Tài liệu thiết kế khuôn mẫu

Tài liệu thiết kế khuôn mẫu

1 Mold.ppt C C ORE ORE T T ECH ECH S S YSTEM YSTEM Mold Design Mold Design Fundamentals Fundamentals 2 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Basic Tasks of a Mold 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 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. 3 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Functional Systems of the Injection Functional Systems of the Injection Molds 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 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. 4 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Structure of A Mold Unit Structure of A Mold Unit Sprue Sprue Primary Runner Secondary Runner /Sub-runner Gate Part Cold-Slug Well Cold-Slug Well Sprue Ejector Pin Sprue Bushing 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. 5 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Mold Design Issues Mold Design Issues mold base cooling channel/lines runner (mainfold) system gate cavity q Mold Design ) No.Cavity ) Cavity Layout ) Runner System Design ) Gating Scheme ) No.Gate ) Gating Location ) Mechanical/Mechanism Consideration q Cooling System Design ) Cooling Channel Layout ) Special Design 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. 6 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Determine Number of Cavities Determine Number of Cavities q Single Cavity vs. Multicavity Mold ) Productivity and complecity consideration q 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) 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. 7 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Cavity Layout Cavity Layout Layout in Series Circular Layout X-style layout H-style bridge (branching) layout 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. 8 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Design of Runner System Design of Runner System Piston or Screw Screw Chamber (Reservoir) Heating Element Nozzle Runner Gate Sprue Cavity Mold Unit q Runner System ) Sprue ) Runner (Primary/Secondary) ) Gate q Goal: ) 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 q Design Consideration ) Quality (filling pattern ) & Economics (cycle time ) 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. 9 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Common Runner Cross Sections Common Runner Cross Sections q Circular Runner ) Full Round Runner q Parabolic Runner ) U-Type or Modified Trapesoidal Runner q Trapezoidal Runner q Half Round Runner q Rectangular Runner 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. 10 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Considerations in Runner Design Considerations in Runner Design q Part Consideration ) Geometry, Volume, Wall Thickness ) Quality (Dimensional,Optical, Mechanical ) q Material Consideration ) Viscosity, Composition, Fillers,Softening Range, Softening Temperature,Thermal Sensivity, Shrinkage, Freezing Time q Machine Consideration ) Type of Clamping, Injection Pressure, Injection Rate q Mold Consideration ) Way of Demolding, Temperature Control 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.