The Influence of Compression Molding on Carbon Fiber Product Quality
Among the various fabrication techniques for carbon fiber products, such as filament winding and extrusion, compression molding stands out as a preferred method for many mechanical components. This process involves curing preimpregnated materials within a mold at a specific temperature, allowing the carbon fiber composite to take shape before the finished product is removed. Ensuring the quality of carbon fiber products in compression molding involves a meticulous consideration of several factors, including the prepreg, press machinery, molds, ambient conditions, temperature, and humidity. The crux of the process lies in the curing of the prepreg within the mold cavity, a result of the interplay between pressure, temperature, and dwell time. Here's an in-depth look at how pressure, temperature, time, and volatiles affect the compression molding process.
Quality Considerations in Compression Molding of Carbon Fiber Products:
1. Molding Duration
The duration of compression molding is influenced by the type of prepreg, volatile content, product geometry, thickness, mold design, molding temperature, and pressure. The length of the molding time significantly impacts the product's performance. Insufficient molding time can result in incomplete curing, leading to subpar physical and mechanical properties, poor surface finish, and susceptibility to deformation. Extending the molding time can reduce shrinkage and deformation, but excessive time can cause over-curing, increasing internal stress within the product. Selecting the optimal molding time is crucial.
2. Volatile Content
The volatile content in the compression molding process greatly affects the flowability of the prepreg. High volatile content can lead to excessive flowability, causing resin loss, bubble formation, and a decline in surface finish. On the other hand, low volatile content can hinder the flowability of the prepreg, complicating the formation of composite products.
3. Molding Pressure
Pressure facilitates the flow of prepreg within the mold cavity, increases density, and counteracts the pressure generated by the release of low molecular weight substances during resin polymerization, preventing swelling and delamination. It also ensures that the mold is properly closed, maintaining the product's dimensions and preventing deformation during cooling.
The required molding pressure is contingent upon the type of prepreg, product shape, and prepreg condition. If the prepreg has low flowability, rapid curing speed, and a high compression ratio, greater pressure is necessary; otherwise, less pressure is needed. In the compression molding process, temperature and pressure are interconnected. Increasing the mold temperature can enhance prepreg flowability, and reducing the molding pressure during the initial stages can achieve the desired outcome.
4. Temperature
The flow, filling, and curing reaction rate of prepreg during compression molding are intimately linked to temperature, which plays a pivotal role in the process. It influences the degree of resin crosslinking and, by extension, the final performance of the composite product.
Empirical evidence suggests that increasing mold temperature can expedite curing speed and reduce curing time. However, excessively high temperatures can lead to rapid curing, which can quickly diminish prepreg flowability and result in incomplete filling, particularly in large, thin-walled, and complex-shaped composite products. When the outer layer cures much faster than the inner layer, it becomes challenging to expel volatiles, which can degrade the product's physical and mechanical properties and lead to defects and deformation. Conversely, low curing temperatures result in slow curing and inadequate curing.
These four factors are paramount in the compression molding process of carbon fiber composite materials. With a well-executed process, the mechanical performance of carbon fiber composite products is significantly enhanced, which is a key reason for the widespread application of carbon fiber across various industries.
