, product design
In this process, we should pay attention to the following problems: we should not change the product structure at will (unless the user has special requirements); Can't change the product size at will (except the special requirements of users); Check whether the dimensions are correct and whether the product materials are indicated before submitting drawings.
, mold design
(a) First of all, we must draw up the mold structure:
A, determine the location of the parting surface: the parting surface should be selected at the largest section of the plastic part; Do not affect the appearance quality of plastic parts, especially those with clear requirements on appearance, but also pay attention to the influence of parting surface on appearance; It is beneficial to ensure the accuracy requirements of plastic parts; Conducive to mold processing, especially mold cavity processing; Conducive to the setting of pouring system, exhaust system and cooling system; In order to facilitate the demoulding of plastic parts, try to keep the plastic parts on the side of the moving mold when opening the mold (some plastic parts need to be fixed and pushed out); Minimize the projected area of plastic parts on the clamping plane to reduce the required clamping force; Easy to install plug-ins; The long core should be placed in the direction of mold opening.
B. Determination of the number of cavities: The number of cavities is mainly determined according to the quality, projected area, geometric shape (with or without core pulling), accuracy, batch size and economic benefits of plastic parts. If these conditions restrict each other, they must be coordinated when determining the design scheme to ensure that the main conditions are met.
C cavity arrangement and mold structure determination: cavity arrangement involves mold size, gating system design, gating system balance, core-pulling mechanism design, insert and core design and temperature regulation system design. These problems are related to the location of parting surface and gate, so in the specific design process, necessary adjustments should be made to achieve a more perfect design structure. 1) large plastic parts or small precision plastic parts adopt lateral parting and core-pulling (parting or core-pulling is divided into several directions), and the core-pulling mechanism is movable, using a single cavity and single parting surface mold; 2) Due to the high requirements of appearance quality and dimensional accuracy of plastic parts, when using point gate, a mold with one cavity and multiple parting surfaces is adopted; 3) Multi-cavity single parting surface or multi-cavity multi-parting surface mold is used for small and medium-sized plastic parts with general dimensional accuracy requirements.
(2) Selection of plastic die steel:
A, the performance requirements of plastic die steel (1) require that the material has high hardness and good wear resistance, and its surface hardness should be 30~60HRC, and its hardening ability should be >; 55HRC, the material center has enough hardening depth and strong toughness to avoid brittle fracture and plastic deformation. (2) It is required that the material has certain heat resistance, can work for a long time at the temperature of 150 ~ 250℃, and has no oxidation, deformation and good dimensional stability. (3) The material is required to have certain corrosion resistance. (4) Good weldability and forging process performance are required.
B. Selection of plastic mold steel Cold-formed plastic molds are mainly low carbon steel, and the models can be 20, 20Cr, 12CrNi3A, 40rC or DTI. The plastic mould cut and formed is mainly quenched and tempered steel, which is quenched and tempered before processing. The model can be 40, 50, 3Cr2Mo, 4Cr3MoSiV, 5CrNiMo, 4Gr5MoSiV 1 or 4c r5 w2 SIF 1. Cold-working die steels, such as Cr 12, 9Mn2V, Cr6 WV or 7 Cr Mo NiMo, are used for thermoplastic and thermosetting plastic dies with strong abrasion. Ultra-low carbon maraging steels, such as 18Ni(250), 18Ni(300) or 18Ni(350), can be used for advanced plastic molds.
(3) The factors that should be considered in template and gate setting:
A, the setting of the gate should achieve balanced mold filling.
B, the gate should be located in the thick wall.
C, the gate should be far away from thin-walled features
D, the gate should be set to achieve the same flow.
E, if necessary, increase the gate to reduce the mold filling pressure.
F, increase the gate to prevent overpressure.
G, the type of mold used, 2-plate mold or 3-plate mold?
H. Hot runner or cold runner or mixed runner
The type of gate I want, such as side gate, submerged gate, etc.
J, due to the limitation of product function and gate position.
There is no fixed principle to decide where the gate should or should not be located in the part. Different designers may recognize the best gate position differently. This section will discuss some principles of gate position design, which should be paid attention to by people concerned with the flow analysis of parts filling.
(4) Selection of templates and accessories:
When designing the mold, we should choose standard mold base and standard parts as far as possible (including general standard parts and mold-specific standard parts, such as fasteners, mold-specific standard parts, such as locating ring, sprue sleeve, push rod, guide post, mold-specific spring, cooling and heating elements), because a large part of standard parts can be bought in the market at any time, which is very beneficial to shorten the manufacturing cycle and reduce the manufacturing cost. After the size of the die frame is determined, the necessary strength or stiffness calculation should be made for the relevant parts of the die to check whether the selected die frame is suitable, especially for large dies.
Before arranging the process, we should first analyze the feasibility of the plastic molding process:
(1) accept the design task (the working drawing of plastic parts, if it is a physical part, it should be drawn into a two-dimensional engineering drawing). The variety, batch number, dimensional accuracy and technical conditions of the plastics used, and the functions and working conditions of the products shall be indicated on the working drawings of the product parts.
(2) Analyze and digest the product drawings or samples provided in detail, and pay attention to the following matters.
A, the accuracy of product size and the correctness of drawing size;
B, the demoulding slope is reasonable;
C, plastic thickness and uniformity;
D, plastic types and their shrinkage;
E, plastic surface color and surface quality requirements.
(3) Understand the mechanical and physical properties of plastic parts and the parameters related to injection molding process.
(4) Review the molding process of plastic parts, and discuss the feasibility of wall thickness, rib, fillet, surface roughness, dimensional accuracy, surface decoration, demoulding slope and insert placement. If the molding process of product structure design is not good, it can be discussed with the designer, who will modify the product structure to meet the needs of injection molding process without affecting the product performance.
(5) Calculate the volume and mass of plastic parts.
According to the process sequence. In the mold processing center of the molding company, there are mainly the following methods for processing molds: lathe processing, milling machine processing, grinding machine processing, numerical control processing, EDM processing, wire cutting processing and so on.
Comparison of several mold processing methods;
1), lathe processing
Machining accuracy: 0.02 mm.
Machining features: it is suitable for a series of forming processes such as holes, steps and grooves, and has a wide machining range.
2), milling machine processing
Machining accuracy: 0.02 mm.
Processing characteristics: suitable for a series of forming processes such as holes, steps and grooves.
3), grinding machine processing
Machining accuracy: 0.00 1 ~ 0.005 mm.
Machining features: Suitable for precise forming of circular arc, inclined plane and groove.
4), numerical control processing
Machining accuracy: 0.0 1 mm.
Machining features: suitable for rough and finish machining of concave-convex die holder, 3D die core and various electrodes.
5), EDM
Machining accuracy: 0.002 ~ 0.0 1 mm.
Machining features: It is suitable for machining grooves, holes and workpieces with complex shapes, and can be machined in mirror.
6), wire cutting processing
Machining accuracy: 0.002 ~ 0.005 mm.
Machining features: high machining precision, good finish and convenient operation, and can process irregular workpieces from top to bottom.
In the process of mold manufacturing, assembly is mainly completed by assemblers.
Assembly technology can be divided into "separation" and "integration" Integrated assembly: A, welding B, fixing C, bonding D, embedding technology E, 90-degree angle buckle; The separating assembly includes: A, a buckle B with an angle less than 90 degrees, a screw assembly C, a center assembly D, and a pressing assembly. Press-piece assembly: Press-piece assembly can carry out high-strength assembly of plastic parts at the lowest cost. For example, for the buckle assembly, due to stress relaxation, the tensile strength of the high-voltage assembly will decrease with time (see Figure 3). This must be taken into account in the design calculation. In addition, it is necessary to carry out the test of periodic change of service temperature to ensure the feasibility of the design; Thread assembly: Thread assembly includes the use of separate screws, combined screws or integral screw inserts. The bending modulus of the material provides guidance for the reasonable assembly of screws. For example, the bending modulus of threaded screws can reach 2800Mpa. If metric screws are needed, or thread assembly needs to be completed many times, metal fine-grained inserts are needed.
Before mold testing, we should know the mechanical and physical characteristics of plastic parts and related parameters related to injection process. The most commonly used plastics in the company are polypropylene (PP) and acrylonitrile butadiene styrene (ABS). In addition, we also need to determine the molding equipment.
(1) Characteristics of common plastics:
(1) polypropylene
Polypropylene (PP), as a thermoplastic polymer, began commercial production on 1957, and is the first stereoregular polymer. Its historical significance is more reflected in that it has been the fastest growing major thermoplastic, and its total world output reached 24 billion pounds in 199 1 year. It is widely used in the field of thermoplastics, especially in fiber and filament, film extrusion, injection molding and so on.
Chemistry and properties
Polypropylene is synthesized by using metal-organic stereoregular catalyst (Ziegler-Natta type) to produce propylene monomer under controlled temperature and pressure conditions. Due to the difference of catalyst and polymerization process, the molecular structure of the obtained polymer has three different types of stereochemical structures, and the number is also different. These three structures refer to isotactic polymers, syndiotactic polymers and random polymers. In isotactic polypropylene (the most common commercial form), methyl groups are all on the same side of the polymer skeleton, and this structure is easy to form crystalline state. Isotactic crystallinity endows it with good solvent resistance and heat resistance. The catalyst technology used in the first ten years minimized the formation of isomers, eliminated the necessity of separating useless random components, and simplified the production steps.
(2) styrene resin ABS
Ternary polymer ABS began to be commercialized in the 1940s, and its sales volume increased year by year. Now it has become the largest engineering thermoplastic in the world, and its sales in the United States alone exceed19891200 million pounds. Between commercial plastics and high-performance engineering thermoplastics, ABS occupies a unique "transitional" polymer position.
Chemistry and properties
The versatility of ABS comes from its three monomer structural units-acrylonitrile, butadiene and styrene. Each component provides a different set of useful properties for the final polymer. Acrylic eye mainly provides chemical resistance and thermal stability; Butadiene provides initial strength and impact strength; Styrene component provides ABS with hardness and processability. There are three production processes-emulsion method, continuous bulk method or suspension method, and the styrene content in ABS raw materials prepared by any process method is 50% or even higher. Usually at least two processes are used in combination to optimize the final product. ABS resin belongs to two-phase system: styrene-acrylic polymer (SAN) is the continuous phase and butadiene rubber is the dispersed phase of elastomer.
In fact, a small amount of styrene and acrylic acid were polymerized (grafted) on butadiene rubber, which was compatible with hard SAN which was incompatible with rubber. Therefore, ABS can be regarded as one of the first polymer alloys to achieve commercial success.
(3) Acrylic Acid-Styrene-Acrylonitrile (ASA for short)
ASA polymer is an amorphous material, which can be made into products with excellent weather resistance by extrusion and injection molding. The mechanical properties of ternary polymer ASA are usually similar to ABS resin, but the difference is that the performance of ASA is much less affected by outdoor climate than ABS resin.
Chemistry and performance
Terpolymer ASA can be produced by patented reaction process or grafting process. The reaction method is to graft acrylate elastomer to prepare ASA during the polymerization of styrene and acrylonitrile (SAN), and the elastomer fine powder is evenly dispersed and grafted on the molecular chain of SAN.
ASA's excellent weather resistance comes from acrylate elastomer. For many plastics, embrittlement and yellowing will occur under the interaction of solar radiation, especially under the interaction between the ultraviolet end of the spectrum and oxygen in the atmosphere. This change of ASA parts takes longer than that of other plastic parts.
(2) Selecting molding equipment:
According to the type of molding equipment, so we must be familiar with the performance, specifications and characteristics of various molding equipment. For example, for the injection machine, we should know the following contents from the specifications: injection quantity, mold locking pressure, injection pressure, mold installation size, ejection device and size, nozzle diameter and nozzle spherical radius, gate sleeve positioning ring size, mold maximum and minimum thickness, template stroke, etc. See related parameters for details. It is necessary to estimate the overall size of the mold and judge whether the mold can be installed and used on the selected injection molding machine.
Through such a long period of study, we use the preparatory knowledge of plastic mold design, mechanical drawing, tolerance and technical measurement, mechanical principle and parts, mold material and heat treatment, mold manufacturing technology and so on to analyze and solve the problems of plastic mold design, and further consolidate, deepen and broaden the knowledge we have learned; So that I can gradually establish a correct design idea, enhance the sense of innovation and competition, basically master the general rules of plastic mold design, and cultivate the ability to analyze and solve problems; Through the calculation, drawing and application of technical standards, specifications, design manuals and other related design materials, the comprehensive basic skills training of plastic mold design is carried out to lay a good foundation for graduation.