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Plastic molding

Plastic molding

Issue Time:2021-04-21

The choice of plastic molding is mainly determined by the type of plastic (thermoplastic or thermosetting), the initial form, and the shape and size of the product. The commonly used methods for processing thermoplastics include extrusion, injection molding, calendering, blow molding and thermoforming. The processing of thermosetting plastics generally uses compression molding, transfer molding, and injection molding. Plastic molding is the process of making various forms (powders, pellets, solutions and dispersions) of plastics into products or blanks in the desired shape. There are as many as thirty kinds of molding methods. Laminating, molding, and thermoforming are forming plastics on a flat surface. The above-mentioned plastic processing methods can all 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 they are also the most basic molding methods.

Process characteristics

Shrinkage

After the plastic part is taken out of the mold and cooled to room temperature, the size shrinkage 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 the plastic part due to thermal expansion and contraction, elastic recovery and plastic deformation during demolding, the size of the plastic part shrinks after being demolded and cooled to room temperature. For this reason, the cavity design must be considered make up.

(2) When shrinking directional forming, the molecules are arranged in the direction, which makes the plastic part present anisotropy. Along the material flow direction (ie parallel direction), the shrinkage is large and the strength is high, and the direction perpendicular to the material flow (ie vertical direction) is small shrinkage. , Low intensity. In addition, due to uneven density and filler distribution at various parts of the plastic part during molding, the shrinkage is uneven. The difference in shrinkage makes the plastic parts prone to warpage, deformation, and cracks, especially in extrusion and injection molding, the directionality is more obvious. Therefore, the shrinkage direction should be considered during mold design, and the shrinkage rate should be selected according to the shape of the plastic part and the direction of the flow material.

(3) When forming post-shrinking plastic parts, due to the influence of forming pressure, shear stress, anisotropy, uneven density, uneven filler distribution, uneven mold temperature, uneven hardening, plastic deformation and other factors, it will cause a The effect of the series of stresses cannot all disappear in the viscous flow state, so there is residual stress when the plastic part is formed under the stress state. After demolding, due to the stress balance and the influence of storage conditions, the residual stress changes and the plastic part shrinks again, which is called post-shrinkage. Generally, plastic parts change the most within 10 hours after demolding, and are basically shaped after 24 hours, but the final stabilization takes 30-60 days. Generally, the post-shrinkage of thermoplastics is larger than that of thermosetting, and extrusion and injection molding are larger than compression molding.

(4) Post-treatment shrinkage Sometimes plastic parts require heat treatment after forming according to performance and process requirements. After treatment, the size of the plastic parts will also change. Therefore, for high-precision plastic parts during mold design, post-shrinkage and post-processing shrinkage errors should be considered and compensated.

2. Calculation of shrinkage The forming shrinkage of plastic parts can be expressed by shrinkage, 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 rate (%)

a —The one-way size of the plastic part at the forming temperature (mm)

b — one-way size of plastic parts at room temperature (mm)

c — one-way size of the mold at room temperature (mm)

The actual shrinkage rate represents the actual shrinkage of the plastic part. Because its value is very different from the calculated shrinkage, the mold design uses Q as the design parameter to calculate the cavity and core size.

3. The factors that affect the change of shrinkage rate are not only different for different types of plastics in actual forming, but also different batches of the same type of plastic or different parts of the same plastic part have different shrinkage values. The main factors that affect the change of shrinkage rate are The factors are as follows.

(1) Plastic varieties. Various plastics have their own shrinkage ranges. The shrinkage and anisotropy of the same type of plastics are also different due to different fillers, molecular weights, and ratios.

(2) Characteristics of plastic parts The shape, size, wall thickness, presence or absence of inserts, the number and layout of the inserts 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 pouring system also have a greater impact on the shrinkage and directionality, especially in extrusion and injection molding.

(4) Molding process Extrusion and injection molding processes generally have larger shrinkage and obvious directionality. The preheating condition, forming temperature, forming pressure, holding time, filling material form and hardening uniformity all affect the shrinkage rate and directionality.

As mentioned above, the mold design should be based on the shrinkage range provided in the specifications of various plastics, and according to the shape, size, wall thickness, presence or absence of inserts, parting surface and pressure forming direction, mold structure and Various factors such as the form, size and position of the feed inlet, and the forming process are comprehensively considered 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, forming shrinkage is also affected by various forming factors, but it is mainly determined by 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 the plastic part.

fluidity

The ability of plastics to fill the cavity under a certain temperature and pressure is called fluidity. This is an important process parameter that must be considered during mold design. High fluidity is easy to cause excessive overflow, inadequate filling of the cavity, loose structure of plastic parts, accumulation of resin and filler separately, easy to stick mold, difficult to demold and clean, and premature hardening. However, if the fluidity is small, the filling is insufficient, forming is not easy, and the forming pressure is high. Therefore, the fluidity of the selected plastic must be compatible with the requirements of the plastic part, the forming process and the forming conditions. When designing the mold, the pouring system, parting surface and feeding direction should be considered according to the flow performance. The fluidity of thermoset plastics is usually expressed in terms of Raschig fluidity (in millimeters). Larger values ​​mean good fluidity. Each type of plastic is usually divided into three different levels of fluidity for different plastic parts and forming processes. Generally, plastic parts with large area, many inserts, thin cores and inserts, and complex shapes with narrow deep grooves and thin walls are not good for filling. Plastics with better fluidity should be used. Plastics with a Raschig fluidity of 150mm or more should be used for extrusion molding, and plastics with a Raschig fluidity of 200mm or more should be used for injection molding. In order to ensure that each batch of plastics have the same fluidity, the batch method is commonly used in practice to adjust, that is, to mix the same variety of plastics with different fluidity, so that the fluidity of each batch of plastics can compensate each other to ensure the quality of plastic parts . The Raschig fluidity values ​​of commonly used plastics are shown in Table 1-1. However, it must be pointed out that in addition to the variety of plastics, the fluidity of plastics is often affected by various factors when filling the cavity, so that the plastic actually fills the cavity. The ability to change. Such as fine and uniform particle size (especially round pellets), high humidity, high moisture and volatile matter, proper preheating and forming conditions, good mold surface finish, proper mold structure, etc., are all conducive to improving fluidity. Conversely, poor preheating or molding conditions, poor mold structure, large flow resistance, or plastic storage period, overdue, high storage temperature (especially for amino plastics), etc. will cause the actual flow performance of the plastic to decrease when filling the cavity and cause filling bad.

Specific volume and compression ratio

Specific volume is the volume occupied by each gram of plastic (in cm3/g). The compression ratio is the ratio of the volume or specific volume between the plastic powder and the plastic part (its value is always greater than 1). They can all be used to determine the size of the die loading chamber. A large value requires a large volume of the charging chamber, and at the same time indicates that the plastic powder is filled with a lot of air, it is difficult to exhaust, the forming cycle is long, and the productivity is low. The opposite is true if the specific volume is small, and it is conducive to pressing and pressing. The specific volume of various plastics is shown in Table 1-1. However, the specific volume value often has errors due to the particle size of the plastic and the unevenness of the particles.

Hardening characteristics

During the forming process, the thermosetting plastic transforms into a plastic viscous state under heating and pressure, and the fluidity increases to fill the cavity. At the same time, a condensation reaction occurs, the crosslinking density continues to increase, the fluidity drops rapidly, and the melt gradually solidifies . When designing the mold, for materials with fast hardening speed and short flow state, attention should be paid to easy loading, loading and unloading of inserts, and selection of reasonable forming conditions and operations to avoid premature hardening or insufficient hardening, resulting in poor molding of plastic parts.

The hardening speed can generally be analyzed from the holding time, which is related to the type of plastic, the wall thickness, the shape of the plastic part, and the mold temperature. However, it is also subject to other factors, especially related to the preheating state. Proper preheating should keep the plastic to maximize its fluidity and increase its hardening speed as much as possible. Generally, the preheating temperature is high and the time is long (when allowed Within the range), the hardening speed is accelerated, especially when the pre-compacted ingot is preheated with high frequency, the hardening speed is significantly increased. In addition, high molding temperature and long press time increase the hardening speed. Therefore, the hardening speed can also be adjusted by preheating or forming conditions to be appropriately controlled.

The hardening speed should also be suitable for the requirements of the forming method. For example, injection and extrusion molding should require slow chemical reaction and slow hardening during plasticization and filling, and should maintain a fluid state for a long time, but when the cavity is filled, it should be at high temperature and high pressure. The bottom should harden quickly.

Moisture and volatile content

Various plastics contain different levels of moisture and volatile content. When they are too much, the fluidity will increase, the material will overflow, the retention time will be long, the shrinkage will increase, and the defects such as ripples and warping will easily occur, which will affect the electrical and mechanical properties of the plastic parts. However, when the plastic is too dry, it will be difficult to form with poor fluidity. Therefore, different plastics should be preheated and dried as required. For materials with strong moisture absorption, especially in the humid season, even the preheated materials should be prevented from re-absorption.

Since various plastics contain different components of water and volatiles, and at the same time condensation occurs during the condensation reaction, these components need to be turned into gases and discharged out of the mold during forming. Some gases have a corrosive effect on the mold and have a corrosive effect on the human body. Stimulating effect. Therefore, in the mold design, we should understand the characteristics of various plastics, and take corresponding measures, such as preheating, mold chrome plating, opening an exhaust groove or setting an exhaust process during forming.

Method editing

Plastic products are made of a mixture of synthetic resin and various additives as raw materials by injection, extrusion, pressing, pouring and other methods. While plastic products are being molded, they also obtain the final performance, so plastic molding is a key process for production.

injection

Figure 1 Injection molding

Injection molding, also called injection molding, is a method of using an injection machine to quickly inject molten plastic into a mold and solidify it to obtain various plastic products. Almost all thermoplastics (except fluoroplastics) can use this method, and it can also be used for the forming of some thermosetting plastics. Injection molding accounts for about 30% of the production of plastic parts. It has the advantages of being able to form complex shapes at one time, precise dimensions, and high productivity. However, the cost of equipment and molds is relatively high, and it is mainly used for the production of large quantities of plastic parts.

There are two commonly used injection molding machines: plunger type and screw type. Figure 1 is a schematic diagram of screw type injection molding. The principle of injection molding: the powdery and granular raw materials are fed into the barrel from the hopper. When the plunger advances, the raw materials are pushed into the heating zone, and then through the shunt shuttle, the molten plastic is injected into the mold cavity through the nozzle, and the plastic product is obtained by opening the mold after cooling. After the plastic injection part is taken out of the mold cavity, proper post-treatment is usually required to eliminate the stress generated during the forming of the plastic part and stabilize the size and performance. In addition, there are cutting off burrs and gates, polishing, surface coating, etc.

Extrusion molding


Extrusion is a process in which plasticized plastic is continuously extruded into a mold by means of screw rotation and pressure, and a plastic profile that is compatible with the shape of the mold is obtained when it passes through a certain shape of the die. Extrusion molding accounts for about 30% of plastic products, and is mainly used for various plastic profiles with a certain cross section and large length, such as plastic tubes, plates, rods, sheets, strips, materials, and special-shaped materials with complex cross-sections. It is characterized by continuous forming, high productivity, simple mold structure, low cost, compact organization and so on. Except for fluoroplastics, almost all thermoplastics can be extruded, and some thermosetting plastics can also be extruded.

Figure 2 is a schematic diagram of spiral extrusion. The granular plastic is sent from the hopper into the spiral chamber, and then sent to the heating zone by the rotating screw to melt and be compressed; under the action of the spiral force, it is forced to pass through the extrusion with a certain shape Mold, obtain a profile consistent with the cross-sectional shape of the die; after it falls on the conveyor belt, it is cooled and hardened by jetting air or water to obtain a solidified plastic part

.

Pressing

Figure 3 Press molding

Figure 3 Press molding

Compression molding, also known as compression molding, compression molding, compression molding, etc., is to add solid pellets or prefabricated pieces into a mold, and then soften and melt it by heating and pressurizing, and fill the mold under pressure. Cavity, the method of obtaining plastic parts after curing. Press molding is mainly used for thermosetting plastics, such as phenolic, epoxy, silicone, etc.; it can also be used to press thermoplastic polytetrafluoroethylene products and polyvinyl chloride (PVC) records. Compared with injection molding, the press molding equipment and mold are simple, and can produce large-scale products; but the production cycle is long, the efficiency is low, it is difficult to realize automation, and it is difficult to produce thick-walled products and products with complex shapes.

Figure 3 is a schematic diagram of press forming. The general press forming process can be divided into several stages: feeding, clamping, venting, curing and demolding. After demolding the plastic part, post-treatment should be carried out, and the treatment method is the same as that of the injection-molded plastic part.

Blow molding


Blow molding (belonging to the secondary processing of plastics) is a processing method in which hollow plastic parisons are blown and deformed by means of compressed air, and the plastic parts are obtained after cooling and shaping. The methods mainly include hollow blow molding and film blow molding.

Figure 4 is a schematic diagram of the extrusion blow molding of a hollow part. The extruded or injected tubular parison with a certain temperature is placed in a split blow mold, the mold is closed, and compressed air is blown in through a blow tube to inflate the parison Then make it close to the mold wall, after holding pressure, cooling and shaping, the mold is opened and the hollow part is taken out.

Casting

The casting of plastic is similar to the casting of metal. The processing method of injecting polymer materials or monomer materials in a fluid state into a specific mold, reacting and curing them under certain conditions, and forming a plastic part consistent with the mold cavity. This forming method has simple equipment, does not require or slightly pressurize, has low mold strength requirements, and has low production investment, and can be applied to various sizes of thermoplastic and thermosetting plastic parts. However, plastic parts have low precision, low productivity, and long forming cycles.

Gas-assisted injection molding

Gas-assisted injection molding (referred to as gas-assisted molding) is a new method in the field of plastic processing. The gas-assisted forming process can be roughly divided into 3 methods: A) Hollow forming, that is, the plastic melt is injected into the mold cavity, and when it is filled to 60%-70% of the cavity volume, the injection is stopped and the gas is injected until the pressure is maintained. Cool down and shape. This process is mainly suitable for thick-walled plastic products such as handles and handles. B) Short shot, that is, when the plastic melt is filled to 90%-98% of the cavity volume, air intake begins. This method is mainly used for thick-walled or partial-walled products with larger planes. C) Full injection, that is, when the plastic melt is filled to completely fill the cavity, the gas is injected, and the space created by the volume shrinkage of the melt is filled by the gas, and the gas pressure and the melt pressure are used together to make the product warp The deformation is greatly reduced, and it is used for the molding of thin-walled products with larger planes, and its process control is more complicated. The first two methods are also called short-material gas-assisted injection, and the latter is called full-material gas-assisted injection.

The gas-assisted process includes the following four stages: the first stage, plastic injection. The melt enters the cavity and meets the lower temperature of the mold wall to form a thinner solidified layer; the second stage: gas is incident. The inert gas enters the molten plastic, pushing the unsolidified plastic in the center into the cavity that has not been filled; the third stage: the gas is incident. The gas continues to push the plastic melt to flow until the melt fills the entire cavity; the fourth stage: gas pressure retention. In the pressure-holding state, the gas in the air passage compresses the melt and feeds to ensure the appearance quality of the part.

Gas-assisted forming has the following advantages: eliminate sink marks on the product surface, improve product surface quality; reduce warpage and deformation, reduce flow streaks; reduce product internal stress and increase product strength; save plastic raw materials and reduce product weight (generally, it can be reduced by 20% -40%); Improve the distribution of materials on the section of the product, improve the rigidity of the product; shorten the molding time, increase production efficiency; extend the service life of the mold.