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Appearance Defects And Process Control Of Composite FRP Pultruded Products

Jun 10, 2020

Defects in the pultrusion process

(1) Bird's nest: The reinforcing fibers are entangled with each other at the entrance of the mold, causing the product to be destroyed in the mold.

Reason analysis: ① The fiber is broken; ② The effect of fiber drape; ③ The resin viscosity is high; ④ The fiber adheres too much resin; ⑤ The traction speed is too high; ⑥ The design of the mold inlet is unreasonable.

(2) Unstable curing performance: Sudden increase in adhesion in the mold can cause product damage in the mold.

Reason analysis: ①The traction speed is too high; ②The hot resin caused by pre-curing suddenly flows back.

(3) Sticky mold: Some products adhere to the mold, causing the product to stretch and break.

Reason analysis; ① The volume fraction of the fiber is small, and the filler is added in a small amount; ② The effect of the internal release agent is not good or the dosage is too small.

(4) Scaling: Poor surface finish.

Analysis of the reasons: ① The stress at the detachment point is too high, causing creeping creep; ② The detachment point is too advanced before the solidification

(5) Incomplete curing: the vapor pressure of the styrene monomer is too high or the condensate is too much, and the product will crack when the styrene flashes.

Reason analysis: ①The speed is too fast; ②The temperature is too low; ③The mold is too short.

(6) Local curing: The internal curing of the profile lags far behind the curing of the profile surface, causing internal cracks in the product.

Reason analysis: The product is too thick.

(7) White powder: white powder is attached to the surface of the product after the product is out of the mold.

Reason analysis: ①The surface finish of the mold is poor; ②When demoulding, the product sticks to the mold, causing damage to the surface of the product.

(8) There are droplets on the surface of the product There is a layer of viscous liquid on the surface layer after the product exits the mold.

Reason analysis: ① The product is not cured completely, the temperature is low or the pulling speed is too high; ② The fiber content is small, the shrinkage is large, and the uncured resin is ejected; ③ The temperature is too high, which degrades the resin on the surface of the product.

(9) Groove marks and unevenness: The flat part of the product is uneven, and there are groove-like marks on the part.

Reason analysis: ①The fiber content is low, and the local fiber yarn is too little; ②Mold sticky products and scratched products.

(10) White spots: The surface layer of products containing surface felt and continuous felt often appears partially whitish or exposed with white yarn.

Reason analysis: ①The resin impregnated by the yarn felt is not complete, the felt layer is too thick, or the performance of the felt itself is not good; ②There are impurities mixed in, forming bubbles between the felt layers; ③The resin layer left on the product surface is too thin.

(11) Cracks: There are tiny cracks on the surface of the product.

Reason analysis: ① The crack is only on the surface layer, and the resin layer is too thick to produce the surface crack; ② The uneven curing of the resin causes thermal stress concentration and stress cracking, which is deep.

(12) Surface fluff: Fibers are exposed on the surface of the product.

Reason analysis: ① Too much fiber; ② Resin and fiber cannot be fully bonded, and the coupling agent is not effective.

(13) Surface peeling and breaking

Reason analysis: ① The resin layer is too thick; ② The pressure in the molding is not enough; ③ The fiber content is too little.

(14) Bending and twisting of products

Analysis of the reasons: ①The product is unevenly cured and is not synchronized, resulting in curing stress; ②The pressure of the product is reduced after the mold is released, and it deforms under the stress; ③The material in the product is uneven, resulting in different degrees of curing shrinkage; ④The product is not It is completely cured and deformed under the action of external traction.

(15) Product corners

Reason analysis: ① The fiber content is insufficient; ② The matching precision between the upper and lower molds is poor or scratched, resulting in the formation and accumulation of solidified materials on the mold clamping line, resulting in missing corners and edges.

The importance of pultrusion process parameters

The forming temperature, pultrusion speed, and traction force are the three most important process parameters of the pultrusion process. It is more difficult to control the molding temperature.

The method to solve the defects of pultrusion FRP products is to analyze the causes of the defects and gradually adjust various process parameters, such as: increasing the number of rovings, forming temperature, pultrusion speed, etc. During the pultrusion process, while adjusting and observing, first fine-tuning and groping based on the pre-selected process conditions, and finally obtaining and using the best process parameters.

There are often signs before defects in products, such as: increased traction. From these signs, we can judge and analyze the problems that will occur in the future, and take timely measures. When a defect occurs, it should be shut down immediately to stop production. Restart the production line after suspending the process to eliminate the fault.

At present, the operation of the pultrusion process relies on the operator to select various process parameters and deal with abnormal conditions to monitor product quality. It is particularly emphasized here that the operator creates optimal operating conditions and uses a special system to automate the process. In this way, the operating set point can be optimized, emergency situations can be handled in time, and qualitative and quantitative information can be used to perfectly control product quality.