiber rovings. C-glass fiber, S-glass fiber, T-glass fiber, AR-glass fiber, etc. can also be used according to the needs of the product. In addition, synthetic fibers such as carbon fiber, aramid fiber, polyester fiber, and vinylon can also be used for special-purpose products. In order to improve the lateral strength of hollow products, continuous fiber mats, cloth, and tapes can also be used as reinforcing materials.

 

Auxiliary Materials

(1) Initiator

The characteristics of the initiator are usually expressed by active oxygen content, critical temperature, and half-life.

 

The commonly used initiators are:

 

MEKP (methyl ethyl ketone peroxide)

 

TBPB (tert-butyl perbenzoate)

 

BPO (benzoyl peroxide)

 

Lm-P (curing agent for pultrusion)

 

TBPO (tert-butyl peroxyethyl octanoate)

 

BPPD (diphenoxyethyl peroxydicarbonate)

 

P-16 [bis(4-tert-butylcyclohexyl peroxydicarbonate)

 

In actual applications, single-component curing agents are rarely used. Usually, two-component or three-component curing agents are used in combination according to different critical temperatures.

 

(2) Epoxy Resin Curing Agents

Commonly used curing agents include anhydrides, tertiary amines, and imidazoles.

 

(3) Colorants

The colorants in pultrusion usually appear in the form of pigment paste.

 

(4) Fillers

Fillers can reduce the shrinkage rate of products, improve the dimensional stability, surface finish, smoothness, and matte or matte properties of products; effectively adjust the viscosity of resins; meet different performance requirements, improve wear resistance, improve electrical conductivity and thermal conductivity, etc. Most fillers can improve the impact strength and compression strength of materials, but cannot improve tensile strength; can improve the coloring effect of pigments; some fillers have excellent light stability and chemical corrosion resistance; can reduce costs. It is best to select a gradient in the particle size of the filler to achieve the best use effect. Now there are also surface treatments for fillers to increase the dosage.

 

 

(5) Release Agent

The release agent has extremely low surface free energy and can evenly wet the mold surface to achieve a demolding effect. Excellent demolding effect is the main condition to ensure the smooth progress of the pultrusion process.

 

In the early pultrusion process, external release agents were used, and silicone oil was commonly used. However, the amount used was large and the surface quality of the product was not ideal. Now internal release agents are used.

 

The internal release agent is directly added to the resin. Under certain processing temperature conditions, it seeps out from the resin matrix and diffuses to the surface of the cured product, forming a layer of isolation film between the mold and the product to play a demolding role.

 

Internal release agents generally include phosphates, egg phosphoric acid, stearates, triethanolamine oil, etc. Among them, zinc stearate has a better demolding effect. In pultrusion production, people usually prefer to use internal release agents that are liquid at room temperature. Currently, the internal release agents on the market are mostly mixtures of primary amines, secondary amines, and copolymers of organic phosphates and fatty acids.

 

Application of Pultruded Products

 

Pultruded products include various rods, flat plates, hollow tubes and profiles, with a wide range of applications, including the following aspects:

 

1. Electrical Market

This is the earliest market for pultruded FRP applications. The products that have been successfully developed and applied include: cable trays, ladder racks, brackets, insulating ladders, transformer isolation rods, motor slot wedges, street lamp posts, electric railway third rail guard plates, optical fiber cable core materials, etc. There are many products worth further development in this market.

 

2. Chemical and Anti-Corrosion Market

Chemical anti-corrosion is a major user of pultruded FRP. Successful applications include: FRP sucker rods, cooling tower brackets, offshore oil production equipment platforms, walking grilles, stair handrails and brackets, structural brackets in various chemical corrosion environments, water treatment plant covers, etc.

 

3. Consumer Entertainment Market

This is a market with huge potential. Currently, the products developed and applied include: fishing rods, tent poles, umbrella frames, flagpoles, tool handles, lamp posts, railings, handrails, stairs, radio antennas, yacht docks, garden tools and accessories.

 

4. Construction Market

In the construction market, pultruded FRP has penetrated into the market of traditional materials, such as: doors and windows, concrete formwork, scaffolding, stair handrails, house partition wall panels, reinforcement materials, decorative materials, etc. It is worth noting that reinforcement materials and decorative materials will have a lot of room for growth.

 

5. Road Traffic Market

Successful applications include: isolation barriers on both sides of highways, road signs, pedestrian overpasses, sound insulation walls, refrigerated truck components, etc.

 

 

Common Problems and Solutions in Pultrusion Process

During the pultrusion process, products often have peeling, cracking, bubbles, color difference and other problems. The following lists 7 major problems and corresponding solutions.

 

1. Peeling

When solidified resin particles come out of the mold on the surface of the part, this phenomenon is called peeling or shedding.

 

Solution:

• Increase the temperature of the inlet feed end of the mold for early curing resin.
• Reduce the line speed to cure the resin earlier.
• Stop the line for cleaning (30 to 60 seconds).
• Increase the concentration of low-temperature initiator.

 

2. Blistering

When blistering occurs on the surface of the part.

 

Solution:

• Increase the temperature of the inlet end mold to cure the resin faster
• Reduce the line speed, which has the same effect as the above measures
• Increase the reinforcement level. Blistering is often caused by voids caused by low glass fiber content.

 

3. Surface cracks

Surface cracks are caused by excessive shrinkage.

 

Solutions:

• Increase mold temperature to cure faster
• Reduce line speed, same as above
• Increase filler loading or glass fiber content to toughen the resin-rich surface, reducing shrinkage, stress and cracks
• Add surface pads or veils to the part
• Increase the amount of low-temperature initiator or use an initiator at a lower temperature than the current one.

 

4. Internal Cracks

Internal cracks are usually related to too thick a section and can appear in the center of the laminate or on the surface.

 

Solutions:

• Increase the temperature at the feed end to cure the resin earlier
• Reduce the mold temperature at the tail end of the mold to act as a heat sink to reduce the exotherm peak
• If changing the mold temperature is not possible, increase the line speed to reduce the temperature of the outer contour of the part and the exotherm peak, reducing any thermal stresses.
• Reduce the initiator level, especially the high temperature initiator. This is the best permanent solution, but some experimentation is required.
• Replace the high temperature initiator with an initiator that has a lower exotherm but better curing effect.

 

5. Color Variation

Hot spots can cause uneven shrinkage, which can cause color variation (also known as color shift)

 

Solutions:

• Check heaters to ensure they are in place so there are no uneven temperatures across the mold
• Check resin mix to ensure fillers and/or pigments are not settling or separating (color variation)

 

6. Low Barcol Hardness

Low reading on the Barcol hardness tester; due to incomplete cure.

 

Solutions:

• Reduce line speed to speed up cure of the resin
• Increase mold temperature to increase cure rate and extent in the mold
• Check mix formulation for overplasticization
• Check for other contaminants such as water or pigments that can affect cure rate

 

Note: Barcol hardness readings can only be used to compare cures using the same resin. They cannot be used to compare cures using different resins because different resins are made with their own specific glycols and have different depths of crosslinking.

 

7. Bubbles or Porosity

Bubbles or porosity can appear on the surface.

 

Solutions:

• Check for excess moisture and solvents that may have been introduced during mixing or due to improper heating. Water and solvents will boil and evaporate during the exotherm, causing bubbles or pores on the surface.
• Reduce line speed, and/or increase mold temperature to better overcome this problem by increasing the hardness of the surface resin.
• Use a surface mask or surface felt. This will strengthen the surface resin and help eliminate bubbles or pores.
• Add surface pads or veils to the part

Precautions for the Use of Release Agents in Pultrusion Molding Process

 

Choice of Internal Release Agent

The internal release agent is directly added to the resin. It is compatible with liquid resin but incompatible with cured resin. Under certain processing temperature conditions, it seeps out and diffuses from the resin matrix to the surface of the cured product, forming a layer of isolation film between the mold and the product, which plays a demolding role.

 

Internal release agents generally include phosphates, egg phosphoric acid, stearates, triethanolamine oil, etc.

 

Among them, zinc stearate has a better demolding effect. Due to the high viscosity of the resin, it is difficult to stir evenly by directly adding powdered zinc stearate, and zinc stearate is loose and large in volume, and carries more air, resulting in more bubbles in the resin. Therefore, zinc stearate is usually added to the cross-linking agent before use to make it a uniform paste, and then added to the resin. The use of zinc stearate as an internal release agent has no significant effect on the color, curing speed and resin viscosity of the product.

 

In pultrusion production, it is usually preferred to use an internal release agent that is liquid at room temperature. At present, the internal release agents on the market are mostly mixtures of primary amines, secondary amines, and copolymers of organic phosphoric acid and fatty esters.

 

The role of Release Agents

During the molding process of the product, a strong adhesion will be generated between the molded product and the mold surface. In addition, from the time the pultruded material enters the mold mouth, as the temperature rises, the resin viscosity decreases, the volume expands, and the pressure acting on the mold wall gradually forms, increases and accumulates, and reaches the maximum value in the gelling zone.

 

In order to prevent the additional load of the molded FRP product from adhering to the mold, a type of isolation film (i.e., release agent) must be applied between the product and the mold so that the product can be easily removed from the mold to ensure the surface quality of the product and the integrity of the mold.

 

All material surfaces have surface free energy. The size varies with different materials. Generally speaking, the surface free energy of metals is relatively high. Organic matter is also a solid, so the liquid will diffuse and evenly distribute on the surface of the solid. The release agent is to have extremely low surface free energy, so as to evenly wet the mold surface and form a layer of low surface energy coating on the mold surface, so as to achieve the effect of easy demolding.

 

Because the production of pultrusion is a continuous process, excellent demoulding effect is the main condition to ensure the smooth progress of pultrusion. According to the different ways of use, demoulding agents are divided into external demoulding agents and internal demoulding agents. In the early pultrusion process, external demoulding agents were used, and silicone oil was commonly used. During use, the demoulding agent is placed in a special groove. When the product is pulled, the demoulding agent is brought to the surface of the glass fiber molded product impregnated with resin, and then enters the molding mold for molding and curing to achieve demoulding. However, the amount of demoulding agent used is large and the surface quality of the product is not ideal. Now internal demoulding agents are used instead.

 

Liquid Internal Release Agents have Many Advantages in Pultrusion Process:

1. It is easy to disperse in the resin;
2. It has the function of cleaning the mold;
3. It can protect the metal mold from corrosion;
4. It can improve production efficiency while reducing pultrusion resistance and mold loss;
5. It can reduce the surface tension of the resin mixture, reduce the viscosity of the resin, improve the wettability of the resin to the reinforcing material and filler, and improve the fluidity of the resin;
6. It assists in defoaming and improves the surface quality of the parts to make the surface of the product smooth;
7. It does not affect the curing characteristics of the resin, does not change the application period of the mixed resin, does not affect the physical and mechanical properties of the product, does not affect the weather resistance of the product, etc.

 

Issues to Note When Using Release Agents

Since most liquid internal release agents are acidic, the following issues should be noted when using them:

1. When using acid-sensitive pigments, it will cause color changes;
2. When using alkaline fillers, such as calcium carbonate, acidic release agents will react with them, causing the viscosity of the mixture to increase, but will not affect the demolding effect;
3. If the filler is aluminum hydroxide, the acidic release agent will not only increase the viscosity of the mixture, but also release water during the curing process of the mixture, causing bubbles, cracks and other problems.

 

Usually, the starting dosage of the internal release agent is 1% of the resin amount, and the effective addition range is 0.75-2% based on the weight of the resin. It should be adjusted appropriately according to actual conditions.

1. For simple thin-walled profiles, the dosage can be appropriately less, such as 0.8% or less;
2. Thick-walled or complex-shaped profiles need to add more.
3. In a high-filler system, the amount of internal release agent added should be increased, but too much internal release agent will delay curing.
4. In pultrusion production, if the resistance is too large and the cause cannot be found, it is necessary to increase the amount of release agent appropriately. When using, attention should be paid to the order of adding materials. When mixing, the internal release agent should be added to the resin system and mixed evenly before adding the curing agent, filler and other resin additives. This can achieve the best demoulding effect.

 

Pultrusion Process Parameter Control and Yarn Rack Layout

 

1. Skill Requirements

Be able to control process parameters such as temperature, pressure, and traction speed, be able to arrange the yarn rack reasonably according to the process documents, and be able to arrange the process for yarn and felt.

Be able to reasonably adjust the process parameters according to the process requirements of different products.

 

1. Related Knowledge

The pultrusion process has a total of 8 processes: fiber extraction, fiber impregnation, preforming, heating, curing, cooling, traction and cutting. In the entire product production process, not all processes can set parameters, which depends on the degree of automation of the equipment used by the operator. In the following table, the process parameters that the operator must determine before the product is produced are listed, and the parameters marked with triangles are alternative parameters.

 

 

The above parameters can be adjusted within a certain range. Some of these process parameters can be adjusted directly through the pultrusion equipment, such as the temperature of the mold, the speed of traction, etc. However, some process parameters, such as the traction force of the product and the mold cleaning time, cannot be adjusted directly for equipment with simple functions. They can only be adjusted by taking auxiliary measures when necessary.

 

(II) Mold Temperature Setting

 

1. Skill Requirements

Be able to set the temperature of each heating zone according to the process file.

 

2. Related Knowledge

Mold temperature setting. The pultrusion mold generally has three heating zones. Its temperature control and distribution are one of the key process parameters of the pultrusion process. The temperature of the three zones not only affects the surface quality of the product but also seriously affects the mechanical properties of the product. If the temperature of the preheating zone is too high, the gel point moves forward, the detachment point is too far from the end of the mold, the traction force increases, local mucosa may occur, foaming is serious during production, and the surface of the product is rough; but if the temperature of the preheating zone is too low, the material is not fully preheated, which will cause demolding difficulties, increased traction, and even mold blocking, and process failure.

 

The temperature of the gel zone must also be controlled within an appropriate range. If the temperature of this zone is too high, the large amount of heat released during the resin curing reaction may cause the resin matrix to crack due to the local high temperature, which will reduce the performance of the composite material; if the temperature of the gel zone is too low, the curing reaction of the resin in the gel zone will not be sufficient, resulting in sticking, increased traction, and poor surface quality of the product.

 

The temperature control of the curing zone is based on the principle of fully curing the resin in this zone. If the temperature is too low, the resin cannot be completely cured; if the temperature is too high, it will waste energy and increase the internal stress of the product, affecting the dimensional stability and mechanical properties of the product, and may even cause the resin matrix to crack and affect the performance of the product.

 

(III) Process Layout of Yarn Racks and Felt Racks

 

The reasonable layout of the yarn rack is one of the important conditions to ensure the smooth progress of production. The method should be based on the requirements of the product process card, select a certain number of yarn balls from the yarn rack, lead them out from the small ring on the yarn rack, pay attention to no entanglement, crossing, etc., and then pass through the yarn separation plate, yarn pressing clamp and preformed plate in turn.

 

During the threading process, the principle of “front and back alignment, up and down, left and right parallel” should always be followed, so that the yarn should be clearly layered at any stage without crossing or entanglement. Some products need to be added with felt, and they should be cut according to the width requirements of the felt on the product process card. After the yarn bundle passes through the preformed plate, the yarn bundle is bundled together with cotton thread or fine fiber bundles, and the operator uses copper wire to pull it through the mold at the mold outlet and pull it to the traction clamp.

 

Use tape to wrap the yarn bundle several times at a certain distance. During the wrapping process, the yarn should be straightened as a whole, and try not to have loose strands. Ensure that after the traction clamp clamps the yarn bundle, all yarn bundles can transmit traction at the same time.

 

In the actual production process, yarn balls often have yarn knots, messy yarns and other phenomena. These phenomena should be discovered and solved in time, and they should not be allowed to enter the yarn pressing clamp and preformed plate to avoid yarn blocking and yarn breakage, which will cause production interruption in severe cases. Smaller yarn knots can be untied, and large yarn knots can be cut off by cutting the yarn knots and messy yarns, and then tied with cotton thread or braided.

 

For products with strict strength requirements, the presence of yarn knots will seriously affect the strength. When joining the yarn, you can braid it first. When the braided joint is about to reach the entrance of the mold, untie the braided yarn, wrap the yarn head in the moving yarn bundle, and clamp it with tweezers until the joint enters the mold. If it is not unfolded, the braided section of yarn is not easy to be soaked by the resin, which will cause the local interlayer bonding strength to decrease. At the same time, due to the increase in local yarn content, it will cause local uneven force when working bending force, and the product is prone to surface fuzzing, cracking, and breaking.

 

When changing the felt, align the joint first, use the butt joint method, sew the front and rear felts together with needle and thread, and use tweezers to straighten the joint when it enters the mold to prevent the joint from being blocked at the entrance. You can also wait until the front section of the felt is about to enter the mold, clamp the joint of the rear section of the felt between the front section of the felt and the yarn, and press the joint with tweezers until the connected felt can automatically move forward synchronously with the yarn.

 

Pultrusion Molding Process Production Operation

 

1. Skill Requirements

Be able to skillfully start or stop equipment such as pultruders and mixers, and be able to complete operations such as felt and yarn connection.

 

1. Related Knowledge

 

(I) Pultrusion Machine Operation Steps and Essentials

Take a 3-ton small pultrusion device as an example, the operation steps are as follows:

1. Turn on the main power, and the main power indicator on the machine will light up.
2. Press the heating button on the equipment and set the mold temperature according to the equipment instructions.
3. According to the requirements of the pultrusion process operation manual, set the temperature of the front area, middle area, and rear area of ​​the mold in turn, and then press the confirmation key to heat the mold.
4. When the temperature measured by the temperature controller is around the set value, the machine can be started for production. At this time, the white yarn pulled through the mold is gathered into a bundle and placed between the upper and lower clamps of the traction. Then, the traction button on the control panel is pressed to start the traction device and start the continuous traction of the fiber. At the same time, the traction speed can be adjusted according to the provisions on the process sheet.

 

5. The adjustment of the traction tension is generally adjusted according to the size and resistance of the drawn product. The display instrument of the tension is set on the control panel of the crawler equipment and the pneumatic reciprocating equipment, but it cannot be adjusted directly. Generally, the traction force is enhanced by the clamping force. There are special tension and clamping regulating valves on the hydraulic pultrusion equipment, which can be easily adjusted separately.
6. During the production process, you should always pay attention to the display value of the tension meter. When it exceeds a certain value, check whether there is any abnormality, including whether the online process is normal, and decisively stop the machine for investigation to prevent the tension from exceeding the equipment load and causing damage. Turn off the heating control button, power control button, and finally turn off the surrounding environmental sanitation in sequence.
7. After the production is completed, turn off the heating control button, power control button, and finally turn off the main power.
8. Clean the equipment and the surrounding environment.

 

(II) Yarn Arrangement and Yarn Connection Operation Steps and Essentials

 

Yarn arrangement means placing the reinforcing materials on the yarn frame and leading these materials out according to the design requirements. Only when the reinforcing materials are placed in the correct position can the smooth progress of pultrusion production be guaranteed, and the comprehensive performance of the pultrusion products can be brought into play and the design purpose of the products can be achieved. The main equipment to complete this process is the yarn feeding device, which includes the yarn frame and the buncher.

 

There are two ways to lead the fiber out of the yarn frame, one is to lead it out in the way of fiber inner withdrawal, and the other is to lead it out in the way of fiber outer withdrawal. The former’s yarn tube is placed statically on the yarn frame. When the fiber is led out from the inner wall, twisting will inevitably occur. In most cases, glass fiber is led out in this way. The latter’s yarn tube is placed on a rotating mandrel, which can avoid fiber twisting. This method is suitable for the lead-out of special fibers such as carbon fiber and aramid fiber.

 

Regardless of the method of drawing out, when performing the yarn threading operation, one principle must be followed, that is, no matter how many bundles of fibers are to be threaded on a layer of yarn rack, the drawn fibers cannot be entangled with each other, and must be strictly independent and non-interfering. In this way, the continuous production can be guaranteed without interruption caused by fiber knotting and disordered yarn. After the fiber is drawn out from one side of the yarn rack, it enters the next production process through a special yarn guide ring.

 

In order to improve the lateral performance of pultruded products, various forms of fabrics are also drawn out from the raw material rack and enter the impregnation process, such as stitched felt, needle felt, surface felt, multi-axial fabric, continuous felt, etc.

 

(III) Fiber Impregnation Process

 

(IV) Preforming and Curing

 

The main function of preforming is to guide the flat ribbon-shaped fibers after impregnation to gradually evolve into the shape closest to the pultruded product. At the same time, the excess resin in the reinforcing material is squeezed out, and the bubbles brought into the material are removed to obtain a pultruded product with a dense structure.

 

The preforming process is completed by the preforming mold, and the preforming mold gradually transitions from simple to complex. The length occupied by its series of templates is about 0.6-1.2m. The reinforcing material is gradually formed into the designed shape during preforming, and the distribution of the fiber in the product meets the design requirements.

 

Usually, a tubular preforming mold is generally used for pultrusion rods. The simplest design is to make a certain number of radially distributed yarn holes on a plate. A mandrel preforming mold is required to produce pipes. When manufacturing special-shaped materials, it is mostly necessary to make about 2 to 6 preforming molds to ensure that the fiber and felt can be smoothly and reasonably transitioned to a suitable shape, close to the cross-sectional shape of the profile.

 

The design of the preforming mold is a very worthy study in the pultrusion process. It is very flexible and requires the designer to have open-minded ideas and rich experience, as well as strong hands-on ability. The normal drawing of a complex pultruded product is inseparable from a preforming system with reasonable design and novel ideas. As long as the important role of preforming is understood, it is not necessary to be limited to any one preforming mode, to innovate and form a system.

 

The following is a design method for pultruded channel preforming:

After the material is pulled out of the preforming die, it enters the heating die, solidifies and forms in the die, and is pulled out of the die. This process is the most important and main process in the pultrusion process.

 

The length of the curing die is generally 0.5m-1.55m, and its specific length depends on factors such as product thickness, pultrusion speed and chemical reaction characteristics of the resin system. The forming die is generally made of die steel, and then its surface is chrome-plated or nitrided to increase hardness, reduce wear, reduce traction and extend die life. The heating methods of the die include steam heating, thermal oil heating, electric heating and other heating methods. Among them, electric heating is easy to control the temperature of different areas in the length direction of the die. The die of the pultruder usually contains one to four heating zones, and the number of heating zones is usually determined by factors such as the resin system, pultrusion speed and the length of the die.

 

In the design of the forming die, in addition to considering any size of the cross section, the following two main factors should be considered: one is the chemical and physical properties of the curing reaction of the resin system; the other is the friction performance between the pultruded material and the die wall. In many cases, according to the reaction characteristics of the resin and the properties of related materials, the mold is designed into three different heating zones, namely the preheating zone, the gel zone and the curing zone, and the temperatures of the three zones are coordinated with each other.

 

The resin-fiber mixed material first enters the preheating zone to reduce the viscosity of the resin, improve the fluidity of the resin and allow the resin to further infiltrate the reinforcing material; then the material enters the gel zone, the resin begins to react, and the resin changes from a viscous liquid to a gel state: finally, the material enters the curing zone to fully cure the material; the reaction of the resin mainly occurs in the gel zone. The position where the resin reacts to the gel state at a higher temperature is called the “gel point”, and the curing reaction process in the gel zone is an exothermic reaction process. The point with the fastest reaction heat release rate is called the “exothermic peak”; when the resin is cured to a solid, the pressure drops due to the curing shrinkage, and the product detaches from the mold surface. This point is called the “detachment point”.

 

A successful pultrusion process is to make the gel point, exothermic peak and detachment point close to and concentrated in the gel zone, otherwise the product may have poor mechanical properties and sticky membranes.

 

(V) Pulling Operation Steps and Essentials

After the product solidifies in the mold, a pulling force is required to pull the product out of the mold. This pulling force comes from the pulling device. In order to meet the needs of pultrusion process, there are several basic requirements for the traction device: During the pultrusion process, the traction device must ensure continuous traction. If the traction of the product is stopped or the downtime is too long, the thermal balance in the mold will be destroyed, causing serious process accidents such as mold blocking; the traction force and traction speed are adjustable, so the traction force required for products of different cross-sections, different sizes and different materials is also different, and the traction speed should be adjusted according to factors such as the chemical reaction characteristics of the resin matrix, the temperature distribution of the mold, and the length of the mold. If the traction speed is too slow, the resin stays in the mold for a long time, and the gel point and the detachment point are forward, which will cause demolding difficulties. On the contrary, the resin will not be fully cured and affect the performance of the product; the clamping force is adjustable, because the traction force is transmitted to the product by the friction force generated by the clamping force, so the clamping force of different traction forces is also different; the chuck can be replaced at will, and the chuck needs to be designed with a pad when clamping to enhance the friction and reduce damage to the outer surface of the product.

 

There are two types of traction devices widely used in the pultrusion process. One is a reciprocating clamping traction device, which has two pairs of traction devices that can move alternately back and forth in the pultrusion direction. When one pair of traction clamps moves forward, the other pair of traction clamps releases and moves backward to the starting position.

 

The two complement each other, clamping the product in turn to achieve a continuous pultrusion process.

 

Safety and Technical Operating Procedures for Pultrusion Production Lines

 

1. Preparation Before Production

 

1. Before starting the machine, the operator should check the lubrication, transmission, electrical control, etc. of the units in each part of the equipment. If any problems are found, they should be communicated and solved in time with the workshop team leader.
2. Check whether the production tools are complete according to the requirements of the product, and the measuring tools should be accurately calibrated.
3. Start the heating system 1-2 hours in advance, set the temperature of each zone according to the process regulations, and check before starting the machine to prevent the temperature from being too high or too low.
4. Configure the appropriate amount of large and small materials according to the product production quota, and check whether the yarn is clearly layered and arranged in order, whether there is any cross-confusion, check whether the surface felt and inner felt are placed well, and whether the shaft can rotate smoothly to produce felt.
5. Prepare the traction rope, and test the machine to observe the air pressure, traction speed, resin tank device, heating control system, electrical switches of various parts, water, electricity, gas, etc., and start production only after confirming that there are no problems.

 

2. Start The Machine

 

2. Add the evenly mixed resin and other ingredients to the resin tank, press the yarn into the resin tank, and pass the yarn through the preforming device according to the process regulations. The zinc rod and the mold sleeve must be firmly connected and the screws must be tightened to prevent the iron galvanized rod core from entering the mold, so that the product cannot be normally discharged from the mold.
3. After the resin, yarn and felt are ready and all requirements meet the process regulations, the personnel should be actively organized to start the machine. When starting the machine, they should divide the work and cooperate closely.
4. Turn on the traction machine and start the traction switch. The operator should pay attention to the resin ingredients and the glass fiber yarn entering the mold, observe the speed of the traction machine, and pay attention to whether the machine has abnormal noise. At this time, the operator is not allowed to leave the machine to prevent problems from being discovered in time.
5. After the glass fiber products are extruded from the die mouth, observe the surface, concentricity, outer diameter, etc. of the glass fiber products. When the glass fiber products are normalized, start to adjust the traction machine speed (temperature is proportional to speed) to adjust the thickness of the glass fiber products evenly to prevent concentricity deviation.
6. Take samples to inspect the glass fiber products according to the process regulations, and check the quality of the glass fiber products after extrusion, such as concentricity, strength, surface gloss, straightness, outer diameter, inner hole to see if they meet the process requirements, and there should be no visible pores in the cross section.

 

6. The Following Points Should be Noted During Normal Production:

 

1. Product quality
2. Mechanical operation of each part of the equipment
3. Control of the heating system
4. Changes in mold temperature and traction speed
5. Be diligent in three aspects: frequently measure the outer diameter, frequently check the quality, and frequently observe the equipment

 

7. Keep records of the start-up inspection form, process sheet, etc.

 

3. Shutdown

During the production process, the machine operator should record the production quantity in time to prevent the number of products from being too large. After the shutdown, the mold power should be turned off in time, and then the mold should be removed, the mold should be taken out of the heating device, the traction machine power should be turned off, and the fiberglass product should be taken out of the traction machine for next use.

1. The machine should be shut down in the following situations:
2. After the production task is completed, the mold and resin tank should be cleaned in time.
3. When the temperature is too high, the surface of the fiberglass product will crack and white dust will appear on the surface. The temperature should be lowered in time to increase the pultrusion speed.
4. If the shutdown time is long, the iron galvanized rod core should be withdrawn from the mold sleeve.
5. If there are other reasons for shutdown, such as power outage, water outage, equipment or personal accidents, the iron galvanized rod core should be withdrawn from the mold sleeve.

 

2. The iron frame and mold should be cleaned and put back into the mold room after cleaning for next use.
3. Keep a record of product production and prepare for production for the next shift. Such as: molds, production felt, semi-finished products, etc.
4. After stopping the machine, check the power supply, water supply, air supply, and all parts of the equipment. After confirming that there are no problems, turn off the power supply and the air supply before leaving the machine.