Transcription
Design and manufacturing of plasticinjection mould
ContentDesign and manufacturing of plastic injection mould . 11Injection molding . 31.1History. 31.2Equipment . 31.3Injection molding cycle . 61.4Molding trial. 71.5Molding defects. 72Injection molding machine. 102.1Types of injection molding machines . 102.2Injection unit . 102.3Clamping unit . 123Injection mould . 154Gate type . 194.1Sprue gate . 204.2Edge gate . 204.3Tab gate . 214.4Overlap gate . 214.5Fan gate . 224.6Film or flash gate. 224.7Diaphragm gate . 234.8Internal ring gate. . 234.9External ring gate . 234.10 Spoke gate or multipoint gate. 244.11 Pin gates . 244.12 Submarine (tunnel) gates. 254.13 Curved tunnel gate. . 264.14 Hot runner gates . 264.15 Valve gates . 275Why Molds are Expensive . 28
1 Injection moldingFrom Wikipedia, the free encyclopediaInjection molding (British: moulding) is a manufacturing technique for making parts fromboth thermoplastic and thermosetting plastic materials in production. Molten plastic isinjected at high pressure into a mold, which is the inverse of the product's shape. After aproduct is designed, usually by an industrial designer or an engineer, molds are made by amoldmaker (or toolmaker) from metal, usually either steel or aluminium, and precisionmachined to form the features of the desired part. Injection molding is widely used formanufacturing a variety of parts, from the smallest component to entire body panels of cars.Injection molding is the most common method of production, with some commonly madeitems including bottle caps and outdoor furniture. Injection molding typically is capable oftolerances equivalent to an IT Grade of about 9–14.The most commonly used thermoplastic materials are polystyrene (low cost, lacking thestrength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a terpolymer or mixture of compounds used for everything from Lego parts to electronicshousings), polyamide (chemically resistant, heat resistant, tough and flexible – used forcombs), polypropylene (tough and flexible – used for containers), polyethylene, and polyvinylchloride or PVC (more common in extrusions as used for pipes, window frames, or as theinsulation on wiring where it is rendered flexible by the inclusion of a high proportion ofplasticiser).Injection molding can also be used to manufacture parts from aluminium or brass (diecasting). The melting points of these metals are much higher than those of plastics; this makesfor substantially shorter mold lifetimes despite the use of specialized steels. Nonetheless, thecosts compare quite favorably to sand casting, particularly for smaller parts.1.1HistoryIn 1868 John Wesley Hyatt became the first to inject hot celluloid into a mold, producingbilliard balls. He and his brother Isaiah patented an injection molding machine that used aplunger in 1872, and the process remained more or less the same until 1946, when JamesHendry built the first screw injection molding machine, revolutionizing the plastics industry.Roughly 95% of all molding machines now use screws to efficiently heat, mix, and injectplastic into molds.1.2EquipmentInjection molding machines, also known as presses, hold the molds in which the componentsare shaped. Presses are rated by tonnage, which expresses the amount of clamping force thatthe machine can generate. This pressure keeps the mold closed during the injection process.Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used incomparatively few manufacturing operations.1.2.1Mold
Mold (Tool and/or Mold) is the common term used to describe the production tooling used toproduce plastic parts in molding.Traditionally, molds have been expensive to manufacture. They were usually only used inmass production where thousands of parts were being produced. Molds are typicallyconstructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copperalloy. The choice of material to build a mold is primarily one of economics. Steel moldsgenerally cost more to construct, but their longer lifespan will offset the higher initial costover a higher number of parts made before wearing out. Pre-hardened steel molds are lesswear resistant and are used for lower volume requirements or larger components. The steelhardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treatedafter machining. These are by far the superior in terms of wear resistance and lifespan.Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can costsubstantially less, and when designed and machined with modern computerized equipment,can be economical for molding tens or even hundreds of thousands of parts. Beryllium copperis used in areas of the mold which require fast heat removal or areas that see the most shearheat generated. The molds can be manufactured by either CNC machining or by usingElectrical Discharge Machining processes1.2.2DesignMolds separate into two sides at a parting line, the A side, and the B side, to permit the part tobe extracted. Plastic resin enters the mold through a sprue in the A plate, branches outbetween the two sides through channels called runners, and enters each part cavity throughone or more specialized gates. Inside each cavity, the resin flows around protrusions (calledcores) and conforms to the cavity geometry to form the desired part. The amount of resinrequired to fill the sprue, runner and cavities of a mold is a shot. When a core shuts off againstan opposing mold cavity or core, a hole results in the part. Air in the cavities when the moldcloses escapes through very slight gaps between the plates and pins, into shallow plenumscalled vents. To permit removal of the part, its features must not overhang one another in thedirection that the mold opens, unless parts of the mold are designed to move from betweensuch overhangs when the mold opens. Sides of the part that appear parallel with the directionof draw (the direction in which the core and cavity separate from each other) are typicallyangled slightly with (draft) to ease release of the part from the mold, and examination of mostplastic household objects will reveal this. Parts with bucket-like features tend to shrink ontothe cores that form them while cooling, and cling to those cores when the cavity is pulledaway. The mold is usually designed so that the molded part reliably remains on the ejector (B)side of the mold when it opens, and draws the runner and the sprue out of the (A) side alongwith the parts. The part then falls freely when ejected from the (B) side. Tunnel gates tunnelsharply below the parting surface of the B side at the tip of each runner so that the gate issheared off of the part when both are ejected. Ejector pins are the most popular method forremoving the part from the B side core(s), but air ejection, and stripper plates can also be useddepending on the application. Most ejector plates are found on the moving half of the tool, butthey can be placed on the fixed half if spring loaded. For thermoplastics, coolant, usuallywater with corrosion inhibitors, circulates through passageways bored through the main plateson both sides of the mold to enable temperature control and rapid part solidification.To ease maintenance and venting, cavities and cores are divided into pieces, called inserts,and subassemblies, also called inserts, blocks, or chase blocks. By substitutinginterchangeable inserts, one mold may make several variations of the same part.
More complex parts are formed using more complex molds. These may have sections calledslides, that move into a cavity perpendicular to the draw direction, to form overhanging partfeatures. Slides are then withdrawn to allow the part to be released when the mold opens.Slides are typically guided and retained between rails called gibs, and are moved when themold opens and closes by angled rods called horn pins and locked in place by locking blocks,both of which move cross the mold from the opposite side.Some molds allow previously molded parts to be reinserted to allow a new plastic layer toform around the first part. This is often referred to as overmolding. This system can allow forproduction of one-piece tires and wheels.2-shot or multi shot molds are designed to "overmold" within a single molding cycle and mustbe processed on specialized injection molding machines with two or more injection units.This can be achieved by having pairs of identical cores and pairs of different cavities withinthe mold. After injection of the first material, the component is rotated on the core from theone cavity to another. The second cavity differs from the first in that the detail for the secondmaterial is included. The second material is then injected into the additional cavity detailbefore the completed part is ejected from the mold. Common applications include "soft-grip"toothbrushes and freelander grab handles.The core and cavity, along with injection and cooling hoses form the mold tool. While largetools are very heavy weighing hundreds and sometimes thousands of pounds, they usuallyrequire the use of a forklift or overhead crane, they can be hoisted into molding machines forproduction and removed when molding is complete or the tool needs repairing.A mold can produce several copies of the same parts in a single "shot". The number of"impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool withone impression will often be called a single cavity (impression) tool. A mold with 2 or morecavities of the same parts will likely be referred to as multiple cavity tooling. Some extremelyhigh production volume molds (like those for bottle caps) can have over 128 cavities.In some cases multiple cavity tooling will mold a series of different parts in the same tool.Some toolmakers call these molds family molds as all the parts are not the same but often partof a family of parts (to be used in the same product for example).1.2.3MachiningMolds are built through two main methods: standard machining and EDM machining.Standard Machining, in its conventional form, has historically been the method of buildinginjection molds. With technological development, CNC machining became the predominantmeans of making more complex molds with more accurate mold details in less time thantraditional methods.The electrical discharge machining (EDM) or spark erosion process has become widely usedin mold making. As well as allowing the formation of shapes which are difficult to machine,the process allows pre-hardened molds to be shaped so that no heat treatment is required.Changes to a hardened mold by conventional drilling and milling normally require annealingto soften the steel, followed by heat treatment to harden it again. EDM is a simple process inwhich a shaped electrode, usually made of copper or graphite, is very slowly lowered onto themold surface (over a period of many hours), which is immersed in paraffin oil. A voltage
applied between tool and mold causes erosion of the mold surface in the inverse shape of theelectrode.1.2.4CostThe cost of manufacturing molds depends on a very large set of factors ranging from numberof cavities, size of the parts (and therefore the mold), complexity of the pieces, expected toollongevity, surface finishes and many others.1.3Injection molding cycleFor the injection molding cycle to begin, four criteria must be met: mold open, ejector pinsretracted, shot built, and carriage forward. When these criteria are met, the cycle begins withthe mold closing. This is typically done as fast as possible with a slow down near the end oftravel. Mold safety is low speed and low pressure mold closing. It usually begins just beforethe leader pins of the mold and must be set properly to prevent accidental mold damage.When the mold halves touch clamp tonnage is built. Next, molten plastic material is injectedinto the mold. The material travels into the mold via the sprue bushing, then the runner systemdelivers the material to the gate. The gate directs the material into the mold cavity to form thedesired part. This injection usually occurs under velocity control. When the part is nearly full,injection control is switched from velocity control to pressure control. This is referred to asthe pack/hold phase of the cycle. Pressure must be maintained on the material until the gatesolidifies to prevent material from flowing back out of the cavity. Cooling time is dependentprimarily on the wall thickness of the part. During the cooling portion of the cycle after thegate has solidified, plastication takes place. Plastication is the process of melting material andpreparing the next shot. The material begins in the hopper and enters the barrel through thefeed throat. The feed throat must be cooled to prevent plastic pellets from fusing togetherfrom the barrel heat. The barrel contains a screw that primarily uses shear to melt the pelletsand consists of three sections. The first section is the feed section which conveys the pelletsforward and allows barrel heat to soften the pellets. The flight depth is uniform and deepest inthis section. The next section is the transition section and is responsible for melting thematerial through shear. The flight depth continuously decreases in this section, compressingthe material. The final section is the metering section which features a shallow flight depth,improves the melt quality and color dispersion. At the front of the screw is the non-returnvalve which allows the screw to act as both an extruder and a plunger. When the screw ismoving backwards to build a shot, the non-return assembly allows material to flow in front ofthe screw creating a melt pool or shot. During injection, the non-return assembly prevents theshot from flowing back into the screw sections. Once the shot has been built and the coolingtime has timed out, the mold opens. Mold opening must occur slow-fast-slow. The mold mustbe opened slowly to release the vacuum that is caused by the injection molding process andprevent the part from staying on the stationary mold half. This is undesirable because theejection system is on the moving mold half. Then the mold is opened as far as needed, ifrobots are not being used, the mold only has to open far enough for the part to be removed. Aslowdown near the end of travel must be utilized to compensate for the momentum of themold. Without slowing down the machine cannot maintain accurate positions and may slam toa stop damaging the machine. Once the mold is open, the ejector pins are moved forward,ejecting the part. When the ejector pins retract, all criteria for a molding cycle have been metand the next cycle can begin.
The basic injection cycle is as follows: Mold close – injection carriage forward – injectplastic – metering – carriage retract – mold open – eject part(s) Some machines are run byelectric motors instead of hydraulics or a combination of both. The water-cooling channelsthat assist in cooling the mold and the heated plastic solidifies into the part. Improper coolingcan result in distorted molding. The cycle is completed when the mold opens and the part isejected with the assistance of ejector pins within the mold.The resin, or raw material for injection molding, is most commonly supplied in pellet orgranule form. Resin pellets are poured into the feed hopper, a large open bottomed container,which is attached to the back end of a cylindrical, horizontal barrel. A screw within this barrelis rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flightsdecreases toward the end of the screw nearest the mold, compressing the heated plastic. Asthe screw rotates, the pellets are moved forward in the screw and they undergo extremepressure and friction which generates most of the heat needed to melt the pellets. Electricheater bands attached to the outside of the barrel assist in the heating and temperature controlduring the melting process.The channels through which the plastic flows toward the chamber will also solidify, formingan attached frame. This frame is composed of the sprue, which is the main channel from thereservoir of molten resin, parallel with the direction of draw, and runners, which areperpendicular to the direction of draw, and are used to convey molten resin to the gate(s), orpoint(s) of injection. The sprue and runner system can be cut or twisted off and recycled,sometimes being granulated next to the mold machine. Some molds are designed so that thepart is automatically stripped through action of the mold.1.4Molding trialWhen filling a new or unfamiliar mold for the first time, where shot size for that mold isunknown, a technician/tool setter usually starts with a small shot weight and fills graduallyuntil the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure willbe applied and holding time increased until gate freeze off has occurred, then holding pressureis increased until the parts are free of sinks and part weight has been achieved. Once the partsare good enough and have passed any specific criteria, a setting sheet is produced for peopleto follow in the future.Process optimization is done using the following methods. Injection speeds are usuallydetermined by performing viscosity curves. Process windows are performed varying the melttemperatures and holding pressures. Pressure drop studies are done to check if the machinehas enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are doneto optimize the holding time. A cooling time study is done to optimize the cooling time.1.5Molding defectsInjection molding is a complex technology with possible production problems. They caneither be caused by defects in the molds or more often by part processing Causes
BlisteringRaised or layeredzone on surface ofthe partBurn marksAir Burn/Gas BurnBlack or brownburnt areas on theTool lacks venting, injection speed is toopart located athighfurthest points fromgateColor streaks(US)Masterbatch isn't mixing properly, or theColourLocalized change ofmaterial has run out and it's starting tostreaks (UK) color/colourcome through as natural onlyDelaminationContamination of the material e.g. PPmixed with ABS, very dangerous if theThin mica likepart is being used for a safety criticallayers formed in partapplication as the material has very littlewallstrength when delaminated as thematerials cannot bondFlashBurrsExcess material inthin layer exceedingnormal partgeometryTool damage, too much injectionspeed/material injected, clamping forcetoo low. Can also be caused by dirt andcontaminants around tooling surfaces.EmbeddedparticulatesForeign particle(burnt material orother) embedded inthe partParticles on the tool surface,contaminated material or foreign debrisin the barrel, or too much shear heatburning the material prior to injectionFlow linesDirectionally "offtone" wavy lines orpatternsInjection speeds too slow (the plastic hascooled down too much during injection,injection speeds must be set as fast asyou can get away with at all times)Deformed part byturbulent flow ofmaterialPoor tool design, gate position or runner.Injection speed set too high.BlisterEmbeddedcontaminatesFlow marksJettingTool or material is too hot, often causedby a lack of cooling around the tool or afaulty heater
Polymerdegradationpolymer breakdownExcess water in the granules, excessivefrom hydrolysis,temperatures in barreloxidation etcSink marksLocalizeddepression (Inthicker zones)Holding time/pressure too low, coolingtime too low, with sprueless hot runnersthis can also be caused by the gatetemperature being set too highPartial partLack of material, injection speed orpressure too lowShort shotNon-fill /Short moldSplay marksCircular patternSplash mark /around gate causedSilver streaksby hot gasStringinessStringingWarpingString like remainNozzle temperature too high. Gate hasn'tfrom previous shotfrozen offtransfer in new shotLack of holding pressure (holdingpressure is used to pack out the partEmpty space within during the holding time). Also mold maybe out of registration (when the twopart (Air pocket)halves don't center properly and partwalls are not the same thickness).VoidsWeld lineMoisture in the material, usually whenhygroscopic resins are dried improperlyKnit line /Meld lineTwistingDiscolored linewhere two flowfronts meetMold/material temperatures set too low(the material is cold when they meet, sothey don't bond)Distorted partCooling is too short, material is too hot,lack of cooling around the tool, incorrectwater temperatures (the parts bowinwards towards the hot side of the tool)
2 Injection molding machineFrom Plastics Wiki, free encyclopediaInjection molding machines consist of two basic parts, an injection unit and a clamping unit.Injection molding machines differ in both injection unit and clamping unit. The name of theinjection molding machine is generally based on the type of injection unit used.2.1Types of injection molding machinesMachines are classified primarily by the type of driving systems they use: hydraulic, electric,or hybrid.2.1.1HydraulicHydraulic presses have historically been the only option available to molders until NisseiPlastic Industrial Co., LTD introduced the first all-electric injection molding machine in 1983.The electric press, also known as Electric Machine Technology (EMT), reduces operationcosts by cutting energy consumption and also addresses some of the environmental concernssurrounding the hydraulic press.2.1.2ElectricElectric presses have been shown to be quieter, faster, and have a higher accuracy, howeverthe machines are more expensive.2.1.3HybridHybrid injection molding machines take advantage of the best features of both hydraulic andelectric systems. Hydraulic machines are the predominant type in most of the world, with theexception of Japan.2.2Injection unitThe injection unit melts the polymer resin and injects the polymer melt into the mold. Theunit may be: ram fed or screw fed.
The ram fed injection molding machine uses a hydraulically operated plunger to push theplastic through a heated region. The high viscosity melt is then spread into a thin layer by a"torpedo" to allow for better contact with the heated surfaces. The melt converges at a nozzleand is injected into the mold.Reciprocating screw A combination melting, softening, and injection unit in an injectionmolding machine. Another term for the injection screw. Reciprocating screws are capable ofturning as they move back and forth.The reciprocating screw is used to compress, melt, and convey the material. The reciprocatingscrew consists of three zones (illustrated below): feeding zonecompressing zonemetering zoneWhile the outside diameter of the screw remains constant, the depth of the flights on thereciprocating screw decreases from the feed zone to the beginning of the metering zone.These flights compress the material against the inside diameter of the barrel, which createsviscous (shear) heat. This shear heat is mainly responsible for melting the material. The heaterbands outside the barrel help maintain the material in the molten state. Typically, a moldingmachine can have three or more heater bands or zones with different temperature settings.Injection molding reciprocating screw An extruder-type screw rotates within a cylinder,which is typically driven by a hydraulic drive mechanism. Plastic material is moved throughthe heated cylinder via the screw flights and the material becomes fluid. The injection nozzleis blocked by the previous shot, and this action causes the screw to pump itself backwardthrough the cylinder. (During this step, material is plasticated and accumulated for the nextshot.) When the mold clamp has locked, the injection phase takes place. At this time, thescrew advances, acting as a ram. Simultaneously, the non-return valve closes off the escapepassages in the screw and the screw serves as a solid plunger, moving the plastic ahead intothe mold. When the injection stroke and holding cycle is completed, the screw is energized toreturn and the non-return valve opens, allowing plastic to flow forward from the cylinderagain, thus repeating the cycle.2.2.1Feed hopperThe container holding a supply molding material to be fed to the screw. The hopper locatedover the barrel and the feed throat connects them.
2.2.2Injection ramThe ram or screw that applies pressure on the molten plastic material to force it into the moldcavities.2.2.3Injection screwThe reciprocating-screw machine is the most common. This design uses the same barrel formelting and injection of plastic.The alternative unit involves the use of separate barrels for plasticizing and injecting thepolymer. This type is called a screw-preplasticizer machine or two-stage machine. Plasticpellets are fed from a hopper into the first stage, which uses a screw to drive the polymerforward and melt it. This barrel feeds a second barrel, which uses a plunger to inject the meltinto the mold. Older machines used one plunger-driven barrel to melt and inject the plastic.These machines are referred to as plunger-type injection molding machines.2.2.4BarrelBarrel is a major part that melts resins transmitted from hopper through screws and structuredin a way that can heat up resins to the proper temperature. A band heater, which can controltemper atures in five sections, is attached outside the barrel. Melted resins are supplied to themold passing through barrel head, shot-off nozzle, and one-touch nozzle.2.2.5Injection cylinderHydraulic motor located inside bearing box, which is connected to injection cylinder load,rotates screw, and the melted resins are measures at the nose of screw. There are many typesof injection cylinders that supply necessary power to inject resins according to thecharacteristics of resins and product types at appropriate speed and pressure. This modelemploys the double cylinder type. Injection cylinder is composed of cylinder body, piston,and piston load.2.3Clamping unitThe clamping unit holds the mold together, opens and closes it automatically, and ejects thefinished part. The mechanism may be of several designs, either mechanical, hydraulic orhydromechanical.Toggle clamps - a type clamping unit include various designs. An actuator moves thecrosshead forward, extending the toggle links to push the moving platen toward a closedposition. At the beginning of the movement, mechanical advantage is low and speed is high;but near the end of the stroke, the reverse is true. Thus, toggle clamps provide both high speedand high force at different points in the cycle when they are desirable. They are actuatedeither by hydraulic cylinders or ball screws driven by electric motors. Toggle-clamp unitsseem most suited to relatively low-tonnage machines.Two clamping designs: (a) one possible toggle clamp design (1) open and (2) closed; and (b)hyd
Injection molding machines, also known as presses, hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold
