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On the Internal Stress of Injection Molded Products

October 14,2022

The so-called stress refers to the force on an object per unit area, and it emphasizes the internal stress of the object. Generally, when an object is subjected to an external force, the stress resisting the external force will be generated inside it; When an object is not affected by external forces, its internal stress is called internal stress, which is caused by uneven plastic deformation of various parts inside the object.

According to the scope of internal stress, it can be divided into three categories:

(a) the first kind of internal stress (macro internal stress), that is, the internal stress within the macro scope caused by uneven deformation of various parts of the material; (2) The second kind of internal stress (microscopic internal stress), that is, the internal stress between grains or sub-grains of an object caused by uneven deformation between grains or sub-grains (in nature, most solid substances are crystals);
(3) The third kind of internal stress (lattice distortion stress), that is, the internal stress caused by some atoms in the crystal deviating from their equilibrium positions due to lattice distortion, is the most important internal stress in deformed objects (damaged objects).
The internal stress of plastics refers to an internal stress caused by factors such as orientation of macromolecular chains and cooling shrinkage during the melting process of plastics. The essence of internal stress is the unbalanced conformation of macromolecular chain formed in the melting process. This unbalanced conformation can’t be immediately restored to the equilibrium conformation suitable for environmental conditions when cooled and solidified. The essence of this unbalanced conformation is a reversible high elastic deformation, and the frozen high elastic deformation is usually stored in plastic products in the form of potential energy. Under suitable conditions, this forced unstable conformation will be transformed into a free and stable conformation, and the potential energy will be converted into kinetic energy and released. When the force between macromolecular chains and the intertwining force can’t bear this kinetic energy, the internal stress balance will be destroyed, and plastic products will have stress cracking and warping deformation.

Almost all plastic products have internal stress in varying degrees, especially the internal stress of plastic injection products is more obvious. The existence of internal stress not only makes plastic products warp and crack during storage and use, but also affects the mechanical properties, optical properties, electrical properties and appearance quality of plastic products. Therefore, it is necessary to find out the causes of internal stress and the ways to eliminate it, reduce the internal stress of plastic products to the greatest extent, and distribute the residual internal stress on plastic products as evenly as possible to avoid stress concentration, so as to improve the mechanical and thermal properties of plastic products.

Causes of internal stress in plastics
There are many reasons for internal stress, such as strong shearing action of plastic melt during processing, orientation and crystallization in processing, extremely difficult uniformity of cooling rate of various parts of the melt, uneven plasticization of the melt, difficulty in demoulding products, etc., which will all lead to internal stress. According to different causes of internal stress, internal stress can be divided into two categories, orientation internal stress and cooling internal stress.
How to deal with the internal stress of plastics
(1) design of raw material formula
1) resin with large molecular weight and narrow molecular weight distribution is selected; 2) resin with low impurity content is selected; 3) blending modification is performed; 4) reinforcement modification is performed; and 5) nucleation modification is performed.
(2) Control of molding processing conditions
① Cylinder temperature ② Mold temperature ③ Injection pressure ④ Holding pressure ⑤ Injection speed ⑥ Holding time ⑥ Residual pressure of mold opening.
Design of plastic products
① Shape and size of plastic products
In the concrete design of plastic products, in order to effectively disperse the internal stress, the following principle should be followed: the shape of the products should be as continuous as possible, and acute angles, right angles, notches and sudden expansion or contraction should be avoided.
② Reasonable design of metal inserts
③ Design of holes on plastic products

The shape, number and position of holes in plastic products will have great influence on the concentration of internal stress.

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