杜邦模具设计.doc

Part DesignPrototypingPrototyping is an important step in the development of any sound design, but particularly important if the designer is unfamiliar with Surlyn(R), or if the new part differs significantly from one having a commercial history. Prototypes can be inexpensively prepared by machining stock shapes, by thermo-forming them from sheet stock, or simple injection molding tools can be made from casting kirksite or aluminum.In addition to providing a three dimensional presentation of the design, a prototype allows: Subjecting the part to the chemical and mechanical environment it will endure in real life Accelerated testing for part life Checking compatibility with anticipated secondary assembly operations Injection molded prototypes from molds that have cooling channels can supply cycle time estimates and guidelines for commercial tooling. Determination of shrinkage factors, optimum gate sizes, vent locations and effect of mold temperature should also be established. Molded prototypes are preferred because flow orientation may influence performance. The latter is particularly important in glass-filled compositions. Also, machining the shape can yield variable behavior because of random matching in areas subjected to stress.Part Geometry and Wall ThicknessWhen designing with Surlyn(R), section thicknesses should be minimized consistent with functional requirements and mold filling.In addition to increasing material costs, thick sections cool slowly and so require longer cycles. This is particularly true with ionomers because of their depressed freezing point and low crystallinity. (Figure 1)Figure 1An optimum design has a uniform wall thickness throughout the part which minimizes molded-in stresses, differential shrinkage (which combines with stress relaxation to cause warping) and void or sink mark formation in thick sections. Often different wall thicknesses cannot be avoided. In this case, walls should be designed to merge gradually and the part should be gated at the thickest section. Coring is a good method to attain uniformity. (Figures 2 and 3)Figure 2 Figure 3 Large, smooth, flat surfaces should be avoided because they: are often difficult to keep flat often lack rigidity and require ribbing that may detract from appearance. are easily marred Arched surfaces often are more structurally effective, and they make slight distortion less obvious and increase rigidity.Size and location of the ejection area are important with Surlyn(R) ionomer resins. At ejection, warm parts are more rubbery than low density polyethylene. Improperly located or inadequately sized ejectors may bind the part and cause distortion or other damage. Ejection will be covered more completely in the mold design section.Ribs and Strengthening MembersBeads, ribs or flanges are commonly used to increase section stiffness. Often, thick sections can be reduced without sacrificing stiffness, but checks should be made to verify that acceptable design stress levels are observed.Ribs and strengthening members should be one-half to two-thirds as thick as the walls they reinforce. Unsupported ribs should be no higher than three times the thickness of the wall they reinforce, and all sections should have no less than 1/20 of taper. Ribs perpendicular to the parting line simplify part ejection. (Figure 4) As the rib thickness approaches that of the adjoining wall, sinks (depressions) in the part surface may become difficult to control. In some cases, sinks may be disguised by incorporating emblems or patterns on the part surface. (Figure 5)Figure 4 Figure 5 Fillets and RadiiSharp corners are perhaps the greatest cause of failure in most plastic parts, and parts of Surlyn(R) ionomer resins are no exception. Eliminating sharp corners reduces the stress concentration factor and produces a part with increased structural strength. Fillets streamline the melt flow path, reduce molded-in stresses and aid in part ejection. Minimum radii also improve mold life by delaying corner fatigue in the metal.A useful optimum fillet radius for most practical purposes is at R = 0.61, where R = fillet radius and T = wall thickness at the radius. (Figure 6)Figure 6 All inside and outside corners should have a minimum radius of 0.38mm (0.015) which is usually permissible even where a sharp edge is required. If this is not acceptable, there are ways to give the required corner without violating the radius rule. (Figure 7)Figure 7 DraftDesigning part walls with ample draft greatly simplifies ejection. It can also help break the vacuum formed when the core is removed from a deep part, or a part is removed from a deep cavity. Drafts below 10 may require special techniques for removing the part, such as air ejection. This will be discussed further under mold design.Parting LinesParting lines are formed at the opening and closing faces of the mold. These lines will normally appear on the surface of the part, but by designing the mold properly, they can be located along inconspicuous edges or sections of the part. Since molds will wear more at these surfaces and could allow flash to appear, the chosen area should not be critical. Flexing of a flashed area could initiate a crack with resultant failure of the part. For best results, parting lines should not be located on any wear surface. (Figure 8)Figure 8 HolesHoles are easily produced in molded parts by providing pins in the mold which protrude into the mold cavity. Through-holes are more easily produced than blind-holes, because the core pin can be supported at both ends. Pins supported at only one end can be deflected by the flow of melt into the cavity. Therefore, hole depth is generally limited to twice the diameter of the core pin. To obtain greater hole depth, a stepped core pin can be used. Counter-boring of a side wall may also be utilized to reduce the length of an unsupported core pin. (Figure 9)Figure 9 Holes which run parallel to the parting line may increase the cost of the mold because of the associated need for retractable core pins or split tools. This problem may be overcome by placing holes in walls perpendicular to the parting line, using steps or extreme taper on the wall.Allowances should be made for a minimum 0.4mm (0.015) vertical step at the open ends of holes. A perfect chamfer or radius at the open end of a hole demands precision in the mold which may be economically unfeasible. (Figure 10)Figure 10 UndercutsUndercuts should be designed for uniform ejection. Because Surlyn(R) ionomer resins have low stiffness at ejection, 10% undercuts can often be stripped without complex split cavity or cam action molds. However, where this is contemplated the mold must be designed so the wall can stretch or compress. That is, the wall of the part opposite the undercut must clear the mold or core before ejection is attempted. It may also be necessary to provide rings or plate ejection, rather than pins.Under certain circumstances undercuts of greater than 10% have been stripped directly. Prototype tooling is strongly recommended in these cases. (Figure 11)Figure 11 Molded-In InsertsSurlyn(R) has been successfully used with molded-in inserts in a number of applications, but thorough prototype testing should be carried out.These points should be considered when designing for inserts. Inserts must be dry and free of oil, grease and dirt. Inserts should not have sharp edges and any knurling should have a rounded profile. To improve adhesion, metal inserts can be post heated to 260C (50F) by induction heating. Priming inserts can also improve adhesion. TolerancesRegardless of the economics, it may be unreasonable to specify close production tolerances on a part when it is designed to operate within a wide range of environmental conditions. Dimensional change due to temperature variations must be considered. Also, in many applications close tolerances with plastics are not as vital as with metals because of the resiliency of plastics.Some general suggestions are: Part design should indicate conditions under which the dimensions shown must be held. (Temperature, humidity, etc.) On a drawing, overall tolerances for a part should be shown in length per unit of length, not in fixed values. A title block should read, all decimal dimensions 0.00 x mm/mm, not 0.00 x mm/mm unless otherwise specified. Only those tight tolerances required for specific dimensions should be specified as such. Mold DesignMachine NozzleThe nozzle connects the high temperature heating cylinder of the machine and the cooled mold, and must keep its contents easy-flowing but not drooling into the mold sprue bushing. Any resin left in the nozzle tip from the last shot is chilled by the cold mold and will be a part of the next shot. A reverse taper nozzle minimizes the cold slug that is being pushed into the next shot. (Figure 12)Figure 12 Unless the nozzle is very short, it should have heater bands to correct for heat lost to the mold. With heaters and a nozzle bore sized slightly less than the opening of the sprue bushing, pressure loss is small. Although a reverse taper nozzle is preferred, Surlyn(R) ionomer resins have been successfully molded in machines having constant bore nozzles.Sprue BushingBecause the sprue is usually soft at ejection and the shrinkage of Surlyn(R) is low, the sprue bushing should be as short as possible. It should also have a smooth bore, a minimum of 30 included angle taper, and an inlet opening of 5.6mm (0.218) minimum. Sprue cooling may limit cycling speed, so the junction of the runner and sprue should be no larger than about 9.Smm (0.375) diameter. By keeping the sprue short, the bore taper can be increased without the outlet diameter becoming excessive. The junction between the sprue and main runner should also have a generous radius to improve flow. (Figure 13)Figure 13 Sprue PullerGenerally, the sucker pin type sprue puller is preferred because it minimizes the volume of resin at the end of the sprue. It helps cool the resin and the resin shrinking around it provides a good grip. (Figure 14) The Z puller and others are also used, but they may require deeper undercuts for Surlyn(R) than for stiffer plastics.Figure 14 RunnersTo provide part-to-part uniformity in a multicavity mold, the runner system should enable each cavity to fill at the same rate.Designing runners involves several compromises: The runner system should be compact to minimize the amount of rework. The runners must deliver melt that has retained maximum heat to completely pack out the cavity. At the same time, runners must cool quickly after delivering the melt to aid ejection. If the flow distances from the sprue to the various cavities are equal, the runner system is balanced. Since balanced runner systems contribute to melt uniformity among cavities, dimensional tolerances are easier to maintain. Unbalanced systems can cause cavity to cavity variations. (Figure 15)Figure 15 Occasionally, balancing the runner system and minimizing runner length conflict. In most cases it is better to accept the additional rework and use a more balanced system. Increased injection pressure can be a disadvantage in balanced systems with their longer runners and more turns.Pressure can be reduced by increasing runner sizes, thereby converting a pressure penalty to a regrind penalty. In the H pattern shown in Figure 15, the H crossbar is the primary runner fed by the sprue. It should be 1.6mm (0.0625) larger in diameter than the runner it feeds. These runners in turn should be 1.6mm (0.0625) larger than the branch runners feeding the gates.When possible full round runners are preferred. They provide minimum surface area per unit volume, and so have minimum pressure drops and reduced heat losses. For most parts the branch runner should be at least as thick as the heaviest section of the part, not less than 2.3mm (0.090) and need not be more than 9.5mm (0.375). (Figure 16)Figure 16 Trapezoidal runners are acceptable if the depth-to-width ratio is about 2:3, for example 4.8mm deep by 6.3mm wide (0.188 deep by 0:25 wide). Trapezoidal runners are often used in three-plate molds.Notes on Three-Plate Design (Figure 17)Figure 17 1. The gate land should have a 150 included angle, draft, to assure proper separation from the part.2. The gate entrance is produced with a 3.0mm (0.125) diameter ball-nose end mill. The entrance to the gate may, of course, be larger but this is about the minimum size for a secondary sprue of the length shown.3. The secondary sprue leading to the gate entrance should be tapered and polished free of tool marks for easy removal. A 150 included draft angle is appropriate.4. The gates are pulled by sucker pins with a spherical end and an undercut of about 0.5mm (0.020) on a side. Note that the area around the sucker pins must be cooled as well as possible to solidify the runner, or they will not pull the gate. Be sure the resin flow paths are not restricted by the sucker pins.5. The runner and sprue are ejected by a spring-loaded, movable stripper plate (not shown) to assure positive separation of the sprue from the nozzle. A mechanical sweep may be necessary to assure faultless clearing of the runner from the mold during automatic operation.Half round and shallow trapezoidal runners can be used only if the runners are short and the flow distance in the part is less than 50 times the part wall thickness.Gate DesignThere are several considerations in picking gate location: Gates should not be placed where part-bonding or impact will take place in use. The gate area will usually have higher residual stresses than the rest of the part due to the packing that takes place. Therefore, it will be weaker than the rest of the part. Position the gate to minimize trimming problems. Scissor-type gate cutters, kept sharp, will work smoothly and leave no blemish marks. If possible, gate into the thickest section to avoid incomplete fill or sink marks. This will also minimize flow marks and distortion associated with setting into a thick section from a thin one. Position the gate so that venting can be provided opposite it, at the parting line or an ejector pin. End gate parts where possible as side gating produces stresses due to the final packing necessary to remove sinks. Improper gating can cause surface defects and excessive stress in parts of Surlyn(R) ionomer resins. If the gate is too small, high temperatures and pressures will be needed to fill the mold and may cause warpage and flow marks. If the gate is too large, the resin will jet into the cavity, entrap air and the part will have surface sinks. These gates are commonly used:1. Sprue Gates leave large trim marks and are slow to cool. They may allow jetting and overpacking but make it easy to fill a part, especially if thick sections are involved, and are inexpensive to machine.2. Full Round Gates may cause flow patterns or surface defects from jetting, or warpage due to the relaxation of packing stresses. Sharp edges entering the cavity should be flared. The minimum diameter is about 1.5mm (0.060) and land should be as short as possible, and not less than 1.0mm (0.040). These gates trim clearly and are inexpensive. (Figure 18)Figure 183. Pinpoint Gates can cause problems. The fault often lies not so much with the gate as with the approach to the gate which is often long, thin, and tapered. (Figure 19) The gate land length appears to be very short, but because the gate is choked off when the resin freezes, the effective land length diameter may vary.Figure 19 Figure 20 shows another approach to the gate which helps to keep the gate hot and open. The land length will change less during injection, and a higher effective cavity pressure will result during fill.Figure 20 4. Tunnel Gates may exhibit some of the same warpage and jetting problems as full round gates. Too small an included angle may result in excessively high pressure drops. They can be used most effectively if the length of the tapered section and the land length are minimized, and the part has a short flow distance (a length to thickness ratio 50).Gates of 1 .5mm (0.060) diameter, minimum can be used. Knockout pins for the part and the runner should be located near the gate. (Figure 21)Figure 21 4. Rectangular Gates have some of the same disadvantages as round gates, but the depth and width can be changed independently to increase area (reduce pressure drop) without increasing gate freeze off time. A typical starting dimension would be 1.3mm (0.050) thick by 3.Omm (0.125) wide with 1.3mm (0.050) minimum land. (Figure 22)Figure 22 5. Fan Gates are a type of rectangular gate. However, because of the shape in transition and decreasing thickness from runner to gate, the fan gate reduces stresses as the melt enters the cavity and so improves part toughness. This type of gate can be useful for parts with thick sections. (Figure 23) A typical starting dimension would be 6.3mm x 1.3mm (0.25 wide by 0.050 thick) with 1.3mm (0.050) minimum land.Figure 23 6. Tab Gates are not really gates, but tabs or extensions of the part that the gate enters. The tab deflects the flow and is then cut from the part to remove any highly stressed areas. The gate into the tab should be flared to avoid jetting at high fill speeds. Tabs can be trimmed cleanly at the machine. (Figure 24)Figure 24 7. Ring and Diaphragm Gates are thin, continuous gates designed to fill tubular parts uniformly. The diaphragm gate has the advantage of low pressure drops and is often capable of faster cycles. Ring gates are less satisfactory because if faster fill