About the post-shrinkage of plastic products
Plastics are divided into two categories: Crystalline plastics and Amorphous plastic.
Plastic products, from the injection molding process to the use of the product, there will be a shrinkage process of dimensional changes. Regardless of the dimensional changes caused by post-moisture absorption of certain materials, most of the materials alone are in the process. Shrinkage due to fluid-to-solid density change and post-shrinkage after cooling and setting.
There are crystalline parts and non-crystalline parts mixed in the molded product of crystalline resin. Since the crystals are random molecular chains, they will be regularly arranged. Therefore, when the resin starts to arrange, the volume will shrink. This phenomenon is called shrinkage. is a relatively large contraction.
In the post-shrinking part, that is, after the plastic product is cooled and set, it has been cooled and set from the molten fluid, and it seems that the size has been stabilized, but the plastic product still has post-shrinkage, crystalline plastic and amorphous plastic, compared between the two However, crystalline plastics have greater post-shrinkage than amorphous plastic due to the relationship of molecular chains.
Crystalline plastic
Crystalline plastic, also known as polymer crystallization, is a process in which polymer chains are partially arranged. During this process, polymer chains are stacked and arranged to form an ordered region, such a region is called a platelet. Crystallization affects the optical, mechanical, thermal and chemical properties of polymer materials. The degree of crystallinity can be measured by various analytical methods. Most crystalline materials are generally between 10% and 80%. Therefore, crystalline polymers are often called semi-crystalline polymers, while liquid crystal materials (LCP) are highly crystalline. Polymer Materials. The properties of crystalline polymers are not only characterized by crystallinity, but also by the stacking size and orientation of molecular chains. However, whether the crystal of a polymer is crystallized depends on its molecular structure—the side groups on the main chain are evenly distributed, which is conducive to crystallization. For example, isotactic polypropylene crystallizes more readily than atactic polypropylene. If the side chain of atactic polymers is small, the crystallinity can also be improved, such as vinyl polymers, if the substituted side chains are large, it is not easy to crystallize, and the crystallinity will be reduced.
Once the molten resin is solidified, its molecular chain is fixed, and the ratio of crystallized part/uncrystallized part in the solid molded product does not seem to change. At high temperature, or under the influence of external force, the molecular chains of the uncrystallized part will sometimes rearrange, resulting in crystallization. As a result, the volume of the molded product will be further reduced. This phenomenon is called post-contraction. Post-shrinkage can easily lead to dimensional changes, dents, warping and other undesirable phenomena.
When the ambient temperature of the molded product is high, it is more likely to cause post-shrinkage. If quenching is used in the process, sometimes the crystallization of the resin will be incomplete, and the crystallinity will not reach the original expected degree, which will easily cause post-shrinkage, and even affect the optical, mechanical, thermal properties and chemical properties of the product.
Factors that affect the crystallization of the process?
1) In the structure of the polymer chain, if the symmetry is good, only the main chain has no branch, or the branch is few or the side chain is small, the macromolecules with large intermolecular force are easy to stick to each other. Crystallization easily occurs.
2) Under the influence of temperature, the polymer moves from disordered random to the crystal surface that is being combined. When the mold temperature is high, it can slowly cool down from the high temperature melt to the shape, thus improving the mobility of the polymer, thereby speeding up the process. Crystallization speed.
3) The influence of pressure, if there is an external force in the cooling process, it can also promote the crystallization of the resin polymer, so the pressure of the melt and the pressure of setting can be adjusted to control the crystallinity of the crystalline plastic during production.
4) Adding crystal nucleating agent, because low temperature is conducive to rapid nucleation, but slows down the growth of crystal grains, so in order to eliminate this contradiction, add crystal nucleating agent and nucleating agent to the molding material, which can make the crystallinity Plastics crystallize rapidly at high mold temperatures.
Ways of post-processing to improve crystallization
To prevent post-shrinkage due to incomplete crystallization, fully crystallize the molded product before actually using it.
Specifically, one of the following conditions can be selected:
1- The temperature should be about 20℃ higher than the ambient temperature when the product is used, but not higher than the temperature of the second condition below.
2- Use about 15°C lower than the heat distortion temperature of the material.
Choose one of the above conditions, let it stand for 2 to 3 hours under such conditions, and then let it cool down to the ambient temperature naturally. This process is called annealing treatment. If the size of the product has fallen within the tolerance range after annealing, in most cases, there is usually no large change in size, and the annealed product, regardless of mechanical, thermal or chemical properties, will higher than that before unannealed treatment.
