How does shape of plastic structure affect quality of product?

Time：2021-08-05 16:14:29 / Popularity： / Source：

1. Crystallization effect

(1) Concept of crystallization

Supramolecular structure of polymer has a very obvious influence on injection molding conditions and product performance. In the past, research on polymer processing was mostly from perspective of molecular weight, molecular weight distribution and molecular chain branching. However, in recent years, people have paid more attention to influence of super-molecular structure (aggregate structure), arrangement of macromolecular chains, stacking methods of various particle shapes, crystallization effect, orientation effect on quality of product.
Polymers can be divided into crystalline and non-crystalline types according to their supramolecular and supramolecular structures. Molecular chains of crystalline polymers are regularly arranged, while molecular chains of amorphous polymers are irregularly arranged. Different forms show different process characteristics, physical and mechanical properties. Generally, crystalline polymers have higher heat resistance and mechanical properties than non-crystalline polymers.
Polymers with simple molecular structure and high symmetry are easy to form crystals, such as polyethylene, polytetrafluoroethylene, polyvinylidene chloride, etc.; although molecular chain is relatively large, interaction between molecules can also generate crystals, such as polyamide and polyoxymethylene. However, if there are large side groups on molecular chain, it is not easy to form crystals, such as polystyrene, polyvinyl acetate and organic glass. Polymers with high molecular chain rigidity cannot be crystallized, such as polysulfone, polycarbonate, and polyphenylene ether.

(2) Influence of polymer crystallinity on product performance

① Density

High degree of crystallinity indicates that most of molecular chains have been arranged in an orderly and compact structure. Intermolecular force is strong, so density increases with increase of crystallinity. For example, density of polypropylene with 70% crystallinity is 0.896g/cm3. When crystallinity increases to 95%, density increases to 0.903g/cm3.

② Tensile strength

High crystallinity and high tensile strength. For example, polypropylene with 70% crystallinity has a tensile strength of 27.5Mpa. When crystallinity increases to 95%, tensile strength can be increased to 42 Mpa.

③ Impact strength

Impact strength decreases with increase of crystallinity. For example, 70% crystallinity polypropylene has a notched impact strength of 14.9KN·m/m2. When crystallinity is 95%, impact strength decreases to 4.77KN·m/m2.

④ Stiffness

Modulus of polypropylene with 70% crystallinity is 4400Mpa, and when it reaches 95%, it drops to 980Mpa.

⑤ Thermal performance

Increase in crystallinity helps to increase softening temperature and heat distortion temperature. For example, polypropylene with a crystallinity of 70% has a heat distortion temperature of 124.9℃ under load and 151.1℃ with a crystallinity of 95%. Rigidity is one of demolding conditions for injection molded products, higher crystallinity will reduce cooling cycle of product in mold. Crystallinity will bring brittleness to low temperature. For example, isotactic polypropylene with crystallinity of 55%, 85%, and 95% has embrittlement temperatures of 0℃, 10℃, and 20℃.

⑥ Warpage

Increase in crystallinity will reduce volume and increase shrinkage. Crystalline materials are more susceptible to warpage than amorphous materials. This is because when product is cooled in mold, difference in temperature causes difference in crystallinity, resulting in uneven density and unequal shrinkage, resulting in higher internal stress, , causing warpage, and reducing stress crack resistance.

⑦ Glossiness

Increase in crystallinity will increase compactness of product and improve surface finish of product, but presence of spherulites will cause scattering of light waves and reduce transparency.

(3) Factors affecting crystallinity

① Temperature and cooling rate

Crystallization has a thermal history, which must be related to temperature. When polymer melt temperature T is higher than melting temperature Tm, thermal motion of macromolecular chain increases significantly. When cohesion is greater than that of molecule, molecules are difficult to form an ordered arrangement and are not easy to crystallize; when temperature is too low, kinetic energy of macromolecular segment is very low, even in a frozen state, and it is not easy to crystallize.
Therefore, crystallization temperature range is between glass transition temperature Tg and melting temperature Tm. In high temperature area (close to Tm), solar nucleus is unstable and number of nucleation per unit time is small, while in low temperature area (close to Tg), free energy is low, crystallization time is long, and crystallization speed is slow, which cannot create conditions for nucleation. In this way, there is a highest crystallization rate (Vmax) and corresponding crystallization temperature (Tvmax) between Tm and Tg.

② Melt stress

Implementation shows that increase of melt stress and strengthening of shearing effect will accelerate crystallization process. This is because stress will cause chain segments to oriented along direction of force to form an ordered region, which easily induces many embryos, increases number of crystal nuclei, shortens time for crystallization, and accelerates crystallization. For example, investigation of polymerized propylene found that when pressure is increased, not only crystallinity and density are increased, but also crystallization temperature is increased.
Through above introduction, we know how important it is to control plastic melt temperature, mold temperature and cooling rate in injection molding, because this will have an important impact on crystallinity and internal quality of product.

How does shape of plastic structure affect quality of product

2. Orientation effect

(1) Orientation mechanism

In process of polymer processing, under action of force, flowing macromolecular segments will definitely be oriented, but nature of orientation and degree of orientation are very different according to orientation conditions. According to force form and nature of macromolecules in melt, it can be divided into "flow orientation" under shear stress and "stretch orientation" under stretching.

(2) Influence of orientation on product performance

Since orientation of amorphous polymer is orientation of macromolecular chain in direction of stress action, mechanical properties in orientation direction are significantly increased, while mechanical properties perpendicular to orientation direction are significantly reduced; tensile strength (σu) and elongation at break (εu) in orientation direction increase as degree of orientation increases. For example, if a high-density polyethylene sample with a thickness of 3mm and a width of 39.6mm is heated to 93°C for tensile orientation, ultimate tensile strength is increased from 16.3MPa to 75.9Mpa, which is a 4 times increase.
Orientation of crystalline polymer is effected by connecting wafer segments, its strength increases with orientation of linear segments. Due to existence of straight segments between wafers, crystalline polymer has toughness and elasticity. As degree of orientation increases, density and strength of material increase correspondingly, while elongation decreases. Orientation is effective only at orientation at melting temperature, and if crystallization temperature is lower than crystallization temperature, there is no shearing effect, so there is no orientation effect.
Mechanical properties of biaxially oriented products are anisotropic and are related to stretching ratio in two directions. Biaxial orientation changes mechanical properties of uniaxial orientation. Under normal injection molding conditions, impact strength of injection molded products in flow direction is about 1-2.9 times that of vertical direction, and impact strength is 1-10 times. It shows that impact strength perpendicular to flow direction is much reduced.
Glass transition temperature of injection molded product increases with increase in degree of orientation, with increase in the degree of orientation and crystallinity, Tg value of polymer can increase by 25°C.
Because there is a certain degree of high elastic deformation in product, oriented molecular chain segment at a certain temperature will have a relaxation effect: molecular chain of amorphous polymer must be curled up again, crystalline polymer will undergo secondary crystallization. In this case, product will shrink, and its thermal shrinkage is directly proportional to degree of orientation.
So degree of shrinkage is a reflection of degree of orientation. Linear expansion coefficient will also vary with degree of orientation, and linear expansion coefficient perpendicular to flow direction is about 3 times larger than orientation direction. Oriented macromolecules are elongated, force between molecules increases, and phenomenon of "stress hardening" occurs, which shows phenomenon that elastic mold of injection molded products is improved. The larger "freeze orientation", the easier it is for stress relaxation (polymer orientation or crystallization) to occur, and the greater shrinkage of product. Therefore, shrinkage of product reflects degree of orientation.

(3) Factors affecting product orientation

In injection molding, orientation process of polymer melt can be carried out in two stages. First stage is filling stage. Its flow is characterized by low melt pressure and high shear rate. Material at cavity wall is carried out under fast cooling conditions. Viscosity of polymer melt at this stage is mainly temperature and shear. Function of rate. Second stage is pressure-holding stage, which is characterized by low shear rate, high pressure, and a gradual decrease in temperature.
Viscosity of polymer melt mainly depends on temperature and injection pressure. But main influence on orientation is melt processing temperature (Tp), and main influence on crystallization is mold temperature (TM).
Orientation is not only related to shearing or pulling, but also to Brownian motion of molecules and free energy of macromolecular chains. According to this mechanism, conditions for controlling orientation have following factors.
①Increase of material temperature and mold temperature will reduce orientation effect. Because viscosity will decrease when melt temperature rises, under a certain constant stress, both high elastic deformation and viscous deformation will increase, but the former will increase only limited, while the latter will increase rapidly. From this point of view, we can see that it is beneficial to orientation effect of polymer; but at the same time, Brownian motion of macromolecules is intensified, and relaxation time of macromolecules is shortened, which strengthens de-orientation effect. Final orientation effect of polymer is determined by synthesis of these two factors.
If melt processing temperature is high, temperature range between solidification temperature and solidification temperature will be widened, relaxation time will be lengthened, and de-orientation will be easy. Relaxation time of amorphous polymers is time from temperature Tp to Tg, while for crystalline polymers, cooling rate is high, relaxation process is short, and freeze orientation is easy to occur. Amorphous polymer has a slow cooling rate, a long relaxation process, is easy to de-orientate, and orientation effect will be reduced. In addition to above, cooling rate is also related to specific heat capacity of polymer, latent heat of crystallization and melting, and thermal conductivity. The larger value of three, the stronger solution orientation effect.
②Increase in injection pressure can increase shear stress and shear speed of melt, and help accelerate orientation effect of polymer. Therefore, increase in injection pressure and holding pressure will strengthen crystallization and orientation effect, density of product will increase rapidly with increase in holding pressure.
③Closing time will affect orientation effect. If thermal motion of macromolecules is still strong after melt flow stops, oriented unit will relax again, resulting in a de-orientation effect. Use of large gates has a slower cooling, longer sealing time and longer melt flow time, thereby enhancing orientation effect, especially orientation at the gate is more obvious, so direct gate is easier to maintain orientation effect than point gate.
④ When mold temperature is low, effect of freezing on orientation increases, while effect of de-orientation decreases.
⑤ Influence of filling speed on product orientation needs specific analysis. Rapid mold filling will cause melt on the surface to be highly oriented, but internal orientation is very small. This is because under certain temperature conditions, rapid filling will maintain a higher temperature in the core of product, prolong cooling time and polymer relaxation time, strengthen de-orientation ability, so degree of core orientation is smaller than that of surface layer.
Under same conditions of injection temperature, slow filling will extend flow time, reduce melt temperature and increase shear stress. Interval between actual temperature (Tp) of melt and glass transition temperature (Tg) or melting point (Tm) is smaller than fast filling interval, and stress relaxation time is also short, so deorientation effect is small; on the other hand, temperature of slow filling melt is lower than that of fast filling, Brownian motion ability of macromolecules is weakened, deorientation effect is reduced, and orientation effect is increased.
As far as structure of core of the product is concerned, rapid filling will cause smaller orientations, while slow filling will cause larger orientations. This situation has been confirmed by experiments. For example, in ABS tensile test, product obtained by fast filling has a smaller shrinkage rate than slow filling, indicating that effect is small, but as far as surface layer is concerned, maximum orientation is still larger than slow filling.
In summary, factors affecting crystallization and orientation of polymers are as follows:
⑴ Temperature
① Temperature of melt processing (Tp)
② Mold temperature (Tm)
③ Melting point Tm (Tf) of polymer
④ Glass transition temperature of polymer (Tg)
⑤ Maximum crystallization rate temperature of melt (Tvmax)
⑵ Time
① Polymer heating time
② Filling time
③ Holding pressure time
④ Gate closing time
⑤ Cooling time (time from melt to solidification)
⑶ Pressure
① Filling pressure
② Holding pressure
⑷ Speed
① Mold filling speed (injection speed)
② Plasticizing speed (screw speed)
Above factors all affect original crystal nucleus number (crystal nucleus), spherulite size, and spherulite distribution of polymer melt; affect cooling rate, crystallinity, melt viscosity, shear stress or shear rate; affect balance of orientation and de-orientation of melt unit; it will ultimately affect density (or specific volume), mechanical properties, stress size and distribution of product; affect deformation, warpage, shrinkage, dimensional accuracy of product, and surface quality determined by filling flow.

Gud Mould Industry Co., Ltd