Molding Process | Erick L. Sokn

This course focuses on establishing a repeatable injection molding process by controlling the material and cavity conditions that determine part quality. It compares velocity-controlled and pressure-limited molding, then develops the relationships among material moisture, melt temperature, fill rate, viscosity, cooling, packing, cycle time, machine condition, and mold performance.

Core Topics

Velocity-controlled and pressure-limited molding
Repeatable material and cavity conditions
Material drying, melt temperature, and residence time
Fill rate, shear, viscosity, and cavity balance
Cooling, packing, hold time, and cycle control
Scientific molding setup, verification, and troubleshooting

Detailed Course Outline

Objective of the Molding Process
- Producing repeatable cavity conditions from shot to shot
- Duplicating the material conditions that produce an acceptable component
- Recognizing that machine settings are inputs used to create the required material and cavity conditions
- Understanding that the machine settings required to reproduce those conditions may change as equipment and material conditions change
- Controlling variation before adjusting the component design or mold tool
- Maintaining dimensional and functional consistency throughout production
Velocity-Controlled versus Pressure-Limited Molding
- Differences between velocity-controlled and pressure-limited fill
- Controlling the rate of cavity filling rather than allowing pressure availability to determine fill rate
- Relationship between injection speed and material viscosity
- Effect of viscosity variation on a pressure-limited process
- Using available pressure as a limit rather than the primary process control
- Recognizing pressure-limited setups as a potential source of process variation
- Selecting the appropriate approach based on equipment, component, material, and application requirements
Repeatable Cavity Conditions
- Correct press and barrel sizing for the component
- Uniform mold temperature
- Adequate and uniform venting
- Balanced cavity filling
- Repeatable material dryness
- Velocity-controlled filling
- Consistent overall cycle time
- Reducing unnecessary operator-dependent operations
- Correcting tooling or design only after major process variables have been stabilized
Establishing a Stable Fill Pattern
- Ensuring cavities fill at the same rate
- Venting the tool before balancing runners
- Adjusting runner sizes to obtain uniform fill
- Recognizing that fill balance changes with viscosity
- Maintaining repeatable barrel residence time
- Maintaining repeatable melt temperature
- Maintaining repeatable fill time
- Maintaining repeatable moisture content
- Using short-shot studies to evaluate cavity filling and balance
Molding Machine and Barrel Requirements
- Matching barrel capacity to shot size
- Using an appropriate percentage of rated barrel capacity
- Effects of excessive barrel residence time
- Multiplication of cycle-time variation by the number of shots resident in the barrel
- Material-specific barrel and nozzle configurations
- Required clamp force
- Dryer-hopper capacity
- Material-specific screw length-to-diameter ratio
- Material-specific compression ratio
Injection Screw and Check-Ring Function
- Plasticizing through screw rotation and shear
- Contribution of screw shear to melt energy
- Material movement through the screw flights
- Screw tip
- Check ring
- Seat
- Material recovery in front of the screw
- Check-ring sealing during injection
- Effects of screw-tip and check-ring wear
- Backflow during injection
- Dimensional instability caused by equipment wear
- Short-shot conditions caused by leakage past worn components
Insert-Molding Machine Configurations
- Vertical-clamp / horizontal-injection machines
- Loading inserts outside the clamp area
- Rotary-table operation
- Unloading, inspection, and reloading while the alternate mold half is cycling
- Operator safety systems
- Light curtains
- Palm-button controls
- Separating operator activity from the active clamp area
Interactive Nature of the Molding Process
- Injection molding as a system of highly interactive variables
- Recognizing that changing one process variable affects others
- Variation within individual material lots
- Lot-to-lot material variation
- Minor dimensional effects versus scrap-producing changes
- Avoiding isolated adjustment of process settings without considering system interactions
Critical Components of a Repeatable Process
- Repeatable cavity conditions
- Material moisture content
- Pre-injection melt temperature
- Barrel residence time
- Volumetric fill rate
- Shear heating and nonlinear viscosity effects
- Mold temperature
- Cooling and heating rate
- Pack pressure
- Pack time
- Overall cycle time
Material Moisture Control
- Controlling moisture content before material enters the barrel
- Material-specific moisture requirements
- Effects of excessive moisture on material viscosity
- Effects of under-dried material
- Effects of over-dried material
- Nylon moisture and viscosity
- Polyester hydrolytic degradation
- Relationship between moisture variation and molding scrap
- Verifying moisture with a moisture analyzer
- Adjusting the drying process based on measured moisture rather than assumptions
Drying-System Variables
- Drying-air temperature
- Dew point
- Airflow rate
- Material residence time in the dryer
- Initial material moisture content
- Airflow restrictions
- Dryer filters and line obstructions
- Supply and return-air leaks
- Hopper sealing
- Fan capability and static pressure
- Temperature loss between dryer and hopper
Reducing Moisture Variation
- Controlling incoming material exposure to atmospheric moisture
- Using smaller sealed containers where appropriate
- Resealing opened material containers
- Supplying dry air to stored material
- Closed-loop drying
- Reducing dryer temperature during extended production shutdowns
- Preventing nylon over-drying
- Draining material from feed tubes during extended shutdowns
- Preventing moisture reabsorption in unheated feed areas
- Drying color concentrate
- Avoiding reintroduction of saturated material into properly dried resin
Melting the Material
- Creating melt temperature through barrel heating and screw shear
- Setting the initial temperature profile using material-manufacturer recommendations
- Setting initial back pressure
- Setting screw speed
- Measuring actual melt temperature with a needle pyrometer
- Centering melt temperature within the recommended processing range
- Returning existing processes to the established melt-temperature target
- Avoiding process changes that compensate for unrelated root-cause problems
Shear Energy and Melt Temperature
- Contribution of screw speed to shear heating
- Contribution of back pressure to shear heating
- Interaction between barrel temperature and mechanical shear
- Material degradation from excessive time at temperature
- Effect of residence time on viscosity
- Using changes in required machine settings as possible indicators of equipment wear
- Correcting the underlying cycle-time or equipment problem rather than continually changing the approved setup
Volumetric Fill Rate
- Maintaining the same fill time on every shot
- Supplying the same amount of material in the same amount of time
- Maintaining consistent ram momentum at transfer
- Effect of ram speed at transfer on packing
- Relationship between injection speed and viscosity
- Faster fill and reduced viscosity
- Slower fill and increased viscosity
- Relationship between skin thickness and fill time
- Effect of fill-rate changes on required fill pressure
- Effect of fill speed on impingement force during insert molding
Non-Newtonian Material Behavior
- Nonlinear relationship between injection rate and viscosity
- Effect of shear rate on molecular alignment
- Effect of increasing shear rate on viscosity
- Effect of excessive shear on shear-sensitive materials
- Shear heating
- Lot-to-lot shifts in viscosity
- Effect of melt temperature on the viscosity curve
- Effect of residence time on viscosity
- Selecting a fill speed near the knee of the viscosity curve
- Balancing low viscosity against insert-loading and component-movement forces
Viscosity versus Fill Time
- Developing viscosity curves from process data
- Relating pressure and fill time
- Using logarithmic fill-time representation
- Identifying the point of diminishing return in injection speed
- Using the viscosity curve to select a repeatable operating region
- Avoiding excessive machine stress
Fill Speed and Fill Pattern
- Effect of injection speed on fill pattern
- Effect of fill speed on boundary-layer thickness
- Changes in the last point to fill
- Changes in venting requirements
- Changes in fiber and molecular alignment
- Maintaining balanced cavities at the selected viscosity and fill speed
- Recognizing that a mold balanced at one viscosity may become unbalanced under different material or process conditions
Cooling Rate and Mold Temperature
- Dynamic heat transfer between melt, mold steel, and cooling system
- Effect of melt temperature on mold temperature
- Cooling-water temperature
- Cooling-water flow rate
- Importance of turbulent coolant flow
- Nozzle-contact time
- Overall cycle time
- Time required for the cooling system to reach equilibrium
- Effects of mold temperature on required fill pressure
- Effects of cooling rate on skin thickness
Cooling Rate and Crystallinity
- Crystal growth during cooling
- Number and size of crystalline regions
- Effect of cooling rate on part dimensions
- Effect on warpage
- Effect on toughness
- Effect on physical properties
- Maintaining consistent cooling for dimensional repeatability
- Cooling-line placement
- Cooling-water routing
- Avoiding daisy-chained cooling lines
- Maintaining coolant flow rate
- Preventing lime and scale buildup
Transfer from Fill to Pack
- Completing most of the cavity fill during the primary fill phase
- Transferring at approximately 95–99% full
- Verifying transfer position with pack pressure removed
- Maintaining a slight short shot at the end of primary fill
- Maintaining consistent shot size
- Maintaining consistent fill time
- Maintaining consistent momentum at transfer
- Avoiding unnecessary transfer-point adjustments when another variable has changed
Pack Pressure and Hold Time
- Applying sufficient pack pressure to eliminate sink away from the gate
- Holding pack pressure until the gate freezes
- Preventing material backflow through an open gate
- Maintaining stable part weight
- Maintaining dimensional repeatability
- Distinguishing insufficient pack pressure from insufficient hold time
- Using sink location as an indicator of the required adjustment
Determining Gate Freeze Time
- Using molded-part weight to determine gate freeze
- Reducing hold time incrementally
- Identifying the point at which part weight begins to fall
- Plotting the gate-freeze curve
- Setting hold time above the measured freeze point
- Maintaining packing pressure until the gate has sealed
Overall Cycle Time
- Maintaining the same total cycle time from shot to shot
- Relationship between cycle time and barrel residence time
- Relationship between cycle time and material degradation
- Relationship between cycle time and mold temperature
- Relationship between cycle time and part cooling
- Setting screw recovery to finish before mold opening
- Coordinating operator activity to the established cycle
- Avoiding process changes made simply to accommodate faster or slower operators
- Understanding the cascading effects created by uncontrolled cycle-time changes
Cycle-Time and Manufacturing Capacity
- Mold-close time
- Injection delay
- Injection time
- Cooling time
- Screw recovery time
- Mold-open time
- Table rotation, ejection, or robotic handling time
- Operator loading and unloading time
- Part throughput
- Machine cost per component
- Effect of cycle-time requirements on equipment and manufacturing planning
Establishing the Approved Molding Setup
- Recording the settings used to produce the approved process
- Recording actual process outputs rather than set points alone
- Melt temperature
- Actual fill time
- Cooling rate and tool temperature
- Moisture content
- Screw recovery time
- Cushion position
- Shot size
- Overall cycle time
- Recognizing that inputs may require minor adjustment as equipment wears in order to maintain the same process outputs
- Replacing worn equipment when adjustments become significant
Process Verification
- Verifying that actual readouts match the approved process conditions
- Verifying material conditions
- Verifying machine conditions
- Verifying mold conditions
- Using cavity-pressure transducers where appropriate
- Process fingerprinting
- Monitoring repeatability of cavity conditions
- Recognizing drift before it creates dimensional or functional failures
Decoupled and Cavity-Pressure-Controlled Molding
- Using cavity pressure to identify the end of fill
- Transferring to pack based on a rise in cavity pressure
- Relationship between transducer location and process control
- Limitations on very small molded components
- Using cavity-pressure monitoring even when full decoupled control is not practical
- Applying the same process-control principles across small and large molded components
Identifying Scrap and Process Variation
- Checking for cavity-dependent failures
- Establishing scrap percentage by cavity
- Material-viscosity variation
- Cycle-time variation
- Raw-material variation
- Material-moisture variation
- Lot variation
- Tool damage
- Inadequate or blocked venting
- Uneven fill patterns
- Incoming insert or component variation
- Pressure-limited processing
Structured Molding Troubleshooting
- Looking for root cause rather than immediately changing settings
- Comparing the current process with the approved setup sheet
- Ensuring mold vents are clean
- Checking cavity dependencies
- Verifying actual machine outputs
- Checking melt temperature
- Checking actual fill time
- Checking tool temperature
- Checking material moisture content
- Checking screw recovery time
- Checking cushion position
- Checking shot size
- Checking overall cycle time
- Considering interactions among variables before making process changes
Common Molding Defects and Investigation Areas
- Nozzle drool
- Nozzle freeze-off
- Discoloration
- Short shots
- Flash
- Sticking in the sprue bushing
- Weld lines
- Sinks and voids
- Poor dimensional control
- Burn marks
- Splay and silver streaks
- Sticking in cavities
- Relating each observed defect back to material, machine, mold, and process conditions
Integrating the Scientific Molding Process
- Control material moisture before processing
- Establish and verify melt temperature
- Establish a repeatable volumetric fill rate
- Operate in a stable region of the viscosity curve
- Balance cavity fill
- Maintain adequate venting
- Control mold temperature and cooling rate
- Transfer consistently from fill to pack
- Maintain pack pressure through gate freeze
- Maintain a stable overall cycle time
- Record actual process outputs
- Monitor equipment and mold condition
- Investigate variation systematically rather than compensating for it with unrelated setting changes
- Maintain repeatable cavity conditions as the basis for consistent part quality

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