Brief Introduction to the Viscoelastic Principle of Hollow Forming
1、 Mathematical description
Polymers are characterized by their long chain structure and the flexibility of their molecular chains In general, when polymers are placed at a certain temperature and subjected to external forces, the conformational process of macromolecules undergoes a series of intermediate states and transitions to an equilibrium state that is compatible with the external forces, which is a relaxation process. The deformation changes with time as follows:
(1) Above the glass transition temperature of the polymer, the proportion of general elastic deformation in the total deformation is very small and can be ignored
(2) Among them, t * is the relaxation moment, which decreases with increasing temperature
η is the viscosity, which decreases with increasing temperature
(2) The formula can also be written as
(3) High elastic deformation when γ 2 ∞=δ/E2 is t →∞
When γ η=δ/η is t=1, the viscous deformation refers to the viscous deformation per unit time
Because in the vicinity of high elasticity or viscous flow temperature, the relaxation time t * of the polymer is very short. If the deformation time t1 during molding is greater than or equal to t *, then equation (3) shows that when the external force acts at time t=t1:
After releasing the external force, the high elasticity deformation recovers
(4) If the temperature is lowered below Tg (or crystallization temperature) before the high elastic deformation recovery and the external force is released, then the relaxation time t * →∞, at which point equation (4) becomes
The high elastic deformation part no longer recovers, the viscous deformation is irreversible, and the release of external force also does not recover.
2、 Principle narration
By utilizing the temperature dependence of the relaxation time of polymer induced high elastic deformation, near Tf above the glass transition temperature of the polymer, the polymer semi-finished product (tube, hollow profile, etc.) is rapidly deformed and then maintained in deformation. It is cooled to below the glass transition temperature or crystallization temperature in a short period of time, so that the deformation of the formed product is frozen. This is the viscoelastic principle of hollow forming The blow molding wheels in Suzhou blow molding processing plant follow this principle.
Inference
The usage temperature of hollow blow molded products should be much lower than the glass transition temperature or crystallization temperature of polymers
2. The higher the molding temperature, the greater the proportion of irreversible deformation in the product, and the better the shape stability of the molded product
