Presentation on theme: "Parameters of the Molding Process"— Presentation transcript:
1Parameters of the Molding Process Professor Joe GreeneCSU, CHICO
2TopicsIdentifying the ParametersTemperaturePressureTimeDistance
3Injection Molding Parameters Typically temperature and pressure are criticalWhich temperatures and pressures? Melt, mold, Barrel,...Other factorshumidity , shift changes, relief operators, fans blowing, etcExample,Injection molding operation was having difficulty with quality between 6 AM and 8 AM.The operators would adjust machine settings for temperature, pressure, hold and back pressure, etc…At 8 AM things would be back to normal and the settings had to be readjusted to original ones.SolutionDrop in water pressure from showers from the towns people caused the cooling devices to not work properly and the machines to heat up. Cycle times were increased and quality decreased.Water demand decreased and the machines cooled down, requiring readjustment on the machine parameters.
4Injection Molding Parameters Four basic categoriesTemperaturePressureTimeDistanceTemperaturePressureTimeDistance
5Injection Molding Parameters TemperatureMelt Temperature ControlFlow history of plastic material includes traveling from hopper into heating cylinder of injection unit. The material is augered in heating cylinder and through machine nozzle and then pushed into mold.The temperature of the melt must be controlled all along pathHeating CylinderHeating bands for 3 zonesRear zoneCenter zone (10F-20F hotter)Front Zone (10F-20F hotter)Fourth zone is the nozzlePlastic is additionally heated from shearing action
9Injection Molding Parameters Mold Temperature ControlPlastic material can flow in the mold where it cools.Critical material parameterRate at which plastic cools affects part quality. (Table III-2)Mold cooling is done with water.Mold Temperature is measured directly from molding surface with data acquisition or surface pyrometer.Object of the cooling process is to lower the temperature of the molded plastic to the point at which it solidifies.Demold occurs after solidifications minimizingwarpage, twisting, or other shrinkage related problems.Shrinkage can last for up to 30 days.95% of total shrinkage occurs while in mold. 99% within 3 hrs
12Injection Molding Parameters Hydraulic System Temperature ControlTarget temperature range between 80ºF and 140ºFIf oil is too cool, the oil viscosity is too high and cause sluggish action of hydraulic components.If oil is too hot, the viscosity will be too low and it will break down, causing components to stick or valves to malfunction.Temperature of oil is regulated with heat exchanger, acting like a radiator in and cools the oil by circulating it around tubes filled with circulating water.The tubes must be kept clean and require preiodic flushing with an acid cleaner.
13Ambient Temperature Control The temperature in the molding room.Cooling fans and loading dock doors affect mold temperature and thus the quality of the parts.Insulation sheets (Figure 3-3)Made from insulatorThermoset polyester, polyurethane,etc.Common thickness of 0.25in and inSheets are cut to fit over outside of moldAll six sides should be coveredSheets can be used between platens and moldInsulators provide lower energy usage (25% lower) and better temperature control
14Pressure Control Two areas require pressure and pressure control Injection unit and Clamp unitInjection unitInitial injection pressureApplied to the molten plastic and resulting from the main hydraulic pressure pushing against the back end of the injection screw (or plunger). Figure 3-4Injection PressureUsually 1,000 psi to 5,000 psiLower than hold and pack pressure which be between 10,000psi and 20,000 psi
16Pressure Control Injection unit (continued) Hold pressure (pack pressure)Used to finish the filling of the mold and pack the part.Rule of thumb: Hold pressure = 50% of injection pressure.Hold pressure applied against a pad or cushion of material.Applied at the end of the initial injection stroke, (Figure 3-5), and is intended to complete the final filling of the mold and hold pressure to solidify while staying dense or packed.
17Pressure Control Injection unit (continued) Back pressure Applied after the injection phases are complete.When holding pressure is complete the screw begins to turn in order to bring new material to the front of the barrel in preparation for next shot.As material fills the cavity, the screw is pushed back (Fig 3-6).Back pressure is small compared to injection pressure (between 50 psi and 500 psi (screw may not turn if exceeded).Procedure is to start with small amount of back pressure and steadily increase in increments of 10 psi.Ensures consistency in part weight, density, and material appearance.Squeezes out any trapped air or moisture.Minimizes voids in molded parts.
18Pressure Control Clamp Unit The purpose of developing clamp pressure is to keep the mold clamped shut against the forces developed when the injection pressure pushes plastic into the closed mold.Clamp pressure is applied mechanically or hydraulically
19Pressure Control Hydraulic Clamp Clamping force is developed by a hydraulic cylinder. A piston from the cylinder is attached to a moving platen on which the mold is mounted.AdvantagesClamp pressure can be regulated over a wide range.Example,Machine is rated at a 250-ton clamp force, the clamp force can be set anywhere from 50 tons to 250 tons.Allows the proper tonnage to be applied to minimize energy usage.DisadvantagesWhen tonnage requirements approach maximum rating, the clamp may open up due to high injection pressures.Mold requiring 225 tons is placed in a 250 ton machine. If the injection pressure is high, the mold will open up causing flash.
20Pressure Control Mechanical Clamp Utilizes a knuckle and scissors (toggle) mechanism to close mold.Toggle is attached to the moving platen on which the mold is mounted. When the clamp is open a small hydraulic cylinder actuates the arms by pushing at centerline.As the piston moves forward, it pulls the arms together, closing the moldThe knuckles must lock to achieve proper tonnage.AdvantageOnce locked in place, it is impossible to blow open mold.DisadvantagesConsiderable wear on knuckles requires replacementLittle accommodation for adjustment on tonnage. Tonnage rating is only force available tons machine delivers 250 tons.
21Pressure Control Pressure Required Total clamp force is determined by projected area.Total force = projected area times injection pressureRule of thumb 4 to 5 tons/in2 can be used for most plastics.Example,Part is 10 in by 10 in by 0.1 inProjected area is all of the sides of the cube.Neglect 0.1 in thickness.Surface area = 10in x 10 in = 100 in2Pressure = 15,000 psi for PCTonnage required to keep mold closed is100 in2 x 15,000 psi= 1,500,000 lbs = 750 tons (note : 2000 lbs = 1 ton)10 in10 in
24Time Gate-to gate Cycle Time Time required to produce a part Average Times for CycleGate Close secMold Close 1 secInjection 2 secPack and Hold 8 secPart Cool secScrew return 2 secMold open 1 secEjection sec
25Cycle Time Gate Close Time Mold Close Time Initial Injection Time Operator closes safety gate in manual demold operationAn increase in 2 seconds in cycle time (Basis = 30 seconds) annually can cost $20,000 additionally.Likewise a reduction of 2 sconds in a 30 second cycle time can net annual savings of $20,000.Mold Close TimeTwo phases; a fast close and a slow close phase (at 1cmapart) for high pressure.Initial Injection TimeInjection in a short as time as possible or as quickly as possible (< 2 sec)Longer fill times could result in higher pressure due to thicker cores region from cooling effects.
26Injection Molding Process Fill timeHow long it takes to fill part. Faster filling rate = shorter fill timeVolume of part divided by volumetric flow rateNote: Pressure is a function of the flow rate. Faster flow rate = higher pressures, except at very slow fill which causes larger core and smaller flow channel and then higher pressures.
27Injection Molding Distance ImportanceControl of distances is critical to producing high quality products at a low cost because longer distances results in longer cycle times.Mold-close Distance (Figure 3-9)Initial close speed is usually fast closeFinal close speed (last 1 cm) is slow close (minimize damage).Injection DistanceSet to ensure 95% of the intended material is injected.Ideal shot size is 50% of barrel capacity.Injection-hold DistanceAfter filling 95% of the required material, the machine switches to holding pressure.This finishes filling and holds pressure against material previously injected
28Injection Molding Distance Cushion (Pad)A Pad or cushion (0.125” to 0.25in) of material should be left in barrel for the hold pressure to be applied against.Cushion is created by creating a total shot size that is slightly larger than that required to fill the mold.Example,Amount of material required to fill mold is 2.9 oz (82.2g), then the total shot size would be 3 oz.Thickness of cushion is criticalMinimum is 1/8” because anything less would be difficult to control and there is a chance it could go to zero from the inconsistencies of the density.Maximum of 1/4” thick because any more than this and the cushion might solidify and block the nozzle.Screw-return DistancePrepare for the next shot. The set point is such that slightly more material in the barrel than is required to fill the mold.RPM should fall within 30 to 160 RPM
29Injection Molding Distance Mold-open DistanceMold open slowly (1/4”) to break vacuum created from filling.Slides or cams may need mold to open slowly for 2 or 3 in.Ejection DistanceAmount of ejection required is only that which will push part free.Rule of thumb: Add 1/8” to 1/4” to measured ejection travelFigure 3-13
30Injection Molding Costs Material CostsRaw and recycled materialScrap allowanceEstimated regrind buildupLabor ChargesStraight time and OvertimeMachine RateSetup chargesScrap allowance and downtimeCycle time and number of cavities per toolTooling ChargesInitial mold costsMaintenance costsProduction volume for tooling costs per part amortization