Workshop Series 2016 Hands-on Approach to Cycle Time Reduction and Productivity Improvement Vishu Shah, Consultek Consulting Group April 21, 2016 ENGEL North America California Technical Center Corona
300 Ton Machine : Machine Hour rate $/Hr 35.00 Current Cycle Time: New Cycle time: Cycle time savings: Cycle time savings: 30 seconds 29 seconds 2 seconds per minute 2 Minutes per hour Cycle time savings: (5000 hrs per year) 10,000 Minutes or 166 hours Total $ amount savings: 166 x 35 = $ 5810 Total $ amount saved: (10 Machine shop) 58,100 Benjamin Franklin once said Beware of little expenses; a small leak can sink a great ship.
Key Considerations Productivity improvement is not all about cycle time reduction Minimizing rejects/rework, automation, reducing down time. What prevents one from speeding up cycle time? a. Dimensional issues b. Aesthetic/Cosmetic issues
Cycle Time Optimization Mold Filling (5) Packing (3) Holding (3) Cooling (25) Ejection (2) Where is the best return on investment of your time?
Cycle Time Optimization Temperatures Melt & mold Fill, Pack and Hold Cooling time Mold Open-Close Ejection Process window
Temperature Melt temperature affects cycle time BTU s (heat) IN = BTU s (heat) OUT Heat always travels from HOT to COLD at a given rate based on each material s rate of transmission or thermal diffusivity Some materials give off heat faster than others Parts Must be cooled below Heat Distortion Temperature (HDT) of the Plastics in order to eject it out of the mold without warpage The Hotter the melt..longer the cooling time Avoid too high or too low Melt temperature.. Refer to material supplier s Data sheet for recommended settings And use it as starting point
Mold Temperature Temperature setting on Mold heater vs. Actual Steel Temperature
Mold filling ( Injection) Inject the material into the mold in fastest possible time. Velocity profiling helps with visual problems associated with too slow or too fast injection speeds. Why can t we inject the material faster in the mold?
Mold Packing and holding The ideal holding time is the gate freeze time and can only be determined by gate freeze study.
A very simple study is performed to determine this time. Samples molded with different holding times are weighed and the time after which the weight remains constant is set as the holding time. As the holding time is increased more and more plastic enters the cavity increasing the weight. But as soon as the gate is frozen, the plastic cannot get into the cavity and the part weight remains constant. This is called the gate freeze time or the gate seal time. See the picture on the next page. You will notice that the part weight remains constant after 9 seconds. The holding time is set to one second higher than the gate seal time to ensure that the gate is frozen during every shot. In the case of the graph below the time was set to 10 seconds. This will ensure consistency and any small variations will be compensated for. Holding Time Procedure for determining the Hold Time. 1. Set the injection speed to the value obtained from the viscosity curve experiment. 2. Set the process at the center of the process window from the process window study. 3. Set the cooling time to a value to ensure that the part is cooled before ejection. 4. Drop the holding time to zero and start molding. Mold approximately 5 to 8 shots. 5. Increase the holding time to one second and collect a shot. 6. Increase the holding time to two seconds and collect a shot. Similarly collect shots at increments of one second. 7. Weigh the shots and plot a graph of part weight versus time similar to the graph in the picture. 8. Determine the gate seal time. (Source: www.fimmtech.com)
Concept of Heat Transfer & Cooling
Cooling time Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts Cooling time is a function of : material properties melt temperature mold wall temperature part wall thickness
Cooling Considerations Molding Cycle.80% is cooling time Flow type.laminar or Turbulent Flow rate GPM Reynolds number of > than 5000 for turbulent flow Thermal conductivity of mold steel Plastic material s Heat Content Waterlines Part Design Rule of Thumb: 7/16 Diameter Waterline requires 1.5 GPM to achieve turbulent flow.
What affects Cooling time? Dimensional tolerance is the controlling factor Visual and functional issues such as warpage Screw recovery time Too thick sprue and runner No water in some parts of the mold such as core pins Type of metal (thermal conductivity)
Cycle time savings from proper Water Management Points to Consider at the Machine Is the supply pressure adequate (50 psi min) Is the return pressure at least 40 psi less than the supply Adequate pipe sizing for the number of machines in service Is the GPM flow adequate to cool the molds properly Is there an adequate number of valves on each manifold Are the supply/return manifolds in close proximity to the mold Are the valves properly sized Source: Pulse cooling-west
Waterlines size and Placement Basics Water pressure differential 25 to 30 psi Water Temperature differential < than 4 F Typical Flow rate 1.5 GPM each line Source: DSM
Special Techniques Heat Pipe (Thermal Pin High thermal conductivity alloys What is a heat pipe? A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Heat pipes have an effective thermal conductivity many thousands of times that of copper. Heat pipes can be built in almost any size and shape.
Heat Pipes
High Thermal conductivity alloys MoldMax Ampcoloy
Water Recirculating System Open loop cooling system Typical closed cooling system.
Mold Open & Mold Close Open and close the mold as fast as possible without damaging the mold or the machine Close the mold as soon as the part is cleared Ejection Limit ejection to single stroke
Key Questions to ask. is runner controlling the cycle? is it cavity in a multi-cavity mold or process related? Part design issues? Too thick/thin walls? Deep ribs? Possible to investigate fast cycling resins? Loosen up tolerances?
Troubleshooting Techniques
Problem Solving (Trouble Shooting) Conventional techniques and troubleshooting charts Sink Marks Description Sink Marks occur during the cooling process if certain areas of the part are not cooled sufficiently causing them to contract. Possible Solutions Decrease amount of regrind use. Decrease back pressure. Confirm that the non-return valve being used is not leaking excessively. Decrease melt temperature. Do this if the sink marks are near the gate or thick walled areas. Decrease mold temperature. Do this if the sink marks are near the gate or thick walled areas. Decrease injection rate. Do this if the sink marks are near the gate or thick walled areas. Dry material. Increase injection pressure. Do this if the sink marks are away from the gate or in thin walled areas. Increase injection speed. Do this if the sink marks are away from the gate or in thin walled areas. Increase mold temperature. Do this if the sink marks are away from the gate or in thin walled areas. Increase injection-hold. Increase shot size and confirm that the a cushion is being maintained. Increase size of sprue and/or runners and/or gates. Relocate gates on or as near as possible to thick sections. Increase cooling time. If possible change the mold design to maintain an even wall thickness throughout the part.
Problem Solving If a part has been produced satisfactorily and it goes bad, something has changed. The principle is not to change conditions immediately. The cause should be identified and rectified. Never adjust one condition to compensate for a failure or change in another condition. For example, if melt temperature has increased, don t reduce speeds or pressures or mold temperature to compensate. Identify problem, that is to say reduce melt temperature to what it was before, by checking and replacing thermocouples, etc. Source: GE Plastics
Problem Solving Ask questions DEFINE THE PROBLEMS COLLECT DATA - ACT what has changed? what is at fault? when did it start? how often does it occur? where is fault? is the fault randomly situated or always in same place? Identify causes of defects test, observe, conclude and study history of faults injection speed: test screw speed: test back pressure: test melt temperature: test machine check mold temperature control material cushion
Factors affecting Quality of Molded parts Where is the problem stemming from? Filling Phase Surface defects, Jetting, Blush, Gate Fracture, burning, flow marks, Molecular orientation, Knit lines Packing Phase Flash, Some surface details Holding Phase Voids, sinks, warpage, part density Cooling Phase Part dimensions, Warpage Source: Bayer
Sink Marks A sink mark is a local surface depression that typically occurs in moldings with thicker sections, or at locations above ribs, bosses, and internal fillets. Gas Assist Pack & Hold Phase Wall Thickness Hot spots in the mold Material change Additives
Jetting Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. The buckled, snake-like jetting stream causes contact points to form between the folds of melt in the jet, creating small-scale "welds" FIGURE 2. Using an overlap gate to avoid jetting
Burning (burn Marks) Burn marks are small, dark or black spots that appear near the end of the flow path of a molded part or in the blind area where the air trap forms. 1. Venting 2. Fast injection speed 3. Too high melt temperature
Weld (Knit) Lines A weld line (also called a weld mark or a knit line) is formed when separate melt fronts traveling in opposite directions meet. A meld line occurs if two emerging melt fronts flow parallel to each other and create a bond between them. Weld and meld lines can be caused by holes or inserts in the part, multiple gates, or variable wall thickness where hesitation or race tracking occurs. If weld or meld lines can't be avoided, position them at low-stress and low-visibility areas by adjusting the gate position and dimension. Improve the strength of weld and meld lines by increasing the local temperature and pressure at their locations.
Weld Lines VENTING SLOW INJECTION SPEED EXCESSIVE MOLD RELEASE LONG FLOW LENGTH TOO LOW A MELT TEMPERATURE
Gate blush Gate Blush is a melt fracture at the gate that is usually caused by sharp corners, excessive injection speed, gate size or gate design. It appears as a dull spot on the part. Source: Bayer
Bubbles Bubbles form due to moisture in the resin or due to trapped air in the mold
Splay (silver streaks) Silver streaks are the splash appearance of moisture, air, or charred plastic particles on the surface of a molded part, which are fanned out in a direction emanating from the gate location.
Voids A void occurring in the center of the thick part is usually caused by excessive shrinkage from thick walls or under packing.
Warpage Dimensional distortion in a plastics part after molding is referred to as warpage. Warpage is caused by uneven post molding shrinkage and Overpacking
Rules to Forget If you twist enough knobs or punch enough numbers the problems will go away. The machine has a mind of its own. All problems are because of the bad part design, bad tooling, or bad set-up. It takes a genius to operate a molding machine. It ain t my $#@% Fault! Rules to Remember The fastest cycle that produces the most parts use a. Minimum allowable melt Temperature b. Minimum pressure for low stress parts c. Minimum time fast fill, fast cool, etc. All problems have a logical cause Data drives decisions Think-Observe-Act Source: Anonymous
Contact Information Vishu Shah Consultek 13933-J Ramona Avenue Chino, CA 91710 PH: 909-465-6699 E-Mail: vishu@consultekusa.com Web: www.consultekusa.com www.theplasticfailureanalysis.com
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