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Among all the methods of plastics processing known in production, one of the most difficult in terms of the used tool is injection molding. When producing parts by this method, for each part (or group of parts) it is necessary to design and manufacture a separate mold.
Clamping force for injection mold can range from tens to thousands of tons; the casting cycle time can only last 5-6 seconds. In addition, there are various complex casting technologies (multicomponent, gas injection, mold assembly, etc.).
According to experts, the annual growth of the market of injection molding machines in some countries exceeds a thousand units. Each injection molding machine must be equipped with at least one mold. The number of molds for one injection molding machine varies depending on the range of products manufactured by the enterprise and the production plans of the company. As practice shows, one injection molding machine accounts for an average of two to five molds, and the tendency to an increase in the number of molds used by the enterprise is becoming more and more obvious from year to year. A similar situation is associated with the desire of manufacturers of plastic products to increase their competitiveness in the market, including with respect to imported products, by expanding the product range, using original design, color, structure in products.
To implement such aspirations, at the beginning of which there is the idea of manufacturing plastic products, and at the end - mass production and the desired sale of plastic products, the enterprise needs a certain set of molds, which differ in design complexity and are individual in their execution for each specific case ...
Design and technological preparation of production
The first stage of mold production includes preparation of product sketches (sketch graphics), construction of a 3D model, photorealistic visualization, etc.
Mold design involves the design study of the model and the division of the assembly model into separate structural elements. Preparation of each specific element, design of the parting line of the part, as well as gates, signs, inserts and mold terrain. The gating system is of particular importance.
It is possible to avoid or minimize warping, streaks, slips and other defects using computer simulation of the form filling process. Also, it is possible to control the distribution of melt temperature, flow rates and other parameters.
At the initial stage of mold production, all existing ideas, drawings are transferred to a computer, on which molds are designed using certain programs.
In the design of molds, CAD / CAM systems are used, automated, through design. The mold design system Mold-Works for SolidWorks is designed to automate the work of the designer and perform the tasks of the automated layout of molds. Checking the mold for "spillage" is carried out on a computer using Mold-flow analysis. The leading suppliers of software for computers in the design and manufacture of molds and dies are Unigraphics, ProEngineer, Cimatron.
The first stage is extremely important for determining the required scope of work at the second stage of mold manufacturing, since the required set of standard and special products required for the manufacture of the mold is determined.
The second stage of mold production includes design (ordering standard parts, machining workpieces, making special mold elements), prototyping, and directly assembling the mold.
Structural elements of special mold parts
The mold consists of a large number of parts that must be precisely mated to each other. Mold parts are divided into normalized and special. The design of normalized parts of the mold can be set in advance, regardless of the design features of the cast products in the mold.
Any mold for 50-95% consists of parts (normals) that are standard in terms of nomenclature and weight. These parts are priced between 10 and 60% of its total cost. Percentage fluctuations depend both on the design of the mold, the geometry of the casting and the requirements of the tool production, as well as on the level of preparedness of the designer (design company) of the mold, his desire and ability to use suitable standard elements. The solution of one and the same technical problem in the design of a mold is almost always structurally invariant in terms of nesting, gating system, ejection system, etc.
Normalized mold parts include:
Companies specializing in the production of normalized elements operate in the world. As a rule, a set of such parts is presented in the catalogs of these companies, and the price for them is predetermined. The degree of readiness is from 10 to 100%. The most famous companies in the world producing standard parts are Hasco Hasenclever GmbH (Germany), FCPK Bytow (Poland), DME Company (USA), EOC Normalien (Germany), Strack Norma GmbH (Germany), Ewikon Heisskanalsysteme (Germany), Mold -Masters Europa GmbH (Canada), Diemould Service Co. Ltd (DMS) (UK), etc.
In addition, there are companies that interface the production of molds with the production of standard mold elements. The most famous among these companies: Schottli AG Mold Technology (Switzerland), FCPK Bytow (Poland), Superior Die Set Corporation (USA), Mold & Hotrunner Technology AG (MHT) (Germany), StackTeck (Canada), Husky (Canada).
Special parts of the mold include plug-in dies, punches, smooth marks, thread marks, rings, etc. The most critical parts of the mold, from the point of view of the stresses that arise in them, are the loading chambers, insert dies and mold holders.
Specialized parts are produced by specialized tool manufacturing, usually the company from which the mold is ordered directly.
The die is the most laborious part of the mold. The mold cavities must match exactly the shape of the molded plastic products.
Punches are used to design the outer or inner contour of a molded plastic product in a mold.
Smooth marks are used to design holes in molded plastic products. Smooth signs can be simultaneously ejectors.
Threaded marks and rings are intended for decoration, respectively, of internal and external threads in molded plastic products.
The ejectors are designed to remove the plastic product from the mold cavity.
Thus, the mold has the following structure: plate pack, centering system; power supply system (gating system); ejection system; cooling system; system of forming parts. Apart from the forming system, all other parts can be standard or can be modified from standard parts supplied as blanks.
Nowadays, the system of rapid production of molds and pilot batches of products using the TAFA technology (the process of making injection molds by electric arc spraying) is widely used. Unified elements made according to catalogs are laid in the design of molds. This allows mold makers to focus on machining mold molds, reduce labor intensity and shorten mold manufacturing times.
In this case, production is divided into three parallel flows: independent production of molds, purchase and refinement of mold normals, production of original mold parts. Through the use of standardized elements for the mold, manufactured by companies specializing in this field, it is possible to increase the quality and durability of the molds.
Gating system of the mold
The gating system of the mold can be of three types: cold-runner, hot-runner and combined. The simplest and cheapest version of the gating system of the mold is cold channel. For a single-cavity mold with a melt flow into the part through the center sprue, it consists only of a sprue sleeve. Advantages: simplicity and low cost. Disadvantages: the entire sprue goes to waste (increased material consumption); the cycle time depends on the cooling time of the gate, and not on the part itself; the need to manually remove the sprue from the casting, store it and recycle it. After the sprue has been removed, the inlet usually needs additional cleaning.
To increase the productivity of the injection molding machine, reduce waste and maintain the required melt processing temperature at
change hot runner gating systems (GKS). Structurally, the simplest GCS consists of only one heated nozzle with one or two heating zones, while the universal GCS consists of a heated manifold, to which one or more nozzles fit. To control the operation of the GCS heaters of the mold, it is necessary to have a thermostat with the number of control zones equal to the number of thermocouples in the system.
The production of the cold channel system is carried out by the forces of a company that produces special mold elements and assembles the mold.
The manufacture of a hot runner system is an expensive undertaking, and at the moment in the world there is a situation where there are certain companies that produce hot runner elements and supply from assembly plants, since it is unprofitable for small production of molds to have their own facilities for the production of hot runner systems.
Manufacturers of hot runner systems can be roughly divided into three groups of companies:
manufacturers of standard elements for molds mentioned above;
large manufacturers of molds with the ability to produce and sell GCS: Schottli AG Mold Technology (Switzerland), Mold & Hotrunner Technology AG (MHT) (Germany), StackTeck (Canada), Husky (Canada);
companies specializing in the production of hot runner systems and controllers for them - Elwik (Poland), GC Heat (Germany), Incoe International (Germany), Guenther HeissKanaltechnik GmbH (Germany), THERMOPLAY S.p.A. (Italy), Heitec Heisskanaltechnik (Germany), Hotset Gmbh (Germany), i-mold GmbH & Co. KG. (Germany), Synventive Molding Solutions (USA, Netherlands), Watlow Company (USA), Heatlock (Sweden), etc.
As a rule, the production of hot runner systems at these enterprises is associated with the production of temperature controllers for them. There are also companies specializing in the production of only controllers for hot runner systems. These companies include: Athena Controls, Inc. (USA), PMS Systems Ltd (UK), Sels (Poland), etc.
Cooling system elements
The casting cycle consists of more than 50% of the cooling time of the casting. To create a reliable mold cooling system that is technologically advanced in manufacture and maintainable during operation, a variety of standard parts are offered: quick-detachable couplings, overflows, fortanating tubes, turbulators, heat pipes, heat dissipation marks, sealing rings, plugs and plugs. It is recommended to connect the mold cooling system to the hoses of the refrigerant source (thermostat, cooler, shop return line) using quick-detachable couplings consisting of a nipple and a coupling.
Cooling system elements are usually found in the catalogs of manufacturers of standard parts for molds. At the same time, there are companies specializing in the production of cooling system elements.
The main part of the machining of mold elements is carried out on CNC milling machines and EDM machines.
The processing technology assumes four stages of mechanical treatment and four stages of heat treatment: 1. Annealing - roughing; 2. Normalization - semi-finishing; 3. Hardening - finishing; 4. Chrome plating.
This processing step is especially important, since it is at this point that all precision machining takes place.
To achieve the required machining accuracy, special expensive tools, mandrels and fixtures are often required, all work is carried out with imported high-quality tools.
Polishing, fitting and assembly. This is the job of a toolmaker. For each specific mold, the content of this work may be different, this is where all errors are found and corrected. Sometimes assembling and testing a mold causes unexpected problems, which can increase lead times.
Chromium plating is necessary to obtain high surface hardness of the parts of the mold, which are subject to increased wear. The matrix having a hardness of 44-46 HRc has a guaranteed service life of 600-700 thousand cycles. The service life of the chrome mold is much longer.
The described stages of manufacturing a mold in general terms reveal the technology and allow
The customer should compare the price of work and its content.
Modification and testing of the mold
At the third stage, the mold is fine-tuned to produce a product of the required quality and with a given production cycle time. Obtaining a mold of a given quality is the main goal of all the processes described above, and if the mold in working conditions does not show the desired results, it is necessary to look for errors in the first two stages.
The duration of the finalization of the mold directly depends on the quality of manufacture of the mold - the more thoroughly the work is done, the less time it will take to fine-tune it.
The test of the mold is carried out under conditions close to the real conditions of the mold operation.
The most important part of this process is the equipment on which the testing of molds is carried out - injection molding machines - ideally, the equipment installed for testing the mold should correspond to the equipment on which it will carry out the production of plastic products.
Mold production time
The usual work at the enterprise is carried out by two teams in two shifts. The production cycle has the following distribution of terms (for molds of medium complexity and size):
Not every enterprise specializing in the manufacture of molds and tooling is capable of designing and manufacturing a mold for a complex plastic product that meets the high requirements of a modern consumer.
At the same time, the costs are very significant - highly qualified personnel, time, expensive materials and specialized equipment for the production of molds are needed. All of this ultimately affects the price.
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