RTM Process Quality Control – Carbon Fiber Composite Material Manufacturing and Processing

 

The factors that affect the product in the advanced composite material molding process are very complex. The traditional process can only follow experience because it is difficult to obtain information about the changes in the internal state of the processed material. Once the raw materials and working environment fluctuate slightly, the performance of the composite material product will expose shortcomings such as poor repeatability and large data discreteness of the furnace parts. Therefore, strict quality control must be carried out on the advanced composite material molding process.

 

1. Preparation and Management of Process Documents

 

Process documents are an important part of technical documents. The preparation and management of process documents are the basis of product quality control and an important part of enterprise quality management. Process documents or specifications are the main basis for product production, and are the basis for guiding production operations, compiling production plans, mobilizing labor organizations, arranging material supplies for product technical inspections, tooling design and manufacturing, etc.

 

Figure 1. Process Document Preparation Control Flow Chart

 

Process documents mainly include process document directory, process master plan, process route table, key process details of various process regulations, special process equipment details, outsourced parts details, material consumption process quota documents, etc. The process document directory refers to the directory of the entire set of documents. The most important thing is to indicate the current valid version of each document.

 

The process master plan is a guiding document that proposes process technology preparation, specific work tasks and measures based on product design requirements, production types and production capacity. It is the basis for compiling process regulations and formulating process technology measures plans. The process route table stipulates what to do first and what to do later. If necessary, it provides the operators in these processes and the quality requirements for the operators, how many people are needed, how much time each process takes, what are the key points of operation, what standards to achieve, and what special tools to use. All of these are based on the flow chart.

 

The process specification is specific to each link, usually used by operators, and stipulates what materials and tools to use, what matters to pay attention to during operation, and what standards to achieve in execution. More content is the sequence and method of operation steps. Material consumption process quota refers to the standard of material quantity of certain varieties and specifications required by production units to produce qualified products under the conditions of saving and rational use of materials, including the amount of material used and the necessary process loss and waste. The formulation of material consumption quota is mainly to use the quota as an economic lever to control and supervise material consumption, so as to achieve the purpose of reducing material consumption and engineering costs.

 

The preparation of process documents should be correct, complete, consistent and clear, and can effectively guide production and ensure stable production quality. The preparation format, signing, modification, and marking of important parts of process documents should all comply with relevant standards

 

The management of process documents mainly includes:

① Preparation and signing of process documents;

② Borrowing and use of process documents;

③ Modification of process documents;

④ Confidentiality of process documents.

 

2. Raw Material Quality Control

 

Raw material quality control mainly involves the acceptance of raw materials entering the warehouse and the inspection of raw materials leaving the warehouse (before use). Different raw materials should establish independent material specifications to ensure the qualification of raw materials entering and leaving the warehouse. In RTM process manufacturing, the main raw materials are fibers (fabrics), resins and curing agents. The fluctuations in the composition and performance of the fiber and resin curing system directly affect the quality of the product. When accepting fibers, the tensile strength, tensile modulus, breaking strain, density and twist of the fibers are generally tested; if the fibers have other special uses, the corresponding test indicators and test methods should also be added to the specifications.

 

When accepting resins and curing agents, according to traditional methods, only the density, viscosity or softening point, moisture, volatiles, epoxy equivalent, etc. of the resin are generally measured. Facts show that these indicators are far from controlling the chemical composition of the resin, and even different types of epoxy resins can have the same viscosity (or softening point) and epoxy equivalent. People have gradually begun to pay attention to controlling the chemical composition and chemical characteristics of resins, and there is a trend to include them in material standards as control indicators. At present, the commonly used methods for identifying the chemical composition of resins at home and abroad include gel permeation chromatography, high performance liquid chromatography, infrared spectroscopy, elemental analysis, etc.

 

3. Process Control

 

Composite material molding has the characteristics of material formation and component molding being completed at the same time, so the process control directly determines the quality of composite material products. The quality of the process can be controlled from two aspects: one is to control the operation of each process to fully comply with the standard action, and each process parameter meets the standard requirements; the other is to strengthen the inspection between processes. When there are items that do not meet the specifications in the previous process, they will never flow to the next process before the problem is solved.

 

These are two different types of control. The former directly controls the factors that affect product performance and is a positive aspect; the latter helps to understand whether the operation and process parameters are correctly mastered and prevent unqualified products from flowing into the next process, so it is also an indispensable process control.

 

Each monitoring point in the RTM process must be permanently recorded in the acceptance record book so that quality problems can be analyzed and responsibilities can be determined when quality problems occur in the product. This monitoring point should be set up at the key control point of the process. For the RTM process, the key control points are:

 

(1) Before the fiber is pre-shaped, the inspector needs to verify the accuracy of the layer reduction

 

(2) Before glue injection, check whether the process parameters of the glue injection machine (such as mixing ratio, glue liquid temperature, glue injection rate, glue injection pressure upper limit setting, etc.), mold temperature, and vacuum leak detection meet the process specification requirements;

 

(3) After the glue injection is completed, whether the heating rate, curing temperature, and insulation time during the curing process meet the process specification requirements.

 

In the RTM process, the Inspections Between Processes Mainly Include:

 

(1) Fiber preforming process, that is, when receiving materials such as fiber fabrics and setting agents, it is necessary to check whether the raw material identification meets the specification requirements, such as whether there are material brands, specifications, suppliers, batch numbers and production dates, acceptance certificates, etc.;

 

(2) Glue injection process, that is, whether the semi-finished product (preform) is transferred from the previous process or received from the warehouse, it is necessary to check whether the preform meets the specification requirements, such as whether there is any form of fiber movement, bridging, wrinkles or sliding on the surface that exceeds the specification;

 

(3) Post-processing process, whether the semi-finished product transferred from the previous process has any form of defects that exceed the specification, such as dry spots, bubbles, etc.

 

4 Finished Product Inspection

 

Finished product inspection is also called final inspection. It is the last checkpoint of quality control of composite parts and is an activity (inspection, testing, measurement or test) that provides the basis for the final release of products (including services). For composite parts, in addition to checking the appearance, size and weight, non-destructive testing methods are also used to check whether there are defects inside the parts that do not meet the requirements.

 

Visual appearance inspection can find common defects of RTM composite parts, such as resin-rich layers, resin cracking, preform movement, wrinkles, wrinkles, dry spots, surface bubbles, etc. Currently, the methods that can be used for non-destructive testing of composite materials include ultrasonic testing, radiographic testing, laser speckle testing, acoustic emission, etc.

 

Usually one or several non-destructive testing methods should be selected according to the characteristics of the material and structure and the design requirements, considering both the effectiveness and reliability of the test, as well as the practicality and economy.