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Common technology and problem analysis of epoxy resin potting

发布时间:2018-03-31 16:12:27  浏览次数:

Encapsulation means that the liquid composite is mechanically or manually filled with electronic components, circuit devices, cured at room temperature or heating conditions to become a thermosetting polymer insulation material with excellent performance. It can strengthen the integrity of electronic devices, improve the resistance to external shocks and shocks, improve the insulation between internal components and lines, which is conducive to miniaturization and lightweight of devices, avoid direct exposure of components and lines, improve the waterproof and moisture-proof performance of devices, and improve the performance and stability parameters.

Sealing technology

The quality of filling products is closely related to product design, component selection, assembly and filling materials. The filling technology is also an important factor.

Epoxy potting has two normal and vacuum sealing processes. Epoxy resin. Amine type ambient temperature curing potting material, generally used in low voltage electrical appliances, mostly using normal potting. Epoxy resin, acid anhydride heat curing filling material, generally used for high-voltage electronic device filling, mostly using vacuum filling process, is the focus of our section. At present, there are two common methods of manual vacuum filling and mechanical vacuum filling, and mechanical vacuum filling can be divided into A and B components mixed defoaming and then filling and respectively defoaming and then mixed filling. The process is as follows:

(1) manual vacuum filling process

(2) mechanical vacuum encapsulation process

First mixing and defoaming Technology

A and B were first mixed and sealed after defoaming.

In contrast, mechanical vacuum sealing, equipment investment, high maintenance costs, but the product consistency, reliability and other aspects of the obvious superior to manual vacuum sealing process. No matter what kind of filling method, we should strictly observe the given technological conditions, otherwise it is difficult to get satisfactory products.

Problems and causes of potting products

(1) The low starting voltage of partial discharge (PD), inter-line firing or breakdown of output transformers of TV sets and monitors, automobile and motorcycle igniters and other high-voltage electronic products, often due to improper filling technology, will occur when working partial discharge (corona), inter-line firing or breakdown, because the high-voltage coil diameter of such products is very small. Generally only 0.02 to 0.04mm, filling materials can not completely penetrate the interturn, leaving gaps between the turns of the coil. Because the porosity permittivity is far less than epoxy filling material, under alternating high voltage conditions, uneven electric field will be generated, resulting in partial discharge at the interface, aging and decomposition of the material, causing insulation damage.

From the technological point of view, there are two reasons for the interspace.

1) the vacuum degree is not high enough when filling, and the air between lines can not be completely eliminated, so that materials can not be completely infiltrated.

2) the preheating temperature of the specimen before filling is not enough, and the viscosity of the material can not be reduced rapidly, which will affect the infiltration.

For manual filling or vacuum filling, the high temperature of material mixing and defoaming, long operation time or beyond the material application period, and the product does not enter the heating and solidification process in time after filling, will cause the increase of material viscosity, affecting the coil immersion. According to the experts of Shanghai Changxiang Industrial Co., Ltd., the higher the initial temperature, the lower the viscosity, and the faster the viscosity increases with time. Therefore, in order to make the material have good impermeability to the coil, we should pay attention to the following points in operation.

1) potting compound should be kept within a given temperature range and will be used during the application period.

2) before pouring, the specimen should be heated to the specified temperature. After the filling is completed, the heating and curing process should be entered in time.

3) the vacuum degree of potting should conform to the technical specification requirements.

(2) There are two kinds of shrinkage in the process of heat curing, namely, chemical shrinkage in the process of phase transformation from liquid to solid and physical shrinkage in the process of cooling. Further analysis shows that there are two processes of chemical shrinkage in the curing process, namely, the shrinkage from the beginning of the chemical crosslinking reaction after the encapsulation to the initial formation stage of the micro network structure, which we call gel precuring shrinkage. Shrinkage from gel to complete curing is called post cure shrinkage. The shrinkage of these two processes is not the same. In the process of the former changing from liquid to reticulated structure, the physical state changes abruptly, the consumption of reaction groups is greater than that of the latter, and the volume shrinkage is also higher than that of the latter. At the gel precuring stage (75 /3h), the epoxy group disappeared more than the post curing stage (110 C /3h), and the results of DTA analysis also proved that the curing degree of the sample was 53% after 750 /3h treatment.

If we adopt a high temperature curing for the encapsulated specimen, the two stages in the curing process are too close, and the gel pre curing and post curing will be completed at the same time. This will not only cause exothermic peaks, damage elements, but also cause huge internal stress of the sealing parts, resulting in defects in the internal and external appearance of the products. In order to obtain good parts, we must focus on the matching of curing speed (A, B composite gel time) and curing conditions when designing the filling material formulation and curing process. The usual method is: according to the nature and usage of potting material, the technology is segmented and solidified according to different temperature regions. According to experts, color TV line output transformer filling according to different temperature section solidification rules and parts internal heat release curve. The curing reaction of the encapsulated material in the gel precuring zone was slow, the reaction heat gradually released, the viscosity increased and the volume contracted slowly. At this stage, when the material is in the flowing state, the volume shrinkage shows that the liquid level drops until the gel can completely eliminate the volume shrinkage inside the stage.



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