Common Problems in Glass Fiber Reinforced Plastic Vacuum Infusion Process

 

The types of defects that usually exist in composite laminates using vacuum infusion technology can be divided into two categories, namely internal quality problems and appearance quality problems

 

Internal Quality Defects

 

Internal quality defects mainly include pores, delamination, inclusions and resin-rich, etc. From the microscopic morphological analysis, pores and delamination defects are mainly caused by the introduction of external gas.

 

Possible causes include fiber moisture absorption, incomplete resin degassing, vacuum system leakage, unreasonable setting of penetration or curing temperature parameters, etc.

 

Inclusion defects are mainly caused by foreign matter falling into the dry fiber stacking process.

 

The possible causes of resin-rich phenomenon are mainly unreasonable process parameter settings, fiber misalignment, stacking or bridging during laying, etc., as follows:

 

Defect Types and Cause Analysis

 

Blowholes/Delamination

 

(1) Fiber moisture absorption

 

(2) Incomplete resin degassing

 

(3) Vacuum system leakage

 

(4) Improper setting of penetration or curing temperature parameters

 

Inclusions

 

Foreign Matter Dropped during Paving

 

Rich Resin

 

(1) Improper setting of process parameters during resin penetration

 

(2) Fiber misalignment, stacking or bridging during paving

 

From the above table, it can be seen that inclusions and rich resin are mainly related to the quality control of the paving process, while other defect types are mainly related to the control of liquid molding process parameters.

 

External Quality Defects

 

 

The appearance quality defects of the parts mainly include excessive thickness uniformity, scratches on the surface, fiber wrinkles, etc. Among them, the reasons for excessive thickness uniformity are relatively complex.

 

The main factors are: insufficient uniformity of pressure applied to the preform, vacuum bag bridging in the package, resin accumulation during resin penetration, fiber rebound, etc.; surface scratches are mainly caused by human factors; and fiber wrinkles are caused by the state of the preform, insufficient uniformity of pressure applied to the preform, and other reasons for process defects, such as interlayer inclusions, foreign matter on the film surface, etc., as follows:

Defect type and cause analysis

 

Excessive Thickness Uniformity

 

(1) Resin accumulation during penetration

 

(2) Preform Insufficient pressure uniformity

 

(3) Fiber rebound

 

(4) Vacuum bag bridging

 

Surface Scratches

 

(1) Artificial scratches during demoulding, movement, and processing

 

Fiber Wrinkles

 

(1) Lack of setting agent between preform layers

 

(2) Lack of pressure uniformity control measures

 

(3) Interlayer inclusions and foreign matter on the film surface

 

Process Defect Control

 

Influence and Control of Fiber Moisture Absorption on Molding Quality

 

Dry fiber moisture absorption has a serious impact on the internal quality of the molded composite laminate, and visual defects will appear on the appearance.

 

The defects are mainly caused by the vaporization of moisture absorbed in the fiber during the curing and heating process; in addition, by measuring the surface, even if the non-destructive testing is normal, the glass transition temperature of the composite laminate will drop significantly, indicating that the residual water molecules in the fiber destroy the resin curing and cross-linking reaction.

 

Therefore, in the process of liquid molding of composite parts, fiber moisture absorption control is a process link that must be considered. The storage and cutting of fiber raw materials, the laying, preparation, and packaging of fiber preforms must be carried out in a controllable environment.

 

The Influence and Control of Resin Degassing on Molding Quality

 

If the resin is not degassed, the appearance of the composite material formed by vacuum infusion technology will not change significantly. However, it will have a significant impact on the internal quality of the molded composite laminate, mainly with defects of local pore density.

 

The degree of influence of resin degassing on the molding quality of parts is related to the molding method, the size and structure of the parts. The simpler the molding process control, the larger the size of the parts, and the more complex the structure, the greater the impact of the residual gas in the resin.

 

Therefore, in the process of liquid molding composite materials, sufficient degassing of the resin is a necessary process step. In order to ensure that the resin is fully degassed, static vacuum can be drawn when the resin is in a low viscosity state, and the vacuum degree is generally not less than -0.095Mpa, and the time is not less than 30min; in addition, when molding larger parts, more resin is needed.

 

It is difficult to fully degas the resin by only using static vacuuming of the resin. The resin can be dynamically degassed by circulating vacuuming or stirring degassing vacuuming to ensure sufficient degassing of the resin.

 

 

The Influence and Control of Vacuum System Leakage on Molding Quality

 

The leakage of the vacuum system is the most common process defect in the process of making composite materials by vacuum infusion process. The penetration type can be divided into three types according to the location: resin inlet pipeline leakage, vacuum bag leakage, and vacuum source (outlet pipeline) leakage. The details are shown in the figure above

 

Inlet Pipeline Leakage

The inlet pipeline leakage test is divided into two cases: slight leakage and severe leakage. The system vacuum range for slight leakage is: -0.095 to -0.1Mpa, and the vacuum range for severe leakage is ≤-0.08Mpa.

 

When the Vacuum Degree Remains basically Unchanged (-0.095 to -0.1Mpa), a small leakage in the glue feed line during the vacuum infusion molding process has a certain impact on the internal quality of the final composite laminate molding. The main defect type is relatively dispersed small pores that do not exceed the standard, but this type of internal defect cannot be identified from the appearance. The cause of the defect is that the resin is mixed with external air when it passes through the leakage point of the glue feed line during the infiltration process.

 

When the Leakage of the Glue Feed line Seriously Affects the Vacuum Degree of the System (≤-0.08Mpa), the resin cannot be completely impregnated into the fiber preform, and large dry spots can be observed visually on the parts after molding. The reason for the defect is that when there is a serious leakage in the glue feed line, a large amount of gas entering the glue feed line forms a cavity area between the resin source and the preform parts, which hinders the resin from entering the preform, so the resin cannot achieve complete impregnation of the fiber preform.

 

Vacuum Bag Leakage

Vacuum bag leakage is also divided into two types: minor leakage and severe leakage. The system vacuum range is -0.095 to -0.1Mpa for minor leakage and ≤ -0.08Mpa for severe leakage.

 

When the System Vacuum Remains Basically Unchanged (-0.095 to -0.1Mpa), minor leakage of the vacuum bag during the liquid molding process has a significant impact on the internal quality of the final composite laminate. The prepared composite laminate shows obvious bottom wave attenuation in non-destructive testing, and there are dispersed dense pores. The average porosity value also exceeds the indicator of general composite components by 1.5%; in addition, obvious point defects are observed from the appearance.

 

When the Vacuum Bag Leakage Seriously Affects the Vacuum Degree (≤-0.08Mpa), although the resin can be impregnated into the fiber preform, the large amount of gas flowing into the vacuum bag seriously hinders the resin from impregnating the preform, and the low pressure causes serious defects such as looseness and delamination in the molded parts. Serious defects can be found from non-destructive testing and external observation.

 

Vacuum Source Leakage

The system vacuum degree of the vacuum induction process cannot be lower than -0.095Mpa. When the vacuum source leakage is reduced to -0.08Mpa, the molding quality of the composite laminate will be significantly reduced. The bottom wave attenuation phenomenon can be seen in the non-destructive testing and dense pores can be detected. The average porosity value is close to the maximum porosity index of general composite materials, 1.5%, and the thickness uniformity shows a downward trend.

 

When the vacuum source leakage is further reduced to -0.06Mpa, although the resin can be impregnated into the fiber preform. However, the prepared composite laminates have serious bottom wave attenuation in non-destructive testing, and there are defects such as dispersed dense pores. Obvious insufficient areas can be observed on the surface, and the thickness uniformity is further reduced.

 

Therefore, vacuum leakage inspection of the system is one of the key links in the vacuum induction process. The conventional detection method is:

 

Observe the System Vacuum Reading, Which is not Less than -0.095Mpa;

 

Turn Off the System Vacuum Source and Observe the Change Value of the System Vacuum Reading.

 

The change value cannot be greater than 0.017Mpa/5min. Once the system vacuum is lower than -0.095Mpa or the reading change value exceeds the range, it is necessary to perform partition inspections from multiple aspects such as the resin storage tank, resin pipeline, vacuum bag, resin glue tank and vacuum source until the vacuum leakage is eliminated.