Presentation on theme: "Dr. Joseph Greene Copyright 2000 all rights reserved 1 Ejection Chapter 12."— Presentation transcript:
Dr. Joseph Greene Copyright 2000 all rights reserved 1 Ejection Chapter 12
Dr. Joseph Greene Copyright 2000 all rights reserved 2 Ejection Definitions Basic Requirements for Any Ejection Method General Ejection Guidelines Ejector Pins and Sleeves Strippers Air Ejection Multiple Ejection Stroke Special Ejection Methods Two-Stage and Multistage Ejection Molding Surface Finish Sequence of Ejection Collapsible Cores Unscrewing Molds
Dr. Joseph Greene Copyright 2000 all rights reserved 3 Background and Definitions After plastic flows into the mold and cools it has to be removed (or ejected) from the mold Parts can be removed –manually –with robotics, –automatically Automatic ejection –Molds have ejection mechanisms to separate the product from the molding surface, usually the core –Free fall- products fall freely into box or conveyer belt –Advantages- Consistent cycle time and uniform products Improves safety of plant since no operators are close to machinery. Faster cycle times
Dr. Joseph Greene Copyright 2000 all rights reserved 4 Basic Requirements Rule of ejection –As mold opens, products must stay on side from which are ejected Sometimes ribs, bosses, undercuts keep part on core side –Molds have election actuator on clamp (moving) side of machine Ejection Features –Mold with ejector pins shown in Fig 12.1 Knock-out pads are used to keep the machine ejectors level with the moving platen surface for ease of mold installation. Stripper plate is actuated from the ejector plate (Fig 12.2) used if ejector plate cannot be directly reached (Fig 12.3) –Length of Machine Ejectors If mold can be inserted by moving it toward the moving platen, –there is no need for knockout pads, and –the machine ejectors can reach all the way to contact the ejector or stripper plate (Fig 12.4) Less expensive than adding the knockout pads More difficult mold installation
Dr. Joseph Greene Copyright 2000 all rights reserved 5 General Ejection Guidelines Stroke, Clearance, and Product Height –Must have clearance between cores and the cavities to permit the products to fall freely. –Stroke should be about S=2.5H (H=product height) for deep products –Method of alignment affects required stroke
Dr. Joseph Greene Copyright 2000 all rights reserved 6 Stroke, Clearance, Product Height Excessive stroke presents no problem other than extending cycle time and reduces productivity. Small strokes are possible for very flat products For servicing of the mold, it is best to keep stroke large enough, minimum of 150 mm for access. Husky guide rails on Husky machines the stroke can be as low as 2.5H. In single level molds, available stroke is usually sufficient.
Dr. Joseph Greene Copyright 2000 all rights reserved 7 Venting Venting is very important where a vacuum occurs –Especially for cup-shaped and flat products –Fig 12.8- A cup-shaped product is pushed off the core –For flexible material, PE, it may return to original shape –For a brittle material, PS, it may break. –An ejector near the center may act as a vent Where to Eject Relative to the Product –Guidelines For deep products, eject at the points where product is stiffest –Fig 12.10- Do not eject at center of cup on inside For products with deep ribs, eject at the points where it is stiff –Fig 12.11
Dr. Joseph Greene Copyright 2000 all rights reserved 8 Venting Where to Eject Relative to the Product –Guidelines Shallow ribs (where the height is less than 1.5 times thickness) usually do not need ribs but need venting to prevent implosion. For pin ejection of shallow products: –Distribute ejectors to lift product without tipping. –Place ejectors at lowest point for ribs or bosses. –Use ejector pins and sleeves as natural vents. Land should be not longer than 2D (Diameter of pin). (Fig 12.12) Witness lines are round marks left on the part by ejector pins. Ejection of bosses depends upon the shape and solidity of boss. –No need for ejection for solid bosses that are shallow (depth=width or D) and there is good draft angle (at least 5° per side)
Dr. Joseph Greene Copyright 2000 all rights reserved 9 Ejection Location Where to Eject Relative to the Product –Guidelines –Diameter of ejector should be standard size and should be smaller than the bottom diameter of the boss. –Avoid solid bosses to prevent sinks. –Width or diameter of boss should be less than wall thickness of the product –Tubular bosses (Fig 12.14) are preferred wherein »a hole passes through a boss, or (Fig 12.14)- No sink marks on end »Blind hole (Fig 12.14)- no sink marks and provides cooling –Place a few ejector pins evenly spaced at the bottom of a large boss »Fig 12.15 –Avoid small ejector pins –Use standard pins and sleeves
Dr. Joseph Greene Copyright 2000 all rights reserved 10 Ejector Pins and Sleeves Pins and sleeves can be used or any shape of product –Advantage of ejectors versus stripper plate Inexpensive and easy to use Good natural, self cleaning venting in areas where the air would otherwise be trapped and require vent pins –Disadvantage of ejector pins and/or sleeves Area where product is pushed is relatively small The area must be well cooled (stiff) so as not to bend Witness lines –Advantage of strippers (plates, rings, bars) or air ejection Surface of ejection is large and vestige is smaller. Only used where product at the parting line has proper contour Air is simple without out moving parts but limited to cup shape
Dr. Joseph Greene Copyright 2000 all rights reserved 11 Clearance (Fit) and Length of Land Clearances for ejector pins and sleeves –Specified by pin manufacturer per vent sizes in Chap 11 –Clearance of pin in bore Too tight a fit may bind the ejector causing it to break. Too loose a fit may create flash –Length of land Land should not be longer than 2D for small pins D up to 6mm Land should not be longer than 1.5D for large pin sizes Too long lands are costlier to produce and may result in poor venting Too short a land may cause excessive flashing and may cause bore to wear out more rapidly.
Dr. Joseph Greene Copyright 2000 all rights reserved 12 Size, Finish, and Shape Size of pins –Use standard sizes. Special sizes are expensive Finish of pins –Pins are made from H13 or similar hard steel, hardened and nitrided, with surface hardness of 70 Rc. High polish. Finish of bore –Must be smooth and free of grinding marks that could act as a file on the pins. Suggested finish of 0.4microns –Produced usually by jig grinding. Avoids bell mouth, which causes flash at location of bore. Bell-mouthing is a funnel or bell-shaped ending of the bore at the molding surface. (Fig 12.16)
Dr. Joseph Greene Copyright 2000 all rights reserved 13 Size, Finish, and Shape Shape of ejector pin (or sleeve) hole in core –Holes must be funnel-shaped from the rear to facilitate entry of the pins (sleeves) during assembly (Fig 12.17). Selection of ejector pin sizes –Make ejector pins as large as possible. –Bore diameters above 6mm are cheaper to produce than smaller ones. –Avoid sizes 3mm diameter and smaller, especially if length is 50 times diameter. –Sizes smaller that 3mm (2.5,2.0, and 1.5mm) are stepped down from 3 mm, which extends to 75mm –Long, slender pins may collapse under ejection force and could wear faster and need to be replaced often
Dr. Joseph Greene Copyright 2000 all rights reserved 14 Special Ejector Shapes Very small ejectors required for bottom of ribs, –Better to provide a flat ejector pin instead of a round pin –Example, Width at the bottom of the rib is 1.5mm, it is better to have a flat ejector (1.5mm x 5mm). The area facing the plastic in the round pin is 1.77 mm 2, whereas for flat pin is 7.5mm 2. –Larger surface area is less likely to penetrate the plastic during ejection –pin is stronger and will last longer Design a slot for the special shape pin. –Use EDM into the solid core or use two-piece construction. Fig 12.18 –Rear of the core under the slot is bored out for clearance of the shank of the pin from which a special ejector was made. –For a 2-piece construction, a shallow U-tube is machined into one portion of core. 6 mm standard pin size 6 mm 1.5 mm
Dr. Joseph Greene Copyright 2000 all rights reserved 15 Special Ejector Shapes Number and location of Ejector pins –No set rules for number of ejector pins per cavity. More pins yield better ejection and product flatness, higher cost Example, –24-cavity mold that has 2 unnecessary pins per cavity results in 48 unnecessary pins per mold and the cost to produce pins. –Guidelines for location of pins Located where they will provide necessary venting of areas which do not vent to the parting line. Located at the lowest points of the products, ribs, rim, bosses. Required at or near the corners of the product. Located symmetrically and evenly spaced around product Located at intersections of rib and rib OR wall and rib. fig12.20 Intersection locations allow increased ejector size. Example, Fig 12.20: Part with 1.5mm ribs, –Rib intersection diagonal dimension would be 1.5x(2) 1/2 =2.12mm (2 rib) –At wall, 1.5x1.25=1.875mm (Use 2mm pin with 0.15mm corners)
Dr. Joseph Greene Copyright 2000 all rights reserved 16 Ejector and Ejector Retaining Plate Two forces tend to deflect the ejector plate: – Ejection and injection Ejection Force –Forces depend upon Finish of core: rougher the core = more force Draft angle: smaller draft = more force Undercuts required from product: heavy undercuts = high force Undercuts required when product remains in cavity due to: –Vacuum under bottom of product, e.g., containers. –Stickiness of plastic, e.g., some adhere more to steel. –Shrinkage forces in cavity, e.g. pins in cavity or too warm a core. –Symmetrical products about parting line, e.g., records, dinner plates, etc Max ejector force is 6-10 tonnes (metric tons =1000kg) small Max ejector force is 10-16 tonnes for larger machines
Dr. Joseph Greene Copyright 2000 all rights reserved 17 Ejector and Ejector Retaining Plate Injection Force –Ejector pins are loaded at their face with the injection pressure during injection, packing, and hold stages. The bigger the area subjected to pressure, the greater the force transmitted through the ejector pin to the ejector plate. –High forces can cause the pins to deflect and bend. Stop pins under or near heavily loaded pins, the effect of plate deflection due to these forces can be eliminated. Ejector plate is more often a plate rather than a simple beam, with ejector pins well distributed over the entire surface. –Complicated plate theory is actual condition. –Simple beam calculations due not apply. –Plate deflection, f, must be kept to a minimum. –Rule: f < 0.1 mm is acceptable.
Dr. Joseph Greene Copyright 2000 all rights reserved 18 Ejector and Ejector Retaining Plate Location of Ejectors Relative to Machine Ejectors –Fig 12.21 –Make the ejector plate thick to keep deflections low –Design ejector as beam. –Deflection depends upon the stiffness of the plate. –The stiffness of the plate is (Flex Mod)(Thickness) 3 Example, –If ejector plate is 25mm thick and the head is 6mm, the stiffness is 25 3 = 15625 mm 3. If the plate thickness is increased to 31mm, then the stiffness is 31 3 = 29,791 or almost twice as stiff. –Thickness of plate does not depend upon tensile strength. f Max. ejection force f f f f
Dr. Joseph Greene Copyright 2000 all rights reserved 19 Ejector and Ejector Retaining Plate Ejector Retainer Plate –Purpose Hold the ejector pins (or sleeves) on the ejector plate. –Bad practice to thread ejectors into ejector plate to eliminate retainer. Sometimes several small retainer plates rather than one large plate are used on one ejector plate. Heads of the ejector plate and return pins or sleeves, should float with lateral play in the retainer plate for proper alignment. Fig 12.22 Thickness of retainer plate at the head need never be more than 3mm. Axial clearance should be very little (0.1mm) Radial clearances, HC and SC, should be a minimum of 0.5mm
Dr. Joseph Greene Copyright 2000 all rights reserved 20 Ejector and Ejector Retaining Plate Preventing Ejector Pins from Turning –Must be prevented from turning when Front of pin in the cavity is shaped to match the product. It carries engraving. A large pin is used for ejection and partly as a return pin. –Preferred method is to key the head of the pin. –Fig 12.23 A and B B is preferred, but milling the cutter in view B is smaller and thereby slower. Thickness of the ejector retainer plate does not add anything to the strength of the ejector plate against deflection. Spacing the screws close to groups of ejectors and return pins, few screw are usually necessary.
Dr. Joseph Greene Copyright 2000 all rights reserved 21 Ejector and Ejector Retaining Plate Return Pins (Fig 12.26) –Ensure all ejector pins are returned to their back position by the time the mold closes. This prevents the pins from contacting (and possibly damaging) the opposite cavity surface. –Design guidelines –Always have 4 return pins, evenly spaced so plate doesn’t cock. –Do not position pins where they could hit a vent channel. –Minimum diameter of return pin should be 12 mm. Preferable sizes are 16 or 19mm. Larger pins have less damage on plate –Always use standard (commercial) pin sizes. –Passage through plate can be larger tolerances than ejector pin –Length of return pin should be less than the theoretical length required to push the ejector plate back all the way. Must protect cavity wall opposite an ejector pin from damage.
Dr. Joseph Greene Copyright 2000 all rights reserved 22 Ejector and Ejector Retaining Plate Ejector Box (Fig 12.29) –Portion of the mold assembly which surrounds, supports, and guides the ejector plate. Consists of a mold mounting plate, two parallels, and supports –Many molds built this way in the past. –Based on certain standard sizes of rectangular plates and round pillars. –Occasionally, ejector plate must have pins in odd locations. (Fig 12.30) Preferred method to some designers. –Machine all supports and parallels into the core backing plate. –Requires few assembly screws required to enter the core backing plate. –More space is available for the cooling and air channels. Tapped holes or slots core mounting ledges can be machined into core backing plate. Important Note: Larger the cutter diameter, the faster the milling operation. Deep pockets require large cutters.
Dr. Joseph Greene Copyright 2000 all rights reserved 23 Ejector and Ejector Retaining Plate Guiding Ejector Plates –All moving plates must be guided in At least 2 and preferably 4 locations, evenly spaced. Standard leader pins (LP) and LP bushings should be used. Clearance C (Figure 12.31) should be about 0.1 mm (0.004in) –Guidelines Retainer plate is inserted first, then the pins are inserted through the plate and into the core plate. Ejector plate is lined up with guide pins, pushed into position, and all the screws are installed to hold retainer plate. Good practice to align ejector retainer and ejector plate with two dowels at opposite corners. Guide pins must be long enough (Dim L in Fig 12.31) to project beyond the bottom face of the ejector box for alignment. There must be at least one tapped hole on the underside of the ejector plate for removal because the plate is in the box.
Dr. Joseph Greene Copyright 2000 all rights reserved 24 Ejector and Ejector Retaining Plate Returning the Ejector Plates –With the use of return pins, there is no need to provide additional features to return ejector plate. As mold closes, the return pins drive ejector plate back into its “rest” position, touching the stop pins. –Acceptable for slow closing speeds of the clamp and short ejector stroke. Normally, ejector pins move so far out that the return pins strike the cavity side of the mold while the clamp is moving fast –Resulting impact could damage return pins and be Noisy. Helpful if ejector plate is returned by an independent method before the return pins strike the opposing mold half. Methods to return the ejector plate. –Tying ejector plate to molding machine –Using linkages attached to the mold –Using return springs –Attaching air cylinders to the ejector plate.
Dr. Joseph Greene Copyright 2000 all rights reserved 25 Ejector and Ejector Retaining Plate Methods to return the ejector plate. –Tying ejector plate to molding machine Push-pull method. Used with hydraulic machine ejectors. –Disadvantage is the additional connection of mold and machine. Time –Using linkages attached to the mold Both ejection and return motions of ejector plate are linked to mold stroke. –Requires exact and repetitive opening stroke. –Must ensure that the clamp does not open beyond design intent. –Using return springs Internal and external return springs placed between core and ejector plates. (Fig 12.34) –Springs must be pre-loaded 10% of maximum stroke to solid. –Spring must not be compressed by more than 25% of this stroke.
Dr. Joseph Greene Copyright 2000 all rights reserved 26 Ejector and Ejector Retaining Plate Methods to return the ejector plate. –Using return springs Effective stroke must be less than 15% of max stroke to solid. »Ejection travel of 30mm, standard medium duty spring = 200 mm long »Tough to package that mush length. Follow spring mfg guidelines Note: unguided (unsupported) length of any spring should never be more than its diameter. –If distance is larger, spring must be supported (internally) with a rod Springs with long strokes should be placed outside mold –Enough space and where broken springs can be identified. –Four springs should be used and located near corners of ejector plate. For small molds Fig 12.36 –One spring, centrally located should suffice. –Disadvantages »The moving platen has a large central opening. »Projecting spring assembly is easily damaged during storage or hoisting of mold
Dr. Joseph Greene Copyright 2000 all rights reserved 27 Ejector and Ejector Retaining Plate Methods to return the ejector plate. –Attaching air cylinders to the ejector plate. Advantage is that the force generated is constant over the whole length of the stroke, unlike springs, which are weaker in the expanded state and stronger in the compressed state. Example 1, –Four cylinders are located near the corners of the ejector plate to provide balanced force on plate. »Typical arrangement for 4 air actuators to return the ejector plate. »Left illustration, the piston holds the plate against the stop pins. »Right illustration, the machine ejectors push the plate forward. Example 2, –One air cylinder is centrally located (small molds only). »The piston is fixed to the core plate, and the cylinder is fastened to and moves with the ejector plate. –Compressed air may be connected permanently to the air cylinders. –Force of the air is less than the machine ejectors.
Dr. Joseph Greene Copyright 2000 all rights reserved 28 Strippers Strippers (plates, rings, or bars) are preferable to ejector pins. –Surface of the product where it is pushed is relatively large, and the ejection force is evenly distributed. –Vestige left from the stripper is usually less noticeable. –Should be used where product at parting line has a shape which can be easily generated when machining. –Must seat on taper which is preloaded to prevent flashing Odd-shaped strippers with tapered shut-offs are CNC or EDM –Should have simple geometric shape. Circular or linear that is easily machined. Fig 12.37- Noncircular product ejected 3 ways –Reference preload on tapers (sec 22.8.2)
Dr. Joseph Greene Copyright 2000 all rights reserved 29 Strippers General Rules for Strippers –Stripper must always clear the core. –All stripper must seat on a taper (tapered shut-off is best) –Cylindrical shut-off is not recommended Guidelines –Minimum clearance between stripper and core is 0.25 mm (0.010 in) Clearance prevents scratching and damage to the core and to the edge of the stripper, especially with little side draft on core. –Fig 12.39 Commercial ejector sleeves are the only exception to rule that all stripper must seat on a taper. –Sleeves are inexpensive and easily replaced –Superior quality surface finish will not easily wear or damage the bore in core
Dr. Joseph Greene Copyright 2000 all rights reserved 30 Strippers Good reasons for not specifying cylindrical shut-off –Shut-off will wear as the stripper slides on core (Fig 12.40) Wear means flashing Good sliding fit when new may be too large to prevent plastic from entering and flashing between two mating surfaces. Gap on one side of the stripper or core may be zero, then the other side can have twice the intended gap and flash. –Shut-off leaves the seat on the core if the product requires a long ejection stoke. When returning, the sharp edge can be damaged easily. –Once stripper is worn, it cannot be reseated but must be replaced
Dr. Joseph Greene Copyright 2000 all rights reserved 31 Strippers Guidelines –Vertical shut-off is inexpensive. –Cylindrical shut-off may be acceptable in prototype molds and used for a few sample parts. –Tapered shut-off method is best for all good molds. Angle a is usually between 5° to 15° (Fig 12.41) Smaller tapers might be needed if there is not enough –thickness in the stripper to accommodate a standard taper without the steel becoming too thin at end with the wider taper dimension. –Manufactured sleeves (Fig 12.42) may not allow enough room for a standard taper, so a smaller taper may be specified. Stripper seat must have pre-load or flashing will occur (22.14) –Advantage is the taper shut-off ensures that the critical edges of stripper and core will not collide as the stripper moves forward on its seat and prevents damage to the edges. –Sharp edge can be damaged but easily recreated –Loose tapers can be reseated by grinding underside of stripper.
Dr. Joseph Greene Copyright 2000 all rights reserved 32 Strippers Guiding the stripper –Stripper must be guided in axial direction of the mold. Ensures that the taper seat is moving properly into engagement without hitting the edge of the core. Methods –Use the leader pins of mold to guide stripper. (Fig 12.44) »Disadvantage is that the alignment of leader pins may fight the alignment of the stripper taper. »Guide bushings on the stripper plate are made looser than standard »Looseness in leader pin bushings must not exceed the total of the positional tolerances of leader pins and cores. »Floating core mounting can overcome alignment problem. –Use separate guide pins for stripper plate. (3-plate molds) Fig 12.45 »Mold leader pins align the mold but also guides the stripper plate. »Disadvantage is floating stripper rings are required, otherwise the taper fit will fight the alignment in leader pin bushings. –Ejector stroke must be less than the stroke limiter stroke so that the stripper bolt will never be stressed by the ejector force.
Dr. Joseph Greene Copyright 2000 all rights reserved 33 Strippers Guiding the stripper –Methods Actuate the stripper from the ejector box (Fig 12.47) –Mold leader pins have the same function as previous example. »Pins should protect the core and do not require a bushing in the cavity plate. »Stripper is guided by pins and bushings in the core backing plate. »Stroke is limited in the ejector box. –No need for stop buttons. –Guide pin made from hardened steel –Guide bushings should be permanently greased ball bushings or plastic bushings. Stripper plates without stripper rings are used only when the cost of supplying a hardened stripper is little different or lower cost than the cost of a softer plate with hardened, inserted stripper ring.
Dr. Joseph Greene Copyright 2000 all rights reserved 34 Strippers Stripper Bars –Can be considered stripper rings with an infinite inside radius. All rules for stripper rings apply –Guiding, limiting of stroke, taper seat. Bars need additional force to make it seat on the core with the use of an outside taper. Stripping from Injection Side –Some cases (cosmetic reasons) to gate from inside of part Product requirements do not allow for gate or vestige –Rules for either ejectors or strippers are similar to before Added depth to injection side increases lengths of drops in hot runner or sprues which increases tool costs.
Dr. Joseph Greene Copyright 2000 all rights reserved 35 Air Ejection –Can be used for practically any cup-shaped product Air enters the space between the core and the product, it –does not immediately exhaust into the open, but –creates pressure under the product and pushes it free from core. –Problems Little draft products with undercuts may build up significant pressure that can burst the product. Product may suddenly let go from undercuts and fly against the cavity causing damage. Product may lodge itself inside the cavity and stop the machine. Reducing pressure may solve one problem, but fail to eject Mechanical ejection may be needed in addition to air ejection
Dr. Joseph Greene Copyright 2000 all rights reserved 36 Air Ejection –Containers with little or no draft, but with no undercuts are very successfully ejected with air. Fig 12.56 –Bernouolli effect must be taken into account Flow past a horizontal surface causes a vacuum to occur. The flow of air out of the cup causes a suction force which keeps the cup on the core. –The air pressure on the bottom of the cup must overcome this suction force to eject the part. –If air pressure isn’t enough the cup may eject some distance but then stop and float in mid-air without falling off core. –Air pressure can be pushed at the center of the cup bottom, or 2 alternative locations at the bottom of the cup.
Dr. Joseph Greene Copyright 2000 all rights reserved 37 Air Ejection Basic Requirements of Air Ejection –Air supply must be sufficiently large Amount of air permitted to escape in each cavity of a multi- cavity mold to push out products and runners. –Dimensional uniformity of air slots or jets is imperative to avoid flashing into them, but still be able to eject parts. –Air must be clean and filtered without dust, oil, or water Special sterile air is needed for medical or food applications
Dr. Joseph Greene Copyright 2000 all rights reserved 38 Air Ejection Advantages of Air Ejection versus mechanical –Simpler core half constructions –Fewer masses to be moved (less energy consumption) –Less shut height –Less mold weight –Absence of impact noise and wear of ejector mechanism –No metal wear, which is present with taper seats of stripper rings –Less downtime required for maintenance, particularly on parting line
Dr. Joseph Greene Copyright 2000 all rights reserved 39 Air Ejection Disadvantages of Air Ejection versus mechanical –Much compressed air required, which can be expensive. –Existing compressed air supply might have to be upgraded to greater capacity. –Balancing of air outlets, particularly in multi-cavity molds, requires intricate channeling in cores. –Mold layout is more complicated due to air channels. –Additional controls, such as air circuits, hoses, and valves, are required. Advantages usually outweigh the disadvantages.
Dr. Joseph Greene Copyright 2000 all rights reserved 40 Air Ejection Poppet Ejection –Oldest method of air ejection, though rarely used alone today, but rather with air slots or mechanical ejection. –During ejection, The poppet is tight on the taper seat in the tip of the core and prevents plastic from entering into the air system. After mold starts opening, the poppet moves forward by amount SE (less than 0.25mm). Fig 12.58 –The forward motion can come from the air pressure on the underside of the poppet, and/or from the pressure of the coolant. –Poppet can be actuated by ejection mechanism in the mold. »Some use double acting air cylinder at base and at stem giving more force and eliminating the need for mechanical action. »Disadvantage is poor cooling of poppet resulting in slower cycle »Need cooling of poppet (Fig 12.59) with cooling water pressure acting of face and keeping poppet open without air pressure
Dr. Joseph Greene Copyright 2000 all rights reserved 41 Air Ejection Rules for All Poppet Designs –Poppet should be opposite gate so that the inrushing plastic will close the poppet and prevent plastic from entering air channels. –If poppet is not opposite gate, springs are used to close it. –Stroke, SE, for poppet opposite gate, must be 50% of the thickness of the product at this spot. There must be enough space for cold slug in the gate to eject freely into the space between gate and the top of the poppet. Stroke is usually 0.25mm. –Commercial poppets with complete assemblies of poppet and spring in one unit Fig 12.60 can be pressed into core. They must be drilled out for removal
Dr. Joseph Greene Copyright 2000 all rights reserved 42 Air Ejection Rules for All Poppet Designs –Size of seat (land) of the poppet is similar to any other seat that comes under injection or clamping pressure Load should be limited to 5% of the yield of the softer of two materials contacting at the seat. Land width in Fig 12.59 –Plastic should never enter the air channels. –For positive return poppets, with a larger poppet stroke, the poppet must be returned to its seat before the cavity approaches the poppet to ensure no damage to poppet or cavity. Forward and return stroke of poppet can be moved –mechanically or by –connecting the poppet to the mold ejector system, or by –independent double acting air (hydraulic) actuators acting directly on poppet
Dr. Joseph Greene Copyright 2000 all rights reserved 43 Air Ejection Poppet with Double-Acting Actuator –Moved by double-acting piston, which is held by a snap ring. (Fig 12.61) –Air sequence is as follows: Piston forward, Blow-off air, and Piston return –Sealing elements must be properly selected for this. T-seals are better than O-rings for sealing in axial direction. Teflon seals (Glyd and Double Delta II) are better still. Others in Fig 12.61 –Double Delta II ring, O-ring, and Glyd ring.
Dr. Joseph Greene Copyright 2000 all rights reserved 44 Air Ejection Blow Jets (Fig 12.62) –Prevents plastic from entering orifice. Intensity or force of the air blast coming from a blow jet (orifice) decreases considerably with the distance from orifice. –Orifice must be far enough from the molding surface so that the plastic cannot enter and plug orifice –Orifice should be uncovered only after the cavity moves away, and/or after the stripper moves forward a certain distance Blow-off on the underside of the product is used to separate the product from ejector pins or stripper if it sticks (Fig 12.63) –Advantage: Blow-off permits a smaller ejector stroke of the stripper. –Disadvantage:Distance from the orifice increases as stripper advances Example, Fig 12.64 –Jet is inside the taper of the stripper ring. –Air blast helps remove part around tight area.
Dr. Joseph Greene Copyright 2000 all rights reserved 45 Air Ejection Blow Down –Air blasts or air streams Blow at the product from the outside, after the ejector stripper (or pins) have come forward, to remove the product if they stick to the ejectors. –Two methods of blow down Individual open jets –Directed against the tip of the product are used to get best leverage for knocking the products off stripper. –Open designs (old method) used open jets by squeezing the ends of flexible copper tubes. »Advantage: simple. Better to use Exair nozzle for 80% less air »Disadvantage: Safety regulations limit pressure to 30 psi, * Noise of escaping air is often above legal limits. * Air through jets is difficult to control * High operating cost. 1/4” nozzle at 80 psi = $2,970/yr
Dr. Joseph Greene Copyright 2000 all rights reserved 46 Air Ejection Two methods of blow down Blow Down Curtains: Continuous sheet of air blows down over surface of mold. –As the mold opens, the products arrive into a moving air stream and are blown down. »Air curtains are used even if products eject easily to prevent them from hitting side of mold or assist in sliding down a sheet –Method 1: High volume, low-pressure air curtain. (Rarely used) »Low-power air blower is mounted above the mold and blows a steady stream of air down over mold. * Advantage- noiseless and negligible power consumption and no timing devices since air is continuous * Disadvantage- Air pressure is low and may not remove products. Blowers are bulky and awkward to move. –Method 2: Exair Knife: Similar to Exair jets »Applied to a continuous narrow gap (0.02 in) along plenum chamber of standard lengths from 6 t o30 in. »Air escaping the gap entrains surrounding air causing fast curtain