Analysis on the cause of rod fracture during pressure welding and tension test

Feature: broken parts of the core rod are in the ball head or inside of the ball and socket, with a low pulling value.

Cause analysis:
1. Crimping force is too large, outnumbering the compressive strength of rods;
2. Improper crimping area; Briquetting should be located in the middle of the connection area;
3. Bad metal fitting processing precision: a. eccentricity b. insufficient cylindricity c. diameter biases of the inner bore;
4. Nonuniform material quality among the same batch of hardware(especially ball and socket), and hardness problem after heat treatment;
5. Terrible burnish of mandrels, resulting in tamper angle in the end or insufficient cylindricity;
6. Crimping machine itself has poor stability, six claws or eight claws being not synchronous;
7. Bending strength under high temperature is too high. (thermostability is too high, the rods are brittle );
8. During the assembly of rods and fittings, the ball head is not in the same line as the ball and socket, so the rods will be exposed to the sheer force resulting from the hardware.

The solution:
1. Choosing big manufacturers products which have steady quality and fewer problems existing in themselves. When reaching the plant, the fittings need to be checked whether their hardness changes among the same batch, and whether the inner hole size and mandrel diameter tolerance are by the technical requirements discussed when placing the order. Other parameters are checked and accepted according to relevant standards.
2. Using advanced equipment and technology to ensure the quality of polished mandrel. (special grinding machine, specialist grinding).
3. Controlling the fitting clearance during the assembly of core rods.
4. Choosing proper crimpling parameters. When hardware and mandrels from different batches or different manufacturers arrived, pick up 5 ~ 8 products to destroy them to determine the best crimping parameters. You better make segmented crimping with large-tonnage products.
5. Guaranteeing the stability of crimping machines. Pay attention to the replacement of hydraulic oil and cooling devices after several work hours in summer.

Several key issues when drawing the product in the manufacturing process of organic composite insulator

The outer insulation coated process of organic composite insulator experienced several steps below: dry umbrella dish > extruded sheath wear the flat moulding > complete injection. The development of the core rod also experienced the following stages: regular mandrel > heat-resistant mandrel > heat-resistant and acid-resistant mandrel > transparent, high-impact -strength, acid-resistant mandrel. All these changes are made to adapt to the production process of insulators and to surpass the world’s advanced levels for the overall development of domestic insulators.

The most respected and concerning part about the development of organic composite insulators is the structure type of whole injection and crimping. The ministerial standard for more than 220 kV organic composite suspension insulator recommends using an acid-resistant mandrel. The great resistance to high temperature of the core rod services for the whole process of injection. Crimping structure requires strong impact strength. According to the structural characteristics of organic polymer materials, the higher heat-resistant property a mandrel has, the lower impact strength it possesses. These two impacts are contradictory, and, to solve this contradiction must, core rod manufacturers and insulator manufacturers must work together to find a common ground. Core rods can’t be broken during injection moulding and have high impact strength during crimping. The structure of the fitting and core rod is firm and not pulled off. Experiments show that to crimp firmly, a core rod with good impact strength must be used and will crack if you are not careful enough when drawing it out from moulding after injection, then the whole mandrel will be damaged. So what insulator manufacturers need to do is, to sum up, experience and pay attention to the standardized operation of production technology and process, and to pursue the accordance of quantity, quality, and benefits. Insulators break when they are taken out and checked after injection, They can’ be broken during injection or moulding, but during the hard prying process after the upper mould and lower mould separate. Hard pry causes rods breaking and silicone rubber umbrella setting apart.
1. mould is one of the main reasons, and glueing a model is related to the following several factors.
(1)mould material and smoothness is not good, or mould release agent is not sprayed timely.
(2)Inappropriate silicon raw material and hardness of rubber. (hardness should be adjusted in summer and winter)
(3) Inappropriate time and pressure, overheat mould, or overture.
2. Another reason is that mandrels are not heat-resistant. According to determination, in general, the internal temperature of a composite insulator mould is about 155 °C and its external temperature is about 170 °C. Measures to prevent insulator fracture when levering it up
1. First, use a good mould with good metal materials, technical content, and a good finish. Using good release agents.
2. Second, stable quality of raw material in silicon rubber ingredients. Operation personnel must understand that the proportion changing of raw materials is always related to temperature, pressure and time in the whole process of production. Of course, it includes the influence of the temperature of the external environment and the hardness degree of rubber.
3. Do not be eager to lever it up after opening mould no matter if it is stuck to the upper mould or the lower mould (of course no dropping down). You’d better stay 35 minutes or blow it for a while with fans, air conditioning, and cool the. the middle of the insulator. The strength of such a core rod will be increased greatly with the drop of temperature, and the risk of fracture will be lowered.
4. For the 110kV composite insulator, generally you lever it up with the angle between FRP rod and mould not being more than 30 degrees after using the third measure. You need to be very careful and try other methods if the insulator doesn’t drop off with more than 30 degrees. At present, the insulator hardware has been pressed into the mandrel through injection and moulding, and the insulator will drop off easily with its own weight and your gentle touch. Otherwise, you have to find the cause.
5. Insulator manufacturers must take equipment situation, personnel quality and raw material into consideration in formulating and practising hour norm and piece-rate system, seeking the accordance of quality, quantity and benefits based on ensuring each finished product is qualified.

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