The emergence and application of the third generation random copolymer PP-R pipes marks the maturity of polypropylene pipes. Its superior performance is that it has the last laugh in the front PK with the traditional galvanized pipe, and has the opportunity to show its talents in fields such as floor heating, central air conditioning, cold and hot water. The processing of PP-R pipes needs to strictly control the processing temperature, cooling rate, die compression ratio and geometric size changes. Let's have a detailed look below.
Influence of processing temperature
Polymer PP-R materials have three states: glass state, high elastic state and viscous flow state at different temperatures, and their mechanical properties are also very different. When the raw material enters the barrel, it is in glass state. From the compression section to the homogenization section, it must change from high elastic state to viscous flow state, and from high elastic state to glass state when it is cooled from the outlet mold to full sizing. Every state change is controlled by temperature. If the temperature is too low, the brittleness of the pipe will be too large at low temperature, and the compressive strength will be reduced; If the temperature is too high, it will cause excessive plasticization and prone to thermal aging. Therefore, strictly controlling the processing temperature is very important to ensure product quality.
Effect of cooling rate
The good impact resistance of PP-R pipes comes from the voids between its internal molecules, which can play an effective buffer role when impacted by external forces. The cooling rate during processing plays a decisive role in the formation of appropriate voids in the pipe. During the cooling process, the relative positions of polymer chains in the material are rearranged. If the cooling speed is too fast, the gaofeizi chain has been frozen before it enters the best position, which not only cannot obtain the ideal impact resistance effect, but also causes the problem of stress concentration.
Usually, constant temperature water tank is used to control spray for cooling during processing, and gradient gradual cooling method is used to make the materials pass through multiple water tanks with different temperatures in turn, so as to slow down the cooling rate, obtain high enough impact resistance and prolong the service life.
Influence of die compression ratio
Pure polypropylene material has obvious pressure orientation characteristics, that is, the polymer is orderly arranged in one direction, and its compression resistance is very strong; And in the direction perpendicular to it, the arrangement is more disordered, and its compressive capacity is much weaker than the former. In order to solve this problem, the compression ratio of the die was adjusted in the production of PP-R pipe, and the random copolymerization method was used to break the original orientation characteristics and make its compressive capacity tend to be consistent in all directions.
The manufacturer uses the inner spiral multi channel head to make the plastic melt cross rotate into a dynamic flow direction before exporting the mold into the blank. Using this mold with good material, high precision and advanced design, combined with the selection of high-quality PP-R raw materials, we can get an isotropic ideal controlled state, without affecting the plasticizing effect, and improve the quality of the pipe.
Influence of geometric dimension change
In the processing technology of PP-R pipe, the geometric size, especially the outer diameter size, has a great impact on the service performance. It is easy to understand that if the wall thickness of the pipe is different for the sizing sleeve of the same size, even if the cooling water temperature, cooling rate and vacuum degree are the same, the shrinkage rate will be different. The thicker the pipe wall, the greater the shrinkage, and the smaller the pipe wall, the smaller the shrinkage. According to previous experience, when the pipe wall thickness is 6.8 mm, the processing error is 0.7 mm; When the thickness of the pipe wall is 12.5 mm, the machining error reaches 1.4 mm.
In addition, the geometric dimension also has a certain impact on the use of pipes. When the outer diameter of the pipe is too large, too many overflow edges will be formed during welding, reducing the diameter of the pipe; When the outer diameter of the pipe is too small, the bonding strength is low when welding, and the overflow is too small, and the peripheral auxiliary effect is poor. Therefore, the outer diameter size should be strictly controlled to ensure the quality of pipes.