Technology of Heat Shrinkable Products
Heat shrink technology is based on selective and controlled crosslinking of molecular chains. Crosslinking can be induced either by chemical means or irradiating the product by an electron beam.

Three dimensional inter-molecular chains are established by exposing the polymer to high speed electron bombardment or by addition of free electron peroxides to the polymer.

The formation of links between molecular chains is called crosslinking.

A direct consequence of crosslinking is that the relative position of each molecule is fixed with the respect to its adjacent molecules. The fixing of the relative position of every molecule in the crosslinked molecular structure amounts to the material achieving "shape memory".

The crosslinked material is characterized by strong binding forces and does not flow on exposure to heat, as the links between molecular chains hold them together. Beyond a certain temperature called the crystalline melting temperature, the crystals disappear and the material becomes soft and malleable.

In this rubbery state it is possible to expand the material using mechanical forces - this process is known as "expansion". The material can be locked in its expanded form by allowing the material to cool to a temperature below its crystalline melting temperature and the material will contract to its original state before expansion.


Cross Linking and Shape Memory
Schematics of the polymer matrix
before cross linking
 
After cross linking







Heat shrink 'shape memory' material consists of long chain molecules called polymers. These polymers are interlinked by means of 'cross links'. These cross links cannot be broken up easily. The polymer chains are loosely bundled together in the form of crystals called crystallites. These crystals can be opened up and formed again rather easily.

This process requires energy in the form of heat. As the material is heated, the crystals open up and disappear. In this state, the polymer chains can be stretched by mechanical force. Subsequently when the material is cooled, the crystals reappear and the material gets frozen in its expanded form. If heating is resorted to once again in this stretched form, without any restraining mechanical force, the crystals open up and the polymer chain regains its original unstretched shape as determined by the cross-links. This property of the heat shrinkable material is refferred to as 'shape memory'.


Heat Shrink versus Push - On and Premoulded Technology
  Heat shrink Tapex Push-On (Premoulded)
Definition Polymeric material which is shaped, crosslinked and thereby imparted with a "Shape Memory". When heated, expanded and cooled, the shaped component remains in the expanded shape. When heated again to more than 120 Deg.C. the component shrinks onto the substrate. Nil Elastic pre-moulded Polymeric material. The hollow core of the shaped component is pushed over the insulated cable core.
Stress Control Provided by means of a shrinkable tube having defined impedance characteristics. The tube is inserted over the core, positioned and shrunk. Formation of airvoids is totally avoided, as the component shrinks circumferentially creating a natural draught for expulsion of air and is restrained from shrinking further by the core ensuring a void-free interface. Provided by a stress grading pad referred to as FSD pad. The pad is applied over the semicon end and held in position with EPR based rubber tapes. The effectiveness of the system depends on the skill of application to achieve the optimum tension.
Inadvertent under - application lowers the discharge inception voltage and hence the application is skill dependent. Stress control being the crux of the system, insufficient or improper application can result in premature failure.		 Provided by a combination of premoulded stress cone, FSD pad and tapes. The diameter of the hollow core is designed to make the stress cone push fit. However depending upon diameter, tolerances of insulated cores, the operation can be difficult and time consuming. The semicon screen of the core is connected to the conductive layer over the moulded cone shape, by application of self amalgamating tapes. Since the tapes are flexible in nature, shifting of the cone during core crossing can occur, leading to void formation at screen cut back. This inturn would lead to ionisation of air voids and hence a permanent failure.
       
 


 
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