Plastic molding process

After the plastic part is taken out of the mold and cooled to room temperature, the dimensional shrinkage occurs, which is called shrinkage. Since shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to various forming factors, the shrinkage of the plastic part after forming should be called forming shrinkage.
1. Forms of forming shrinkage Forming shrinkage is mainly manifested in the following aspects:
(1) The linear size shrinkage of plastic parts shrinks due to thermal expansion and cold shrinkage, elastic recovery and plastic deformation when the plastic parts are demolded and cooled to room temperature. Therefore, the cavity design must be considered. compensate.
(2) Shrinkage directional When forming, the molecules are arranged in the direction, so that the plastic part is anisotropic, and the shrinkage is large and the strength is high along the material flow direction (ie, the parallel direction), and the shrinkage is small in the direction perpendicular to the material flow (ie, the vertical direction). , low intensity. In addition, due to the uneven distribution of density and filler in each part of the plastic part during molding, the shrinkage is also uneven. The resulting shrinkage difference makes the plastic parts prone to warping, deformation and cracks, especially in extrusion and injection molding, the directionality is more obvious. Therefore, the shrinkage direction should be considered in the mold design, and the shrinkage rate should be selected according to the shape and flow direction of the plastic part.
(3) When the post-shrinkage plastic parts are formed, due to the influence of forming pressure, shear stress, anisotropy, uneven density, uneven distribution of fillers, uneven mold temperature, uneven hardening, plastic deformation and other factors, a The effect of the series of stress cannot be completely disappeared in the viscous flow state, so there is residual stress when the plastic part is formed in the stress state. After demolding, due to the balance of stress and the influence of storage conditions, the residual stress changes and the plastic parts re-shrink is called post-shrinkage. Generally, the plastic parts change the most within 10 hours after demoulding, and they are basically finalized after 24 hours, but the final stability will take 30-60 days. Generally, the post-shrinkage of thermoplastics is larger than that of thermosets, and that of extrusion and injection molding is larger than that of compression molding.
(4) Post-treatment shrinkage Sometimes the plastic parts need to be heat treated after forming according to the performance and process requirements, and the size of the plastic parts will also change after treatment. Therefore, when designing the mold, the error of post-shrinkage and post-processing shrinkage should be considered and compensated for high-precision plastic parts.
2. Calculation of shrinkage rate The shrinkage of plastic parts can be expressed by shrinkage rate, as shown in formula (1-1) and formula (1-2).
(1-1)Q real=(a-b)/b×100
(1-2) Q meter=(c-b)/b×100
In the formula: Q real - actual shrinkage rate (%)
Q meter - Calculate shrinkage (%)
a—One-way dimension of plastic parts at forming temperature (mm)
b—One-way size of plastic parts at room temperature (mm)
c—The unidirectional dimension of the mold at room temperature (mm)
The actual shrinkage rate represents the actual shrinkage of the plastic part, because the difference between the value and the calculated shrinkage is very small, so the mold design uses Q as the design parameter to calculate the cavity and core size.
3. Factors affecting the change of shrinkage rate In actual molding, not only the shrinkage rates of different varieties of plastics are different, but also the shrinkage values ​​of different batches of the same variety of plastics or different parts of the same plastic part are often different, and the main factors affecting the shrinkage rate change are The factors are as follows.
(1) Plastic varieties Various plastics have their own shrinkage ranges. The same type of plastics have different shrinkage rates and anisotropy due to different fillers, molecular weights and ratios.
(2) Characteristics of plastic parts The shape, size, wall thickness, the presence or absence of inserts, the number and layout of inserts of plastic parts also have a great influence on the shrinkage rate.
(3) Mold structure The parting surface and pressure direction of the mold, the form, layout and size of the gating system also have a great influence on the shrinkage rate and directionality, especially in extrusion and injection molding.
(4) The extrusion and injection molding processes of the forming process generally have a large shrinkage rate and obvious directionality. Preheating conditions, forming temperature, forming pressure, holding time, filling material form and hardening uniformity all affect shrinkage and directionality.
As mentioned above, the mold design should be based on the shrinkage rate range provided in the specifications of various plastics, and according to the shape, size, wall thickness, the presence or absence of inserts, parting surface and pressure forming direction, mold structure and The form, size and position of the feeding port, the forming process and other factors are considered comprehensively to select the shrinkage value. For extrusion or injection molding, it is often necessary to select different shrinkage rates according to the shape, size, wall thickness and other characteristics of each part of the plastic part.
In addition, the forming shrinkage is also affected by various forming factors, but mainly depends on the type of plastic, the shape and size of the plastic part. Therefore, adjusting various forming conditions during forming can also appropriately change the shrinkage of plastic parts.
Plastic molding is the process of making various forms (powders, pellets, solutions and dispersions) of plastics into articles or blanks of desired shapes. There are as many as thirty kinds of molding methods.
The choice of plastic molding is mainly determined by the type of plastic (thermoplastic or thermoset), the starting form, and the shape and size of the product. Commonly used methods for processing thermoplastics include extrusion, injection molding, calendering, blow molding and thermoforming. Thermosetting plastics are generally processed by molding, transfer molding, and injection molding. Lamination, molding and thermoforming are the shaping of plastics on a flat surface. The above plastic processing methods can be used for rubber processing. In addition, there are castings using liquid monomers or polymers as raw materials. Among these methods, extrusion and injection molding are the most used and the most basic molding methods.