Pultrusion Production Process Operation

 

 

Section 1 Raw Material Preparation

 

1. Skill Requirements

 

Be able to prepare glue liquid and reasonably select reinforcing materials according to process documents

 

1. Related Knowledge

 

(I) Preparation Method of Glue Liquid

 

Glue preparation is one of the key processes in the pultrusion production process. Whether its operation is reasonable and whether the ingredients are accurate will determine the quality of the final product. Therefore, the process control of this process should be strengthened, and the operation should be accurate, the records should be clear, and it should be traceable.

 

The raw materials used in the pultrusion product formula mainly include: resin, low shrinkage agent, initiator, release agent, filler, color paste and auxiliary agent (such as defoamer, dispersant, etc.). When preparing glue, the following steps should be strictly followed:

 

1. Put the filler in a tray and put it in an oven at a temperature of (110 ± 5℃) for about 0.5h.

 

2. Calibrate weighing instruments such as scales, balances, etc.

 

3. Measure or weigh the resin according to the requirements of the process documents. ＿

 

4. Add dispersant and other components according to the proportion of the pultrusion process formula and stir for 5-10 minutes;

 

5. Add low shrinkage agent, color paste and other components in turn, stir for about 5-10 minutes: weigh the internal release agent and curing agent at the same time;

 

6. Add the internal release agent, then add the curing agent, and keep the mixer stirring;

 

7. Take out the baked filler from the oven, weigh and add it, and continue to stir for about 5-10 minutes;

 

8. Finally, turn off the mixer and clean up the glue preparation site.

 

The stirring time mentioned above is only a reference time. The operator can adjust it according to the speed of the stirrer used, the actual situation of the formula, and the amount of glue. If the stirring time is too short, it is not conducive to the uniform mixing of various raw materials.

 

If the stirring time is too long, it will cause the temperature of the glue liquid to rise and affect the storage period of the glue liquid. Under normal production conditions, depending on the size of the product, it is generally appropriate to configure 10-15kg of resin. If the amount of resin configured at one time is too large, it will increase the burden on the operator and affect the operating efficiency.

 

 

During the Mixing Process, the mixer must be operated strictly in accordance with the operating procedures and pay attention to safety. Each time the liquid component is poured in, the liquid in the weighing container should be poured out as much as possible.

 

And during the production process, the next bucket of glue should be prepared in time before the glue in the glue tank is almost used up to avoid production pauses.

 

Learn to operate and use the balance correctly during the glue preparation process. First of all, keep the weights and trays clean. If there are resins, color pastes, etc., they should be wiped clean; before weighing, be sure to adjust the level of the balance so that the pointer is aligned with the dial.

 

The scale or left and right swing amplitude is consistent: put the object to be weighed on the left tray (usually in a beaker), put the weights on the right tray, and place the weights in order from large to small. Finally, adjust the floating weights on the beam until the balance is balanced, accumulate the total weight of the weights, subtract the weight of the beaker, and the difference is the weight of the object to be weighed.

 

Table 5-1 Necessary tools in the process of preparing glue

 

 

 

Section 2 Pultrusion

 

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

 

Taking 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 control meter is around the set value, the machine can be turned on for production.

At this time, the white yarn pulled through the mold is gathered into a bundle, placed between the upper and lower traction clamps, and 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 regulations on the process sheet.

 

5. The adjustment of traction force 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 instrument.

 

When it exceeds a certain value, check whether there is any abnormality, including whether the online process is normal, and stop the machine for investigation decisively to avoid the tension exceeding the equipment load and causing damage. Turn off the heating control button, power control button, and finally turn off the surrounding environment.

 

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 is to place the reinforcing material on the yarn frame and lead these materials out according to the design requirements. Only when the reinforcing material is placed in the right position can the smooth progress of pultrusion production be ensured, and the comprehensive performance of pultrusion products can be brought into play to achieve the design purpose of the product. The main equipment to complete this process is the yarn feeding device, which includes a yarn rack and a buncher.

 

There are two ways to lead the fiber out of the yarn rack, one is to lead it out by fiber inner withdrawal, and the other is to lead it out by fiber outer withdrawal. The former’s yarn tube is placed on the yarn rack statically. 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 lead-out method, 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 fibers drawn out cannot be entangled with each other, and must be strictly independent and non-interfering. In this way, the continuous production can be ensured without interruption caused by fiber knotting and disordered yarn. After the fibers are drawn out from one side of the yarn rack, they enter the next production process through a special yarn guide ring.

 

In order to improve the lateral performance of the 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

 

The fiber impregnation process is detailed in Section 4, Part 5 of Chapter 4.

 

(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 a preforming mold, and the preforming mold gradually transitions from simple to complex. The length occupied by a 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 used for pultrusion rods. The simplest design is to make a certain number of radially distributed yarn holes on a plate. The production of pipes requires a mandrel preforming mold. When manufacturing special-shaped materials, it is mostly necessary to make about 2 to 6 preforming molds to ensure that the fiber and felt material can transition to a suitable shape smoothly and reasonably, close to the cross-sectional shape of the profile.

 

The design of preforming molds is a very worthy subject of study in the pultrusion process. It is very flexible and requires designers to have open minds 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 you understand the important role of preforming, you don’t have to be limited to any one preforming mode, and you can innovate and form your own system.

 

The following Is a design method for pultruded channel steel preforming:

 

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

 

The length of the curing molding mold 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 molding mold is generally made of mold steel, and then its surface is chrome-plated or itride to increase hardness, reduce wear, reduce traction, and extend mold life.

 

The heating methods of the mold 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 mold and is more common. The mold 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 mold.

 

In the design of the molding mold, 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 pultrusion material and the mold 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, increase 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”. The curing reaction process in the gel zone is an exothermic reaction process, and the point with the fastest 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) Traction Operation Steps and Essentials

 

When the product is solidified in the mold, a traction force is required to pull the product out of the mold. This traction force comes from the traction device. In order to meet the needs of the 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 completely 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 forward and backward in the pultrusion direction. When one pair of traction clamps moves forward, the other pair of traction clamps loosens and moves backward to the starting position.

 

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

 

Section 3 Product cutting and equipment working principle and operation

 

1. Skill requirements Be able to start and stop equipment such as cutting machines, pultruders, mixers, felt cutters, etc., be able to cut products as required, and understand the working principle of the equipment.

 

1. Related Knowledge

 

(I) Start and stop steps and operating instructions for cutting machines

 

When the product reaches the required size for production, it is cut to a fixed length. There are two ways of cutting: automatic cutting by the equipment and manual cutting. For equipment without a cutting system, manual cutting can be used. Generally, a marble machine is used as a cutting tool. During the cutting process, hold the handle of the cutting machine with the right hand, hold the product with the left hand and keep a certain distance from the saw blade, and pay attention to safety.

 

It is required that the product should not have a split head, and the dimensional error should not exceed the allowable range. The cut products should be neatly stacked at the designated location, and at the same time, pay attention to protecting the product surface. The product name, quantity, batch and production shift should be marked on the product so that when there is a problem with the product, these marks can be used to effectively discover and deal with it. Its technical requirements:

 

1. The temperature of each zone can be set on the pultruder temperature control instrument according to the process documents, and the pultrusion machine pulling speed, mold cleaning time, and mold cleaning frequency can be set according to the process documents.

 

2. The stirring speed and operation time can be adjusted according to the process documents, and the impeller can be cleaned after production is completed.

 

3. The cutting machine can be tested before use to check whether the electrical system and mechanical transmission system are safe and reliable, the felt roll can be installed on the core, and the cutting tool can be installed and replaced.

 

(II) Pultrusion Machine Structure and Operating Procedures

 

1. The Pultrusion Molding Equipment Consists of the Following Parts:

 

(1) Reinforcement Material Transmission System:

such as yarn rack, felt spreading device, yarn hole, etc.

 

(2) Resin Impregnation Area:

The straight groove impregnation method is most commonly used. During the entire impregnation process, the fibers and felt should be arranged neatly and orderly.

 

(3) Preforming Area:

The impregnated reinforcement material passes through the preforming device in turn, moves in a gradual manner, gradually approaches the final shape of the product, and squeezes out the excess resin, and then enters the mold for molding and curing.

 

(4) Mold Placement and Heating Zone:

According to the resin curing exotherm curve and the friction performance between the material and the mold. The mold is divided into 2 to 3 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. The length of a typical mold ranges from 0.5m to 1.2m.

 

(5) Traction Zone:

The traction device itself can be a crawler puller or two reciprocating clamping devices to ensure continuous movement.

 

(6) Cutting Zone:

On pultrusion equipment with a high degree of automation, this zone uses an automatically synchronized cutting saw to cut the product to the required length. Considering cost reasons, most pultrusion equipment uses manual cutting.

 

(III) Working Principle and Operation of Mixer

 

Mixer, also known as disperser, is a common machine in chemical, food, construction and other industries. Its core component is the rotor structure. The high tangential speed and high-frequency mechanical effect generated by the high-speed rotation of the rotor bring strong kinetic energy, which makes the material subject to strong mechanical shear, hydraulic shear, centrifugal extrusion, liquid layer friction, impact tearing and turbulence.

 

The combined effects make the immiscible solid and liquid phases dispersed evenly in an instant, and finally obtain a stable mixture after high-frequency reciprocating. The mixer is mainly used to prepare glue in the pultrusion process. Its operation is as follows:

 

1. When using the mixer, first check the power plug and check whether the protective cover on the motor transmission device is firmly covered.

 

1. When placing the batching barrel on the support platform, the rotor must be adjusted to the center of the barrel and the batching barrel must be clamped with a clamp.

 

1. After the mixing is completed, wait until the mixing stops completely before lowering the support platform and removing the batching barrel.

 

(IV) Working principle and operating procedures of felt cutter

Felt cutter is one of the main auxiliary equipment of pultrusion production line, mainly used to cut reinforced felt materials of different widths. There are two main types of felt cutters. One is to fix the felt on the rotating shaft. After starting the motor, the felt roll rotates with the shaft. On the upper part of the felt layer, there is a movable downward pressure cutting saw blade. By measuring the cutting width of the felt in advance, fixing the position of the saw blade, and controlling the downward pressure of the saw blade, the felt roll can be cut.

 

The other Is designed according to the winding principle. Multiple blades are set before winding. The winding can achieve cutting of multiple widths at the same time, and multiple small felt rolls can be cut at one time. The first type of felt cutting method is more commonly used in pultrusion. This method is more flexible and has a wide range of applications.

 

Its Operation is as Follows:

 

1. It should be run idling before use to check the overall coordination and safety when no load.

 

2. Wrap the outside of the felt roll with tape to prevent the felt roll from falling apart during the cutting process, then align the felt roll paper tube with the support rod of the felt cutter and insert it. After tightening it with the push rod, fix the positioning disk at one end; adjust the size of the expansion clamp at the other end to make it clamped with the inner wall of the felt roll paper tube, and start the power switch.

 

3. Choose different speeds according to the tightness of the felt roll and the specifications of the felt. The surface felt can be high speed, the continuous felt medium speed or high speed, and the short-cut fiber felt low speed.

 

4. After cutting as required, place the cutting saw blade in a safe place, remove the cut felt, mark the specifications, and place it in the specified area for production personnel to choose.

 

5. Turn off the felt cutting equipment and clean up the site.

 

 

Section 4 Pultrusion process parameter control and yarn rack layout

 

(I) Skill Requirements

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

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

 

1. Related Knowledge

 

There are 8 processes in the pultrusion process: fiber extraction, fiber impregnation, preforming, heating, curing, cooling, traction and cutting. In the entire product production process, not all processes can be set with 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 of the triangle are alternative parameters.

 

The above parameters can be adjusted within a certain range. Some of these process parameters can be directly adjusted by the pultrusion equipment, such as the temperature of the mold, the speed of traction, etc. However, there are some process parameters, such as the traction force of the product, the mold cleaning time, etc., which cannot be directly adjusted for equipment with simple functions. They can only be adjusted by 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. Pultrusion dies generally have 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 preheated sufficiently, which will cause demolding difficulties, increased traction, and even mold blocking, resulting in 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, and the large amount of heat released during the resin curing reaction may cause the resin matrix to crack due to excessively high local temperature, reducing 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 is not sufficient, resulting in mucosa, 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 the Yarn Rack and Felt Rack

 

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, and pay attention to not having entanglement, crossing, etc., and then pass through the yarn separation plate, yarn pressing clamp and preformed plate in turn. In the process of threading, always follow the principle of “front and back alignment, up and down, left and right parallel”, so that the yarn should be clearly layered at any stage without crossing or entanglement.

 

Some products need to add felt, and they are 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 thin fiber bundles, and the operator uses copper wire to pull it through the mold at the mold outlet and drag it to the traction clamp. Use tape to wrap the yarn bundle several times at intervals. 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 must be discovered and solved in time. They must not be allowed to enter the yarn pressing clamp and preformed plate to avoid yarn blocking and yarn breaking, which will cause production interruption in severe cases. Smaller yarn knots can be untied, and large yarn knots can be cut off, 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 woven 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 the local yarn content, the local force will be uneven when the working bending force is applied, and the product is prone to surface fuzzing, cracking, and breaking.

 

When changing the felt, first align the joints, use the docking method, and sew the front and back felts together with needle and thread. When the joint enters the mold, use tweezers to straighten it to prevent the joint from being blocked at the mold entrance. You can also wait until the front section of the felt is about to enter the mold, clamp the joint of the back 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.

 

 

Section 5 Trial production and development of new products

 

1. Skill requirements Able to test molds and produce samples according to the trial production plan, able to propose process operation plans for the purpose of improving product output and quality, reducing energy consumption and protecting the environment, and able to undertake the research and development and production operations of new products.

 

1. Related Knowledge

Understand the current development status of pultrusion technology and pultrusion product development in China.

 

(I) Development Status of Pultrusion Technology at Home and Abroad

 

The glass fiber reinforced plastic (FRP) pultrusion industry began to develop in the United States in the 1950s. Initially, it only produced simple rods. In the 1970s, the pultrusion industry developed rapidly worldwide.

 

my country began to study pultrusion technology in the 1970s, and research institutions include Beijing No. 251 Factory and Wuhan University of Technology. Many domestic manufacturers have introduced advanced pultrusion technology and equipment from abroad. For example, Beijing Glass Steel Institute Composite Materials Co., Ltd. introduced 8-ton pultrusion equipment from Pultrex, UK, for the production of tent poles, electrician ladders and other profiles; Qinhuangdao Yaohua Glass Company Glass Steel Branch introduced pultrusion equipment from Pultrex, UK, to produce oil extraction sucker rods and other products; Sino-Italian Glass Steel Co., Ltd. introduced 5 pultrusion production lines from TOp Glass, Italy, one of which is the first pultrusion equipment for optical cable reinforcement core introduced in my country, and its pultrusion speed can reach 15m/min-35m/min.

 

Due to the continuous development of pultrusion technology, products with small cross-sectional size, simple shape and symmetrical structure have developed into large, complex and asymmetrical pultruded products. In terms of the width of pultruded products, it can produce building panels of about 1.5m. Beijing Glass Steel Institute has used pultrusion technology to develop sandwich panels with a width of 1.2 meters for use in civil engineering projects. In terms of cross-sectional shape, as the application field becomes more and more extensive, various profiles with complex cross-sections have been introduced, which has also brought about continuous leaps in pultrusion technology.

 

(II) Current Status of Development and Research of FRP Pultruded Products

 

In the past process, when the material or structure on the cross section of the pultruded product is asymmetrical, it is bound to be distorted after solidification. Therefore, the material design principle of pultruded products is considered to be that the material or structure in the cross section of the product must be symmetrical, but now this limitation has been broken by advanced pultrusion process technology. Designers can design asymmetrical cross-sectional structures according to needs, or use symmetrical materials for laying design. Process designers can use advanced pultrusion equipment and computer control methods to keep pultruded products straight.

 

The development of a new pultruded product includes many contents such as product structure design, mold design, and process parameter determination. Most manufacturers basically do not need to design product structures when receiving R&D tasks from customers, and customers often consult products with already determined structures. All the pultrusion manufacturer needs to do is to consider whether the pultrusion process is suitable for the production of a certain product, and then proceed to the design and production of the mold. Finally, according to the performance indicators proposed by the customer, a suitable configuration plan for the process parameters is proposed, and then after repeated online adjustments and tests, the customer’s requirements are finally met. Among them, mold design is very important in the pultrusion molding process. Here we focus on the mold design and the formulation of process parameters.

 

1. Mold Design Requirements

 

The mold is an important tool in pultrusion molding technology. The basic requirements for molding mold design are as follows:

 

(1) The ratio of the cross-sectional area of ​​the molding mold to the cross-sectional area of ​​the product should generally be greater than or equal to l0. The purpose is to ensure that the mold has sufficient strength and rigidity; the second is to ensure that the heat distribution is uniform and stable after heating;

 

(2) The length of the pultrusion mold is related to the resin curing speed, the mold heating conditions and the pultrusion molding speed. The most important thing is to ensure that the product reaches a certain degree of curing when it is pulled out. The mold length can be selected between 500-1500mm. Except for thin-walled products and small rods, ordinary pultrusion product molds can be designed according to a length of 800-900mm to meet normal production requirements.

 

2. During the mold processing, the requirements for cavity processing accuracy should be emphasized, mainly including the following aspects:

 

(1) Mold deviation is an indicator that affects the quality of the product mold line, and is generally required to be less than ±0.05mm.

 

(2) The roughness of the mold cavity is an important indicator that directly affects the appearance of the product. For pultrusion molds, the roughness requirement should be above 0.2; the mold after processing should be bright and flat, without obvious scratches, processing marks, black spots, and smooth to the touch, with no burrs on the edges.

 

(3) The mold cavity hardness is generally required to be greater than HRC50. This indicator directly affects the service life of the mold. The higher the hardness, the more products the mold can draw. Usually, the pultrusion mold should have a service life of 50,000-80,000 meters.

 

(4) The cavity straightness requirement is less than ±0.55mm. This indicator must be strictly controlled to ensure that the pultrusion mold can smoothly pull out the product. Molds with insufficient straightness accuracy often have a phenomenon: that is, under normal process parameter settings, there will be a blockage phenomenon without any signs. The reason is often that the thickness of the mold outlet is smaller than the thickness of the inlet, which causes the product to be strongly squeezed near the mold outlet after solidification in the mold, resulting in severe wear on the product surface and even failure to eject the mold.

 

(5) When designing the mold cavity size, the shrinkage rate should be taken into consideration and enlarged at a certain ratio. Under different formulas and process parameters, the shrinkage rate will vary, so this value cannot be determined in general, but should be formulated based on the product precision requirements and specific process parameters combined with the operator’s experience. At the same time, the tolerance of the basic size of the cavity should be determined.

 

(6) In addition to the above more important indicators, the designer should also consider the mold temperature measurement hole setting, the position of the mold line, the positioning slot setting, the mold inlet and outlet chamfering and other related factors. These designs should be specifically formulated according to the product process parameters and production equipment conditions.

 

2. Process Parameter Formulation

The pultrusion process parameters mainly include curing temperature, curing time, traction tension and speed, and the number of yarn balls.

 

(1) Molding Temperature

 

During the pultrusion process, the changes that occur when the material passes through the mold are the most critical. Generally speaking, it is believed that after the glass fiber is impregnated with resin, it passes through the heated metal mold. According to its different states in the mold, the mold is divided into three parts, namely the preheating zone, the gel zone and the curing zone. Three pairs of heating plates are used on the mold to heat separately, and the temperature is controlled by a computer. The separation point refers to the point where the resin separates from the mold. During the heating process, the temperature of the resin gradually increases and the viscosity decreases.

 

After passing through the preheating zone, the resin system begins to gel and solidify. At this time, the viscous resistance at the interface between the product and the mold increases, and the boundary condition of zero velocity on the wall is broken. At the separation point, the resin has a sudden change in speed. The resin and the reinforcement material move evenly at the same speed. In the curing zone, the product continues to solidify under heat to ensure that there is sufficient curing degree when it is ejected from the mold.

 

(2) Determination of Temperature

 

The heating conditions of the mold are determined according to the resin system and the type of curing agent. Taking the polyester resin formula as an example, the resin system is first dynamically scanned by a differential scanning calorimeter (DSC) to obtain the exothermic peak curve. Generally speaking, the mold temperature should be greater than the exothermic peak of the resin, and the upper temperature limit is the degradation temperature of the resin. At the same time, the gel experiment of the resin should be carried out, and the temperature, gel time and pultrusion speed should be matched. The temperature of the preheating zone can be lower, and the temperature of the gel zone and the curing zone is similar. The temperature distribution should make the product curing exothermic peak appear in the middle of the mold, and the gel curing separation point should be controlled in the middle of the mold. Generally, the temperature difference of the three sections is controlled at about 20-30℃, and the temperature gradient should not be too large.

 

Pultrusion Speed

 

The length of the pultrusion die is generally 0.5-1.2m. The mold temperature is preliminarily determined by the curing exothermic curve of the resin system, and other relevant factors must be fully considered at this temperature.

 

Under certain temperature conditions, the gel time of the resin system is very important for the determination of the process parameter pultrusion speed. Generally speaking, the selection of the pultrusion speed should fully consider the gel curing of the product in the middle of the mold, which directly affects the quality of the product, and the surface of the product will have a thick, uncured resin layer; if the pultrusion speed is too slow, the profile stays in the mold for too long, the product is over-cured, and the production efficiency is reduced.

 

The typical experimental pultrusion speed is about 300mm/min. When the pultrusion process starts, the speed should be slowed down and then gradually increased to the normal pultrusion speed. The general pultrusion speed is 300-500mm/min. One of the development directions of modern pultrusion technology is high speed. At present, the fastest pultrusion speed is said to be 1.5m/min. However, there are no manufacturers in China that can achieve this level.

 

(4) Traction

 

The traction is the key to ensure the smooth demolding of the product. The magnitude of the traction is determined by the shear stress on the interface between the product and the mold. The shear stress on the above interface can be measured by measuring the traction of the resin-impregnated reinforcing fiber being pulled through a short distance of the mold. The traction is very important in process control. If you want to make the surface of the product smooth during molding, the shear stress at the detachment point of the product 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. It is related to many factors, such as: fiber content, product geometry and size, release agent, temperature, pultrusion speed, etc.

 

(5) Correlation of Various Pultrusion Process Variables

 

Among the three process parameters of heating temperature, pultrusion speed and traction force, temperature is determined by the characteristics of the resin system and is the primary factor that should be solved in the pultrusion process. The temperature values ​​of each section of mold heating are determined by the peak value of the DSC curve of the resin curing system and related conditions. The principle of determining the pultrusion speed is the gel time at a given mold temperature to ensure that the product gels and solidifies in the middle of the mold. There are many constraints on traction force, which is closely related to the mold temperature and is controlled by the pultrusion speed. The increase in pultrusion speed directly affects the size of the shear stress, that is, the shear stress at the breakaway point, which leads to an increase in traction force.

 

The mold temperature distribution determined by the exothermic peak curve of the resin system curing is the premise for us to determine other process parameters. The pultrusion speed selected must match the temperature. When the mold temperature is high, the traction speed should be appropriately increased. The gel point of the resin can be determined by adjusting the mold temperature and traction speed. When the mold temperature is too high or the reaction is too fast, it will cause thermal cracking of the product. Therefore, by using zoned heating molds to divide the heating area into preheating zone, gel zone and curing zone, the purpose of optimizing the pultrusion process and reducing product thermal cracking can be achieved.

 

In order to improve production efficiency, the pultrusion speed is generally increased as much as possible. For thicker products, a lower pulling speed or a longer mold should be selected, and the mold temperature should be increased. The purpose is to make the product better solidified, thereby improving the performance of the product.

 

 

Section 6 Commissioning and Acceptance of Equipment Overhaul

 

1. Skill Requirements

Be able to formulate quality acceptance standards after overhaul, be able to check whether the equipment status meets the standards item by item according to the standards, and be able to correctly fill in the acceptance report.

 

1. Related Knowledge

 

(I) Knowledge of acceptance and archiving of equipment overhaul

From the date when the new pultrusion equipment is manufactured, the manufacturer must prepare for the equipment acceptance. The acceptance of equipment is divided into acceptance at the manufacturer’s factory and acceptance after the equipment is installed and put into operation. Generally speaking, the following principles should be followed:

 

1. Preparation before Equipment Acceptance

After signing the equipment purchase contract, the user unit should pre-arrange or train full-time technical personnel to be familiar with the technical information provided by the manufacturer. For equipment with special requirements, the user unit should send technical personnel to prepare for the installation of the equipment according to the requirements of the purchased equipment for operating conditions.

 

Capable manufacturers can set up a special acceptance team and formulate an acceptance plan. If there are difficulties in installation and acceptance, relevant experts and engineering and technical personnel can be invited to assist in installation and acceptance.

 

2. Acceptance Requirements

 

(1) Appearance Inspection

 

1. Check whether the packaging inside the equipment is intact and whether there is any damage, bruise, deformation, etc.

 

1. Check whether the equipment and accessories are damaged, rusted, bruised, etc.

 

1. If the above problems are found, detailed records should be made and photos should be taken for evidence.

 

(2) Quantity Acceptance

 

1. Based on the supply contract and packing list, check the specifications, models, configurations and quantities of the main machine and accessories, and check and verify them piece by piece.

 

1. Carefully check whether the accompanying materials are complete, such as equipment manual, operating procedures, maintenance book, product inspection certificate, etc.

 

1. Make a record of quantity acceptance, indicating the acceptance location, time, personnel, box number, product name, expected and actual quantity, etc.

 

(3) Quality Acceptance

 

1. The installation and test machine should be carried out strictly in accordance with the provisions and procedures of the contract terms, instrument manual and operation manual.

 

1. Check the instrument manual and carefully conduct various technical parameter tests to check whether the technical indicators and performance of the equipment meet the requirements.

 

1. Keep careful records during quality inspection. If there are any quality problems with the equipment, the supplier should be notified of the details in writing. Decide whether to return, replace or ask the manufacturer to send someone to inspect and repair it according to the situation.