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Mold and injection molding(2)

2025,08,26
 
Introduction
 
Process characteristics
contraction
After the plastic part is removed from the mold and cooled to room temperature, it produces size shrinkage, which is called shrinkage. Because shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to various forming factors, so the shrinkage of plastic parts after molding should be called molding shrinkage.
 
The form of forming contraction is mainly manifested in the following aspects:
 
Plastic parts due to thermal expansion and contraction, elastic recovery, plastic deformation and other reasons caused by linear size shrinkage, so that plastic parts after mold cooling to room temperature after the size of the shrinkage, in the cavity design must be compensated for this.
In shrinkage oriented molding, the molecules are arranged in the direction, so that the plastic parts show anisotropy. Along the flow direction (parallel direction), large shrinkage, high strength; Along the direction at right angles to the material flow (vertical direction), shrinkage, low strength. In addition, due to the uneven distribution of density and filler in each part of the plastic parts during molding, it also causes uneven shrinkage. Shrinkage difference makes plastic parts prone to warping, deformation, cracking and other phenomena, especially in extrusion, injection molding, directionality is more obvious. Therefore, the direction of shrinkage should be considered in the design of the mold, and the appropriate shrinkage rate should be selected according to the shape of the plastic part and the direction of flow.
Sometimes plastic parts need heat treatment after molding according to performance and process requirements, and the size of plastic parts will also change after treatment. Therefore, when designing high-precision plastic parts molds, we should consider and compensate for post-shrinkage and post-processing shrinkage errors.
Shrinkage calculation The shrinkage of plastic parts in the molding process can be expressed by the shrinkage rate, as shown in Equations (1-1) and (1-2).
 
(1-1) Qact= (ab) /b×100
 
(1-2) Q count = (cb)/b×100
 
Where: Q actual - actual shrinkage rate (%)
 
Q-meter - Calculation of shrinkage rate (%)
 
a -- Unidirectional dimension of plastic parts at molding temperature (mm)
 
b - Unidirectional dimensions of plastic parts at room temperature (mm)
 
c - Mold unidirectional dimension at room temperature (mm)
 
The actual shrinkage represents the actual shrinkage of plastic parts. Since its value is very different from the calculated shrinkage, Q is used as the design parameter to calculate the size of cavity and core when designing the mold.
 
Factors influencing the change in systolic rate. In the actual molding, not only the shrinkage rate of different types of plastics is different, but also the shrinkage value of different batches of the same kind of plastics or different parts of the same plastic part is often different. The main factors affecting the shrinkage rate change include the following aspects.
 
All kinds of plastics have their own shrinkage range. The same kind of plastic will have different shrinkage rate and anisotropy due to different fillers, molecular weights and ratios.
Plastic parts characteristics, shape, size, wall thickness, the presence or absence of inserts, the number of inserts and the way of layout also have a great impact on the shrinkage rate of plastic parts.
The parting surface and pressure direction of the mold structure, the form, arrangement and size of the pouring system also have a great influence on the shrinkage and directionality, especially in extrusion and injection molding.
Molding process extrusion, injection molding process generally shrink-rate is large, and the direction is obvious, preheating conditions, molding temperature, molding pressure, holding time, filling material form, hardening uniformity and so on all affect the shrinkage rate and direction.
As mentioned above, the mold design should be based on the shrinkage range provided by 
 
In addition, the molding shrinkage rate is also affected by various molding factors, but mainly depends on the type of plastic, the shape and size of plastic parts. Therefore, adjusting various molding conditions during molding can also appropriately change the shrinkage rate of plastic parts.
 
liquidity
The ability of a plastic to fill a cavity at a certain temperature and pressure is called fluidity. This is an important process parameter that must be considered when designing molds. Too much fluidity, easy to cause excessive overflow, insufficient cavity filling, loose plastic parts, resin and filler separation and accumulation, easy to stick to the mold, demolding cleaning difficulties, premature hardening and other shortcomings. However, the fluidity is small, the filling is insufficient, the molding is difficult, and the molding pressure is large. Therefore, the fluidity of the selected plastic must be adapted to the plastic part requirements, molding process and molding conditions. When designing the mold, the gating system, parting surface, feeding direction, etc. should be considered according to the flow performance. The flow properties of thermosetting plastics are usually expressed in terms of the Lassig flow rate (in millimeters), with higher values indicating better fluidity. Each type of plastic is usually divided into three different fluidity grades to accommodate different plastic parts and molding processes. Generally, when the plastic area is large, the inserts are more, the core and inserts are thin, and the shape is complex, there is a narrow deep groove, thin wall and other situations that are not conducive to filling, the plastic with better fluidity should be selected. For extrusion molding, a Rasig flow rate greater than 150mm should be used, and for injection molding, a Rasig flow rate greater than 200mm should be used. In order to ensure that each batch of plastic has the same fluidity, in practical applications, the parallel batch method is often used to adjust, that is, the same type of plastics with different fluidity are used together, so that the fluidity of each batch of plastic compensates each other to ensure the quality of plastic parts. Rasig fluidity values for commonly used plastics are detailed in Tables 1-1. However, it must be pointed out that in addition to the type of plastic, the fluidity of plastic is often affected by many factors when filling the cavity, and the actual filling amount of the cavity is affected by the plastic capacity. If the particle size is small and uniform (especially round particles), high humidity, contains a lot of water and volatiles, suitable preheating and molding conditions, good surface finish of the mold, and suitable mold structure, it is conducive to improving the fluidity. On the contrary, poor preheating or molding conditions, poor mold structure, flow resistance, or plastic storage period is too long, expired, storage temperature is too high (especially amino plastics) will lead to the decline of the actual flow performance of plastic when filling the cavity, resulting in poor filling.
 
Specific volume and compression rate
Specific volume is the volume occupied per gram of plastic (in cm3/g). The compression ratio is the ratio of the plastic powder to the volume or specific volume of the plastic part (its value is always greater than 1), both of which can be used to determine the size of the mold charging chamber. The value is large, and the volume of the charging chamber is large, which also means that the plastic powder contains a lot of air, and the exhaust is difficult, resulting in a long molding cycle and low productivity. The opposite is true when the specific volume is small, which is beneficial to the pressing of the ingot. Specific volumes of various plastics are detailed in Table 1-1. However, the specific volume value is often inaccurate due to the particle size of plastic and the inhomogeneity of particles.
 
Characteristic of hardening
During the molding process, the thermosetting plastic changes into a plastic viscous flow state under heating and pressure, and then the fluidity increases and fills the mold cavity. At the same time, the condensation reaction occurs, the cross-link density continues to increase, the fluidity decreases rapidly, and the melt gradually solidify. In the design of the mold, for materials that harden quickly and maintain a short flow state, attention should be paid to facilitate the loading and unloading of inserts, select reasonable forming conditions and operations, and avoid premature hardening or insufficient hardening, resulting in poor molding of plastic parts.
 
The hardening rate can be generally analyzed from the holding time, which is related to the type of plastic, wall thickness, shape of plastic parts, mold temperature, etc. But it is also affected by other factors, especially related to the preheating state. Appropriate preheating should maintain the conditions that maximize the fluidity of the plastic, and try to improve its hardening rate, generally high preheating temperature, long time (within the allowable range), hardening rate will be accelerated, especially if the high frequency of prepressed billet preheating, hardening rate will be significantly accelerated. In addition, if the forming temperature is high and the pressing time is long, the hardening rate will also be accelerated. "Therefore, the hardening rate can also be appropriately controlled by adjusting the preheating or forming conditions."
 
The hardening speed should also meet the requirements of the molding method, such as injection and extrusion molding, requiring slow chemical reaction when plasticizing and filling, slow hardening speed, and long flow state maintenance time, but when filled with the cavity, to withstand high temperature and high pressure, should be fast hardening.
 
Moisture and volatiles content
All kinds of plastics contain different degrees of moisture and volatiles, when too much, the fluidity increases, easy to overflow, long retention time, increased shrinkage rate, easy to produce ripples, warping and other defects, affecting the mechanical and electrical properties of plastic parts. But the plastic too dry, will also lead to poor fluidity, molding difficult. Therefore, different plastics should be preheated and dried as needed. For materials with strong moisture absorption, especially in the wet season, even if the material is preheated, it should be prevented from moisture absorption again.
 
Due to the different components of moisture and volatiles contained in various plastics, and the condensed water will be produced during the condensation reaction, these components need to be transformed into gas to discharge the mold during the molding. Some gases have a corrosive effect on the mold and also have a stimulating effect on the human body. Therefore, the characteristics of various plastics should be understood in the design of the mold, and corresponding measures should be taken, such as preheating during molding, chrome plating of the mold, opening of the exhaust groove or setting up the exhaust process.
 
Methods
Plastic products are made from a mixture of synthetic resins and various additives as raw materials, and are made by injection, extrusion, pressing, casting and other methods. Plastic products are also in the molding process to obtain the final performance, so plastic molding is a key process in production.
 
Injection moulding
Injection molding, also known as injection molding, is a molding method that uses an injection machine to quickly inject molten plastic into the mold and obtain various plastic products after curing. This method can be used for almost all thermoplastics (except fluorine plastics), and can also be used for the molding of some thermosetting plastics. Injection molding accounts for about 30% of the production of plastic parts. It has the advantages of complex shape, precise size and high productivity. However, the cost of equipment and molds is high, which is mainly used for the production of large quantities of plastic parts.
 
 
Injection molding
C
 
Extrusion molding
Extrusion molding is the use of screw rotation and pressure will be plasticized plastic continuous extrusion into the mold, when through a certain shape of the mouth die, to obtain the shape of the Plastic profile to adapt to the shape of the mouth. Extrusion molding accounts for about 30% of plastic products. It is mainly used for various plastic profiles with a certain cross section and a large length, such as plastic tubes, plates, rods, sheets, strips, materials and special-shaped materials with complex cross sections. It is characterized by continuous molding, high productivity, simple mold structure, low cost, and close organization. In addition to fluorine plastics, almost all thermoplastics can be extruded, and some thermosetting plastics can also be extruded.
 
The granular plastic is sent into the screw propulsion chamber by the hopper, and sent to the heating zone by the rotating screw for melting and compression; Under the action of the screw force, the extrusion die with a certain shape is forced to obtain the profile consistent with the section shape of the mouth die. After falling on the conveyor belt, spray air or water to make it cool and harden, and get cured plastic parts.
 
Molding by mould
 
Pressing molding process can be generally divided into feeding, mold, exhaust, curing, stripping several stages. Plastic parts need to be processed after demolding, and the processing method is the same as that of injection molded parts.
 
Blow molding
 
 
Casting.
 
Gas-assisted injection molding
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