The factors of deformation of plastic injection products are summarized
The factors of deformation of plastic injection products are summarized
According to the molecular structure of resin in plastics, plastic can be divided into two categories: crystalline plastic and amorphous (also known as amorphous) plastic. The main difference between crystallized plastic and non-crystalline plastic is whether the resin molecules can form some kind of regular arrangement from melting state to condensation. If a certain degree of regulation is formed, it is called the crystalline type, whereas it is called a non-crystalline form.
Crystallization type plastic and an amorphous type plastic mechanical properties not only have significant differences, and from the viscous flow transition to the glassy state their difference in shrinkage rate, crystal type of plastic shrinkage rate is usually crystallization type plastic shrinkage ratio of 210 times. At the same time, for example, in the case of crystalline plastics, if the cooling rate of each part on cooling, can also cause products within the crystallinity of each part are different, the difference of crystallinity will produce internal stress in the products, causing deformation and cracking. Therefore, for crystalline plastic, the design of the cooling system of mold is an important aspect of the deformation degree of products.
Resin raw materials, in addition to outside, usually also will add some filler to improve the mechanical, electrical, optical and thermal plastic, such as performance, these packing during injection molding due to shear flow can form molecular orientation, given the fiber orientation distribution by Moldflow software analysis. The withdrawal rate of the products in different directions can be significantly different. For example, of 30 glass fiber reinforced PA66, the shrinkage in the direction of flow is 0.4, and in a direction perpendicular to the direction of flow is 1.1, not same direction shrinkage difference creates an internal stress products inside. Therefore, in the case of fiberglass reinforced plastic products, the non-uniform orientation of filler is often the main cause of the deformation of products. In the production practice, in order to improve or eliminate the deformation caused by this product, people usually adopt measures such as changing gate position and adjusting injection speed.
The forming method
At present, there are many kinds of forming methods, such as ultra-high speed injection molding and gas auxiliary forming, which are based on traditional injection molding.
Especially the application of gas-assisted molding has from the initial shape of handles, handrails and simple products, gradually transition to home appliances, audio, car, office supplies and toys and other industries of complex products production, gas production more and more kinds of products, auxiliary forming shape is becoming more and more complex.
Gas-assisted molding broke through the traditional injection molding technology limited, is the innovation of the traditional injection molding technology, it not only save plastic raw materials, to eliminate products surface shrink mark, shorten the molding cycle, simplifying the product design, reduce the mold cost advantages, at the same time also can reduce injection pressure, thereby reducing the pressure in the die differences, so as to reduce the internal stress and deformation. Auto parts bumper with traditional injection molding, the thin plate surface deformation is serious, and is irregular S shape. The whole plate is not deformed by the use of air - assisted forming, and the thin plate surface is a smooth curve.
Product design
The influence of product design on deformation mainly focuses on the thickness of the products, the uniformity of wall thickness, the shape of products and the rigidity of structure.
The influence of the thickness of the product wall on deformation is usually different due to the type of plastic. For non-crystalline plastic, the wall thickness is too thin and the orientation is serious.
For this kind of plastic products, therefore, to reduce the deformation is usually appropriate increase wall thickness, when the design purpose is to reduce the adverse impact of molecular orientation, and appropriately increase wall thickness to some extent increase the rigidity of the products and thus reduce the deformation of the products.
In the case of crystalline plastic products, the measures to reduce deformation in design are often to reduce wall thickness and improve the uniformity of wall thickness. The reason for reducing wall thickness is the large shrinkage of crystalline plastic, which reduces the volume shrinkage of the product. Can see that using software Moldflow analysis to improve the wall thickness uniformity of the reason is: if the wall thickness is uneven, thin wall and thick wall there are differences between the crystallinity, the difference of the different parts of the crystallinity that there are differences between the different parts of the contract, so as to produce internal stress and deformation of products.
Mold design
Reasonable gate position can not only reduce the difference of shrinkage of different parts of the product, but also reduce the negative impact of uneven orientation.
Usually gate opening in the wall thickness of the products was quite thick parts, it is advantageous to the plastic melt flow and filling materials, avoid the opposite case for distal wall thickness thin wall fill material and make the gate end in produced the phenomenon of pressure.
The design of runner and gate size relates to the degree of shear when the product is filled with pressure and plastic after injection. If the flow channel and gate size are too small, the products may not be sufficient. Excessive size, products may overpressure. Underpressure and overpressure can cause deformation of products.
The key of cooling system layout and control is to make the products uniformly cool everywhere, so the products should be considered in the first place to achieve uniform cooling effect. When the piping arrangement is difficult to be realized, it can be considered that different inlet temperature can be used for the mold, and it is difficult to open the core of the cooling line to consider the adoption of high thermal conductivity materials such as beryllium copper.
Ejection system design must ensure that products do not deformation due to launch, therefore should be careful analysis and calculation of mold enclasp force and adhesive force of size, reasonably choose the way to introduce and launch position.
Injection molding process conditions
Mold temperature, melt temperature, pressure protection time and pressure pressure and other forming conditions are closely related to the deformation of products.
Mold temperature directly affect the cooling rate, usually rapid cooling can reduce shrinkage of products, but for crystalline plastics, negative effects caused by the rapid cooling should be paid attention to: the size of the products poor stability, large difference of products of different parts of the contract, the buckling deformation. Therefore, the mould temperature can be increased appropriately for the crystalline plastic products, so that the cooling rate is slow but the purpose of uniform cooling can be achieved.
Melt temperature, holding time and holding pressure bears on the pressure level of the products, it has already been mentioned and low pressure and pressure maintaining excessive can cause deformation, therefore, reasonable choice should make the holding appropriate process.
In addition, the injection time is also related to the deformation of products in the forming process. The reason is that the injection time will affect the orientation degree, thus affecting the deformation of the products.
The deformation of plastic injection is the result of five factors, such as forming raw material, forming method, product design, mould design and injection molding process condition.
To solve or reduce the deformation of the plastic forming methods should be determined firstly, forming method for reasonable design of products for different shapes and sizes, in product design and under the condition of forming methods have been identified to deal with the relationship between mold and forming technology, in view of the specific situation analysis to determine the main factors and take feasible measures, do be done in this way can solve the problem of deformation.
