Knowledge on Quality Inspection and Repair of FRP Pultrusion Products

 

Section 1 Product Appearance and Dimension Inspection

 

1. Skill Requirements

 

(I) Be able to visually inspect whether the appearance of the product, such as shape and color, meets the requirements of the production task book, identify surface defects of the product, determine the type of surface defects and control their use, and determine defects such as pinholes, cracks, and poor curing of the product.

(II) Be able to use measuring tools such as rulers and vernier calipers to measure the size of the product and confirm whether it is within the error range.

(III) Be able to inspect the quality of FRP pultruded products, analyze the causes of defects such as burrs, bubbles, cracks, and incomplete curing, and propose solutions.

 

1. Related Knowledge

 

(I) Surface Characteristics of Pultruded Products

 

Qualified pultruded products should have a smooth and flat surface without defects such as pores, cracks, bubbles, delamination, and whitening. However, the appearance of pultruded products will also show different effects when using different reinforcing materials. The surface of pure glass fiber products should be the best, with a delicate and smooth feel. However, due to considerations of lateral strength, most pultruded products will be laid with materials such as stitched felt and continuous felt on the surface. This surface appears rough due to the random arrangement of stitching lines or chopped fibers on the felt. In this case, a layer of surface felt can be added to the outermost layer. In addition to improving the appearance of the product, it can also reduce the friction between the pultruded product and the mold and improve the corrosion resistance of the product.

 

 

(II) Knowledge of Surface Defects of Pultruded Products

 

The inspection contents of pultruded products mainly include: appearance inspection, integrity inspection and dimension inspection. There are three main reasons for defects in pultruded products: material composition, process parameters and process methods. Material composition refers to defects caused by quality factors such as resin formula, roving, glass fiber mat, surface mat, etc.; process parameters refer to defects caused by improper settings of parameters such as mold temperature and pultrusion speed; process methods refer to defects associated with resin impregnation method, preforming mold, forming mold and pultrusion equipment. See the table below for inspection and defect classification of pultruded products.

 

 

(III) Use of Dimension Measuring Instruments

 

In Pultruded Products, the Following Measuring Instruments are Commonly Used:

 

1. Box ruler is used to measure the length of pultruded products. The measuring range is generally 3m, 5m, etc., and the accuracy is 1 mm.
2. Vernier caliper is used to detect the contour dimensions of the product, such as thickness, width, etc. The measuring range is generally 250mm, and its accuracy is 0.02mm.
3. Wide angle ruler is used to detect the verticality between the cutting surface and the length direction of the product.
4. Square ruler is used to check the deviation of the internal and external right angles of the product. It is marked with scales and can also be used in post-processing line drawing work.
5. Level ruler is generally used when the mold is leveled to ensure the consistency between the mold and the traction table.

 

(IV) Quality Inspection Technology of Pultruded Products

 

The quality inspection content of pultruded products is relatively large, including appearance, dimensional accuracy, performance indicators and other aspects.

 

1. Appearance mainly refers to whether the product color is uniform, whether there are cracks on the surface and end face of the product, whether the surface is smooth, etc. Appearance is the most basic and most direct inspection item for pultruded products, and generally adopts visual inspection method.

 

2. Dimensional accuracy mainly refers to whether the thickness and angle of each part of the product cross section are within the tolerance range required by the customer. It is generally measured using a vernier caliper and a square ruler. Products with higher requirements will also strictly control the flatness and straightness of the product, which requires sending it to a special testing agency for testing. If it is a large-scale production, a tooling for measuring flatness and straightness can also be specially made to facilitate inspection at any time during daily production to ensure product quality.
3. Performance indicators There are many performance test items for pultruded products. Generally, standard samples are processed and tested in accordance with national standards. There are special testing agencies.

 

 

Section 2 Analysis, Judgment and Repair of Product Surface Defects

 

1. Skill Requirements

Be able to judge bubbles, pinholes, cracks and incomplete curing caused by factors such as on-site conditions and process parameters recorded in the test records, and be able to use tools and auxiliary tools to repair the identified problems.

 

1. Related Knowledge

 

(I) Analysis and Judgment of Product Defects

 

1. Knowledge of defects and corrections of pultruded products

In pultrusion production, the operator should pay close attention to the online quality of the product, respond promptly when the product appearance and performance are abnormal, observe whether the production site conditions have changed, whether the process parameters are stable, etc., and make adjustment plans. During the production process, the types and causes of defects in pultruded products are analyzed as follows:

 

(1) Fiber and Felt Accumulation on the Surface

Result: The reinforcing materials are entangled and squeezed at the entrance of the mold, resulting in increased resistance to the product in the mold, and in severe cases, the product will be damaged. Causes:

1. Fibers are interrupted and accumulated.
2. There are too many elastic strands in the yarn bundle.
3. The fiber viscosity is high, resulting in excessive cavity pressure.
4. The pulling speed is too high.
5. The design of the mold entrance is unreasonable, and the reinforcement material is not smoothly fed into the mold.
6. The design of the preforming system is unreasonable, resulting in the reinforcement material not moving smoothly.

 

(2) The Product is not Sufficiently Cured

Result: The product has a dull appearance and insufficient strength. Cause analysis:

1. The traction speed is too fast.
2. The temperature setting is too low.
3. Insufficient amount of curing agent.

 

(3) The Surface is Sticky to the Mold and Local Peeling Occurs

Result: The product is partially adhered to the mold, resulting in increased resistance and a sharp increase in traction tension, which eventually blocks the mold.

Cause analysis:

1. The fiber content is too low or the amount of filler added is small.
2. The internal release agent is not effective or the amount is too small.
3. The speed is too fast.
4. The temperature setting of the front zone is too high.

 

(4) The Surface is Scaly

Result: The surface finish is poor.

 

Cause Analysis:

1. The stress at the detachment point is too high, resulting in creeping.
2. The detachment point is too advanced from the curing point.

 

(5) Cracks Appear on the Surface or Inside of the Product.

Result:

poor appearance and reduced performance. Cause analysis: a. The speed is too fast, and the product is not cured consistently inside and outside.

1. The temperature setting is unreasonable, and the product is cured too violently.
2. The mold is too short, and the product is not cured well.
3. The matching between the reinforcing material and the matrix material is poor.
4. The matrix material has poor adhesion.
5. The product is thick and the curing heat is too high.

 

(6) Powder Attached to the Surface of the Product.

Result:

The resistance inside the mold increases, resulting in mold blockage. Cause analysis: a. The surface finish of the mold is poor, or the coating is partially peeled off. b. There is sticking in the mold, resulting in damage to the surface of the product.

 

(7) Liquid and Sticky Substances Seep Out of the Surface of the Product.

Result:

The product has poor appearance and surface cracks. Cause analysis: a. The product is not fully cured, the temperature is low or the pulling speed is too fast. b. The fiber content is low, the shrinkage is large, and the uncured resin is sprayed out. c. The temperature setting is unreasonable, and the curing inside and outside is not synchronized.

 

(8) The Surface is Uneven and has Dark Grooves

Phenomenon: The product appearance is poor. Cause analysis:

1. The fiber content is low in some parts.
2. There is a sticky film phenomenon.
3. The surface felt layer is folded.

 

(9) White Yarn or Whitening Appears on the Surface

Result: The product surface is poor.

 

Cause Analysis:

1. The yarn and felt are not fully impregnated, the felt layer is too thick or the impregnation performance of the felt material is poor.
2. Impurities are mixed in, forming bubbles between the felt layers.
3. The fiber content is too high.

 

(10) Surface Fuzzing

Result: The fibers are exposed on the surface.

 

Cause Analysis:

a: The fiber content is too high.

b: The resin and fiber are not well matched, and the impregnation is poor.

 

(11) The Product is Bent, Twisted and Deformed

 

Cause Analysis:

a: The product solidifies asynchronously, generating solidification stress.

b: The pressure of the product decreases after it is ejected from the mold, and it deforms under the action of stress.

c: The materials of various parts of the product are uneven, resulting in inconsistent solidification shrinkage.

d: The product is not completely solidified when it is ejected from the mold, and it deforms under the action of external traction.

e: The relative position of the mold and the traction direction needs to be adjusted.

f: The temperature and speed parameter settings do not match.

 

(12) The Product is Missing Corners

Cause Analysis:

a: The local fiber content is insufficient.

b: The matching accuracy between the upper and lower molds is poor or there are scratches, resulting in knots and accumulation on the load and mold lines, causing the product to be missing corners and less deformed.

 

(II) Pultrusion Product Repair Knowledge

For thermosetting pultrusion products, under certain process parameter settings, after the product is pulled out of the mold, the product’s appearance structure and internal performance basically become an unchangeable state, and there is little room for repair. Of course, the appearance defects can be improved by adopting certain remedial measures such as spraying treatment, but it will greatly increase the cost of the product. If the customer does not have special requirements, this method is generally not recommended. For internal defects, they can only be improved by adjusting the process parameters. Therefore, for normal pultrusion products, all the operator can do is to perform appropriate post-processing to make the product more perfect.

 

1. Two-Stage Sealing of the Product

Some products need to be used in a relatively harsh corrosive environment. In order to increase the service life of the product, the product section needs to be sealed to reduce the damage to the product performance caused by the corrosive medium through the cracks or defects in the section. The sealing material can be ordinary resin.

 

2. Methods for Cleaning Burrs of Pultruded Products

Pultruded burrs are generally rare, and most of them appear on the cut end surface of the product. In the post-processing process, they can be properly repaired with tools such as round files and flat files, or polished with sandpaper.

 

(III) Product Inspection Report

1. Skill Requirements

Be able to reasonably sample the product for inspection and be able to write a product quality report.

 

2. Related Knowledge

(1) Pultruded Product Inspection and Sampling Rules

Product inspection rules are a method and means used to assess and determine whether the product quality meets the standard requirements. It is a basic principle that product manufacturing departments and users jointly abide by to determine whether the product is qualified. In most of my country’s product standards, inspections are unified into two categories: type inspection (routine inspection) and factory inspection (delivery inspection). In all product standards, the rules and test items for factory inspection should be specified, while for type inspection (routine inspection), its conditions, rules and test items should be clearly specified in the specified product standards.

 

Generally speaking, type inspection is a comprehensive inspection of various quality indicators of products to assess whether the product quality fully meets the standards and whether it meets all design quality requirements. Factory inspection is the final inspection that must be carried out on formally produced products at the time of delivery to check whether the product quality at the time of delivery has the quality confirmed in the formal inspection. Products can only be delivered as qualified products after passing the factory inspection. Factory inspection items are part of the formal inspection items.

 

For pultruded products, formal inspection is carried out in one of the following situations:

 

1. When the material, structure and process are significantly changed during the first production or after normal operation, which may affect the product performance.

 

1. After production, the product structure, material, second process and parameters have changed significantly, which may affect the product performance.

 

1. After 12 months of normal production.

 

1. When the factory inspection results are significantly different from the last type inspection.

 

1. When the national quality supervision agency proposes type inspection requirements.

 

Sampling of pultruded products can be done by random sampling from the inspection batch of products. The number of samples can be determined according to the specific situation of the product, or it can be agreed upon by both the supply and demand parties. If all the specified inspection items of the sample are qualified, then the corresponding batch of products can be judged to be qualified, otherwise they are unqualified.

 

(2) Preparation Method of Pultruded Product Quality Test Report

Whether the pultruded product meets the customer’s requirements, in addition to the manufacturer’s own inspection of its size and appearance, in most cases, the material performance test or the overall performance test of the product must be carried out according to the indicators provided by the customer. The results obtained through the test report can largely reflect the quality level of the product. It is also the basis for proving the qualification of the product. There are many institutions that conduct performance testing of FRP products. If it is a document provided to customers, a larger and authoritative testing department with certain qualifications should be selected.

 

If the manufacturer only uses it to compare the performance of products with different formulas as a technical accumulation or to explore material performance, a universal testing machine can be used for testing. For the preparation of product samples, if standards can be used, try to use them. If there are no standards, a test plan can be made according to the actual situation, but the conditions and environment of multiple tests must be consistent to reflect the performance changes to the greatest extent.

 

3. Quality Control and Improvement

Whether the product can stably achieve the required performance and quality level depends largely on the manufacturer’s efforts in quality control. There are many quality control points in pultrusion production, but due to the investment in test equipment and personnel, few manufacturers have achieved this to the greatest extent, and many manufacturers do not even have the most basic quality control capabilities. In fact, as long as the key links in the quality chain are simply understood, relying on their own efforts and external support (such as strong raw material suppliers), manufacturers with limited conditions can still achieve good results. The quality control points and responsibilities in pultrusion are as follows. If conditions permit, each link should be controlled.

 

 

(1) Raw Material Acceptance

The first step in controlling raw materials is the raw material supplier. Raw material suppliers should provide stable raw materials. The most important focus of pultrusion manufacturers should be to grasp the characteristics of resins. Each batch of resin entering the factory should be sampled and inspected to determine whether it meets the specification requirements. The inspection items include appearance, viscosity, acid value, solid content, and reactivity. The quality records of resin testing should be kept to provide valuable original data for judging process problems.

 

Reinforced plastics are as important as resins in function. Reinforcement materials mainly include roving, continuous felt and various forms of fabrics. For roving, moisture content, combustible content and linear density should be tested. For felt, in addition to testing the unit area mass and adhesive content, attention should also be paid to the problem of uneven fiber distribution. Determining the moisture content of the reinforcing material is very useful for analyzing the causes of certain process quality problems. For fillers, the particle size that can meet the product performance requirements should be selected, and it should be noted that the particle size distribution should not be too wide; for pigments, special attention should be paid to the uniformity of their grinding and dispersion, otherwise it will cause serious color difference in the product. Attention should also be paid to their compatibility with the resin and their impact on the resin reactivity.

 

(2) Raw Materials On The Process Line

The raw materials on the process line refer to the prepared resin mixture and the reinforcing materials that will or are being converted into products. The main focus at this control point should still be on the resin performance. Mix the various components in a mixer to make a uniformly dispersed resin mixture. The temperature should be prevented from rising too much during the mixing process. Before removing the resin from the mixing equipment, the dispersion and contamination of the filler should be observed with the naked eye. Whether the fiber is sufficiently impregnated in the resin tank is extremely important for pultrusion. The viscosity of the resin is the most critical parameter for determining whether it can be sufficiently impregnated. The width of the continuous raw fiber felt in the process should be checked frequently to see if it is appropriate. When using a too narrow felt, exposed coarse yarn will appear on the surface of the component, and other defects may also occur, such as local excess resin and unsightly appearance. When using a felt that is too wide, it may cause the positioning of the reinforcement layer to be uncontrollable because it will fold to adapt to the mold cavity. In addition, since the felt must be pleated to enter the mold, the wear of the mold will increase at the pleated area.

 

(3) Process Parameter Control

The main process parameters of pultrusion molding include cavity temperature, resin temperature, cavity pressure, curing speed, curing degree, traction force and traction speed. The most important of these is cavity temperature. The resin system used for pultrusion is very sensitive to temperature, and the cavity temperature should be strictly controlled. If the temperature is low, the resin cannot be cured. When the temperature is too high, the glue will solidify as soon as it enters the mold, making molding and traction difficult. In severe cases, it will also produce waste and even damage the equipment. The cavity temperature distribution should be low at both ends and high in the middle. The second is cavity pressure. Cavity pressure is caused by the viscosity of the resin, the friction between the product and the cavity wall, the volume expansion of the material caused by heating, and the vaporization of some materials caused by heating. Therefore, cavity pressure is a comprehensive reaction parameter of the product’s behavior in the cavity. Generally, cavity pressure is between 1.7MPa and 8.6MPa. Another important parameter is the pulling speed, which is a parameter that balances the degree of curing and the production speed. The pulling speed should be increased as much as possible while ensuring the degree of curing.