Plastic molding process and mold design Plastic molding technology and mold design
Important conditions for molding high-quality plastic parts: advanced mold design, high-quality mold manufacturing, high-quality mold materials, reasonable processing technology and modern molding equipment
Classification according to different molding methods of plastic parts: injection mold, compression mold, injection mold, extrusion mold, pneumatic molding mold
Injection molds are mainly used for molding thermoplastic plastic parts. Compression molds are mostly used for molding thermosetting plastic parts
The structure of solid polymers can be divided into crystalline and amorphous according to the geometric characteristics of molecular arrangement
Three physical states commonly found in linear amorphous polymers: glass state, high elastic state, and viscous flow state
Almost all plastics are based on various resins, and then various additives are added to improve their performance
Types of additives: fillers, plasticizers, stabilizers, lubricants, colorants, curing agents
Classification according to the molecular structure and characteristics of synthetic resins: thermoplastics, thermosetting plastics
Classification according to the application range of plastics: general plastics, engineering plastics, special plastics
Thermoplastic molding process performance: thermodynamic properties, crystallinity, orientation, shrinkage, fluidity Compatibility, hygroscopicity, thermal stability
The factors that affect the shrinkage of plastic parts are mainly: plastic type, plastic part structure, mold structure, molding process conditions
Plastic fluidity: the ability of plastic melt to fill the mold cavity under certain temperature and pressure during the molding process
Melt index: under certain temperature and pressure, the plastic weight value of the melt passing through the standard capillary within 10 minutes is used to determine its fluidity
The factors that affect fluidity are mainly: temperature, pressure, mold structure
Compatibility refers to the ability of two or more different types of plastics to not produce phase separation in the molten state
Hygroscopicity refers to the affinity of plastics to water
Thermosensitivity refers to the tendency of some plastics with poor thermal stability to discolor, degrade, and decompose when the material temperature increases when the heating time is long at high temperature or the gate cross-section is too small and the shearing effect is large
.https://solidcomould.com/product-category/injection-mould/electric-box-mould-2/
The main factors affecting the fluidity of thermosetting plastics: plastic type, mold structure, and molding process
The complete injection process should include, in order: preparation before molding, injection process, and post-processing of plastic parts
The injection process generally includes feeding, plasticization, injection, cooling, and demolding
Regardless of the form of injection molding machine, the injection process can be divided into mold filling, pressure holding, backflow, cooling after gate freezing, and demolding
Important parameters affecting the injection molding process: plasticization flow, cooling temperature, pressure, and the corresponding action time
The most suitable temperature range of the barrel should be between the viscous flow temperature Qf and the thermal decomposition temperature Qd
The pressure in the plasticization process of injection molding includes plasticization pressure and injection pressure
Plasticization pressure: Plasticization pressure, also known as back pressure, refers to the pressure on the molten material at the head of the screw when the screw rotates backward when using a screw injection machine
Injection pressure: The injection pressure of the injection machine refers to the pressure exerted by the plunger or screw head on the plastic melt
The main contents of the structural process design of plastic parts include: size and accuracy, surface roughness value, plastic part shape, wall thickness, slope Reinforcement aid, support surface, fillet, hole, thread, gear, insert
Text, symbol and mark, etc.
For small thermosetting plastic parts, the wall thickness is 1.6~2.5mm, and for large plastic parts, it is 3.2~8mm 1.6–8mm
Thermoplastics are easy to form thin-walled plastic parts, and the minimum wall thickness can reach 0.25mm, but generally should not be less than 0.6~0.9mm, and it is often 2~4mm
The main function of reinforcement aid is to enhance the strength of plastic parts and avoid deformation and warping of plastic parts
Injection molds can be divided into thermoplastic injection molds and thermosetting injection molds according to the materials of the plastics they mold; they can be divided into injection molds for horizontal injection machines according to the type of injection machine they use
Injection molds for vertical injection machines Injection molds for angle injection machines
The basic structure of injection molds consists of two major parts: the movable mold and the fixed mold. The fixed mold part is installed on the fixed template of the injection machine; the movable mold part is installed on the movable template of the injection machine
The injection mold can be composed of the following parts: molding parts, mold guide mechanism, pouring system, lateral parting and core pulling mechanism, ejection mechanism, heating and cooling system, exhaust system, support parts
When designing an injection mold, the designer first needs to determine the structure, type and some basic parameters and dimensions of the mold, such as the number of mold cavities, the required injection volume, the projection area of the plastic part on the parting surface, the mold clamping force required for molding, the injection pressure, the thickness of the mold, the installation fixed dimensions and the mold opening stroke, etc.
The maximum mold opening stroke Smax of the injection machine is not related to the mold thickness. This mainly refers to the clamping mechanism of hydraulic and mechanical combined action. The injection machines using this clamping mechanism are: XS-Z30
XS-ZY-60 XS-ZY-125 XS-ZY-350 XS-Z-500 XS-Z-1000 G54-S200/400
Verification when the maximum mold opening stroke of the injection machine is related to the mold thickness This mainly refers to the injection machine with a safety hydraulic clamping system (XS-ZY-250) and a screw drive clamping system
Right-angle injection machines (SYS-45 and SY-60), whose maximum mold opening stroke is directly related to the mold thickness
The pouring system refers to the channel through which the plastic melt flows in the mold after it is ejected from the injection machine nozzle and before it reaches the cavity. The pouring system is divided into a common runner pouring system and a hot runner pouring system
The common runner pouring system generally consists of four parts: main runner, branch runner, gate and cold material hole
The commonly used branch runner cross-sectional shapes can generally be divided into circular, trapezoidal, U-shaped, semicircular and rectangular
The gate, also known as the feed port, is the channel connecting the branch runner and the cavity. Except for the direct gate, it is the smallest part of the pouring system, but it is the key part of the pouring system. The position, shape and size of the gate have a great influence on the performance and quality of the plastic part.
The layout of the runner is divided into two categories: balanced and unbalanced. The balanced type refers to the runners from the main channel to each cavity, and their length, cross-sectional shape and size are all corresponding and equal. The unbalanced
type refers to the length of the runners from the main channel to each cavity may not be all corresponding and equal
According to the different combinations, the combined die can be divided into integral embedded, partial inlay, bottom inlay and four-wall splicing
The die is the main part of the outer surface of the molded plastic part. According to its different structures, it can be divided into integral and combined types
Both the punch and the core are parts of the inner surface of the molded plastic part. The punch generally refers to the larger, main internal shape of the molded plastic part, also known as the core; the core generally refers to the smaller hole
groove part on the molded plastic part
The working size of the molded part refers to the size on the molded part that is directly used to form the plastic part, mainly including the radial size of the cavity and the core, the depth size of the cavity and the height size of the core, and the position size between the core
and the core, etc.
The main factors affecting the dimensional accuracy of plastic parts are as follows: the influence of the shrinkage rate of plastic parts, the manufacturing error of mold forming parts, the wear of mold forming parts, and the error of mold installation and matching
The mold closing guide mechanism mainly has two forms: guide column guide and cone surface positioning
The role of the guide mechanism: positioning role, guiding role, and bearing a certain lateral pressure
Simple ejection mechanisms include: push rod ejection mechanism, push tube ejection mechanism, push plate ejection mechanism, movable insert and die ejection mechanism, multi-comprehensive ejection mechanism, etc.
The form of the single push plate two-level ejection mechanism: hook type two-level ejection mechanism, wedge slider type two-level ejection mechanism, swing block pull rod type two-level ejection mechanism, U-shaped limit frame type two-level ejection mechanism
Components of compression mold: cavity, feeding chamber, guide mechanism, lateral parting core pulling mechanism, demoulding mechanism, heating system
Classification according to the form of the mold feeding chamber: overflow compression mold, non-overflow compression mold, semi-overflow compression mold
Considerations for selecting the pressurization direction of plastic parts in the mold: convenient for feeding, conducive to pressure transmission, convenient for placing and fixing inserts, convenient for plastic flow, and ensuring the strength of the punch Ensure the accuracy of important dimensions. The long core is located in the direction of pressure. 2. Brief description questions and questions and answers. 1. Briefly describe the advantages and disadvantages of injection molding: Advantages: short molding cycle, can mold plastic parts with complex shapes, precise dimensions, and metal or non-metal inserts at one time; strong adaptability to molding various plastics; high production efficiency and easy to achieve fully automated production, etc. Disadvantages: The equipment price and mold manufacturing cost of injection molding are high, which is not suitable for the production of single pieces and small batches of plastic parts. 2. The reason why the wall thickness of the same plastic part should be as consistent as possible: inconsistency will cause additional internal stress due to different cooling or solidification speeds, causing warping, shrinkage, cracks and even cracking of the plastic part. If the plastic part is too thick locally, dents will appear on the outer surface and bubbles will appear inside. If the structure requires different wall thicknesses, the ratio of different wall thicknesses should not exceed 1:3, and an appropriate modification radius should be used to slow down the sudden change of the thick and thin transition part. 3. The purpose of inserting parts into plastic parts: either to increase the local strength, hardness, wear resistance, magnetic conductivity, etc. of the plastic parts; or to improve the precision, increase the size and shape stability of the plastic parts; or to meet certain special performance requirements for plastic parts.
- Reasons for considering exhaust problems when designing cavities: If the gas generated in the cavity due to various reasons cannot be completely discharged, on the one hand, bubbles, seams, unclear surface contours and filling defects will be formed on the plastic parts. On the other hand, the gas is compressed, the volume is reduced, and the high temperature will cause local carbonization or burning of the plastic parts. At the same time, the accumulated gas will also produce reverse pressure and reduce the filling speed. Therefore, exhaust problems must be considered when designing the cavity. 5. Exhaust methods during injection molding: exhaust by using the matching gap, exhaust by opening exhaust grooves on the parting surface, exhaust by exhaust plugs, and forced exhaust. 6. The parts in the mold that determine the geometric shape and size of the plastic parts are called molding parts, including concave molds, cores, inserts, molding rods and molding rings. 7. Briefly describe the guide structure and technical requirements: length, shape, material, quantity and layout, matching accuracy. 8. Briefly describe the design principle of the ejection mechanism: The ejection mechanism should be set on the side of the movable mold as much as possible to ensure that the plastic parts are not deformed or damaged due to ejection. The mechanism is simple and reliable. Good appearance of the plastic parts. Correct reset when closing the mold
- Briefly describe the design principles of the cooling system: There should be as many cooling channels as possible and the cross-sectional size should be as large as possible. The distance from the cooling channel to the cavity surface should be as equal as possible. Strengthen cooling at the gate. The temperature difference between the inlet and outlet of the cooling channel should be as small as possible. The cooling channel should be set along the direction of the plastic part shrinkage.
