【Technical Article】Written Report on Uneven Inner Diameter of Rubber Expansion Joints

Shanghai Songjiang Group’s official website presents the information on a written report regarding the uneven inner diameter of rubber expansion joints. Songjiang Group offers various rubber expansion joint products to customers. Below is the feedback from customers regarding the uneven inner diameter of rubber expansion joints and the written response from the technical department of Songjiang Group.

Rubber expansion joints are placed into molds before vulcanization. The outer shell is made of metal molds, while the inner diameter molds are made of rubber, also known as rubber bladder molds, commonly referred to in the industry as air blowing molds.

The outer shell molds of rubber expansion joints are made of mold steel, with a general service life ranging from 3 to 5 years. Prolonged use inevitably leads to unavoidable collisions on the surface of the metal molds. The vulcanized surface of the rubber expansion joint will also bear the marks of collisions with the surface of the metal molds. These marks are not indicative of incomplete vulcanization of the product but are imprints left behind. Such marks do not affect the performance and quality of the product, nor do they have any impact on its service life.

The inner bore molds of rubber expansion joints are made of aging-resistant rubber, with a general service life of about 100 uses. For sizes above DN300, the service life is only about 50 uses, and for sizes above DN500, it’s only about 20 uses. These rubber bladder molds all have a finite lifespan. For small-caliber rubber bladder molds, they can be used about 100 times. When used about 80 times, cracks will appear on the surface of the rubber bladder, and these crack patterns will be imprinted on the inner diameter of the rubber expansion joint. These patterns are impressions left by the surface of the rubber bladder during the vulcanization process. The subsequent production of 20 rubber expansion joints will exhibit these patterns on their inner diameters. However, these patterns will not flake off, cause quality issues, or affect the service life.

Shanghai Songjiang Vibration Absorber Group Co., Ltd.

Technical Department

2018.8.29