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A Typical Injection Mold Design Guide
This checklist can be used as a general reference guide for injection mold
design
engineers. It is divided into 3 parts of a mold design process.
Part 1 - Requirements to start your mold design:
Check the injection machine where the mold is to be mounted. This will help
you
decide the size and structure of the mold. for ease of
installation and other factors.
Important notes:
Locating ring size (or other positioning method)
Nozzle size
Method of clamping (Auto or manual)
Temperature control system
Determine the number of cavities and volume requirements.
This will help you
decide the material that you are going to use and other mold components that you
will
choose for cost effective design.
Determine the gate location and size.
Determine the location where ejector pin marks are prohibited.
Part 2 - Mold base layout:
Place cavities close to the center of the mold to minimize
base size and runner
length.
Ensure that the molded part remains on the movable half
(ejector half) upon
opening of PL to facilitate proper ejection.
Waterlines should be placed as evenly as possible to the contours of the
cavity.
Use support pillars underneath the cavity pockets.
Use ejector guides for molds with small ejector pins and rectangular ejector
pins.
Provide eye-bolt hole for ease of mounting and dismounting.
Install mold opening prevention locks on the operator side.
Establish pry bar groove on the corners of the mold parting line to
facilitate ease of
mold opening during assembly and maintenance.
By this time you may ask for the mold layout approval from the customer.
Part 3 - Cavity/core details:
Check material shrinkage. Locate portions (corners) for
possible significant
deflection and deformation.
Maintain uniform wall thickness.
Draft angle should be within dimension tolerance.
Divide core blocks to simplify machining and provide gas vent path.
Gate, small cores, and cores with shut-off fittings are better designed as
insertable
components for easy modification and repair.
Watch out for possible deformation of core pins.
Position the ejector pins on the ribs and other high
strength locations. Ensure
ejector balance.
Detailing/part drawing: Include all parameters needed for
processing -material,
quantity, surface finish/texture, dimensions, tolerances and
many more. Do not
assume the machinist understands everything.
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Any design change and amendments to the mold must be
re-approved by the
customer or mold owner.
Standard horizontal clamp presses deliver molten resin to the mold through a
hole in
the center of the stationary press platen. A material-delivery
system — usually
consisting of a sprue, runners, and gates — then leads the resin through the
mold and
into the cavity. These components of the material delivery system are discussed
in this
section.
Sprues
The sprue, oriented parallel to the press injection unit,
delivers resin to the desired
depth into the mold, usually the parting line. Though they can be cut directly
into the
mold, sprue bushings are usually purchased as off-the-shelf items and inserted
into the
mold (see figure 7-18). The head end of the sprue bushing comes premachined with
a
spherical recess — typically 0.5- or 0.75-inch radius — to receive and seal off
against
the rounded tip of the press injection nozzle. The sprue
bushing flow-channel
diameter typically tapers larger toward the parting line at a rate of 0.5 inch
per foot.
This eases removal of the molded sprue. The sprue orifice size, the diameter at
the
small end, comes standard in odd 1/32s from 5/32 to 11/32
inch. Sprue design can
affect molding efficiency and ease of processing. In many
molds, the greatest
restriction to material flow occurs at the press nozzle tip and sprue orifice.
These areas
see the highest volumetric flow rate of the entire system. An excessively small
sprue orifice can generate large amounts of material shear
and lead to material
degradation, cosmetic problems, and elevated filling pressure. The
problem can be
worse in the press nozzle tip because the tip orifice must be slightly smaller
than the
sprue orifice to avoid forming an undercut. The volumetric flow
rate used during
filling largely determines the correct sprue orifice size. Shot size and filling
speed, as
well as the flow properties of the specific resin, govern the required flow
rate.
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* Large parts and/or parts needing fast filling speeds require
large sprue orifice
diameters to avoid problems associated with excessive flow shear.
* As a general rule, amorphous resins and blends such as Makrolon
polycarbonate,
Lustran ABS, and Bayblend PC/ABS resins require larger sprues
and runners than
semicrystalline resins such as Durethan PA 6 and Pocan PBT.
The diameter at the base of the sprue increases with
increasing sprue
length.Standard sprue taper, typically one-half inch per foot,
leads to large base
diameters in long sprues. This large base diameter lengthens cooling and cycle
times
and also leads to regrind problems.
Figure 7-19 shows typical sprue sizes for Bayer amorphous resins as a
function of
shot size and filling time.Because the maximum shear rate in a
sprue occurs at the
orifice and the majority of shear heating and pressureloss takes place in the
first two
inches,these guidelines should apply to sprues of various
lengths. Part geometry
influences filling time to some extent.For example, parts with a mix of thick
and thin
features may need a fast filling speed to prevent premature cooling of the thin
features.
Other geometries may require slower filling speeds to prevent
problems such as
cosmetic defects or excessive clamp tonnage requirements.
Hot sprue bushings provide one solution to this problem. Hot sprue bushings
have a
heated flow channel that transports material along its
length in molten
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form,eliminating or shortening the molded cold sprue. Additionally, some molds
rely
on extension press nozzlesthat reach deep into the mold to reduce sprue length.
Runners
Unlike sprues, which deliver material depthwise through the center of the mold
plates,
runners typically transport material through channels machined into the parting
line.
Runner design influences part quality and molding efficiency.
Overly thick runners
can lengthen cycle time needlessly and increase costs
associated with regrind.
Conversely, thin runners can cause excessive filling pressures and related
processing
problems. The optimum runner design requires a balance between ease of
filling,mold
design feasibility, and runner volume. Material passing through
the runner during
mold filling forms a frozen wall layer as the mold steel draws heat from the
melt. This
layer restricts the flow channel and increases the pressure
drop through the runner.
Round cross-section runners minimize contact with the mold surface and generate
the
smallest per-centage of frozen layer cross-sectional area. As
runner designs deviate
from round, they become less efficient (see figure 7-20).
Round runners require
machining in both halves of the mold, increasing the potential for mismatch and
flow
restriction. A good alternative, the “round-bottomed” trapezoid, requires
machining in
just one mold half. Essentially a round cross section with
sides tapered by five
degrees for ejection, this design is nearly as efficient as
the full-round design. The
runner system often accounts for more than 40% of the pressure required to fill
the
mold. Because much of this pressure drop can be
attributed to runner length,
optimize the route to each gate to minimize runner length. For
example, replace
cornered paths with diagonals or reorient the cavity to shorten the runner.
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Figure 7-23 Family Mold
The runner diameter feeding the smaller part was reduced to balance
filling.
Runners for multicavity molds require special attention.
Runners for family molds, molds
producing different parts of an assembly in the same shot,
should be designed so that all parts
finish filling at the same time. This reduces over- packing and/or flash
formation in the cavities
that fill first, leading to less shrinkage variation and fewer
part-quality problems. Consider
computerized mold- filling analysis to adjust gate locations and/or
runner section lengths and
diam-eters to achieve balanced flowto each cavity (see
figure 7-23). The same computer
techniques balance flow within multi-gated parts. Molds producing
multiples of the same part
should also provide balanced flow to the ends of each cavity. Naturally balanced
runners provide
an equal flow distance from the press nozzle to the gate on each cavity.
Spoked-runner designs
(see figure 7-24) work well for tight clusters of small cavities. However they
become less efficient
as cavity spacing increases because of cavity number or size.
Often, it makes more sense to orient cavities in rows rather than circles. Rows
of cavities generally
have branched runners consisting of a primary main feed channel and a network of
secondary or
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tertiary runners to feed each cavity. To be naturally balanced, the flow path to
each cavity must be
of equal length and make the same number and type of turns
and splits. This generally limits
cavity number to an integer power of two — 2, 4, 8, 16,
32, etc. —as shown in figure 7-25.
Generally, the runner diameter decreases after each split in response to the
decreased number of
cavities sharing that runner segment. Assuming a constant
flow rate feeding the mold, the
flow-front velocity in the cavity halves after each split. The molding press
flow-rate performance
may limit the number of cavities that can be simultaneously molded if the press
cannot maintain
an adequate flow-front velocity.
Artificially balanced runners provide balanced filling and can
greatly reduce runner volume.
Artificially balanced designs usually adjust runner-segment
diameters to compensate for
differences in runner flow length. For instance, in ladder runners, the most
common artificially
balanced runner design, a primary runner feeds two rows
of cavities through equal-length
secondary runners. The diameters of these secondary runners are made
progressively smaller for
the cavities with shortest runner flow distance (see figure 7-
26). These designs require enough
secondary runner length to flow balance using reasonable runner diameters.
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As a general rule, secondary runner length should be no less than 1/5 the flow
distance from the
inboard secondary/primary runner junction to the gates on the
outboard cavities. Runners for
three-plate molds initially convey material along the runner-split
parting line and then burrow
perpendicularly through the middle plate to the cavity parting line. Tapered
drops typically project
from the main runner to pinpoint gates on the part surface. To ease removal from
the mold, these
drops taper smaller toward the gate at a rate of about 0.5 inch per foot. Avoid
long drops because
the taper can lead to excessive thickness at the runner junction or flow
restriction at the thin end.
Three-plate runners usually require sucker pins or some other
feature to old the runner on the
stripper plate until the drops clear the center plate during mold opening. Be
sure these features do
not restrict flow. See figure 7-27 for three-plate runner and gate-design
guidelines.
Gates
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Except for special cases, such as sprue-gated systems which
have no runner sections, gates
connect the runner to the part. Gates perform two major
functions, both of which require the
thickness to be less than the runner and part wall. First, gates freeze-off and
prevent pressurized
material in the cavity from backing through the gate after
the packing and holding phases of
injection. Applied pressure from the press injection unit can stop earlier in
the cycle, before the
part or runner system solidifies, saving energy and press wear-and-tear.
Secondly, gates provide a
reduced-thickness area for easier separation of the part from the runner system.
A variety of gate
designs feed directly into the parting line. The common edge
gate(see figure 7-28) typically
projects from the end of the runner and feeds the part via
a rectangular gate opening. When
designing edge gates, limit the land length, the distance from the end or edge
of the runner to the
part edge, to no more than 0.060 inch for Bayer
thermoplastics. Edge gates generate less flow
shear and consume less pressure than most self degating designs. They are
therefore preferred for
shear-sensitive materials, high-viscosity materials, highly cosmetic
applications, and large-volume
parts.
Fan gates and chisel gates, variations of the edge gate, flare wider from the
runner (see figure 7-29)
to increase the gate width. Chisel gates can provide better packing and
cosmetics than standard
edge gates on some thick-walled parts. Like the standard edge gate, the land
length for fan gates
should not exceed 0.060 inch at the narrowest point. Chisel gates taper from the
runner to the part
edge with little or no straight land area.
Edge gates can also extend to tabs (see figure 7-30) that are removed after
molding or hidden in
assembly. These tab gates allow quick removal of the gate without concern about
gate appearance.
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Edge gates may also extend from the side of a runner
oriented parallel to the part edge (see
figure7-31). This design, coupled with a “Z”-style runner, tends to reduce gate
blush by providing
uniform flow along the width of the gate and a cold-slug well at the end of the
runner. To hide the
large gate vestige left by large edge gates, the gate can extend under the edge
as shown in figure
7-32.
Because they extend under the mold parting surfaces, tunnel gates can reach
surfaces or features
that are not located on the parting line. The gates typically feed surfaces
oriented perpendicular to
the mold face. Depending upon their design, they degate during
ejection or mold opening (see
figures 7-33 and 7-34). Tunnel gates that degate during mold opening often
require a sucker pin or
a feature similar to a sprue puller to hold the runner on the ejector half of
the mold. The runner
must flex for the gate to clear the undercut in the mold steel. The gate may
break or lock in the
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mold if the runner is too stiff or if the ejector pin is too close to the gate.
Normally, the ejector pin
should be at least two runner diameters away from the base of the gate. The
orifice edge closest to
the parting line must remain sharp to shear the gate cleanly.
When molding abrasive materials
such as those filled with glass or mineral, make the gate of hardened or
specially treated mold
steel to reduce wear. Also, consider fabricating the gate on an insert for easy
replacement. The
drop angle and conical angle must be large enough to facilitate easy ejection
(see figure 7-35).
Stiff materials, glass-filled grades for example, generally require drop angles
and conical angles at
the high side of the range shown in the figure. The modified-tunnel gate design
(see figure 7-36)
maintains a large flow diameter up to the gate shear-off point to reduce
pressure loss and excessive
shear heating.
Curved-tunnel gates permit gating into the underside of surfaces that are
oriented parallel to the
parting plane (see figure 7-37). Unlike mold fabrication for conventional tunnel
gates, the curved,
undercut shape of this design must be machined or EDM burned on
the surface of a split gate
insert. The curved gate must uncurl as the runner advances on guided posts
during ejection. This
gate design works well for unfilled materials that remain
somewhat flexible at ejection
temperature such as Makrolon PC, Lustran ABS, and amorphous blends such as Bay
blend and
Makroblend resins. Avoid this gate for filled materials, brittle
materials, or materials with very
high stiffness. See figures 7-38 and 7-39 for curved-tunnel gate design
guidelines.
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Pinpoint gates feed directly into part surfaces lying parallel to the mold
parting plane. On the
ends of three-plate runner drops, multiple pinpoint gates can help reduce flow
length on large parts
and allow gating into areas that are inaccessible from the part perimeter. For
clean degating, the
gate design must provide a positive break-off point (see figure 7-40) to
minimize gate vestige. Set
in recesses or hidden under labels, properly designed and
maintained pinpoint gates seldom
require trimming. Because gate size must also be kept small,
typically less than a 0.080-inch
diameter, pinpoint gates may not provides ufficient packing for parts with thick
wall sections.
Parts with holes in the center such as filter bowls, gears, and fans often
use the “filter-bowl”gate
design to provide symmetrical filling without knit lines. Typically, the gate
extends directly from a
sprue and feeds the cavity through