Gates
Each injection mold design must have a gate, or an opening that
allows the molten plastic to be injected into the cavity of the mold. Gate
type, design and location can have effects on the part such as part packing,
gate removal or vestige, cosmetic appearance of the part, and part dimensions
& warping.
Gate Types
There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates.
There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates.
Manually Trimmed Gates:
These type of gates require an operator to separate the aprts from
the runners manually after each cycle. Manually trimmed gates are chosen for
several reasons:
·
The gate is too bulky to be automatically sheared by the machine
·
Shear-sensitive materials such as PVC cannot be exposed to high
shear rates
·
Flow distribution for certain designs that require simultaneous
flow distribution across a wide front
Automatically Trimmed Gates
These type of gates incorporate features in the tool to break or
shear the gates when the tool opens to eject the part. Automatically trimmed
gates are used for several reasons:
·
Avoiding gate removal as a secondary operation, reducing cost
·
Maintaining consistent cycle times for all parts
·
Minimizing gate scars on parts
The largest factor to consider when choosing the proper gate type
for your application is the gate design. There are many different gate designs
available based on the size and shape of your part. Below are four of the most
popular gate designs used by Quickparts customers:
The Edge Gate is the most
common gate design. As the name indicates, this gate is located on the edge of
the part and is best suited for flat parts. Edge gates are ideal for medium and
thick sections and can be used on multicavity two plate tools. This gate will
leave a scar at the parting line.
The Sub Gate is the only
automatically trimmed gate on the list. Ejector pins will be necessary for
automatic trimming of this gate. Sub gates are quite common and have several
variations such as banana gate, tunnel gate and smiley gate to name a few. The
sub gate allows you to gate away from the parting line, giving more flexibility
to place the gate at an optimum location on the part. This gate leaves a pin
sized scar on the part.
The Hot Tip Gate is the most
common of all hot runner gates. Hot tip gates are typically located at the top
of the part rather than on the parting line and are ideal for round or conical
shapes where uniform flow is necessary. This gate leaves a small raised nub on
the surface of the part. Hot tip gates are only used with hot runner molding
systems. This means that, unlike cold runner systems, the plastic is ejected
into the mold through a heated nozzle and then cooled to the proper thickness
and shape in the mold.
The Direct or Sprue Gate is a manually
trimmed gate that is used for single cavity molds of large cylindrical parts
that require symmetrical filling. Direct gates are the easiest to design and
have low cost and maintenance requirements. Direct gated parts are typically
lower stressed and provide high strength. This gate leaves a large scar on the
part at the point of contact.
To avoid problems from your gate location, below are some
guidelines for choosing the proper gate location(s):
·
Place gates at the heaviest cross section to allow for part
packing and minimize voids & sink.
·
Minimize obstructions in the flow path by placing gates away from
cores & pins.
·
Be sure that stress from the gate is in an area that will not
affect part function or aesthetics.
o If you are using
a plastic with a high shrink grade, the part may shrink near the gate causing
“gate pucker” if there is high molded-in stress at the gate
·
Be sure to allow for easy manual or automatic degating.
·
Gate should minimize flow path length to avoid cosmetic flow
marks.
·
In some cases, it may be necessary to add a second gate to
properly fill the parts.
·
If filling problems occur with thin walled parts, add flow
channels or make wall thickness adjustments to correct the flow.
Gates vary in size and shape depending upon the type of plastic
being molded and the size of the part. Large parts will require larger gates to
provide a bigger flow of resin to shorten the mold time. Small gates have a
better appearance but take longer time to mold or may need to have higher
pressure to fill correctly.
Prior to ejection from the mold, injection molded parts are cooled
down from manufacturing temperatures so that they hold their shape when
ejected. During the part cooling step of the molding process, changes in
pressure, velocity and plastic viscosity should be minimized to avoid defects.
Few aspects are more crucial during this period than wall thickness. This
feature can have major effects on the cost, production speed and quality of the
final parts.
Proper Wall Thickness:
Choosing the proper wall thickness for your part can have drastic
effects on the cost and production speed of manufacturing. While there are no
wall thickness restrictions, the goal is usually to choose the thinnest wall
possible. Thinner walls use less material which reduces cost and take less time
to cool, reducing cycle time.
The minimum wall thickness that can be used depends on the size
and geometry of the part, structural requirements, and flow behavior of the
resin. The wall thicknesses of an injection molded part generally range from
2mm – 4mm (0.080” – 0.160”). Thin wall injection molding can produce walls as
thin as 0.5mm (0.020”). The chart below shows recommended wall thicknesses for
common injection molding resins.
Uniform Wall Thickness:
Thick sections take longer to cool than thin ones. During the
cooling process, if walls are an inconsistent thickness, the thinner walls will
cool first while the thick walls are still solidifying. As the thick section
cools, it shrinks around the already solid thinner section. This causes
warping, twisting or cracking to occur where the two sections meet. To avoid
this problem, try to design with completely uniform walls throughout the part.
When uniform walls are not possible, then the change in thickness should be as
gradual as possible. Wall thickness variations should not exceed 10% in high
mold shrinkage plastics. Thickness transitions should be made gradually, on the
order of 3 to 1. This gradual transition avoids stress concentrations and
abrupt cooling differences.
Alternatives:
If your part is so complex that you need variations on your wall
thickness, look for an alternative. You may want to use design features such as
coring or using ribs. At the very least, try not to make the transitions
between thicker and thinner sections too abrupt. Try using a gradual transition
or chamfered corners to minimize the dramatic change in pressures inside the
mold.
Most injection molded plastic parts include features such as
outside walls and internal ribs that are formed by opposing surfaces of tool
metal inside a closed mold. To properly release the part when the mold opens,
the side walls of the mold are tapered in the direction that the mold opens.
This tapering is referred to as “draft in the line of draw”. This draft allows
the part to break free of the mold as soon as the mold opens. The amount of
draft required can depend on the surface finish of the mold. A smooth, polished
tool surface will allow the part to eject with less draft than a standard tool
surface.
Consider the fabrication of the hollow plastic box seen to the
right. Once the plastic has hardened around the mold, the mold must be removed.
As the plastic hardens, it will contract slightly. By tapering the sides of the
mold by an appropriate "draft angle", the mold will be easier to
remove.
The amount of draft required (in degrees) will vary with geometry
and surface texture requirements of the part. Below are several rules for using
draft properly:
·
Be sure to add draft to your 3D CAD model before
creating radii
·
Use at least 1 degree of draft on all "vertical" faces
·
1 ½ degrees of draft is required for light texture
·
2 degrees of draft works very well in most situations
·
3 degrees of draft is a minimum for a shutoff (metal sliding on
metal)
·
3 degrees of draft is required for medium texture
When the hot melt flows into the injection mold, the thick
sections don’t cool as fast as the rest of the part because the thicker
material becomes insulated by the outside surface of faster cooling plastic. As
the inner core cools, it shrinks at a different rate than the already cooled
outer skin. This difference on cooling rates causes the thick section to draw
inward and create a sink mark on the outside surface of the part, or worse,
completely warp the part. In addition to being unattractive, the mark also
represents added stress that is built into the part. Other less conspicuous
areas where sink occurs include ribs, bosses and corners. These are often
overlooked because neither the feature nor the part itself is too thick;
however, the intersection of the two can be a problem.
One way to avoid sink marks is to core out the solid sections of
the part to reduce thick areas. If the strength of a solid part is required,
try using cross hatched rib patterns inside the cored out area to increase
strength and avoid sink. As a rule-of-thumb, make sure that all bosses and
locating/support ribs are no more than 60% of the thickness of the nominal
wall. Also, textures can be used to hide minor sink marks.
Texturing is a process used to apply patterns to a mold surface.
This process allows flexibility in creating the final appearance of your parts.
Texturing is an integral piece in overall product development and should be
considered during the design process to achieve the desired results. Texture
can be a functional component of design as well. Imperfect parts can be
camouflaged by the right texture. Is the part designed for frequent handling?
Texture can be used to hide finger prints and improve the grip for the end
user. Texture can also be used to reduce part wear from friction.
A wide variety of textures are available for injection molded
parts such as:
·
Natural/Exotic
·
Matte Finishes
·
Multi-Gloss Patterns
·
Fusions
·
Graphics
·
Leather Grains/Hides
·
Woodgrain, Slate & Cobblestone
·
Geometric & Linens
·
Layered Textures to Create New Looks
·
Images or Logos Incorporated into the Pattern
When applying a texture to a part, the CAD drawing must be
adjusted to accommodate for this surface variance. If the texture is on a
surface that is perpendicular or angled away from the mold opening then no
draft changes are necessary. If the texture is on a parallel surface with the
mold opening, however, increased draft is necessary to prevent scraping and
drag marks that could occur during part ejection. Different textures have
different impacts on the molded part. The rule-of-thumb when designing for
texture is to have 1.5 degrees of draft for each 0.001” of texture finish
depth.
A “parting line” is the line of separation on the part where the
two halves of the mold meet. The line actually indicates the parting “plane”
that passes through the part. While on simple parts this plane can be a simple,
flat surface, it is often a complex form that traces the perimeter of the part
around the various features that make up the part’s outer “silhouette”. Part
lines can also occur where any two pieces of a mold meet. This can include side
action pins, tool inserts and shutoffs. Parting lines cannot be avoided; every
part has them. Keep in mind when designing your part, that the melt will always
flow towards the parting line because it is the easiest place for the displaced
air to escape or “vent”.
Injection molding is a complex technology with possible production
problems. They can either be caused by defects in the molds or more often by
part processing (molding)
Molding Defects
|
Alternative Name
|
Descriptions
|
Causes
|
Blister
|
Blistering
|
Raised or layered zone on surface of the Plastic part
|
Tool or material is too hot, often caused by a lack of cooling
around the tool or a faulty heater
|
Burn marks
|
Air Burn/Gas Burn
|
Black or brown burnt areas on the plastic part located at
furthest points from gate
|
Tool lacks venting, injection speed is too high
|
Color streaks (US)
|
|
Localized change of color
|
Plastic material and colorant isn't mixing properly, or the
material has run out and it's starting to come through as natural only
|
Delamination
|
|
Thin mica like layers formed in part wall
|
Contamination of the material e.g. PP mixed with ABS, very
dangerous if the part is being used for a safety critical application as the
material has very little strength when delaminated as the materials cannot
bond
|
Flash
|
Burrs
|
Excess material in thin layer exceeding normal part geometry
|
Tool damage, too much injection speed/material injected,
clamping force too low. Can also be caused by dirt and contaminants around
tooling surfaces.
|
Embedded contaminates
|
Embedded particulates
|
Foreign particle (burnt material or other) embedded in the part
|
Particles on the tool surface, contaminated material or foreign
debris in the barrel, or too much shear heat burning the material prior to
injection
|
Flow marks
|
Flow lines
|
Directionally "off tone" wavy lines or patterns
|
Injection speeds too slow (the plastic has cooled down too much
during injection, injection speeds must be set as fast as you can get away
with at all times)
|
Jetting
|
|
Deformed part by turbulent flow of material
|
Poor tool design, gate position or runner. Injection speed set
too high.
|
Polymer degradation
|
|
polymer breakdown from oxidation, etc.
|
Excess water in the granules, excessive temperatures in barrel
|
Sink marks
|
|
Localized depression
(In thicker zones) |
Holding time/pressure too low, cooling time too short, with
sprueless hot runners this can also be caused by the gate temperature being
set too high
|
Short shot
|
Non-Fill/Short Mold
|
Partial part
|
Lack of material, injection speed or pressure too low
|
Splay marks
|
Splash Mark/Silver Streaks
|
Circular pattern around gate caused by hot gas
|
Moisture in the material, usually when resins are dried
improperly
|
Stringiness
|
Stringing
|
String like remain from previous shot transfer in new shot
|
Nozzle temperature too high. Gate hasn't frozen off
|
Voids
|
|
Empty space within part
(Air pocket) |
Lack of holding pressure (holding pressure is used to pack out
the part during the holding time). Also mold may be out of registration (when
the two halves don't center properly and part walls are not the same
thickness).
|
Weld line
|
Knit Line/Meld Line
|
Discolored line where two flow fronts meet
|
Mold/material temperatures set too low (the material is cold
when they meet, so they don't bond)
|
Warping
|
Twisting Part
|
Distorted part
|
Cooling is too short, material is too hot, lack of cooling
around the tool, incorrect water temperatures (the parts bow inwards towards
the hot side of the tool)
|
Keep these factors in mind when designing your injection molded
part, and remember that it is easier to avoid problems in the beginning than
change your design down the line.
hi
ReplyDeleteNice article, thanks for sharing this informative article with us. Injection Molding Company
ReplyDelete
ReplyDeleteJohn and his brother Isaiah patented this process of manufacturing celluloid in 1870 and continued by making dentures from their new material which replaced dentures made from rubber. Thus began the manufacturing process of celluloid plastics. John was quite just like the Leonardo of commercial invention because he also was credited with the invention of the stitching machine and roller bearings all of which contributed heavily to manufacturing.
China Custom Plastic Injection Molding Maker