Analysis of Pultrusion Process of Carbon Fiber Epoxy System

 

 

In terms of carbon fiber grading, carbon fiber is divided into T300 ~ T1000, and the higher the grade, the higher the carbon fiber grade.

 

Each grade is divided into 1K, 3K, 6K, 12K, 24K, 48K, 60K, 80K, K represents 1000, that is, 1k represents 1000 monofilaments in each bundle of carbon fiber, and 80k represents 80000 monofilaments in each bundle of carbon fiber.

 

Generally speaking, less than 12k is collectively referred to as small tow carbon fiber, also known as “aerospace grade” carbon fiber, and more than 12k is called large tow carbon fiber, or industrial grade carbon fiber. The pultruded composite material introduced in this article is large in size, so large tow carbon fiber is selected.

 

Process Route

 

The composite material pultrusion process is shown in the figure below. The equipment used mainly includes carbon fiber supply device, resin impregnation tank, preforming device, heating forming die, drawing device, cutting device, etc.

 

 

Yarn Drawing and Impregnation

 

The continuous carbon fiber placed on the shaft frame is led out through the guide and arrangement device and sent to the resin impregnation unit. The design requirement of the guide device is to keep the carbon fiber straight from the shaft frame to the die, and to apply equal force to all fiber bundles to avoid distortion of the pultruded product due to changes in tension between fiber bundles.

 

During the impregnation operation, firstly, the thermosetting resin and curing agent are accurately measured in the resin impregnation tank, and the resin, curing agent and release agent are mixed in a certain mass ratio, stirred and evenly injected into the resin impregnation tank. Generally, resin manufacturers will give a guiding ratio, in which a small amount of internal release agent is essential to prevent clogging of the mold during the thermosetting molding process.

 

During the entire impregnation process, the fiber impregnation must be complete, and there should be no dry fiber. The presence of dry fiber will cause defects in the pultrusion. An important process parameter for controlling the degree of carbon fiber impregnation is the viscosity of the resin system, which is called the initial viscosity. Its size is not only related to the resin itself, but also to additives and temperature.

 

In addition to the initial viscosity, the carbon fiber impregnation effect is also related to the impregnation time, the temperature of the resin during impregnation, and the working state of the carbon fiber in the impregnation tank. Generally speaking, given the initial viscosity, the carbon fiber working state is normal, the impregnation time is extended, and the resin temperature is increased, the fiber impregnation degree can be improved.

 

In this process, the following points should also be noted:

 

(1) Since there are more carbon fibers in each bundle of large-tow carbon fiber, it is necessary to always pay attention to the resin impregnation of the carbon fiber monofilament. Not only the surface of the carbon fiber tow should be well impregnated, but also the inside of the carbon fiber tow should be fully impregnated. Otherwise, internal defects will be caused and the performance of the final product will be affected.

 

(2) The viscosity of the resin should be kept within a certain range, and the indoor temperature should be well controlled. If the weather temperature is too low, the resin tank can be heated to ensure that the resin can smoothly enter the mold mouth with the carbon fiber without being squeezed out or blocked.

 

Heating and Curing

 

Generally speaking, the pultrusion process is divided into three heating areas. The first is the preheating zone, which is used to prepare for the reaction in the next zone; the second is the gel zone, in which the resin undergoes solidification and cross-linking and undergoes phase change, from a viscous state to a gel state; the last is the constant temperature zone, which prevents sudden temperature changes from causing cracks in the composite material.

 

The temperature control in the die area is related to the pultrusion production rate and the quality of the product. The key to temperature control is to make the material solidification rate consistent with the profile pulling speed. At the same time, it is also necessary to ensure uniform heating and the uniformity of the material solidification rate at all locations. Plate or cylinder contact heaters are usually used at the die.

 

At present, high-frequency preheating and high-frequency curing solve the problem of rapid curing. For different types of resins, the temperatures of the three zones are different. Generally speaking, the rule is to increase in sequence.

 

On the other hand, for composite materials of different sizes, the temperatures of the three zones also vary. For example, for large and thick pultruded rods, the curing temperature must be higher than the curing temperature of the resin, because the temperature is transferred from the surface of the composite material to the inside, so that the resin inside can be completely cured and well combined with the carbon fiber.

 

Traction

 

Pultrusion molding usually uses a clamping and traction device, which has two sets of furniture whose internal shapes match the workpiece. The pultruded object is in the middle of the clamp, which is opened and closed by compressed air. The clamp is installed on the support seat, and the hydraulic cylinder drives the clamping device to reciprocate. In order not to damage the surface of the pultruded object, the contact between the clamp and the pultruded object is often lined with polyurethane material.

 

The linear speed of the traction device is matched with the mold temperature and the resin system to ensure a good curing reaction inside the mold. Different traction speeds should be selected to produce different products. Therefore, the speed of the traction device is required to be adjustable.

 

Post-curing

 

The control of the pultrusion process should also consider the problem of post-curing, which is particularly important for large parts. Therefore, no matter how slow the speed is in the continuous molding process, it is difficult to produce a curing state like intermittent production. The simple way to cure pultrusion molding later is to install a heating and curing chamber, and more often a separate heating chamber is used for post-curing to ensure the degree of curing.