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Parameters of the Molding Process Chapter 3

Parameters of the Molding Process Chapter 3. Professor Joe Greene CSU, CHICO. MFGT 144. September 14, 1999. Chapter 3 Topics. Identifying the Parameters Temperature Pressure Time Distance. Injection Molding Parameters. Typically temperature and pressure are critical

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Parameters of the Molding Process Chapter 3

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  1. Parameters of the Molding ProcessChapter 3 Professor Joe Greene CSU, CHICO MFGT 144 September 14, 1999

  2. Chapter 3 Topics • Identifying the Parameters • Temperature • Pressure • Time • Distance

  3. Injection Molding Parameters • Typically temperature and pressure are critical • Which temperatures and pressures? Melt, mold, Barrel,... • Other factors • humidity , shift changes, relief operators, fans blowing, etc • Example, • 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. • Solution • Drop 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.

  4. Injection Molding Parameters • Four basic categories • Temperature • Pressure • Time • Distance Temperature Pressure Time Distance

  5. Injection Molding Parameters • Temperature • Melt Temperature Control • Flow 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 path • Heating Cylinder • Heating bands for 3 zones • Rear zone • Center zone (10F-20F hotter) • Front Zone (10F-20F hotter) • Fourth zone is the nozzle • Plastic is additionally heated from shearing action

  6. Injection Molding Parameters • Melt Temperature(Table III-1. Suggested Melt Temp at nozzle) • Acetal (coploymer) 400 F • Acrylic 425 F • ABS 400 F • Liquid Crystal Polymer 500 F • Nylon 6 500 F • Polyamide-imide 650 F • Polyarylate 700 F • Polycarbonate 550 F • Polyetheretherketone 720 F • Polyethylene LDPE 325 F • Polyethylene HDPE 350 F • Polypropylene 350 F • Polystyrene 350 F • Thermoplastic polyester (PBT) 425 F • Urethane elastomer 425 F

  7. Temperature History

  8. Injection Molding Parameters • Melt Temperature

  9. Injection Molding Parameters • Mold Temperature Control • Plastic material can flow in the mold where it cools. • Critical material parameter • Rate 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 minimizing • warpage, 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

  10. Injection Molding Parameters • Mold Temperature(Table III-2. Suggested mold Temperatures) • Acetal (coploymer) 200 F • Acrylic 180 F • ABS 180 F • Liquid Crystal Polymer 250 F • Nylon 6 200 F • Polyamide-imide 400 F • Polyarylate 275 F • Polycarbonate 220 F • Polyetheretherketone (PEEK) 380 F • Polyethylene LDPE 80 F • Polyethylene HDPE 110 F • Polypropylene 120 F • Polystyrene 160 F • Thermoplastic polyester (PBT) 180 F • Urethane elastomer 120 F

  11. Injection Molding Parameters • Mold Temperature

  12. Injection Molding Parameters • Hydraulic System Temperature Control • Target temperature range between 80ºF and 140ºF • If 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.

  13. Ambient Temperature Control • Ambient temperature • 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 insulator • Thermoset polyester, polyurethane,etc. • Common thickness of 0.25in and 0.375 in • Sheets are cut to fit over outside of mold • All six sides should be covered • Sheets can be used between platens and mold • Insulators provide lower energy usage (25% lower) and better temperature control

  14. Pressure Control • Two areas require pressure and pressure control • Injection unit and Clamp unit • Injection unit • Initial injection pressure • Applied to the molten plastic and resulting from the main hydraulic pressure pushing against the back end of the injection screw (or plunger). Figure 3-4 • Injection Pressure • Usually 1,000 psi to 5,000 psi • Lower than hold and pack pressure which be between 10,000psi and 20,000 psi

  15. Pressure Control

  16. Pressure 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.

  17. Pressure 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. • Back pressure • Ensures consistency in part weight, density, and material appearance. • Squeezes out any trapped air or moisture. • Minimizes voids in molded parts.

  18. Pressure 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

  19. Pressure 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. (Figures 3-7 a and b) • Advantages • Clamp 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. • Disadvantages • When tonnage requirements approach maximum rating, the clamp may open up due to high injection pressures. • Example, • Mold requiring 225 tons is placed in a 250 ton machine. If the injection pressure is high, the mold will open up causing flash.

  20. Pressure 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 (figure 3-8 a), a small hydraulic cylinder actuates the arms by pushing at centerline. • As the piston moves forward, it pulls the arms together, closing the mold (figure 3-8 b). • The knuckles must lock to achieve proper tonnage. • Advantage • Once locked in place, it is impossible to blow open mold. • Disadvantages • Considerable wear on knuckles requires replacement • Little accommodation for adjustment on tonnage. Tonnage rating is only force available. 250 tons machine delivers 250 tons.

  21. Pressure Control • Pressure Required • Total clamp force is determined by projected area. • Total force = projected area times injection pressure • Rule of thumb 4 to 5 tons/in2 can be used for most plastics. • Example, • Part is 10 in by 10 in by 0.1 in • Projected area is all of the sides of the cube. • Neglect 0.1 in thickness. • Surface area = 10in x 10 in = 100 in2 • Pressure = 15,000 psi for PC • Tonnage required to keep mold closed is • 100 in2 x 15,000 psi= 1,500,000 lbs = 750 tons (note : 2000 lbs = 1 ton) 10 in 10 in

  22. Pressure Control • Pressure Required Ref: http://cadpc3.mem.drexel.edu/mfgcourses/injectionmold/sld027.htm

  23. Pressure Control • Pressure Required Ref: http://cadpc3.mem.drexel.edu/mfgcourses/injectionmold/sld027.htm

  24. Time • Gate-to gate Cycle Time • Time required to produce a part • Average Times for Cycle • Gate Close 1 sec • Mold Close 1 sec • Injection 2 sec • Pack and Hold 8 sec • Part Cool 11 sec • Screw return 2 sec • Mold open 1 sec • Ejection 1 sec

  25. Cycle Time • Gate Close Time • Operator closes safety gate in manual demold operation • An 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 Time • Two phases; a fast close and a slow close phase (at 1cmapart) for high pressure. • Initial Injection Time • Injection 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.

  26. Injection Molding Process • Fill time • How long it takes to fill part. Faster filling rate = shorter fill time • Volume of part divided by volumetric flow rate • Note: 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.

  27. Injection Molding Distance • Importance • Control 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 close • Final close speed (last 1 cm) is slow close (minimize damage). • Injection Distance • Set to ensure 95% of the intended material is injected. • Ideal shot size is 50% of barrel capacity. • Injection-hold Distance • After filling 95% of the required material, the machine switches to holding pressure. • This finishes filling and holds pressure against material previously injected

  28. Injection 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 critical • Minimum 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 Distance • Prepare 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

  29. Injection Molding Distance • Mold-open Distance • Mold 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 Distance • Amount of ejection required is only that which will push part free. • Rule of thumb: Add 1/8” to 1/4” to measured ejection travel • Figure 3-13

  30. Injection Molding Costs • Material Costs • Raw and recycled material • Scrap allowance • Estimated regrind buildup • Labor Charges • Straight time and Overtime • Machine Rate • Setup charges • Scrap allowance and downtime • Cycle time and number of cavities per tool • Tooling Charges • Initial mold costs • Maintenance costs • Production volume for tooling costs per part amortization

  31. Injection Molding Costs • Material Costs • Figure 3.14 Determining volume of parts and runner system

  32. Injection Molding Costs • Spreadsheet

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