Times for Plastic Molding
General
Labor Time = Cycle Time * Number of Operators
Labor time is the product of the following:
• Cycle time (see formula below)
• Number of operators (specified by user or defaulted to machine property, typically 0.5—see User Inputs for Plastic Molding)
Batch Setup Time
Setup Time property for the machine as defined in the VPE.
Ejection Time = Mold Open Time + Hydraulic Slide Time + Ejector Plate Time + Part Removal Time + Mold Close Time
Ejection time is the sum of the following:
• Mold open time (see formula)
• Hydraulic slide time: (see formula)
• Ejector plate time (see formula)
• Part removal time: If the setup option Part Eject Method is set to Gravity (the default in starting point VPEs), the part removal time is the value of the cost model variable gravityDropTime (1.25 seconds in starting point VPEs). Otherwise, part removal time is the value of the cost model variable robotGrabTime (2.5 seconds in starting point VPEs).
Mold Open Time = Travel Distance / Max Machine Velocity
Mold open time depends on the following:
• Travel distance: see Travel Distance, below.
• Max machine velocity: see formula.
Travel Distance
Travel distance for mold open or close is generally 3 or 4 times the part height:
• If the setup option Part Eject Method is set to Gravity (the default in starting point VPEs), the travel distance is generally the product of part height and the cost model variable nominalHeightFactor (3 in starting point VPEs):
Travel Distance = Part Height * Nominal Height Factor
To accommodate small parts, this value is bounded below by the product of smallHeightPartFactor (1.1 in starting point VPEs) and the length of the plastic forming region (PFR--see Dimensions of the Plastic Forming Region):
Travel Distance = PFR Length * Small Height Factor
In starting point VPEs, this applies when the part height is less than roughly 1/3 of the PFR length.
• If the setup option Part Eject Method is set to Robotic, the travel distance is generally the product of the following:
o Part height
o Sum of the cost model variables nominalHeightFactor (3 in starting point VPEs) and roboticClearanceFactor(1 in starting point VPEs)
Travel Distance = Part Height * (Nominal Height Factor + Robotic Clearance Factor)
To accommodate small parts, this value is bounded below by the product of the following:
o Length of the plastic forming region (PFR--see Dimensions of the Plastic Forming Region)
o Sum of the cost model variables smallHeightPartFactor (1.1 in starting point VPEs) and roboticClearanceFactor (1 in starting point VPEs)
Travel Distance = PFR Length * (Small Height Factor + Robotic Clearance Factor)
In starting point VPEs, this applies when the part height is less than roughly 1/2 the PFR length.
See also Part Eject Method in User Inputs for Plastic Molding.
Max Machine Velocity =
(Dry Cycle Stroke Factor * Max Tie Bar Distance * 2) / Dry Cycle Time
The speed of travel during mold open and close is derived from the following:
• Dry cycle stroke factor: this is the fraction of the maximum tie bar distance that equals the stroke length. The fraction is specified by the cost model variable dryCycleStrokeFactor (0.7 in starting point VPEs—see the Euromap 6 technical recommendation).
• Max tie bar distance: this the larger of the machine properties Tie Bar Distance, H and Tie Bar Distance, V.
• Dry cycle time: specified by the machine property Dry Cycle Time.
Hydraulic Slide Time = (Hydraulic Slide Length / Slide Cycle Rate) * 2
A slide for a given undercut feature is required to be hydraulic (as opposed to mechanical) if the platen movement required to clear the mechanical slide from the undercut exceeds the mold open stroke. The presence of hydraulic slides contributes to the total ejection time because, unlike mechanical slides, hydraulic slides are retracted only after the mold is opened.
Hydraulic slide time depends on the following:
• Hydraulic slide length: this is 0 if hydraulic slides are unnecessary. If hydraulic slides are necessary, the hydraulic slide length is the maximum undercut depth plus some clearance (0.5 inch if there are multiple cavities, 5mm otherwise).
• Slide cycle rate: this is the rate at which the hydraulic slides can move, specified by the cost model variable slideCcleRate (100mm per second in starting point VPEs).
Hydraulic slides are necessary if and only if the following condition holds:
Max Pin Travel Distance > Machine Opening Stroke
Here, Machine Opening Stroke is the value of the machine property Opening Stroke.
Max Pin Travel Distance is the distance along the draw direction that the platen would have to travel in order to retract a mechanical slide from the part’s deepest undercut. The cost model assumes that a retracting mechanical slide is guided by a pin that makes an angle of 15 degrees with the draw direction. If U is the maximum undercut depth (perpendicular to the draw direction) plus clearance (0.5 inch if there are multiple cavities, 5mm otherwise), then the required platen motion along the draw direction is U/tan(15):
Max Pin Travel Distance = (Max Undercut Depth + Extra Clearance) / tan(15)
If the motion required for a mechanical slide (Max Pin Travel Distance) exceeds the value of the machine property Opening Stroke, then hydraulic slides are necessary for the part.
Ejector Plate Time = (Part Height * Ejector Distance Factor) / Max Machine Velocity
Ejector plate time depends on the following:
• Part height: determined by geometry extraction. Part height is measure along the draw direction.
• Ejector distance factor: this is the multiple of part height that equals the distance an ejector pin should move in order to eject the part. It is specified by the cost model variable ejectorDistanceFactor (3 in starting point VPEs).
• Max machine velocity: see formula.
Mold Close Time = Travel Distance / Max Machine Velocity
Mold close time depends on the following:
• Travel distance: see Travel Distance, above.
• Max machine velocity: see formula.
IM and SFM Times
Times such as labor time and cycle time are calculated as described below.
Cycle Time = (Injection Time + Cool Time + Ejection Time ) / Number of Cavities
Cycle time is a function of the following:
• Injection time (see formula below)
• Cool time (see formula below)
• Ejection time (see formula below)
• Number of cavities (see Number and Layout of Mold Cavities)
Injection Time = ((Number of Cavities * Part Volume) + Runner Volume) / Adjusted Injection Rate
Injection time is function of the following:
• Number of cavities (see Number and Layout of Mold Cavities)
• Part volume (obtained from GCD extraction)
• Runner volume (see Runner System Model)
• Adjusted injection rate (see formula below)
Adjusted Injection Rate = Injection Rate * Injection Rate Multiplier
Adjusted injection rate is the product of the following:
• Injection rate: specified by the machine property Injection Rate.
• Injection rate multiplier: specified by the cost model variable injectionRateMultiplier (0.25 in starting point VPEs).
Cool Time = 0 – ((Nominal Wall Thickness^2) / (2 * π * Thermal Diffusivity)
* ln((π/4) * (Eject Deflection Temperature – Mold Temperature) /
(Melting Temperature – Mold Temperature)))
Cool time is a function of the following:
• Nominal wall thickness: see Nominal Wall Thickness.
• Thermal diffusivity: see formula, below.
• Eject deflection temperature: temperature at which the part can be ejected from the mold without deformation, specified by the material property Eject Deflection Temp.
• Melting temperature: specified by the material property Melting Temp.
• Mold temperature: required temperature of the mold to support proper molten polymer flow. This is specified by the material property Mold Temp, increased by the amount specified by the cost model variable defaultMoldTemperatureIncrease (0 in starting point VPEs) or by the override specified by the user with the setup option Local Mold Temperature Increase—see User Inputs for Plastic Molding.
Thermal Diffusivity = Thermal Conductivity / (Density of Melt * Specific Heat)
This is the thermal diffusivity of the current material, in mm2/s. The value is converted from m2/s to mm2/s so that it can be used in the Cool Time formula. The value in m2/s is determined according to this formula by using the following:
• Thermal conductivity: this is the thermal conductivity of the current material, specified in watt/m°C by the material property Thermal Conductivity of Melt.
• Density of melt: this is the melt density of the current material, specified in kg/m3 by the material property Density of Melt.
• Specific heat: this is the specific heat of the current material, specified in joule/g°C by the material property Specific Heat of Melt, and converted to joule/kg°C for use in this formula.
Note: Versions of aPriori prior to 2019 R2 used a value for thermal diffusivity that was provided by the material property Thermal Diffusivity, rather than computing it from thermal conductivity and specific heat. The cost model will continue to use the material property Thermal Diffusivity, if the new material properties Density of Melt, Thermal Conductivity of Melt, and Specific Heat of Melt are not provided; however aPriori recommends that you update older VPEs to use the new properties, as they are reliably found on spec sheets, while thermal diffusivity generally is not.
RIM Times
Times such as labor time and cycle time are calculated as described below.
Cycle Time = (Injection Time + Cool Time + Ejection Time +
Part Handling Time + Mold-release Application Time) / Number of Cavities
Cycle time is computed by default, but can be specified by the user—see User Inputs for Plastic Molding. If calculated, it is a function of the following:
• Injection time (see formula below)
• Reaction/cool time (see formula below)
• Ejection time (see formula below)
• Part handling time (based on part mass, according to the table below)
• Mold-release application time (part surface area divided by plant variable applyMoldReleaseRate)
• Number of cavities (see Number and Layout of Mold Cavities)
Handling Time (s)
Weight (kg)
5
0.45
5
4.55
15
22.73
20
45.45
23
113.64
27
227.27
30
454,545,454.09
Injection Time = ((Number of Cavities x Part Volume) + Runner Volume) / Adjusted Injection Rate
Injection time is function of the following:
• Number of cavities (see Number and Layout of Mold Cavities)
• Part volume (obtained from GCD extraction)
• Runner volume (see Runner System Model)
• Adjusted injection rate (see formula below)
Adjusted Injection Rate = Injection Rate * Injection Rate Multiplier
Adjusted injection rate is the product of the following:
• Injection rate: specified by the machine property Injection Rate.
• Injection rate multiplier: specified by the cost model variable injectionRateMultiplier (0.25 in starting point VPEs).
Cool Time = 0 – ((Nominal Wall Thickness^2) / (2 * π * Material Thermal Diffusivity)
* ln((π/4) * (Eject Deflection Temperature – Mold Temperature) /
(Melting Temperature – Mold Temperature)))
Cool time is a function of the following:
• Nominal wall thickness: see Nominal Wall Thickness.
• Thermal diffusivity: see formula, below.
• Eject deflection temperature: temperature at which the part can be ejected from the mold without deformation, specified by the material property Eject Deflection Temp.
• Melting temperature: specified by the material property Melting Temp.
• Mold temperature: required temperature of the mold to support proper molten polymer flow. This is specified by the material property Mold Temp, increased by the amount specified by the cost model variable defaultMoldTemperatureIncrease (0 in starting point VPEs) or by the override specified by the user with the setup option Local Mold Temperature Increase—see User Inputs for Plastic Molding.
Thermal Diffusivity = Thermal Conductivity / (Density of Melt * Specific Heat)
This is the thermal diffusivity of the current material, in mm2/s. The value is converted from m2/s to mm2/s so that it can be used in the Cool Time formula. The value in m2/s is determined according to this formula by using the following:
• Thermal conductivity: this is the thermal conductivity of the current material, specified in watt/m°C by the material property Thermal Conductivity of Melt.
• Density of melt: this is the melt density of the current material, specified in kg/m3 by the material property Density of Melt.
• Specific heat: this is the specific heat of the current material, specified in joule/g°C by the material property Specific Heat of Melt, and converted to joule/kg°C for use in this formula.
Note: Versions of aPriori prior to 2019 R2 used a value for thermal diffusivity that was provided by the material property Thermal Diffusivity, rather than computing it from thermal conductivity and specific heat. The cost model will continue to use the material property Thermal Diffusivity, if the material properties Thermal Conductivity of Melt and Specific Heat of Melt are not provided; however aPriori recommends that you update older VPEs to use the new properties, as they are reliably found on spec sheets, while thermal diffusivity generally is not.