Introduction to the Progress of Epoxy Mandrel Pultrusion Process

 

Abstract

This paper introduces and reviews the current status and latest progress of core rod production for composite insulators, as well as the process principles and product characteristics of traditional pultrusion, vacuum-assisted pultrusion and injection pultrusion.

 

Compared with traditional processes, vacuum-assisted pultrusion has improved the transparency and stability of the product, while injection pultrusion fundamentally solves the shortcomings of open-type dipping and ensures the consistency of product characteristics. This paper also proposes the problems that need to be studied and solved in domestic core rod production.

 

1. Introduction

One of the most important components of organic composite insulators is the core rod, which is not only the guarantee of the internal insulation of the composite insulator, but also the mechanical load-bearing component of the composite insulator. Its quality is an important guarantee for the reliability of the composite insulator.

 

Both composite insulator manufacturers and core rod manufacturers recognize this. On the one hand, composite insulator manufacturers have put forward more specific and higher requirements for insulating core rods; on the other hand, core rod manufacturers have also continuously introduced new processes to improve product quality.

 

In recent years, heat-resistant core rods, acid-resistant core rods, transparent core rods and injection pultrusion core rods have been developed. Vacuum pultrusion process and injection pultrusion process have been developed in production technology, and great progress has been made. This article will first introduce the development of pultrusion process and the characteristics of core rods produced by different processes.

 

2. Progress of Pultrusion Process of Epoxy Core Rods in My Country

 

Pultrusion technology began in the 1950s, first registered in the United States, applied in the 1960s, developed in the 1970s, and developed at the fastest speed in the 1980s. In recent years, the growth rate of pultrusion products has remained above 10%.

 

The pultrusion process has been valued by peers in various countries, and various pultrusion equipment and processes have emerged, such as vertical pultrusion, horizontal pultrusion, curved profile pultrusion, injection pultrusion, etc.

 

The rise of epoxy pultrusion core rods for insulators in my country is synchronized with that of composite insulators. They all began to rise in the early 1980s. Pultrusion equipment includes yarn guides, preforming molds, epoxy resin dipping tanks, molding molds, and traction machines. The process is as follows: the glass fiber passes through the yarn guide and pre-forming mold (some manufacturers do not have it), and then passes through an open dipping tank to soak the glass fiber under normal pressure. After being solidified and formed by the forming mold, it is pulled out by a traction machine.Most of the equipment used is a crawler traction device, as shown in Figure 1.

 

 

Figure 1 Schematic Diagram of Traditional Pultrusion Process

 

This process has several obvious disadvantages. First, the product has a large porosity, making it difficult to obtain a transparent product, making it difficult to find the internal defects of the core rod (such as yarn ends, inclusions, insufficient dipping, etc.); second, the product quality is affected by the environment, especially by the ambient temperature and humidity; third, the curing characteristics of the glue in the glue tank change with time, and the consistency of product characteristics cannot be well guaranteed.

 

Therefore, some manufacturers stop production in seasons with high humidity and high temperature or adopt methods such as installing air conditioners, dehumidifiers, and frequent glue changes in the dipping part to try to solve these problems, but the effects of these methods are limited.

 

1) Vacuum-Assisted Pultrusion Process

In order to solve the problem of large porosity and opaque products of ordinary pultrusion core rods, our institute developed a vacuum-assisted pultrusion process in 2001. The characteristics of this process are multiple dipping, vacuum assistance, and minimizing the contact area between glue and the atmosphere, thereby improving product performance.

 

After the first dipping of glass fibers in multiple layers, they enter the preforming die for glue extrusion. Glass fibers containing a certain amount of glue enter the vacuum dipping system, where pores are removed and further dipping is performed under vacuum conditions to reduce pores and ensure that the glass fibers are fully impregnated, as shown in Figure 2.

 

 

Figure 2 Schematic diagram of vacuum-assisted pultrusion

 

This process has achieved relatively good results in the relatively dry environment of the northwest. The performance of the products produced has been significantly improved in the following aspects compared with the ordinary pultrusion process:

 

1. a) Fewer pores and high product transparency make it easy to remove product defects, such as yarn ends, inclusions, and lack of glue;

 

1. b) Multi-layer dipping reduces the contact area between the glue and the atmosphere, thereby reducing the impact of environmental conditions on the product;

 

1. c) Multiple vacuum-assisted dipping allows the glass fiber to be fully impregnated;

 

1. d) The above aspects ensure that the characteristics of the vacuum pultrusion core rod are more stable than those of ordinary pultrusion.

 

Undoubtedly, this process has made great progress compared with the traditional process. Its emergence has gradually led to the rise of epoxy transparent core rods in China, but this process has not completely solved the impact of environmental temperature and humidity on product characteristics and the problem of “new and old glue” (the problem of the applicable period of glue).

 

New glue refers to the glue that has just been added to the glue tank, and its curing characteristics are better; old glue refers to the glue remaining in the glue tank, and the curing characteristics of the glue change due to the reaction of the anhydride (curing agent) with H2O in the air and the curing reaction of the glue itself.

 

2) Injection Pultrusion Process

In order to fundamentally solve the shortcomings of open-type dipping at normal pressure, such as glass fiber impregnation and entrained bubbles, product performance being affected by environmental factors, and the production process having high requirements for the service life of the resin base, domestic units have developed a new injection pultrusion process and applied for a national patent and passed the appraisal. Figure 3 is a schematic diagram of injection pultrusion.

 

After passing through the yarn guide and preforming mold, the glass fiber enters the continuous resin transfer molding mold, and the special resin is injected into the mold at a stable high pressure and flow rate to fully saturate the glass fiber and remove bubbles; the fully saturated and bubble-free glass fiber enters the curing molding mold of the mold under the traction of the traction machine, thereby realizing continuous resin transfer molding (Continuous Resin Transfer Molding Pultrusion Process CRTM) or injection pultrusion.

 

 

Figure 3 Schematic Diagram of Injection Pultrusion

 

Its Characteristics are:

1. a) The glass fiber is fully impregnated, the microbubble content in the produced FRP products is small, and the electromechanical properties are excellent;
2. b) The resin supply system is easily isolated from the atmosphere, and the product performance is not easily affected by the environment;
3. c) The injected resin always maintains the same curing characteristics (always “new glue”);
4. d) It is easy to obtain transparent products, making product defects (such as inclusions, yarn knots, etc.) easy to find and remove.
5. e) Little impact on the environment and operators.

 

The stability and consistency of core rod performance has always been a difficult and headache problem for open-type dipped core rod manufacturers. This problem has also caused certain losses to synthetic insulator manufacturers, especially from May to October when the humidity is high and the temperature is high (in the south, the problem may be greater and last longer). In addition to solving the problem of environmental impact, the injection pultrusion process also solves the problem of the applicability of the glue. It does not have the problem of “new and old” glue, and it is always “new glue”, so the stability and consistency of the product are guaranteed.

 

3. Problems with Core Rods and Prospects for Pultrusion

 

At present, the production process of composite insulator core rods in my country is still mainly traditional. The biggest problem is that the product characteristics vary greatly with time and seasons. Core rod manufacturers should strengthen control and closely monitor the changes in product characteristics. A relatively simple method is to timely monitor the high-temperature mechanical properties and color changes of the product, such as hot bending strength. When the value is found to be lower than a certain value, active measures should be taken, such as changing the glue, etc. Dyes cannot be added to the resin to ensure color consistency. Of course, if conditions permit, vacuum or injection pultrusion is a better solution;

 

Secondly, since most core rods are opaque, core rod manufacturers should try to remove defects in the core rods, such as yarn heads, inclusions, lack of glue, hard glue blocks, local cracks, etc.;

 

Third, the control of glass fiber and raw materials, especially curing agents, should be strengthened to standardize the process; Fourth, special standards for core rods for composite insulators should be compiled. The technical conditions and requirements currently cited are not complete enough.

 

In addition to the above problems, there are some basic issues that should be paid attention to. The first is the glass fiber content. According to the commonly accepted view in China, a higher glass fiber content is better. The glass fiber content of a core rod with a diameter of 18mm (weight percentage) is 81-83%, while the glass fiber content of foreign products is lower. For example, Glassforms: the fiber content of the core rod for line suspension insulators is 73-80%, and the fiber content of the core rod for line support and cross arm is 70-77%. The core rods with such low glass fiber content cannot be produced in China.

 

Here a question is raised, that is, what does the high or low glass fiber content mean; secondly, the domestic core rod production uses internal release agents, which do not participate in the curing reaction. As time goes by, the internal release agent may diffuse to the core rod surface or the interface between the core rod and the silicone rubber or the interface between the core rod and the metal accessories, thereby affecting the long-term applicability of the insulator.

 

There are vertical pultrusions abroad that use liquid metal as a curing mold. The glass fiber content of the core rod produced can be adjusted within a large range, and no internal release agent is added to the resin.

 

How to ensure the quality stability of core rods, study new process methods to remove internal release agents and obtain core rods with different glass fiber contents, and standardize the core rod pultrusion process are issues that need to be solved in the future pultrusion process; studying the influence of glass fiber content on various properties of core rods (including fatigue characteristics) and formulating standards for core rods are basic issues that the composite insulator industry and core rod manufacturers need to pay attention to.