Precision injection molding process and mold design of diverter

Time:2021-03-28 17:29:45 / Popularity: / Source:

Flow divider is a structural part on car controller, which is used to divide flow of liquid. Precision injection mold of splitter is introduced, design plan of mold is worked out by analyzing characteristics, materials, molding process, and mold flow of plastic part.

1 Introduction

Flow divider is a structural part on car controller, which is used to divide flow of liquid. Nowadays, a large number of plastic parts are used in auto parts, which can achieve effects of light weight, low cost, high precision, etc., and properties of plastics bring richer functions to automobiles. It can be said that it is a breakthrough in automobile manufacturing industry. Nowadays, most of plastic machinery parts and components used are injection molding. Structure of modern plastic products is becoming more and more complex, wall thickness of plastic parts is becoming thinner, appearance and precision requirements are getting higher and higher, people's requirements for quality of plastic parts are becoming more and more stringent, technical requirements for injection molding are also getting higher and higher; injection molding industry has entered era of precision injection molding. Precision injection molding means that appearance accuracy of plastic products should meet strict dimensional tolerances, form and position tolerances, surface roughness. Due to high design accuracy of plastic parts, special engineering plastics are used for processing. Conventional injection molding cannot be used for plastic parts, but precision injection molding technology must be used. In order to ensure performance, quality and reliability of these precision plastic parts, stability of long-term use, plastic products with high quality, meeting design requirements of plastic parts must be injection molded. Plastic materials, injection molding equipment, mold design, injection molding process and on-site management must be improved .

2 Current status of field at home and abroad

In recent years, in order to meet requirements of IT and other industries for plastic parts with high precision, high functionality, miniaturization, light weight, low cost, and high added value, development of precision injection molding machines and molds is the key and difficult issue. In development of precision injection molding machines, manufacturers representing world's advanced level are mainly KraussMaffei, Demag, Arburg in Germany and Nissei, Nippon Steel, Toshiba Machinery and Sumitomo Heavy Industries in Japan.
Germany's KraussMaffei was the first to launch a two-platen injection molding machine and attracted attention of its counterparts in the world. Demag has long-term cooperation with Haitian, their technical features and advantages have long been known to domestic counterparts. Here only take Arburg as an example to briefly introduce characteristics of its precision injection molding machine. Clamping mechanism of Arburg precision injection molding machines adopts a box-type design to improve clamping accuracy. Since front and rear plates of three-plate injection molding machine are both fixed on frame, when clamping force is applied, elongation of four tie rods is restricted by frame, so that tie rods tend to "arch bridge shape" and affect accuracy of clamping. Result of formula design improves clamping accuracy from perspective of limiting undesirable deformation. Two-way pressure servo control of injection cylinder accurately positions screw position to double accuracy of injection volume control. Use of frequency converter to optimize control of main pump motor not only improves control accuracy of hydraulic system, but also has significant energy-saving effects. In addition, Arburg precision injection molding machine also adopts a modular design. Each movement system of injection molding machine can adopt a combination of hydraulic and electric according to actual needs of user between two extremes of full hydraulic to full electric. There are also many options for relative position of clamping system and injection system.
In the design of three-plate clamping system for precision injection molding machines of Japan’s Toshiba Machinery, connection between front and rear plates and frame has been improved from usual bottom fixed connection to a waist hinge connection, so that tension rod deformation when clamping force is applied can be stretched freely and always remain parallel, deformation smoothing method is adopted to ensure clamping accuracy. Nissei's comprehensive optimization of traditional hydraulic press has greatly improved its disadvantage compared with all-electric injection molding machine. It adopts new control system TACT (improved response speed, operation stability, multi-language control interface displayed on LCD screen), new injection molding mechanism (large screw length-to-diameter ratio enhances plasticizing effect, and five temperature zones optimizes plasticizing temperature control ), optimized hydraulic circuit, and developed a new hydraulic main model FN series precision injection molding machine. Taking 110t clamping force model as an example, optimized design of precision injection molding machine has increased its control response speed by 30%, increased its quality stability by 50%, reduced impact of oil temperature fluctuations by 50%, and saved energy by 30%. Recently, company has developed a precision small electric molding machine "ELJECT NEX150" suitable for molding extremely small precision parts with a clamping force of 15t. This precision injection molding machine is mainly used for molding liquid crystal polymer (LCP), polyamide (PA), polyphenylene sulfide (PPS), etc. Typical injection molded products are key parts of digital products such as shutters of digital cameras. In injection molding process of such tiny precision parts, injection volume is only 0.1~5g. Japan Steel Works has greatly improved quality and stability of injection molded products through use of advanced holding pressure control technology. Product weight variation range under standard pressure holding conditions is 0.022g, and after adopting new pressure holding control, product weight variation range is reduced to 0.006g. Japan Steel Institute has also researched and developed J-ELII-UPS ultra-high-speed precision injection molding machine.
With expansion of application range of high-precision plastic parts and reduction of manufacturing cost of injection molding machines, there is a lot of room for development of fully automatic injection molding machines. Electric/hydraulic injection molding machines have advantages of hydraulic and high performance and all-electric injection molding machines. Advantages of energy-saving injection molding machines have become development direction of today's precision injection molding machines.

3 Analysis of molding process of plastic parts

3.1 Plastic parts characteristics

Plastic part shown in Figure 1 is a flow divider, which is a structural part on car controller, used to divide flow of liquid. ABS material, opaque, black, mass production. Plastic part is composed of a buckle, a shunt part, and a base. Holes at both ends are formed in different cavities, and coaxiality of plastic parts must be ensured. Therefore, there must be precise positioning measures and good processing technology in design and manufacture of mold to ensure accuracy of shunt. In addition, general shape of plastic part is a stepped shaft-like part, and structure is relatively complicated. Therefore, it is necessary to conduct an overall observation and analysis of plastic part to determine structural form of mold manufacturing. Three-dimensional shape is shown in Figure 1.
Precision injection molding process 
mold design of diverter 
Figure 1 Plastic parts

3.2 Analysis of plastic molding processability

Plastic part uses precision injection, so accuracy grade is IT5. Wall thickness of plastic part is roughly 2~4mm. Due to material reasons, cross-section of runner and gate should be large. Pay attention to position of gate to prevent welding marks. During molding, draft angle is> 2°. In general, shrinkage rate is set to> 0.5%, but because mold is changed to precision injection, shrinkage rate of plastic part in this design is set to 0.3%. In addition, because ABS material is easy to absorb moisture, it needs to be fully dried before molding to avoid defects such as silver wire, markings and bubbles on the surface of plastic part. Formal injection molding process generally includes several steps of feeding, plasticizing, injection, cooling and demolding. Process parameters of plastic part in injection molding process need to be finalized according to characteristics of material, precision requirements of plastic part, and results after trial mold. Preliminary settings are:
Temperature: mold temperature: 80℃; barrel temperature: 190℃; pressure: injection pressure: 200MPa (In order to reduce shrinkage rate of precision injection, injection pressure must be increased), holding pressure = (25%~65%), injection pressure =100MPa.

3.3 Mold flow analysis of plastic parts

In order to optimize this design process and shorten mold design time, MoldFlow is now used to optimize and analyze plastic parts. Due to complexity and particularity of plastic parts, gate position is relatively single, so gate position is not analyzed, and filling situation is mainly analyzed to provide a basis for following mold design..
After dividing part with FUSION mesh by finite element method, material of LUSTRAN ABS EliteHH 189 produced by USA BAYER company is used.
Set process parameters according to material, mold temperature: 80℃; barrel temperature: 190℃; injection pressure: 200MPa (In order to reduce shrinkage rate of precision injection, injection pressure must be increased), holding pressure = (25%~65%) injection pressure = 100MPa. After setting parameters, analysis can be carried out.
(1) Filling pressure. As shown in Figure 2, maximum filling pressure reaches 124.1MPa, which meets injection requirements of precision injection, and injection molding machine must be selected as a reference.
mold design of diverter 
Figure 2 Filling pressure diagram
(2) Shrinkage rate. Shrinkage is a very important issue in precision injection, which directly affects molding quality and dimensional accuracy of plastic part. Shrinkage of this plastic part is shown in Figure 3.
mold flow of plastic part 
Figure 3 Shrinkage analysis diagram of plastic parts
From results of analysis, it can be seen that most of plastic parts have a shrinkage rate below 0.8774%, and shrinkage is relatively satisfactory. except that there will be more obvious pits in thicker area at the bottom of plastic part, surface of plastic part will be uneven after molding, but because this part is not main working part of shunt, requirements for its surface quality are not too high, so such a shortcoming will not have any negative impact on application of plastic parts.
(3) Weld marks. Since ABS material has characteristics of relatively easy to produce weld marks, mold flow analysis is used to observe weld marks of plastic parts. In analysis of possible weld marks during injection molding process, in order to cooperate with analysis of reasons for weld marks, molecular orientation distribution map on the surface of part is intercepted at the same time. Analysis is shown in Figure 4 and Figure 5.
mold flow of plastic part 
Figure 4 Schematic diagram of weld line
mold flow of plastic part 
Figure 5 Molecular flow diagram
Combining molecular orientation maps of Figures 2 to 5, it can be found that weld marks produced in plastic part are mainly caused by inconsistent molecular orientation and peak flow of melt during casting process. Therefore, it can be considered to use local heating and other means to reduce occurrence of weld marks in parts that have surface requirements or strength requirements and are prone to weld marks.

4 Mold design

Mold adopts two cavities, side gates, and uses mutually perpendicular double parting surfaces for injection molding. Among them, side parting adopts form of petal-clamping oblique sliding block, adopts rectangular guide chute and limit pin to guide sliding. Mold selects a standard mold base of 250*315mm according to size and distribution of plastic parts. There are two guiding mechanisms in mold. One guiding mechanism is mainly used to ensure accurate alignment between two major parts of movable mold and fixed mold or other parts in mold, plays role of positioning and orientation; another guiding mechanism is arranged at push plate, which not only plays a guiding role, but also supports supporting plate, thereby improving force of supporting plate and greatly increasing rigidity of supporting plate. In demolding mechanism, in addition to setting 4 push rods at the bottom of each plastic part, it also pushes out petal clamping mold, sets 4 push rods at the bottom of petal clamping mold. At the same time, in order to prevent slider from sticking to fixed mold when mold is opened, 4 sets of springs and spring ejector pins are set in fixed mold part. In order to ensure locking between two clamping molds when clamping mold, upper part of outer mold sleeve of clamping mold must be 0.5mm lower than upper part of clamping mold. Since plastic part requires precision injection, in addition to difference in process parameters from ordinary injection molds, accuracy of mold is also improved accordingly, which is taken as IT6.

4.1 Selection of Parting Surface

Selection of parting surface should take into account that it is beneficial to ensure appearance quality of plastic part and dimensional accuracy of plastic part. Keep plastic part on the side of movable mold to meet use requirements of plastic part as much as possible. Long core should be placed in mold opening direction, which is conducive to exhausting, simplifying mold structure and other factors.
Scheme 1: Parting surface in radial direction is perpendicular to mold opening direction; parting surface in axial direction is parallel to direction in which plastic part is pushed out. Fixed mold core is extracted from plastic part by mold opening action, convex and concave outside of plastic part is molded by flap clamping or sliding block, and movable mold core is extracted together. Molding accuracy of entire plastic part is relatively high, and mold structure is relatively simple.
Scheme 2: Parting surface in radial direction is parallel to mold opening direction; parting surface in axial direction is perpendicular to mold opening direction. Concave and convex parts are separated by mold opening, left and right core pulling is completed by lateral core pulling mechanism of inclined guide post. However, since core on one side of plastic part is longer (about 29mm), you must move 29+2 (safety margin) = 31mm, set installation slope of inclined guide post to 22°, and calculate thickness of slider as 76.7mm. If it is set as a mold with multiple cavities, slider volume will be relatively large, and in order to ensure balance of the mold, plastic parts must be prevented in a symmetrical manner. In this way, mold structure will be quite large, due to large difference in the length of cores on both sides, it will cause waste of mold materials and driving force, mold structure will be relatively complicated.
To sum up, parting surface should adopt scheme 1, so that mold structure is relatively simple, molding accuracy of plastic part is reliable, and cost is saved.

4.2 Determine number and arrangement of cavities

After determining parting surface, axial ends of plastic part are not suitable for direct gate positions. Since plastic part is mass-produced and is also due to structural considerations, a one-mold two-cavity mold structure is adopted. Its general layout is shown in Figure 6.
Precision injection mold 
Figure 6 Cavity distribution map

4.3 Determination of mold structure

This plastic part requires high appearance quality and high dimensional accuracy. A step-shaped shaft part can be drawn from appearance characteristics of plastic part. Although there are many lateral convexities and grooves, it can be found after observation that these concaves and convexes do not require additional inserts but can be directly obtained in all required structural forms in sliding block of sub-type scheme. Therefore, a mold structure with one mold, two cavities and two parting surfaces can be preliminarily drawn up, where two parting surfaces are horizontal and vertical parting surfaces respectively.

4.4 Determine model of injection molding machine

Injection mold is installed on injection molding machine for use. Therefore, when designing an injection mold, you should understand technical specifications of injection molding machine in detail in order to design a mold that meets requirements.
Specification of injection molding machine is determined mainly based on size of plastic part, number and arrangement of cavities, under premise of determining mold structure and preliminary estimation of external dimensions, selecting one that matches mold according to a series of mold parameters.  
(1) Calculation of required injection volume. After plastic part passes three-dimensional modeling, relevant value can be calculated by software. Plastic part volume: V 1 =6.89cm 3 (value calculated by MoldFlow software). Weight of plastic parts: m 1 =6.89*1.41=9.72g. Weight of runner condensate can be estimated as 0.6 times weight of plastic part, and in above analysis, it is determined to be two cavities, so injection volume m=1.6nV 1 =1.6*2*6.89=22.04cm 3.
(2) Determination of model of injection molding machine. Maximum injection volume G≥V/α=22.04/0.75=29.39cm 3. Therefore, horizontal injection molding machine SZ-160/100 with a rated injection volume of 160cm 3 is initially determined according to data.

4.5 How to push out plastic parts

Form of ejection mechanism of this set of molds is relatively complicated, and all ejection rods are used. Petal clamping is pushed out by 4 round push rods, each plastic part is pushed out by 4 stepped push rods. However, due to structure of ejection surface of plastic part, push rod of plastic part must be inserted into core, ejection action must be carried out from core. Considering that mold does not interfere during demolding process, plastic part is formed completely, according to plastic part, two push rods with a diameter of ϕ 2mm are used in one direction, and two push rods with a diameter of ϕ 1mm are used in the other direction. In the first parting, under action of spring ejector pin, plastic part and movable mold part are released together from core of fixed mold part; in the second parting, when flap clamping push rod pushes out split module to complete lateral core pulling and parting, plastic part push rod also pushes out plastic part, so that plastic part is separated from core.

4.6 Determination of lateral parting and core pulling mechanism type

This set of molds adopts a motorized side-drawing mechanism, and its driving mode is an oblique slider. Oblique slider drives lateral parting and core pulling mechanism. Usually, oblique slider is locked by a tapered die sleeve, which can withstand large lateral forces, but shaft pulling distance is not large. Undercut of this plastic part is shallow, required core pulling distance is not large, but undercut has a larger molding area, which requires a larger core pulling force. Therefore, it is more appropriate to use an oblique slider mechanism.
According to characteristics of lateral parting and core-pulling of inclined slider, push-out mechanism is used to inferentially drive inclined sliding block to move diagonally. When plastic part is ejected and demolded, inclined sliding block completes lateral parting and core-pulling action.
(1) Comparison of several schemes of oblique sliding block (flap closure module).
Scheme 1: Flap module slides in fixed mold (locking block). This kind of structure is difficult to control outer dimension relative to center of plastic part, but it is convenient for plastic part to fall automatically. In plastic part ejection method, plastic part can be ejected by using a push plate, a push rod, and a push sleeve.
Scheme 2: Flap module slides on pusher plate. This structure is used to complete two parting at the same time, it is easy to control outer dimension relative to center of plastic part, but plastic part is not easy to fall automatically, and it may fall in flap module.
Scheme 3: Flap module slides in movable mold. It is easy to control size of plastic parts. In order to make plastic parts fall automatically, structure in which plastic parts remain at the bottom of  flap module should be avoided in design.
In summary, this set of molds is suitable to adopt structure of Scheme 3, which is easy to control size and facilitate automatic falling of plastic parts. Based on this scheme, side parting of mold is improved, push rod is used to push out mold and close mold.
(2) Combination form of inclined slider. When designing its combination method, direction requirements of parting and core-pulling should be considered, and plastic part should have a good appearance quality. In addition, combined part of slider should also have sufficient strength. Sub-mold adopts structure of two-part combined module combination.
(3) Assembly requirements of inclined slider. In order to ensure that joint surfaces of inclined slider are tightly closed when mold is closed, and to avoid flash during injection molding, there should be a gap of 0.2~0.5mm between bottom of inclined slider and mold plate after mold is closed, and inclined slider must be 0.2~0.5mm higher than mold sleeve. When there is abrasion between inclined slider and guide chute, grinding lower end surface of inclined slider can continue to maintain closeness of vertical parting surface.
For this mold, if there is a gap of 0.2~0.5mm between bottom of inclined slider and support plate, ABS high-pressure injection molding is likely to produce flashing, so gap between bottom of inclined slider and fixed plate of movable mold should be <0.015 mm, inclined slider should be 0.5mm higher than mold sleeve to ensure that inclined slider is tightly locked when mold is closed.
(4) Selection of push rod position of inclined slider. In the process of side core pulling, care should be taken to prevent inclined slider from moving out of top of push rod, causing inclined slider to fail to complete expected lateral parting and core pulling actions.
(5) Limit position when inclined slider is pushed out. There are two grooves on each oblique slide block, and screw pins are added to limit position of die sleeve.

4.7 Cooling system design

Temperature of plastic in mold is about 200°, while temperature of plastic part is below 60° when it is taken out of mold cavity after curing. After injection molding of thermoplastic, mold must be effectively cooled, so that heat of molten plastic can be transferred to mold as soon as possible, so that plastic can be reliably cooled, shaped and can be demolded quickly.
Molding temperature and mold temperature of ABS are 150°~200° and 50°~80° respectively, mold is cooled with water at room temperature. Since plastic part is a stepped shaft part, it can be roughly divided into upper and lower parts based on a flange with a diameter of ϕ 20mm. Upper part has a sub runner, which should focus on strengthening cooling, and should be arranged at upper part of diameter ϕ 20mm.
For cooling water channel of core, diaphragm diversion type can be used, but calculation above shows that total heat released by mold plastic is not large, and only a cooling water channel can be opened around mold cavity.

4.8 Mold structure

Assembly drawing of mold structure is shown in Figure 7.
Precision injection mold 
Figure 7 Mould structure assembly drawing
1. Movable mold seat plate 2. Push plate 3. Push rod fixing plate 4. Support plate 5. Mould sleeve 6. Fixed mold fixing plate 7. Fixed mold seat plate 8. Core Ⅰ 9. Core Ⅱ 10. Main runner lining Set of 11. Positioning ring 12, 17, 21, 22. Hexagon socket screw 13. Water nozzle 14. Clamping mold 15. Core Ⅲ 16, 20. Plastic push rod 18. Push plate guide sleeve 19. Push plate guide post 23. Spring 24. Round head pin 25. Guide post 26. Guide sleeve 27. Limit pin 28. Plug 29. Valve closing push rod 30. Spacer 31. Sealing ring

5 Mold manufacturing process

5.1 Main processing methods of mold parts

Machining methods of metal parts include metal cutting, electro-erosion machining (also known as electric discharge machining), extrusion and other cutting-less machining. Metal cutting processes include turning, milling, planing, and grinding. Electro-erosion machining includes EDM machining, wire cutting machining and electrochemical corrosion machining. Extrusion processing includes cold extrusion, warm extrusion and hot extrusion processing. According to machining control method of machine tool, it can be divided into manual, automatic and numerical control.
Factors that affect determination of processing technology of mold parts include shape of part, material hardness, precision requirements, and equipment conditions. To design processing technology of a part, first consider shape of part and characteristics of various processing methods, scope of application and economy, then comprehensively consider other factors. Processing technology of mold parts is divided into milling processing, drilling processing, turning processing, grinding processing, electro-erosion processing and polishing processing according to processing method.

5.2 Technical requirements for plastic mold manufacturing

Mold accuracy is one of important factors affecting accuracy of plastic molded parts. In order to ensure accuracy of mold, following main technical requirements should be met during manufacturing:
(1) All parts that make up plastic mold should meet corresponding requirements in terms of materials, processing accuracy and heat treatment quality.
(2) Parts that make up mold base should meet specified processing requirements, assembled mold base should be free to move, meet specified parallelism and perpendicularity requirements.
(3) Function of mold must meet design requirements: ①Action of core-pulling slider and ejector device must be normal; ②Heating and temperature adjustment parts can work normally; ③Cooling water path is unblocked and there is no water leakage.
(4) In order to identify quality of plastic molded parts, assembled mold must be tested under production conditions (or with a test mold machine), trimmed according to problems of test mold, until a qualified molded part is tested.

5.3 Manufacturing process of cavity and core

Forging blanks are used for cavity and core processing. Since there is an oxide layer on the surface of forging, annealing treatment is needed to improve quality of forging for surface processing. Use a milling machine to mill upper and lower surfaces, mill out a datum, then use a grinder to grind to meet accuracy requirements. Before heat treatment, drill screw holes and push rod through holes with a drilling machine to prevent high hardness and difficult processing after heat treatment. Then it is formed by EDM. EDM can be formed at one time using formed electrodes, or it can be formed multiple times using simple electrodes. In this processing, forming electrode is processed, formed in two rough and fine stages. Machining accuracy of EDM is relatively high, grinding machine can be omitted after machining is completed. After end of electric spark, forming surface is processed. Because forming surface is relatively simple, milling can be used to form, then fine milling can be used to meet roughness requirements. Finally, polishing and grinding can be carried out, and cavity processing is completed. Carry out inspection of dimensional accuracy, and put it into storage after passing test.

Go To Top