About the internal stress of plastic products

Talking about the internal stress of plastic products?

The internal stress of plastic/plastic products/plastic stress marks refers to the plastic material after high-temperature melting, extrusion, blow molding, blown film, injection blowing, injection and other molding methods, due to the impact of most of the molecular chain A kind of internal stress rebound in the product material caused by factors such as different orientation and cooling shrinkage is the rearrangement of molecular chains or polymer entanglement rebound and pull. The essence of internal stress is the unbalanced structural phenomenon formed by the molecular chain during the melting process. This unbalanced phenomenon cannot immediately return to the balance suitable for the environmental conditions during the cooling and solidification process. The essence of this unbalanced surface It is a reversible high elastic deformation, and the high elastic deformation of the frozen molecular chain is stored in the plastic product in the form of potential energy at ordinary times. Under suitable conditions, this forced unstable structure phenomenon will slow down. Slowly transform back to the free and stable structural phenomenon, and the potential energy begins to transform into kinetic energy and release outward. When the force between the molecular chains and the intertwined entanglement force cannot withstand this kinetic energy, when the corresponding limit is exceeded, the internal stress balance in the plastic product will be destroyed, and the plastic product will have excessive stress release. and cracking and warping deformation.

Almost all plastic products will have internal stress to varying degrees, that is, the residual stress inside the plastic, called plastic stress, especially the plastic injection molding products, the internal stress is more obvious. The existence of internal stress not only causes stress cracking and warping deformation of plastic products during storage and use, but also affects the mechanical properties, optical properties, electrical properties and appearance quality of plastic products. Products with a high-gloss surface are likely to have plastic stress marks, and these marks are difficult to eliminate unless the plastic stress can be eliminated in a timely manner during the production process.

Therefore, in the process of mass production, it is necessary to find out the cause of internal stress and the method of eliminating internal stress. It is necessary to carry out plastic stress analysis, first find out the main cause of stress, and then improve the main factors that cause high stress. , use various methods to minimize the internal stress of plastic products, and distribute and reduce the residual internal stress on plastic products as evenly as possible. The most important thing is to avoid excessive concentration of internal stress, because plastic stress If the distribution of the difference is too large, it is easy to cause cracks and traces to appear. To avoid the problem of plastic stress concentration and eliminate stress residue, it is necessary to carry out plastic stress analysis and plastic stress detection to improve the mechanical and thermal properties of plastic products.

Analysis of Internal Stress Generation of Injection Molded Products/Plastic Stress Analysis

Generation of internal stress of plastic product/Causes of Plastic Stress

In plastic injection molded products, the local stress state is different everywhere, and the degree of deformation of the product will be determined by the stress distribution. If the product is cooling. If there is a temperature gradient and the gradient is very large, this type of stress will develop and exist in the product, so this type of stress is also called "forming stress".

The internal stress of plastic injection molded products includes two situations: one is the molding stress of the injection molded product, and the other is the temperature stress. When the melt enters the mold with a lower temperature, the melt near the cavity wall of the mold will rapidly cool and solidify, so that the chain segments of the molecular chain are "frozen". Due to the solidified polymer layer, the thermal conductivity is poor, resulting in a large temperature gradient across the thickness of the article. The core of the product solidifies quite slowly, so that when the gate is closed, the melt core layer in the center of the product has not yet solidified, and the injection molding machine cannot supplement the cooling shrinkage. In this way, the internal shrinkage of the product is opposite to the action direction of the solidified layer of the skin layer of the product; the core is in static tension and the surface layer is in static compression.

In addition to the stress caused by the effect of volume shrinkage in the flow of the melt filling the mold. There is also the stress caused by the expansion effect of the runner and gate outlet; the stress caused by the former effect is related to the direction of melt flow, and the latter is due to the outlet expansion effect, which will cause stress perpendicular to the flow direction.

Process Factors Affecting Plastic Internal Stress / internal Stress of plastic product

The influence of the stress in the flow direction Under the condition of rapid cooling, the orientation of the molecular chain will lead to the formation of internal stress in the polymer. Due to the high viscosity of the polymer melt, the internal stress cannot be quickly rebounded and relaxed, which affects the physical properties and dimensional stability of the product.

The influence of various parameters on the orientation stress: plastic melt viscosity, melt temperature, filling speed, molding pressure, holding pressure change, mold temperature, cooling rate, product thickness, etc. The above conditions will affect the flow direction stress. , thereby changing the physical properties and dimensional stability.

Effect of Temperature on Plastic Product Stress

As mentioned above, due to the large temperature gradient between the melt and the mold wall when filling the mold, the outer layer melt that solidifies first reduces the shrinkage of the core layer melt that solidifies later, resulting in compressive stress (shrinkage stress) in the outer layer, The inner layer produces tensile stress (orientation stress).

If the mold is filled for a long time under the action of the holding pressure, the plastic polymer melt will be filled into the mold cavity again, so that the cavity pressure will increase, and this pressure will change the internal stress caused by uneven temperature. However, when the holding time is short and the cavity pressure is low, the interior of the product will still maintain the original stress state during cooling.

If the melt pressure in the mold cavity in the early stage of product cooling is insufficient, the outer layer of the product will form a depression due to solidification shrinkage; if the melt pressure in the mold cavity in the later stage when the product has formed a solidified layer, the core layer of the product will be It shrinks and separates, or forms a cavity; if the cavity pressure is maintained before the gate is closed, it will help to increase the density of the product and eliminate the cooling temperature stress, but there will be a large stress concentration in the area near the gate, but If the cavity pressure is maintained too high before the gate is closed, the internal stress in a large area will be too high, which will affect the physical properties.

Therefore, when the thermoplastic polymer is molded, the greater the in-mold pressure, the longer the holding time, which helps to reduce the shrinkage stress caused by the temperature, and conversely increases the compressive stress.

The relationship between internal stress and product quality

The existence of internal stress in the product will seriously affect the mechanical properties and performance of the product; due to the existence and uneven distribution of the internal stress in the product, cracks will occur in the product during use, starting from small cracks to cracking of the product. When used below the glass transition temperature, irregular deformation or warpage often occurs, and the surface of the product is "whitened", turbid, and the optical properties are deteriorated, and the refractive index and the refractive angle are inconsistent.

Try to reduce the temperature at the gate and increase the slow cooling time, which is beneficial to improve the uneven stress of the product and make the mechanical properties of the product more uniform.

Whether for crystalline polymers or non-crystalline polymers, tensile strength exhibits both longitudinal and transverse characteristics. For amorphous polymers, the tensile strength will vary depending on the position of the gate; when the gate is in the same direction as the filling cavity, the tensile strength decreases with the increase of the melt temperature; when the gate is perpendicular to the filling direction , the tensile strength increases with increasing melt temperature.

From this point of view, an increase in melt temperature leads to a decrease in tensile strength for both crystalline and non-crystalline polymers, but the mechanical properties are different; the former is due to the effect of the decrease through orientation.