Silane Grafting and Extrusion Molding: Problems and Solutions

The processes of grafting silane and extrusion in two sequential stages are termed the two-step method, otherwise known as SioplasE; the amalgamation of grafting and extrusion molding in a single process is referred to as the one-step method, otherwise known as Monosil. In the two-step process (SioplasE), the silane-grafted resin and catalyst masterbatch are solely combined prior to entering the extruder. Once joined, the material possesses a very brief shelf-life, particularly when exposed to humidity. Extruded cables are commonly cooled with an aqueous bath, which provides the moisture necessary for curing. The rate-limiting step in the actual curing of the polymer is diffusion. Typically, a hot water bath, steam bath or low pressure steam autoclave is employed to accelerate curing. The Spherisil method (one-step method) has been globally applied for its processing patent by Constab Company and Crompton Company. These patents relate to the processing of crosslinked polymers where the silane is fed into the extruder in a solid state. The foundation of the Spherisil process is the absorption of liquid silanes during metering with suitable carrier resins (powder or granulate) using suitable apparatus. The Spherisil method encompasses all the necessary elements allowing a continuous one-step operation.

For the cable materials produced by the above two disparate methods, in the process of insulating extrusion molding, they often encounter problems that cannot meet the process requirements. Therefore, operators often use gas flames to directly heat the mold cavity according to productive needs (but not necessarily allow the flame to completely surround the cavity). When the heat transfer effect of the heating belt to the polyethylene melt is inadequate, resulting in poor surface finish of the insulation, it is necessary to take flaming heating measures. The specific situation is exhibited in the following aspects.

Silane grafting and extrusion in two processes are called two-step method, also known as SioplasE ; grafting and extrusion molding in one process is called one-step also known as Monosil . In the two-step process ( SioplasE ), the silane-grafted resin and catalyst masterbatch are only mixed before entering the extruder. Once mixed, the material has a very short shelf life , especially when exposed to humidity. Extruded cables are usually cooled with a water bath, which provides the moisture needed for curing. The rate-limiting step in the actual curing of the polymer is the diffusion . Typically a hot water bath, steam bath or low pressure steam autoclave is used to accelerate curing. The process of Spherisil method (one-step method) has been applied for global process patent by Constab Company and Crompton Company . These patents relate to the processing of crosslinked polymers where the silane is fed into the extruder in a solid state. The basis of the Spherisil process is the absorption of liquid silanes during metering with suitable carrier resins (powder or granulate) using suitable equipment . The Spherisil method contains all the necessary elements allowing a continuous one-step operation.

　　For the cable materials produced by the above two different methods, in the process of insulating extrusion molding, they often encounter problems that cannot meet the process requirements. Therefore, operators often use gas flames to directly heat the mold cavity according to production needs (but not necessarily Allow the flame to completely surround the cavity). When the heat transfer effect of the heating belt to the polyethylene melt is not good, resulting in poor surface finish of the insulation, it is necessary to take flame heating measures. The specific situation is shown in the following aspects.

1. Melt fracture

The surface is very rough with a surface finish (orange peel), yet the insulating interior has a good texture.

Solution:

①Increase the temperature of the machine head;

②The flame directly heats the mold cavity;

③Reduce traction speed;

④Reduce the screw speed.

2. Pre-crosslinking

The surface is smooth but there are intermittent lumps.

Solution:

①Reduce the temperature of compression section and metering section;

② Reduce the number of filter screens and meshes to reduce the residence time.

3. Insufficient cross-linking

The insulation is still thermoplastic and fails the hot extension test.

Solution:

①Reduce the temperature of compression section and metering section;

②Increase the number of filter screens to increase the back pressure.

4. Insufficient bonding of insulation and conductors

The conductor is not covered, the surface is smooth but there is a large area of ​​alternating exposure.

Solution:

Reduce flame temperature.

5. The surface of the pit is smooth but there are pits

Solution:

①Reduce traction speed;

② Lower the cavity temperature slightly;

③Prevent pre-crosslinking.

6. There is material accumulation at the die mouth

Solution:

①The flame directly heats the mold cavity;

② Use an extrusion die (or reduce the length of the shaping section);

7. There is burnt material on the insulating surface

Solution: reduce the temperature at the machine neck.

8. Microporous

The insulation is microporous inside.

Solution:

①Increase the flame temperature;

②The sink should be as close as possible to the die opening.