Parameters of the Molding Process
Chapter 3
Professor Joe Greene
CSU, CHICO
1
MFGT 144
September 14, 1999
Chapter 3 Topics
• Identifying the Parameters
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–
–
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Temperature
Pressure
Time
Distance
2
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.
3
» Water demand decreased and the machines cooled down, requiring
readjustment on the machine parameters.
Injection Molding Parameters
• Four basic categories
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–
–
–
Temperature
Pressure
Time
Distance
Temperature
Pressure
Time
Distance
4
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
5
Injection Molding Parameters
• Melt Temperature (Table III-1. Suggested Melt Temp at nozzle)
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•
•
•
•
•
•
•
•
•
•
•
•
•
•
Acetal (coploymer)
Acrylic
ABS
Liquid Crystal Polymer
Nylon 6
Polyamide-imide
Polyarylate
Polycarbonate
Polyetheretherketone
Polyethylene LDPE
Polyethylene HDPE
Polypropylene
Polystyrene
Thermoplastic polyester (PBT)
Urethane elastomer
400 F
425 F
400 F
500 F
500 F
650 F
700 F
550 F
720 F
325 F
350 F
350 F
350 F
425 F
425 F
6
Temperature History
7
Injection Molding Parameters
• Melt Temperature
8
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.
9
• 95% of total shrinkage occurs while in mold. 99% within 3 hrs
Injection Molding Parameters
• Mold Temperature (Table III-2. Suggested mold Temperatures)
•
•
•
•
•
•
•
•
•
•
•
•
Acetal (coploymer)
Acrylic
ABS
Liquid Crystal Polymer
Nylon 6
Polyamide-imide
Polyarylate
Polycarbonate
Polyetheretherketone (PEEK)
Polyethylene LDPE
Polyethylene HDPE
Polypropylene
200 F
180 F
180 F
250 F
200 F
400 F
275 F
220 F
380 F
80 F
110 F
120 F
• Polystyrene
• Thermoplastic polyester (PBT)
160 F
180 F
• Urethane elastomer
120 F
10
Injection Molding Parameters
• Mold Temperature
11
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.
12
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
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–
–
–
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 13better
temperature control
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
14
Pressure Control
15
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.
16
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.
17
– Minimizes voids in molded parts.
• Clamp Unit
Pressure Control
– 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
18
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
19
injection pressure is high, the mold will open up causing flash.
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 rating20is only
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,
10 in
• 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
10 in
• 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 lbs21= 1
Pressure Control
• Pressure Required
22
Ref: http://cadpc3.mem.drexel.edu/mfgcourses/injectionmold/sld027.htm
Pressure Control
• Pressure Required
23
Ref: http://cadpc3.mem.drexel.edu/mfgcourses/injectionmold/sld027.htm
Time
• Gate-to gate Cycle Time
– Time required to produce
a part
– Average Times for Cycle
•
•
•
•
•
Gate Close
Mold Close
Injection
Pack and Hold
Part Cool
– Screw return
• Mold open
• Ejection
1 sec
1 sec
2 sec
8 sec
11 sec
2 sec
1 sec
1 sec
24
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.
25
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.
26
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.
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– This finishes filling and holds pressure against material previously
•
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
28
the barrel than is required to fill the mold.
• RPM should fall within 30 to 160 RPM
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
29
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
30
Injection Molding Costs
• Material Costs
– Figure 3.14 Determining volume of parts and runner system
31
Injection Molding Costs
• Spreadsheet
------------------------------------------------- --------------------- --------------------- ------------ --------------------- ------------------------------------------------- --------------------- --------------------INJECTION MOLDING TECHNICAL COST MODEL
INJECTION MOLDING TCM: COST SUMMARY
IBIS Associates, Inc.
Copyright (c) 1997
IBIS Associates, Inc.
Copyright (c) 1997
------------------------------------------------- --------------------- --------------------- ------------ --------------------- ------------------------------------------------- --------------------- --------------------Updated: 2/4/98
per piece
per year
PRODUCT SPECIFICATIONS
VARIABLE COST ELEMENTS
--------------------- --------------------Part Name
flying disk
NAME
Material Cost
$0.18
$9,054
Weight
200 grams
WGT
Direct Labor Cost
$0.08
$3,954
Maximum Wall Thickness
6 mm
THKM
Utility Cost
$0.02
$1,212
Average Wall Thickness
3 mm
THKA
External Surface Area
500 sq cm
SAREA
FIXED COST ELEMENTS
--------------------- --------------------Projected Area
450 sq cm
PAREA
Equipment Cost
$0.07
$3,290
Tooling Cost
$0.60
$30,000
Number of Cavities
1
CAV
Building Cost
$0.00
$126
Number of Actions in Tool
1
ACT
Maintenance Cost
$0.06
$2,942
Surface Finish [3=best]
2 [1,2 or 3]
FIN
Overhead Labor Cost
$0.03
$1,267
Cost of Capital
$0.11
$5,605
Annual Production Volume
50 (000/yr)
NUM
=========== ===========
Length of Production Run
1 yrs
PLIFE
TOTAL OPERATION COST
$1.15
$57,450
MATERIAL SPECIFICATIONS
Material Type
Material Price
Scrap Credit Value
Density
Thermal Conductivity
Heat Capacity
Melt Temp
Tool Temp
Eject Temp
PROCESS RELATED FACTORS
Dedicated Investment
Operation Rejection Rate
Material Scrap Rate
Average Equipment Downtime
Direct Laborers Per Station
------------------------------------------------- --------------------- --------------------LDPE
$0.90
$0.00
0.94
0.24
1675
260
45
80
$/kg
$/kg
g/cm^3
W/mK
J/kgK
C
C
C
0 [1=Y 0=N]
0.1%
0.5%
20.0%
0.5
MAT
PRICE
SCPRI
DENS
TCOND
HTCAP
MTEMP
TTEMP
ETEMP
DED
REJ
SCR
DOWN
NLAB
INTERMEDIATE CALCULATIONS
Part Name
Material Designation
Product Weight
Raw Material Price
Material Scrap Price
Material Density
Adjusted Material Scrap
Cumulative Rejection Rate
Effective Production Volume
Tool Complexity Factor
Energy Adjustment Factor
Clamping Force
Cooling Time
flying disk
LDPE
200
$0.90
$0.00
0.94
g
/kg
/kg
g/cm^3
0.005
0.001
50050 /yr
22942
3.232
1482 kN
9.5 sec