Plastic molding is the process of molding plastics in various forms (powder, pellets, solutions and dispersions) into products or blanks of the desired shape. There are as many as thirty types of molding methods.
Introduction
The choice of plastic molding mainly depends on the type of plastic (thermoplastic or thermoset), the starting shape, and the shape and size of the product. Commonly used methods for processing thermoplastic plastics include extrusion, injection molding, calendering, blow molding, and thermoforming. Thermosetting plastics generally use molding, transfer molding, and injection molding. Lamination, molding, and thermoforming shape plastic onto a flat surface. The above plastic processing methods can all be used for rubber processing. In addition, there are also castings using liquid monomers or polymers as raw materials. Among these methods, extrusion and injection molding are the most commonly used and 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 dimensional shrinkage occurs. This property 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.
Forms of forming shrinkage Forming shrinkage is mainly manifested in the following aspects:
- The linear dimension shrinkage of plastic parts due to thermal expansion and contraction, elastic recovery, plastic deformation during demoulding, etc. causes the plastic parts to shrink in size after they are demolded and cooled to room temperature. This must be considered when designing the cavity. compensate.
- During shrinkage directional molding, the molecules are arranged in directions, making the plastic part anisotropic. Along the direction of the material flow (i.e., parallel direction), the shrinkage is large and the strength is high, and in the direction at right angles to the material flow (i.e., the vertical direction), the shrinkage is small., low intensity. In addition, due to uneven density and filler distribution in various parts of the plastic part during molding, the shrinkage is also uneven. The shrinkage difference makes plastic parts prone to warping, deformation, and cracks, especially in extrusion and injection molding, where the directionality is more obvious. Therefore, the shrinkage directionality should be considered when designing the mold and the appropriate shrinkage rate should be selected according to the shape of the plastic part and the flow direction.
- When post-shrinkage plastic parts are formed, 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, a certain The effects of a series of stresses cannot all disappear in the viscous flow state, so there are residual stresses when plastic parts are formed under stress conditions. When the plastic parts are demoulded due to stress equilibrium and the influence of storage conditions, the residual stress changes and the plastic parts shrink again, which is called post-shrinkage. Generally, plastic parts change the most within 10 hours after demoulding, and are basically finalized after 24 hours, but it takes 30-60 days to finally stabilize. Generally, the post-shrinkage of thermoplastics is greater than that of thermosets, and that of extrusion and injection molding is greater than that of compression molding.
- Post-processing shrinkage Sometimes plastic parts need to be heat treated after forming according to performance and process requirements. After treatment, the dimensions of the plastic parts will also change. Therefore, when designing molds for high-precision plastic parts, post-shrinkage and post-processing shrinkage errors should be considered and compensated for.
Shrinkage rate calculation The shrinkage of plastic parts during molding can be expressed by the shrinkage rate, as shown in formula (1-1) and formula (1-2).
(1-1) Q act=(ab)/b×100
(1-2) Q meter = (cb)/b×100
In the formula: Q actual – actual shrinkage (%)
Q meter—calculate shrinkage rate (%)
a — One-way dimension of plastic parts at forming temperature (mm)
b — One-way dimension of plastic parts at room temperature (mm)
c — One-way dimension of the mold at room temperature (mm)
The actual shrinkage rate represents the actual shrinkage of the plastic part. Since its value is very different from the calculated shrinkage, Q is used as the design parameter when designing the mold to calculate the cavity and core dimensions.
Factors affecting the change of shrinkage rate. In actual molding, not only the shrinkage rates of different types of plastics are different, but also the shrinkage values of different batches of the same type of plastic or different parts of the same plastic part are often different. The main factors affecting the change of shrinkage rate are The factors include the following aspects.
- Plastic types Various plastics have their own shrinkage ranges. The same type of plastic will have different shrinkage rates and anisotropy due to different fillers, molecular weights and ratios.
- Characteristics of plastic parts The shape, size, wall thickness, presence or absence of inserts, the number and layout of inserts also have a great impact on the shrinkage rate of plastic parts.
- 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.
- Molding process Extrusion and injection molding processes generally have larger shrinkage rates 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 range provided in the instructions 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, molding shrinkage is also affected by various molding 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 molding can also appropriately change the shrinkage of the plastic part.
Liquidity
The ability of plastic to fill the cavity under certain temperature and pressure is called fluidity. This is an important process parameter that must be considered when designing the mold. High fluidity can easily lead to excessive overflow, loose filling of the mold cavity, loose plastic parts, separate accumulation of resin and fillers, easy mold sticking, difficulty in demoulding and cleaning, premature hardening and other disadvantages. However, if the fluidity is small, the filling will be insufficient, it will be difficult to form, and the forming pressure will be 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, feeding direction, etc. should be considered based on the flow performance. The flow properties of thermoset plastics are usually expressed in terms of Lasig flow (in millimeters). The larger the value, the better the fluidity. Each type of plastic is usually divided into three different levels of fluidity for different plastic parts and forming processes. Generally, when plastic parts have a large area, many inserts, thin cores and inserts, and complex shapes with narrow deep grooves and thin walls that are unfavorable for filling, plastics with better fluidity should be used. When extrusion molding, plastics with a Rasig flow rate of more than 150mm should be used, and for injection molding, plastics with a Rasig flow rate of more than 200mm should be used. In order to ensure that each batch of plastics has the same fluidity, in practice, the parallel batch method is often used to adjust, that is, plastics of the same type but with different fluidities are used together to make the fluidity of each batch of plastics compensate for each other to ensure the quality of plastic parts.. The Rasig flowability values of commonly used plastics are detailed in Table 1-1. However, it must be pointed out that in addition to the plastic type, the flowability of plastics is often affected by various factors when filling the cavity. The actual filling of the cavity by the plastic capabilities change. If the particle size is fine and uniform (especially round pellets), the humidity is high, contains a lot of moisture and volatile matter, the preheating and forming conditions are appropriate, the mold surface has good surface finish, and the mold structure is appropriate, etc., it will help improve the fluidity. On the contrary, poor preheating or forming conditions, poor mold structure and large flow resistance, or the plastic storage period is too long, expired, and high storage temperature (especially for amino plastics) will lead to a decrease in the actual flow performance of the plastic when filling the cavity and cause filling bad.
Specific volume and compression rate
Specific volume is the volume occupied by each gram of plastic (measured in cm3/g). The compression rate is the ratio of the volume or specific volume of plastic powder and plastic parts (its value is always greater than 1). They can both be used to determine the size of the die loading chamber. A large value requires a large volume of the charging chamber. It also means that there is a lot of air in the plastic powder, making it difficult to exhaust, resulting in a long molding cycle and low productivity. The opposite is true when the specific volume is small, and it is conducive to ingot pressing and suppression. The specific volumes of various plastics are detailed 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 properties
During the molding process, thermosetting plastics transform into a plastic viscous flow state under heating and pressure, and then the fluidity increases to fill the mold cavity. At the same time, a condensation reaction occurs, the cross-linking density continues to increase, the fluidity decreases rapidly, and the melt gradually solidifies.. When designing the mold, for materials that harden quickly and maintain a 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 plastic type, wall thickness, plastic part shape, and mold temperature. But it is also affected by other factors, especially related to the preheating state. Appropriate preheating should maintain the conditions that allow the plastic to exert its maximum fluidity, and try to increase its hardening speed. Generally, the preheating temperature is high and the time is long (within the allowable Within the range), the hardening speed will be accelerated, especially if the pre-pressed ingot blank has been preheated with high frequency, the hardening speed will be significantly accelerated. In addition, if the forming temperature is high and the pressing time is long, the hardening speed will also increase. Therefore, the hardening speed can also be appropriately controlled by adjusting preheating or forming conditions.
The hardening speed should also meet the requirements of the molding method. For example, injection and extrusion molding should require slow chemical reactions and slow hardening during plasticization and filling. The flow state should be maintained for a long time. However, when the cavity is filled, it will undergo high temperature and high pressure. It should harden quickly.
Moisture and volatile content
Various plastics contain different levels of moisture and volatile matter. When too much, the fluidity increases, it is easy to overflow, the retention time is long, the shrinkage increases, and it is prone to corrugation, warping and other defects, which affects the mechanical and electrical properties of plastic parts. However, when the plastic is too dry, it will also lead to poor fluidity and difficulty in forming. Therefore, different plastics should be preheated and dried as required. For materials with strong hygroscopicity, especially in humid seasons, even preheated materials should be prevented from absorbing moisture again.
Since various plastics contain different components of moisture and volatiles, and condensation moisture occurs during the condensation reaction, these components need to be turned into gas and discharged out of the mold during molding. Some gases have a corrosive effect on the mold and are also harmful to the human body. Stimulating effect. For this reason, the characteristics of various plastics should be understood when designing the mold, and corresponding measures should be taken, such as preheating, chromium plating of the mold, opening exhaust slots or setting up an exhaust process during forming.
Methods
Plastic products are made of a mixture of synthetic resin and various additives as raw materials, using injection, extrusion, pressing, casting and other methods. Plastic products also obtain final properties while being molded, so plastic molding is a key process in production.
Injection molding
Injection molding, also called injection molding, is a method that uses an injection machine to quickly inject molten plastic into a mold and solidify it to obtain various plastic products. This method can be used for almost all thermoplastics (except fluoroplastics), and 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 parts with complex shapes in one go, precise dimensions, and high productivity. However, equipment and mold costs are 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. Principle of injection molding: Add powdery and granular raw materials from the hopper to the barrel. When the plunger advances, the raw materials are pushed into the heating zone, and then pass through the diverter shuttle. The molten plastic is injected into the mold cavity through the nozzle. After cooling, the mold is opened to obtain plastic products. Injection plastic parts usually require appropriate post-processing after being removed from the mold cavity to eliminate the stress and stabilize the size and performance of the plastic parts during molding. In addition, there are also removal of burrs and gates, polishing, surface coating, etc.
Extrusion molding
Extrusion molding is a process that uses screw rotation and pressure to continuously squeeze the plasticized plastic into the mold. When passing through the die of a certain shape, a plastic profile suitable for the shape of the die is obtained. 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 pipes, plates, rods, sheets, strips, materials and special-shaped materials with complex cross-sections. Its characteristics are continuous forming, high productivity, simple mold structure, low cost, tight organization, etc. Except for fluoroplastics, almost all thermoplastics can be extruded, and some thermosetting plastics can also be extruded.
The granular plastic is fed from the hopper into the spiral propelling chamber, and then sent to the heating zone by the rotating screw for melting and compression; under the action of the spiral force, it is forced to pass through the extruder with a certain shape. Mold to obtain a profile consistent with the cross-sectional shape of the die; after falling onto the conveyor belt, spray air or water to cool and harden it to obtain a solidified plastic part.
Press molding
Press molding, also known as compression molding, compression molding, molding, etc., is to add solid pellets or prefabricated pieces into the mold, soften and melt them through heating and pressure, and fill the mold under the action of pressure. cavity, a method to obtain plastic parts after solidification. Press molding is mainly used for thermosetting plastics, such as phenolic, epoxy, silicone, etc.; it can also be used for pressing thermoplastic polytetrafluoroethylene products and polyvinyl chloride (PVC) records. Compared with injection molding, press molding has simple equipment and molds and can produce large-scale products; however, the production cycle is long, the efficiency is low, it is difficult to automate, and it is difficult to produce thick-walled products and products with complex shapes.
Generally, the press forming process can be divided into several stages: feeding, mold closing, exhausting, curing and demoulding. Plastic parts should be post-processed after demoulding. The processing method is the same as that of injection molded plastic parts.
Blow molding
Blow molding (a secondary processing of plastics) is a processing method that uses compressed air to inflate and deform the hollow plastic parison, and then obtains plastic parts after cooling and shaping. The main methods include hollow blow molding and film blow molding. The extruded or injected tubular parison with a certain temperature is placed in the split blow mold, the mold is closed, compressed air is blown into the parison through the blow pipe, and the parison is inflated. Then make it close to the mold wall, open the mold and take out the hollow part after maintaining pressure, cooling and shaping.
Casting
The casting and forming of plastics is similar to the casting and forming of metals. A processing method in which polymer materials or monomer materials in a flowing state are injected into a specific mold, reacted, solidified, and formed into plastic parts consistent with the mold cavity under certain conditions. This forming method has simple equipment, requires no or little pressure, has low requirements on mold strength, and has low production investment. It can be applied to thermoplastic and thermosetting plastic parts of various sizes. However, plastic parts have low precision, low productivity, and long forming cycle.
Gas-Assisted Injection Molding
Gas-assisted injection molding (gas-assisted molding for short) is a new method in the field of plastic processing. The gas-assisted forming process can be roughly divided into three methods:
- Hollow forming, that is, the plastic melt is injected into the mold cavity. When the cavity volume is filled to 60%-70%, the injection is stopped and the gas is started to be injected until the mold cavity is maintained. Press and cool to set. This process is mainly suitable for thick-walled plastic products such as handles and handles.
- 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.
- Full injection, that is, the plastic melt is filled until the mold cavity is completely filled before gas is injected. The gas fills the space caused by the shrinkage of the melt volume, and the gas pressure holding and melt pressure holding are used together to make the product warp. The bending deformation is greatly reduced, and it is used to form thin-walled products with larger planes, and the process control is more complicated. The first two methods are also called short-material gas-assisted injection methods, and the latter are called full-material gas-assisted injection methods.
The gas-assisted process includes the following four stages: The first stage is plastic injection. The melt enters the mold cavity and encounters the mold wall with a lower temperature, forming a thin solidified layer; the second stage: gas injection. The inert gas enters the molten plastic and pushes the unsolidified plastic in the center into the cavity that is not yet filled; the third stage: gas injection. The gas continues to push the plastic melt to flow until the melt fills the entire cavity; the fourth stage: gas pressure maintenance. In the pressure-holding state, the gas in the air channel compresses the melt and replenishes the material to ensure the appearance quality of the parts.
Gas-assisted forming has the following advantages: eliminates product surface sink marks and improves product surface quality; reduces warping deformation and flow streaks; reduces product internal stress and improves product strength; saves plastic raw materials and reduces product weight (generally 20% reduction) -40%); improve the distribution of materials on the cross-section of the product and improve the rigidity of the product; shorten the molding time and improve production efficiency; extend the service life of the mold.
Equipments
Plastic molding usually uses an injection molding machine, also known as injection molding machine or injection machine. It is the main molding equipment for making thermoplastic or thermosetting materials into plastic products of various shapes using plastic molding molds. Divided into vertical, horizontal and all-electric types. The injection molding machine heats the plastic and applies high pressure to the molten plastic, causing it to be injected and fill the mold cavity.
Blow molding machines are also very important plastic molding machines, which are divided into extrusion blow molding machines and injection blow molding machines, as well as PET bottle blow molding machines specifically used for producing plastic water bottles.
In addition to the above two types of plastic machines, plastic extruders are also very common plastic machinery, which are commonly used for plastic pipes and other plastic products.
