-
Bong-Kee LeeSchool of Mechanical Engineering
Chonnam National University
Mold Design
7. Mold Design – Runner & Gate
School of Mechanical EngineeringMold Design
Delivery System
Delivery System (Feed System)– sprue (for a cold runner mold)–
cold slug well (for a cold runner mold)– runner– gate
basic feed system
-
School of Mechanical EngineeringMold Design
Delivery System
Sprue– should be tapered 3~5° inclusive angle
• being pulled out of the mold more easily
– should also be highly polished in the line of draw• to assist
withdrawal• to induce more efficient flow
– diameter at the narrow end• should be larger than the machine
cylinder nozzle opening
School of Mechanical EngineeringMold Design
Delivery System
Cold Slug Well– to trap the cooler advancing front of the melt,
thus
permitting hotter melt to reach the cavities and gates– snatch
or pull pin or sucker pin underneath it
• to positively pull the sprue out of the sprue bush• diameter
design based on the diameter of the sprue where it
meets the runner• to avoid any obstruction to the melt flow
-
School of Mechanical EngineeringMold Design
Delivery System
Cold Slug Well– for ejection pin systems
• with a hardened bush to minimize wear
normally preferred one
the least desirable one due to large mass and long cooling time
capture the colder advancing front well
sometimes tends to hang up, but preferred for brittle
materials
School of Mechanical EngineeringMold Design
Delivery System
Cold Slug Well– for stripper plate ejection
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– purpose of runner
• to transport the melt from the sprue to the gates
– basic parameters for runner geometry• cross-sectional shape•
diameter• cavity layout
School of Mechanical EngineeringMold Design
Runner System
Runner Design– cross-sectional shape
• full round cross-section: the most efficient design•
trapezoidal one: for three-plate molds• semi-circular or half round
one: severely restricts flow although
frequently used• square and rectangular ones: should never be
used
• formation of dead zones in the runner design→ rheologically
inefficient and wasteful on material and energy
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– cross-sectional shape
School of Mechanical EngineeringMold Design
Runner System
Runner Design– diameter (size of runners)
• based on the thickness of the molding wall section• should be
large enough to provide adequate pressure to all the
cavities → no packing pressure shortfall and adequate control
over the molding conditions
• alternative method: based on an appropriate pressure drop
along the length of the runner
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– cavity layout
• beneath runner intersections– cold slug well for improving
melt flow: length of cold slug well ~
runner diameter– ejection pin
• variation of runner diameters
School of Mechanical EngineeringMold Design
Runner System
Runner Design– cavity layout
incorrect design correct design
incorrect design correct design
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– design rules
• runners must be designed to fill the cavity rapidly• runner
design must provide for easy ejection and easy removal
(de-gating) from the molded part• for a multi-cavity system,
balanced runner layout is preferred
for the best uniformity and part quality– runner balancing may
be achieved by changing the runner size
and length– changing the gate dimensions may seem to give a
reasonably
balanced fill but this will affect the gate freeze-off time
• smaller runner sizes are preferred to larger ones to minimize
scrap volume and generate viscous heating
– high barrel temperature may cause material degradation
School of Mechanical EngineeringMold Design
Runner System
Runner Design– design rules (continued)
• the cross-sectional area of the runner should not be smaller
than that of the sprue, to permit rapid, unaltered flow to the
gates
• the diameter of the branch runner should be smaller than that
of the main runner as more economical way
• the depth of a trapezoidal runner is approximately equal to
its width with a 5~15° taper or draft angle on each sidewall
• the minimum recommended runner diameter for most materials is
1.5mm
• the runner surface and sprue should be polished
313/1 runners)branch ofnumber diameter)(runner (branch
diameter)runner (main : /dND
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– design rules (continued)
• it is desirable to have multiple sprue pullers and ejection
locations in extended runner systems
• the selection of a cold runner diameter should be based on
standard machine tool cutter sizes
• for hot runner systems, it is advisable to consult the
suppliers for availability and recommendations for the correct
manifold and drops
School of Mechanical EngineeringMold Design
Runner System
Runner Design– calculation of runner length
• (assumption) maximum pressure drop along the length of the
runner ~ 70MPa (50MPa for filled material in most cases)→ safe
working figure with which to calculate runner lengths
[m]length runner : [MPa] stressshear :
[MPa] drop pressure : s][Pa raturemelt tempeat viscosity:
[m] radiusrunner : /s][m rate flow : ][s rateshear :
24
3
1-
3
L
P
rQ
rLP
rQ
-
School of Mechanical EngineeringMold Design
Runner System
Runner Design– calculation of runner length
• (example)– polycarbonate using a melt temperature of 310°C
(viscosity of
1000Pa·s) and a flow rate through the runner of 2.85cm3/s–
runner length of 120mm and diameter of 4mm
[MPa]48.54102
10120454.022[MPa]454.04541000
][s454102
1085.244
24
3
3
1-33
6
3
3
rLP
rQ
rLP
rQ
School of Mechanical EngineeringMold Design
Runner System
Runnerless Molding– runnerless molding usually includes
• sprueless molding– basic antechamber designs: melt flows
through an insulated cold
slug well– heated hot sprue bushes or nozzles: internal or
external heating
• insulated runner systems– insulated– semi-insulated
• hot runner systems
-
School of Mechanical EngineeringMold Design
Runner System
Runnerless Molding
typical antechamber design semi-insulated runner design
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems– advantages over cold runner molds
• no runner system to be removed from the mold• shorter cycle
time with no cold runner to be cooled• reduced mold opening stroke•
reduced (or eliminated) cost for storing and regrinding runners•
lowered risk of material contamination• gates may be balanced more
easily• lowered injection pressure using the larger runner
diameters ~
greater number of impressions, utilization of smaller machines•
smaller shot weight ~ reduced metering times and injection
times
-
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems– disadvantages
• significantly more expensive• 24-hour operation is required
for maximum economic
production• heat-sensitive materials may be difficult to
process• gate blockages can be time-consuming and expensive to
remedy
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems– hot runner manifold
• separated unit carrying the runner and nozzle gating systems•
insulated from the main body of the mold
-
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems– hot runner manifold
• nozzles and gate bushes• pin and edge gating, valve gating,
thermal sealing• heating with band, coil, cartridge, tubular
heaters• temperature sensing and control• thermal expansion and
efficiency issues
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems
insulation
water coolingin the core
ejection
support and guideof stripper plate
-
School of Mechanical EngineeringMold Design
Runner System
Hot Runner Systems– hybrid hot runner/cold runner system
mold design for a suitcase half
flash gate ~ uniform melt flow efficient air venting through a
parting line(cf. multi-point gating ~ complex flow) evenly spread
force due to the molded part off-centered hot runner manifold large
number of cooling channels
School of Mechanical EngineeringMold Design
Gate Design
Gate– small opening (or orifice) through which the polymer
melt enters the cavity– gate design: gate type, dimensions, and
locations
• part geometry (wall thickness, etc.)• part specifications
(appearance, tolerances, etc.)• material used• fillers used• cycle
time• de-gating requirements
-
School of Mechanical EngineeringMold Design
Gate Design
Gate– number of gate for the cavity
• single gate• multiple gates ~ if the length of melt flow
exceeds practical
limits → weld and meld lines
– cross-section of the gate• typically smaller than that of the
runner and the part• related to the de-gating (separation from the
molded part), the
material freezing off (during the post-filling stage), and the
viscous heating
School of Mechanical EngineeringMold Design
Gate Design
Basic Gate Terminology
-
School of Mechanical EngineeringMold Design
Gate Design
Gate Location– should be selected in such a way that rapid
and
uniform mold filling is ensured and weld/meld lines and air
vents are positioned properly
– should be positioned away from load-bearing areasbecause the
high melt pressure and high velocity of flowing material causes the
area near the gate to be highly stressed
School of Mechanical EngineeringMold Design
Gate Design
Common Types of Gatesdirect or sprue gate tab gate edge or side
gate overlap gate
• direct feed of materialinto the cavity rapidly and with
minimum pressure drop• the gate has to be trimmed off and a large
gate witness is left on the part
• typically used for flat, thin parts to reduce the shear stress
in the cavity• high shear stress is confined to the tab which is
trimmed off after molding
• located at the parting line of the mold and fills the cavity
from the side, top or bottom of the part
• similar to the edge gate except the overlaps the wall or
surfaces
-
School of Mechanical EngineeringMold Design
Gate Design
Common Types of Gatesfan gate disc or diaphragm gate ring gate
spoke or spider gate
• similar to a wide edge gate with a variable thickness• permits
rapid filling of large parts or fragile mold sections through the
large entry area• creates a uniform flow front
• frequently used for gating cylindrical or round parts that
have an open inside diameter• for concentricity and unacceptable
weld line
• for gating round or cylindrical parts• material flows freely
around the core before it flows down as a uniform front to fill the
cavity
• a four point or cross gate• for tubular parts and offers much
easier de-gating than the ring gate• possibility of weld lines and
out of roundness
School of Mechanical EngineeringMold Design
Gate Design
Common Types of Gatesfilm or flash gate hot probe gate pin gate
submarine or tunnel gate
• similar to a ring gate but is used for flat, straight parts•
consists of a straight runner and a gate land across either the
entire length or width of the cavity or a portion of it
• normally used to deliver material directly into the cavity
through heated runners resulting in runnerlessmoldings
• generally used in three plate and hot runner molds to permit
rapid gate freeze off and easy de-gating
• mainly used in two plate molds• enables automatic de-gating of
the part from the runner during the ejection stage
-
School of Mechanical EngineeringMold Design
Gate Design
Common Types of Gatessprue gate
tab gate
fan gateflash gate
School of Mechanical EngineeringMold Design
Gate Design
Common Types of Gatesring gate disc gate
pin gate submarine gate
-
School of Mechanical EngineeringMold Design
Gate Design
De-gating– manually trimmed gates– automatically trimmed
gates
• for two-plate molds• submarine gates
School of Mechanical EngineeringMold Design
Gate Design
De-gating• hook gates
– gating onto the top surface or side of a component is not
acceptable
– difficult and expensive to make
– material must be flexible– C: cavity insert– B: separate
additional insert– cross-section of the gate mustdecrease toward
the parts– A: extended runner to enablethe full length of the gate
to bewithdrawn from the mold
-
School of Mechanical EngineeringMold Design
Gate Design
Design Rules– gate design should deliver a rapid, uniform
and
preferably unidirectional mold filling pattern– gate location
should allow the air present in the cavity
to escape during the filling stage– if the gate location is
likely to cause weld or meld lines,
it should be positioned so that these will occur at appropriate
positions to preserve part quality and appearance
– gate location and size should avoid the possibility of
jetting
School of Mechanical EngineeringMold Design
Gate Design
Design Rules– freeze-off time at the gate should ensure
maximum
cavity packing time and also prevent back flow– gate location
should be at the thickest area of the part– gate length should be
as short as possible to avoid an
excessive pressure drop across the gate– gate thickness is
normally 50~80% of the gated wall
section thickness– fiber-filled materials require larger gates
to minimize
breakage of the fibers
-
School of Mechanical EngineeringMold Design
Gate Design
Flow Analysis
direct sprue gate (center-gated design)~ part warpage
double edge gate~ weld line at the center of the part
single edge gate~ unidirectional filling pattern with
uniformmolecular and fiber orientation
School of Mechanical EngineeringMold Design
Gate Design
Gate Sizing– length : should be as short as possible–
diameter
• rough approximation
• flow analysis using computer software• empirical
approximation
33
44
Qr
rQ
][mmproduct theof area surface total: 24
AANCd
wall thickness [mm] 0.75 1.00 1.25 1.50 1.75 2.00
C 0.178 0.206 0.230 0.242 0.272 0.294
N: 0.6 (PE, PS); 0.7 (PC, PP, acetal); 0.8 (Nylon); 0.9
(PVC)
