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Mold.ppt CORETECH SYSTEM Mold Design Fundamentals 1 Mold.ppt Basic Tasks of a Mold q Accomodation and Distribution of the Melt q Shaping of the Molded Part q Cooling/Heating and Solidification of the Melt q Ejection (Demolding) of the Molding q Mechanical Functions Accomodation of forces ) Transmission of motion ) Guidance of the mold components ) CORETECH SYSTEM The mold is probably the most important element of a molding machine. It is a arrangement, in one assembly, of one (or a number of) hollow cavity spaces built to the shape of the desired product, with the purpose of producing large numbers of plastic parts. Thus the primary purpose of the injection mold is to determine the final shape of the molded part (shaping function). In addition to give the final shape of the molding, the mold performs several other tasks. It conducts the hot melt from the heating cylinder in the injection molding machine and distributes the melt to the cavity (or cavities), vents the entrapped air or gas, cools the part until it is ejectable, and ejects the part without leaving marks or causing damage. The secondary tasks of a mold derived from these primary tasks include several mechanical functions such as accommodation of forces, transmission of motion, guidance and alignment of the mold components. The mold design, construction, the craftsmanship largely determine the quality of the part and it manufacturing cost. 2 Mold.ppt Functional Systems of the Injection Molds q Melt Delivery System: Sprue/Runner/Gate q Cavity (with Venting) q Tempering/Heat Exchange System q Ejection System q Guiding and Locating System q Machine Platen Mounts q Force Supplier q Motion Transmission System CORETECH SYSTEM An injection mold is composed of several functional units. Each unit performs one or several task of the mold. The melt delivery system or runner system performs the task of receiving and distribution of the melt. The runner system is in fact a set of flow channels that lead the melt into the cavities. Forming/shaping the molten material into the final shape of the part is the job of the cavity. During the filling and packing/holding stages, melt is forced by injection/holding pressure to completely fill the cavity (or cavities). Mold tempering or heat exchange system is used to control the mold temperature, cool down the molten melt (or,if thermosets or elastomer are used, heat the melt and cross-link the material) uniformly, solidify the molding to an ejectable state. Mold tempering system design has direct impact to the production cycle time and the quality of the molded part. Ejector system is utilized to open the mold and remove the molded part from the cavity. Mold mounting, alignment, and guiding are accomplished by the guidance/ locating system and machine platen mounts. Other auxiliary units such as force supplier and movement transmission unit are essential to accomplish the functions of an injection mold. 3 Mold.ppt Structure of A Mold Unit Sprue Ejector Pin Cold-Slug Well Sprue Bushing Secondary Runner /Sub-runner Cold-Slug Well Gate Part Primary Runner Sprue Sprue CORETECH SYSTEM Above figure shows the layout af a typical simple injection mold, which has four identical cavities. Melt from the nozzle enters the mold via the spure, which has a divergent taper to facilitate removal when demolding. Opposite the sprue is a cold slug well, which serves both to accept the first relatively cold portion of the injected material, and to allow a re-entrant shape on the end of an ejector pin to grip the sprue when the mold opens. The melt flows along a system of runners leading to the mold cavities. In general, for a single cavity mold, only the sprue or primary runner appears in the mold; whereas for a multicavity mold, secondary runners or subrunners are needed to distribute the melt into each cavity. The gates at the entries to the cavities are very narrow passages in at least one directions, so that the molded part can be readily detachable from the runners after removal from the mold. Sometimes additional cold slug wells are added in the end of primary runners to trap the cold slug during the filling stage. The mold is aligned with the nozzle on the injection cylinder by means of the locating ring surrounds the sprue bushing. 4 Mold.ppt Mold Design Issues q Mold Design ) ) ) ) ) ) ) q No.Cavity Cavity Layout Runner System Design Gating Scheme No.Gate Gating Location Mechanical/Mechanism Consideration runner (mainfold) system gate cavity Cooling System Design ) ) Cooling Channel Layout Special Design mold base cooling channel/lines CORETECH SYSTEM The primary tasks of an injection mold include the accomodation and distribution of the melt, the shaping and cooling/heating of the molding, solidification of the melt, as well as ejection of the molded part. Besides, a mold has to provide mechaincal functions such as accomodation of forces, transmission of motion, and guidance of mold components. Hence the primary functional systems of a injection mold include the melt delivery system ( sprue/runner/gate ), cavity (single-cavity or multicavity), ejection system, guiding and locating system, as well as mold temperature control unit (cooling system). From the view point of mold design, we have to evaluate the suitable size and layout of runner system and cavity, number of cavity, cooling system, etc. We will propose a few examples to illustrate how these design parameters influence the productivity and quality of the moldings. 5 Mold.ppt Determine Number of Cavities q Single Cavity vs. Multicavity Mold ) q Productivity and complecity consideration Determination of Number of Mold Cavities ) ) ) ) ) ) ) Number of moldings required and period of delivery Quality control requirements (dimensional tolerance,etc.) Cost of the moldings Shape, dimensions, and complexity of the molding (position of parting line and mold release) Size and type of the injection molding machine machine (shot capacity, plasticizing capacity, mold release..) Plastics used (gating scheme and gate location) Cycle time (increase in recovery time of plasticating unit, injection time, pressure drop, and mold opening time) CORETECH SYSTEM The multiple mold cavities can produce several article at the same time and hence has a higher output speeds and improved productivity. However, the greater complexity of the mold also increases significantly the manufacturing cost. The problems arising from a multicavity mold includes cavity layout, flow balance, balanced cooling channels layout, etc. Theoretically, for the same product, cycle time do not increase prorate with the number of cavities because th cooling time does not change. However, one often find that cycle time will increase as the number of cavities increases, for the following reasons: -Increase in recovery time of plasticating unit for the next shot and injection time because the total shot volume is increased. These increases in time are significant for large shots. -Increase in pressure drop becaused of the increased flow length from sprue, through runner system, to each cavity. The pressure drop can be a determining factor in the evaluation of numbers of cavity. -Increase in mold opening time because of the increased complexity. Both the technical and economic criteria have to be considered in determining the number of mold cavity, such as the numbers of moldings required, the cost and time of mold construction, the complexity of the molding, cycle time, quality requirements and the plasticating capacity of the available machine equipment, etc. 6 Mold.ppt Cavity Layout Circular Layout Layout in Series H-style bridge (branching) layout X-style layout CORETECH SYSTEM When the number of parts produced in each cycle exceeds one, a multicavity mold have to be used. Many cavity layouts can be adopted in the production. For example, layout in series has the advantage that there is no space restriction for each cavity; however, the unequal flow lengths to individual cavities may lead to unbalanced flow and differential part weights in each cavity. Circular layout has the advantage of equal flow length and uniform part quality; however, only limited number of cavities can be accomodated by this layout. H-style layout and X-style layout belongs to the so-called symmetrical layout. They are good in flow balance. Their disadvantage is that more larger runner volume and much scrap will be generated. Hot runner system can be adopted to conquer this drawback. Layout of cavities not only influence the filling pattern and extent of pressure packing, but also determines the equilibrium of injection force and clamp force during the molding cycle. 7 Mold.ppt Design of Runner System q Runner System Sprue ) Runner (Primary/Secondary) ) Gate ) q Goal: ) ) q Heating Element Nozzle Runner Piston or Screw Sprue Gate Screw Chamber (Reservoir) Cavity Mold Unit Accommodates the molten plastics material coming from the screw chamber and guides/distributes it into the mold cavity Raises the melt temperature to the proper processing range by viscous (frictional) heating while the melt is flowing through the runner Design Consideration ) Quality (filling pattern...) & Economics (cycle time...) CORETECH SYSTEM A runner system is composed of the sprue, the runner(s), and the gate(s) that connecting the runner with the cavity. The primary task of a runner is the delivery and distribution of melt from the screw chamber into the mold cavity. The runner system must be designed in such a way that the melt fills all cavities simultaneously and uniformly under uniform pressure and temperature. This design criterion is referred to as the flow balance of the runner system. Melt temperature may be significantly increased as it passes througn the narrow runner passage or gate due to friction effect. This viscous heating is important in raising the melt temperature and reducing the flow resistance because of the shear-thinning character of plastic material. The runner system has significant impact on the part quality and the economics of manufacture. Problems such as weld lines, pressure drop, material waste, removability of moldings, etc.,are related to the design of runner system. 8 Mold.ppt Common Runner Cross Sections q Circular Runner ) Full Round Runner q Parabolic Runner ) U-Type or Modified Trapesoidal Runner q Trapezoidal q Half Runner Round Runner q Rectangular Runner CORETECH SYSTEM There are several types of cross section can be adopted for a runner. The selection of the runner cross section depends on its efficiency and ease or difficulty of tooling. Circular or full round cross section provides a maximum volume-to-surface ratio and hence offers the least resistance to flow and least heat loss from the runner. However, it requires a duplicate machining operation in the mold, since two semi-circular sections have to be cut for both mold halves and aligned as the mold is closed. Parabolic or U-type runner represents a best approximation of circular runner, although more heat losses and scrab produced (mass is 35% greater), it needs simpler machining in one (movable) mold half only. Trepezoidal runner is an alternative modification of circular runner, its performance is similar to that of the parabolic runner. Trapezoidal runner is often used in three-plate molds since sliding movements are required across the parting-line runner face. Half round and rectangular cross section may lead to larger flow resistance and are unfavorable in the runner cross section. Normally, full round or trapesoidal runners are adopted in most practical cases. 9 Mold.ppt Considerations in Runner Design q Part Consideration ) ) q Material Consideration ) q Viscosity, Composition, Fillers,Softening Range, Softening Temperature,Thermal Sensivity, Shrinkage, Freezing Time... Machine Consideration ) q Geometry, Volume, Wall Thickness Quality (Dimensional,Optical, Mechanical...) Type of Clamping, Injection Pressure, Injection Rate... Mold Consideration ) Way of Demolding, Temperature Control... CORETECH SYSTEM Key factors affecting the design of a runner are summarized here. In the aspect of part consideration, the geometric dimensions of the runner should be such that flow restriction is at a minimum, that is, the runner should convey melt rapidly and unrestricitly into the cavity in the shortest way and with a minimum heat and pressure losses. The runner system should allow cavity filling with a minimum numbers of weld line so that the mechanical and surface properties of moldings can be improved. The runner should permit the transmission of holding pressure during the packing/holding stage so that the dimensional accuracy can be ensured. In the aspect of material consideration, the flow character and the thermal properties of material are related to the sizing of runner diameter and the runner length. Long or small runner should be avoided for material with short flow length (high viscosity). Runner should be properly sized to minimize material waste while not cause significant pressure loss. In the aspect of machine consideration, we should note the allowable injection pressure, injection rate, type of clamping, etc. The runner should be design so that demolding and removal from the molded is easy. Location and number of runner ejectors should be considered in the mold design phase. 10 Mold.ppt Flow Balance in the Runner Design q Flow Balance in Multi-Cavity Molds: ) Increase in recovery time of plasticating unit, injection time, pressure drop, and mold opening time PLAY412 CORETECH SYSTEM Consider the runner system design in the multicavity mold case. In a symmetric, naturally balanced cavity layout, all flow lengths from the sprue to each cavity are of the same length. In this ideal case the plastic melt will fill all cavities simultaneously under the same pressure and temperature conditions. The molded part in each cavity has the same weight and final properties. Unfortunately not all runners can be naturally balanced, especially for large parts where multiple gating may be needed to produce a proper part. Moreover, the natural flow balance is difficult for molds with a large number of cavities and is even impossible for the so-called family mold (combination mold) where each of the cavities is of different size and forms one component part of the assembled finished product. In these cases we have to balance the flow artifically. Balancing ensures virtually equal flow of plastic through each gate of a multicavity mold, and/or through each gate (if there is more than one) into each cavity. The melt should arrive at all gates/cavities at the same time and with the same properties so that all molded parts have uniform characteristics. This type of runner system is called the artifically balanced runner systems. On the other hand, even though the cavity layout is virtually balanced, the desired balanced flow may not be achieved since the flow depends on the plastic material used, the process condition setting, the accuracy of machining and the finish inside the channel, temperature difference due to unbalanced cooling/heating, , uneven venting, mold surface quality, etc. 11 Mold.ppt Runner Design and Part Shrinkage Part Shrinkage Runner cross-sectional Area Part Shrinkage Runner Length CORETECH SYSTEM The runner system design has a significant impact on the quality of moldings. For example, the part shrinkage increases as the runner length is increased since more pressure drop in the runner system and the melt is less packed within the mold. In general, the runner length should be as short as possible in order to reduce the pressure drop and amount of scrap. However, the runners must be of adequate length to satisfy the other conditions such as flow balance consideration, accommodation of cooling lines and ejector pins, etc. The part shrinkage reduces as the runner cross section is increased since the filling process is promoted and the effective holding pressure is higher. However, increase the runner size also produces more scrap and material waste. The size of the runner depends on the size of the part and its wall thickness, the design of the mold and the type of plastic being processed. Plastics with low viscosity (high melt flow index or long flow length) permit a longer or thinner runner. The runner cross section should be as small as possible but still compatible with the melt flow requirement such as pressure drop consideration. 12 Mold.ppt Design of Runner P la s tic M a te r ia ls ABS, SAN R ecom m ended R u n n e r D ia m e te r s 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) A c e ta l 0 .1 2 5 -0 .3 7 5 ” (3 .1 -9 .5 m m ) A c r y lic 0 .3 1 2 -0 .3 7 5 ” (7 .5 -9 .5 m m ) B u ty r a te 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) C e llu lo s ic s 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) F lu o r o c a r b o n 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) Io n o m e r 0 .0 9 3 -0 .3 7 5 ” (2 .3 -9 .5 m m ) N y lo n 0 .0 6 2 -0 .3 7 5 ” (1 .5 -9 .5 m m ) P o ly a m id e 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) PC 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) P o ly e s te r 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) PE 0 .0 6 2 -0 .3 7 5 ” (1 .5 -9 .5 m m ) PP 0 .1 8 7 -0 .3 7 5 ” (4 .7 -9 .5 m m ) PPO 0 .2 5 0 -0 .3 7 5 ” (6 .3 -9 .5 m m ) P o ly s u lfo n e 0 .2 5 0 -0 .3 7 5 ” (6 .3 -9 .5 m m ) PS 0 .1 2 5 -0 .3 7 5 ” (3 .1 -9 .5 m m ) PU 0 .2 5 0 -0 .3 1 3 ” (6 .4 -8 .0 m m ) PVC 0 .1 2 5 -0 .3 7 5 ” (3 .1 -9 .5 m m ) For most thermoplastics, minimum recommended runner size=1.5mm (0.06”) CORETECH SYSTEM This table lists the recommended runner diameters for different thermo-plastics in injection molding industry. For most thermoplastics, the minimum recommended dimension of runner is 1.5mm (0.06”), too small the dimension may lead to excessive presure drop and filling difficulty. The recommended runner size also reveals the flow ability (processability) of the plastic material. Plastics with low viscosity (high melt flow index or long flow length) such as polyethylene (PE) permit a smaller runner. Larger runner should be adopted for plastics that have shorter flow lengths (higher viscosity values), such as polycarbonate (PC). This table serves as an initial guess for runner sizing. 13 Mold.ppt Design of Runner q Location and Number of Runner Ejectors Stiffer Plastics Ejector Pin Softer/Flexible/Sticky Plastics CORETECH SYSTEM Both the number and location of ejectors depend on the plastic being processed. The stiffer the plastic is (at the moment of ejection), the fewer ejectors are needed; also, the designer has higher degree of freedom to determine the ejector locations. For example, the ejectors can be placed under the connecting runners (bridge runners) . For soft, flexible, or sticky plastics, more ejectors have to be adopted. Care must be taken in the ejector location so that the part can be ejected without leaving marks or causing damage. In general, more ejectors lead to an increase in the comlexicity of mold and the cost of the hardware and of machining. In the design phase of the runner system, one should consider the ease of demolding and removal from the molded part. The runner system should provide sufficient spacing for cavity in order to accommodate cooling lines and ejector pins and leave adequate cross section to withstand the injection pressure force. 14 Mold.ppt Runnerless Molding Technology Moldings Runner System: •Scrap and material waste •Pressure drop q Runnerless Molding Technology: ) ) q runners and sprues are kept a molten state during the processing runner systems are never actually ejected with the molded parts. Types of Runnerless Molding Technology: ) ) Insulated Runner System Heated/Hot Runner System CORETECH SYSTEM The conventional runner systemare referred to as cold runner systems since the runners solidifies during the cooling phase of the injection molding cycle and is ejected with the part. During the molding cycle the pressure drop increas as the runner is cooled down gradually. Degating is required during mold opening (for three-plate molds) or separately afterwards (for two-plate molds) and the runner system is regarded as scrap. The runner material may be reground and recycled again, but it may have some physical properties degraded from the original, virgin material. For small products the mass of cold runners may be as much as 80% of the mass of the total shot. On the other hand, the so-called runnerless molding technology has been developed to circumvent the drawbacks encountered in the cold runner systems. In these special mold designs the runners and sprues are kept a molten state during the processing and are never actually ejected with the molded part. There are no runners to be reground and recycled, thus, savings in material, labor, and/or overhead are realized. Typical examples of runnerless molding methods include insulated runners, heated/hot runner systems. 15 Mold.ppt Insulated Runner System Cooling Lines Solidified resin shell Emergency parting line Parting line Molten state melt q q q Oversized the runner diameter (15~30mm) Insulation effect of frozen skin shell Works for most olefinic resins(PE,PP...) and PS CORETECH SYSTEM In the insulated runner system, the runner diameter is oversized (say, 15~30mm) in order to maintain the molten state of the material. The large diameter runner allows an inner molten melt to pass through during the molding cycle because of the insulation effect of frozen skin shell surrounding the melt core. The insulation runner system has the advantage of extremely simple construction, low cost tooling, and high efficiency, provided the system can be left running undisturbed for long periods. This design is suitable for most olefinic plastics (such as polyethylene (PE), polypropylene (PP)... ) and polystryene (PS). The disadvantages of the insulated runner system includes: - it requires fast cycle to maintain molten state within runner (at least 5 shots/min). - it requires long start-up periods (15-25min) to stabilize the runner temperature (up to 150 oC) - it needs a long color change time - it needs very accurate gate temperature control in order to have a satisfactory production rate. - Additional emergency parting line is required to facilitate the removal of the frozen runner in the case of prolonged delay in the cycle time. 16 Mold.ppt Internally Heated Hot Runner System q q Material is heated by the heating element in the center of the runner Annular gap for melt flow Heater Cartridge Heated Probe (Torpedoe) Melt Part Vlocity Profile Tempertature Profile CORETECH SYSTEM In the internally heated hot runner system, the material is heated and kept at a molten state by the heated probe (torpedoe) in the center of the runner. The melt is allowed to flow in the cross section of the annular gap of the runner. The advantages of the internally heated hot runner systems include: -Less heat loss and lower heating power required since the thermal insulation of polymer melt -Less mold components mis-matching problem arising from thermal expansion -Inexpensive (as compared with the external heated runner system) -Little space required. The disadvantages of this design include: -Higher shear rate and pressure drop since the restricted flow area -Sophicated heat control required (temperature profile exists in the cross section of the annular gap of the runner). 17 Mold.ppt Externally Heated Hot Runner System q q Material is heated by the cartridge-heating manifold in the housing of the runner Circular cross section for melt flow Insulation Blocks Heater Cartridge Heated Manifold Hot Runner Air gap insulation Cooling Lines Vlocity Profile: plug-like flow Part Temperature Profile: constant temperature profile CORETECH SYSTEM In the externally heated hot runner system the material is heated by the cartridge-heating manifold in the housing of the runner. Thus a plug-like flow profile and an approximately constant temperature profile across over the circular flow area is developed. Thus the flow resistance is smaller than that of the internally heated system. The advantages of this design are: -More uniform temperature distribution. -Better temperature control -Lower melt stresses and pressure drop -Color/material changes easily The disadvantages of the externally heated hot runner system include: -More complicated design -More Expensive -Significant thermal-expansion-induced mis-match problems for various mold components. 18 Mold.ppt Design of Gate Part Design •geometry •wall thickness •direction of mechanical loading •quality demands (dimensions,cosmetics, mechanics...) •Flow length Generalities •ease of demolding •ease of degating •weld lines •distortion •molding defects •cost Plastic Material •viscosity (MFI) •processing temperature •flow characteristic •fillers •shrinkage behavior CORETECH SYSTEM Then gate provides the connection between the runner and the mold cavity. It must permit enough material to flow into the mold to fill out the cavity, raises melt temperature by viscous (frictional) heating, and freezes-off when the holding stage is over. It should be smaller in the cross section so that it can be easily separated from the molded part (degated). The type of the gate and its size and location in the mold strongly affect the molding property and the quality of the molded part. The factors which determine the gate design is summarized here briefly. General speaking, the gate should be small, simple to demold and easily separated from the part. The gate should be connected to the molding in such a manner that the latter is not distorted (the molding tends to deform concave to the feed ) and does not exhibit blemishes. Cost of tooling is also a consideration factor. The location of the gate must be such that weld lines are avoided or shifted to a less critical position. Molding defects such as jetting, burning, thermal degradation, short shot, etc. should be avoided in the production. Gating scheme and location of gates are crucial to the quality of the molding. Filling pattern and cavity pressure profile are closely related to the final properties of molded parts, such an mechanical properties, cosmetics (surface properties), dimensional accuracy. A gate should provide appropriate filling pattern and viscous heating effect, permit effective packing and holding of the material within the mold. These criteria depend on both part design as well as physical properties of the plastic material. 19 Mold.ppt Gating Scheme Direct/Sprue Gate Side/Edge Gate Pin Gate ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧‧ ‧ ‧‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ CORETECH SYSTEM There are several gate type can be adopted in the mold design, and each has its own advantage for application. The direct gate or sprue gate feeds material directly into the cavity. It is used for temperature-sensitive or high viscosity materials, and is suitable for producing part with heavy sections. The direct gate can be applied in high quality part because it allows effective holding (minimum pressure loss) and exact dimensions can be obtained. However, it is suitable only for single-cavity molds. Visible gate mark and the high stress concentration around the gate area are the disadvantges. The side gate or edge gate is the standard gate for injection molding. It is used wherever the product can be or must be gated from the parting line and where self-degating is not required or practical. It is carried out at the side of the part and is easy to construct and degate. The pin gate or pinpoint gate is a kind of restricted gates that are usually circular in cross section and for most thermoplastics do not exceed 1.5mm (0.06 in.) in diameter. It is generally used in three-plate molds (with automatic gate removal) and hot runner construction. It provides rapid freeze-off and easy degating of the runner from the gate. Flexibility in gate location is another advantage of the pin gate. It can easily provide multiple gating to a cavity for thin-walled parts. Viscous heating as the melt passing through the restricted pinpoint gate raises melt temperature and improves the filling process since the melt viscosity is lowered. Higher pressure drop is a drawback. 20
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