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Processes:
Injection Molding (IJ)
Injection molding
(IJ) (British:moulding) is a high
volume, high speed production manufacturing
technique derived from casting for making parts from metals and both thermoplastic and
thermosetting plastic materials using a core and cavity or clam shell
molding process.
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IJ Molding Process
In injection molding the raw materials is heated to a molten flow or plasticized
state, using a fluid screw barrel
device to inject the material into the core and cavity mold, which is the inverse of the product's shape.
After a product is designed, usually in 3D CAD, molds are then designed
and developed by an experienced Moldmaker or Toolmaker. Because of the
high clamping forces to hold the mold together during mold material
injection pressures, IJ molds are typically constructed from metal, usually either steel or
aluminum and CNC precision-machined and surface textured to form the features of the desired part.
Injection molding is used for manufacturing a multitude of parts, products and market
applications from engine blocks (See
Die casting) to micro gears to large refuse bins,
from aluminum or zinc to most commodity and engineering thermoplastics.
In general, the core and cavity mold one half is fixed with the other
half moving. When closed the tool half's are clamped together,
under extremely high pressure to seal the tool cavity, allowing for the
material filling or injection mold process. This open and close clamp
and fill, cool, open and eject the part is called the molding cycle.(
See below)
High Volume
Injection molding
is best applied to high volume parts, based on the high tooling
investment costs. The melting points of metals are much higher than those
of thermoplastics; leading to substantially high mold cost due to heat
treatment and shorter mold longevity, despite
the use of specialized steels. The larger the production quantity the
better the fit for injection molding. Investment costs compare quite
favorably to
most other molding process including: reaction injection molding, blow
molding, thermoforming, particularly for smaller parts and based on the
shorter mold cycle times.
Plastics
Molding
The most commonly used
injection molded thermoplastic materials are polystyrene (low cost, lacking the strength
and longevity of other materials), ABS or
acrylonitrile butadiene styrene (a co-polymer or mixture of
compounds used for everything from Lego parts to electronics housings),
nylon (chemically resistant, heat resistant, tough and flexible -
used for combs),
polypropylene (tough and flexible - used for containers),
polyethylene, and
polyvinyl chloride or PVC (more common in
extrusions as used for pipes, window frames, or as the insulation on
wiring where it is rendered flexible by the inclusion of a high
proportion of
plasticiser).
Molding Equipment
Injection molding machines, also known as mold presses,
hold the molds or tools in which the components are shaped. Presses are
rated by tonnage, which expresses the amount of clamping force that the
machine can generate. This pressure keeps the mold closed during the
injection process. Tonnage can vary from less than 5 tons to 6000 tons,
with the higher figures used in comparatively very large part
manufacturing operations such as a 55 gallon garbage can or automotive
car hood. The general rule is the larger the part, the higher the
tooling costs, the more raw materials and higher machine operator costs.
Example; CNC machining- drilling out a hole in a tool
Mold – Tooling
Mold or Mold Tool or
Tooling are common terms used to describe the production tooling used to
produce plastic parts in injection molding. Traditionally, molds are
very expensive to manufacture. Typically only justified for mass
production where thousands of parts. Molds are typically constructed
from aluminum, hardened steel, pre-hardened steel, and/or
beryllium-copper alloy. The choice of material to build a mold is
primarily one of economics. Steel molds generally cost more to
construct, but their longer lifespan will offset the higher initial cost
over a higher number of parts made before wearing out. Pre-hardened
steel molds are less wear resistant and are used for lower volume
requirements or larger components. The steel hardness is typically 38-45
on the Rockwell-C scale. Hardened steel molds are heat treated after
machining. These are by far the superior in terms of wear resistance and
lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).
Example: Aluminum injection mold with cooling
lines
Molds
Aluminum molds can cost substantially less, and when designed and machined with modern
computerized equipment and can be economical for molding tens or even
hundreds of thousands of parts. Beryllium copper is used in areas of the
mold which require fast heat removal or areas that see the most shear
heat generated. High performance alloys such as MoldMax® and
Ampcoloy® have also been developed especially for optimum
heat transfer. Such alloys are considered in mold construction when
conventional heat removal methods are unsuitable or when cycle time is a
critical consideration.
Part - Mold
Design
Molds separate into at
least two halves (core and
cavity) to allow the part to be extracted from the molds.
The core and cavity, along with internal cooling
lines and hoses, sprues, filler gates, vents, and ejection pins form the mold tool
or tooling. Large
tools are very heavy weighting up to 60 tons require a fork lift or
hydraulic or electric crane to hoisted and secure the into molding
machine. For production the tool needs to be removed when molding is complete or the tool
needs repairing or maintenance. A mold can produce
several copies of the same parts in a single "shot". The number of
"impressions" in the mold of that part is referred to as
cavitations. A
tool with one impression will often be called a single cavity
(impression) tool. A mold with 2 or more cavities of the same parts will
likely be referred to as multiple cavity tool. Some extremely high
production volume molds (example: bottle caps or syringes) can have over
128 cavities. In some cases multiple cavity tooling will mold a series
of different parts in the same tool called a family molds.
Mold Draft
In general, the shape of a part must have slightly angled walls or
draft. For example, the vertical sides of objects typically cannot be
parallel with the direction of draw (the direction in which the core and
cavity separate from each other).
Shrinkage
Heated polymer plastics expand up and shrink in
size when it cools. This is an important consideration for 2 reasons.
One is the material part mold must be proportionally sized slightly
larger than the original part in order to allow for the part to cool or
shrink to the final intended dimensions. This
shrinkage rate is typically calculated into the mold design in the 3D
CAD modeling process. The second issue is ejecting the part from the
mold. Parts that are "bucket-like" tend to shrink onto the core while cooling,
locking them or making them difficult to remove from the mold. Ejection
pins are the most popular method of removal, using a small diameter pin
to push and separate the part from the core mold. Other methods
include air ejection and stripper plates, depending on the application. Most ejection plates are found on the
moving half of the tool, but they can be placed on the fixed half.
Applying a 1 - 5 degree or more mold draft will allow the part to cool
and shrink and be ejected from the tool.
Complex Parts Molds
More complex parts are formed using more complex molds, which may have
movable sections called slides which are inserted into the mold to form
features that cannot be formed using only a core and a cavity. Slides
are then withdrawn to allow the part to be released.
Mold
Making
Molds are built through
three main methods: standard machining, EDM machining and 3D CAM - CNC
machining out of aluminum, steel, bronze and related alloys. Standard
manual machining has traditionally been the method of
building injection mold tools. With technological development, electrical discharge machining and
(EDM), and CAM - CNC machining have became the primary method of making very complex molds, with
extremely accurate mold details, in a very rapid time.
Typically once machined a mold must be heat treated to add the required
hardness and toughness (ductility to withstand the high pressure forces
and maintain a long tool life.
(See also
Manufacturing Engineering Metal
Working)
EDM
Electrical Discharge Machining
Electrical discharge machining
(EDM)or spark erosion process is also widely used in mold
making. EDM is a simple process in which a shaped designed electrode, usually made of copper or
graphite, is very slowly lowered with pressure to burn or erode onto the mold surface (many hours), while immersed in paraffin oil. A voltage applied
between the shaped electrode tool and metallic mold causes erosion of the mold surface in the inverse
shape of the burning tool. This process allows for pre-hardened
molds to be shaped or tooled so that no post heat treatment is required.
Changes to a hardened mold by conventional drilling and milling normally
require annealing to soften the steel, followed by heat treatment to
harden it again.
Injection Mold Tooling
Materials
Molds are typically constructed from aluminum , hardened steel,
pre-hardened steel, and/or beryllium-copper alloy.
Costs
The cost of manufacturing injection molds and parts depends on a very large set of factors ranging from
number of cavities, size of the parts (and therefore the mold),
size and complexity of the pieces, expected tool longevity, surface finishes and
many others. Tooling costs are based on the size of the blocks of steel or aluminum
and machining time it will take to build the core and cavity molds.
Piece price is typically based on the raw material part weight x machine
mold cycle time. Considerable thought and
experience is put into the design of molded parts and their tooling molds, to
ensure that the parts will be moldable. Molds must be designed to insure that t does not
get trapped in the mold, compensate for material cooling shrinkage
with adequate water or oil cooling lines for part temperature controls,
that the molds can be completely filled before the molten resin
solidifies and to minimize imperfections in the parts.
Resins (See
Polymers & Plastics Materials
Selection)
The resin, or raw material for injection molding, is usually in pellet
or granule form polymer specially formulated for molding. The resin
pellets are melted by heat and
mechanical screw flow, shearing forces. Resin pellets are
gravity poured into the hopper, a large funnel bottomed container, which feeds the granules
into the molding machine screw.
The Melt - Mold flow process
The depth of the screw
thread depth flights decreases or spirals towards
the end nearest the mold, compressing the heated plastic. As the screw rotates
by motor moving the pellets forward in the screw, undergoing extreme pressure and friction
and heat in the screw grooves,
reaching a melt
temperature before being injected
through a sprue and runner gates to fill the mold cavity.
Heaters on either side of the screw assist in the heating and
temperature control during the melting process.
The channels through which the plastic flows toward the chamber
will also solidify, forming an attached frame. This molded frame
is composed of the sprue, which is the main channel from the reservoir
of molten resin, parallel with the direction of draw, and runners, which
are perpendicular to the direction of draw, and are used to convey
molten resin to the gate(s), or point(s) of injection. The sprue and
runner system can be cut or twisted off and recycled. Some molds are designed so that the
part is automatically stripped through the mold cycle action. This
material melt mold flow process provided a continuous flow of molten
material which is started and stopped in the close, fill, solidify, open
and eject part process or molding cycle.
Injection Molding
Cycle
The basic injection
mold cycle is as follows: The core mold is typically held stationary and
the cavity mold which moves to open and close and clamp the mold
together. The molds are closed under high clamping pressures and
the heated plastic is forced by the pressure of the injection screw to
take the shape of the mold.
The mold clamping forces are dependent on the size
of the part mold (weight/size) and melt filling pressures. The
mold closes - the injection carriage moves forward - injected plastic is
metered - the carriage retracts - the mold opens - the part(s) is
ejected. Water or oil cooling channels within the mold
control the mold temperature and assist in cooling the mold until the
molten plastic solidifies into the part. Improper cooling
can result in distorted or warped parts or burning. The cycle is
completed when the mold opens and the part is ejected with the
assistance of ejector pins within the mold. The ejected part is usually
manually off loaded into a staging area.
The additional of robotic off loading, stacking, boxing and pallet
loading.
Molding Trial
Setting up an injection
mold machine when filling a new or unfamiliar mold for the first time,
where shot size for that mold is unknown is conducted by experience
machine operators and plastics engineers. Once the parts are good
enough and have passed any specific criteria, a specification- setting
sheet is produced for the production run. Process optimization is done
using the following methods. Injection speeds are usually determined by
performing viscosity curves. Process windows are performed varying the
melt temperatures and holding pressures. Pressure drop studies are done
to check if the machine has enough pressure to move the screw at the set
rate. Gate seal or gate freeze studies are done to optimize the holding
time. A cooling time study is done to optimize the cooling time.
2 Shot or Coinjection Mold
Some molds allow previously molded parts to be reinserted to allow a new
plastic layer to form around the first part. This is often referred to
as overmolding. This system can allow for production of one piece tires
and wheels. 2-shot or multi shot molds are designed to "overmold" within
a single molding cycle and must be processed on specialized injection
molding machines with two or more injection units. This can be achieved
by having pairs of identical cores and pairs of different cavities
within the mold. After injection of the first material, the component is
rotated on the core from the one cavity to another. The second cavity
differs from the first in that the detail for the second material is
included. The second material is then injected into the additional
cavity detail before the completed part is ejected from the mold. Common
applications include "soft-grip" toothbrushes and freelander grab
handles.
Common applications include "soft-grip" toothbrushes and freelander grab
handles.
Cleanroom Molding
Cleanroom molding is required when FDA and hygienic quality control
standards are applied. Manufacturing
Engineering & Processes (ManufE)
ManufE Key Vocabulary
Manufacturing – Production Processes
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