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Transcript of advances in injection molding
T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
1. Introduction
Injection molding is the most commonly used manufacturing process for the fabrication
of plastic parts. A wide variety of products are manufactured using injection molding, which
vary greatly in their size, complexity, and application. The injection molding process requires the
use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in
the injection molding machine and then injected into the mold, where it cools and solidifies into
the final part.
Fig. 1.1 Injection molding overview
Injection molding is used to produce thin-walled plastic parts for a wide variety of
applications, one of the most common being plastic housings. Plastic housing is a thin-walled
enclosure, often requiring many ribs and bosses on the interior. These housings are used in a
variety of products including household appliances, consumer electronics, power tools, and as
automotive dashboards. Other common thin-walled products include different types of open
containers, such as buckets. Injection molding is also used to produce several everyday items
such as toothbrushes or small plastic toys. Many medical devices, including valves and syringes,
are manufactured using injection molding as well.
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
2. History and Development
The first man-made plastic was invented in Britain in 1851 by Alexander Parkes. He
publicly demonstrated it at the 1862 International Exhibition in London; calling the material he
produced "Parkesine." Derived from cellulose, Parkesine could be heated, molded, and retain its
shape when cooled. It was, however, expensive to produce, prone to cracking, and highly
flammable.
In 1868, American inventor John Wesley Hyatt developed a plastic material he named
Celluloid, improving on Parkes' invention so that it could be processed into finished form.
Together with his brother Isaiah, Hyatt patented the first injection molding machine in 1872.
This machine was relatively simple compared to machines in use today. It worked like a large
hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The
industry progressed slowly over the years, producing products such as collar stays, buttons, and
hair combs.
The industry expanded rapidly in the 1940s because World War II created a huge demand
for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry
built the first screw injection machine, which allowed much more precise control over the speed
of injection and the quality of articles produced. This machine also allowed material to be mixed
before injection, so that colored or recycled plastic could be added to virgin material and mixed
thoroughly before being injected. Today screw injection machines account for the vast majority
of all injection machines. In the 1970s, Hendry went on to develop the first gas-assisted injection
molding process, which permitted the production of complex, hollow articles that cooled
quickly. This greatly improved design flexibility as well as the strength and finish of
manufactured parts while reducing production time, cost, weight and waste. The plastic injection
molding industry has evolved over the years from producing combs and buttons to producing a
vast array of products for many industries including automotive, medical, aerospace, consumer
products, toys, plumbing, packaging, and construction.
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
3. Process Cycle
The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes,
and consists of the following four stages:
1. Clamping - Prior to the injection of the material into the mold, the two halves of the mold
must first be securely closed by the clamping unit. Each half of the mold is attached to the
injection molding machine and one half is allowed to slide. The hydraulically powered clamping
unit pushes the mold halves together and exerts sufficient force to keep the mold securely closed
while the material is injected. The time required to close and clamp the mold is dependent upon
the machine - larger machines (those with greater clamping forces) will require more time. This
time can be estimated from the dry cycle time of the machine.
2. Injection - The raw plastic material, usually in the form of pellets, is fed into the injection
molding machine, and advanced towards the mold by the injection unit. During this process, the
material is melted by heat and pressure. The molten plastic is then injected into the mold very
quickly and the buildup of pressure packs and holds the material. The amount of material that is
injected is referred to as the shot. The injection time is difficult to calculate accurately due to the
complex and changing flow of the molten plastic into the mold. However, the injection time can
be estimated by the shot volume, injection pressure, and injection power.
3. Cooling - The molten plastic that is inside the mold begins to cool as soon as it makes contact
with the interior mold surfaces. As the plastic cools, it will solidify into the shape of the desired
part. However, during cooling some shrinkage of the part may occur. The packing of material in
the injection stage allows additional material to flow into the mold and reduce the amount of
visible shrinkage. The mold cannot be opened until the required cooling time has elapsed. The
cooling time can be estimated from several thermodynamic properties of the plastic and the
maximum wall thickness of the part.
4. Ejection - After sufficient time has passed, the cooled part may be ejected from the mold by
the ejection system, which is attached to the rear half of the mold. When the mold is opened, a
mechanism is used to push the part out of the mold. Force must be applied to eject the part
because during cooling the part shrinks and adheres to the mold. In order to facilitate the ejection
of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
injection of the material. The time that is required to open the mold and eject the part can be
estimated from the dry cycle time of the machine and should include time for the part to fall free
of the mold. Once the part is ejected, the mold can be clamped shut for the next shot to be
injected.
Fig.3.1 Injection molded part.
After the injection molding cycle, some post processing is typically required. During cooling, the
material in the channels of the mold will solidify attached to the part. This excess material, along
with any flash that has occurred, must be trimmed from the part, typically by using cutters. For
some types of material, such as thermoplastics, the scrap material that results from this trimming
can be recycled by being placed into a plastic grinder, also called regrind machines or
granulators, which regrinds the scrap material into pellets. Due to some degradation of the
material properties, the regrind must be mixed with raw material in the proper regrind ratio to be
reused in the injection molding process.
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
4. Advanced Injection Molding Processes
Following are the advance injection molding processes used in industries
Reaction injection molding
Gas-assisted injection molding
Hobby injection molding
Fusible core injection molding
Matrix molding
Micro molding
Water Injection Technology
Liquid Silicone Rubber/Liquid Injection Molding
4.1 Gas-Assisted Injection Molding (GAIM)
Recently, an innovative injection molding process, called gas-assisted injection molding
(GAIM) was developed for producing parts with hollow shapes. The original idea of GAIM
came from the “injection blowing” method, which is widely used, particularly for the fabrication
of bottles and other relatively small hollow bodies. The use of pressurized gas for a conventional
plastic injection molding process is believed to have been first made commercially available by
the invention of Friederich.This solved the problem of molding hollow shape bodies in a single
injection molding operation.These parts are light in weight and have acceptable surface finish,
i.e., without sink marks that are associated with conventional plastic injection molding. In recent
years, attention has been concentrated on the use of gas assistance with conventional plastic
injection molding to achieve high product quality and productivity. Good surface quality, short
cycle times, lower clamp tonnage, material saving, weight reduction and minimization of part
distortion or warpage can all be achieved with proper utilization of gas assistance into a
conventional plastic injection molding process. There are two methods in conventional GAIM.
The one is “short shot”. The short shot is sequentially done by following a simple three-step
process. In the short shot processing, a molten polymer is initially filled in cavity about 75–98%
by ram speed control of the injection molding machine. After a short delay period, compressed
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
nitrogen gascores out the molten polymer, filling the remainder of the mold. The next step is the
gas packing stage that compensates for the volumetric shrinkage of the polymer melt. As the
plastic solidifies, the gas expands into volume created by shrinkage, locally packing out the part.
The short shot method is used for thick section moldings, typically handles and tubular
components. The advantage of the short shot is reduction in molded plastic weights. However,
surface defects such as hesitation mark [2] may be visible when the gas is injected too late or the
initial gas pressure is too low.
Fig. 4.1 Schematic of the RGIM system.
The other is “full shot”. The full shot is injected to fill or nearly fill the mold cavity, but the
plastic is not packed by an injection molding machine. After a selected time delay, first phase gas
is injected. Second phase gas penetration occurs to compensate for volumetric shrinkage of the
plastic as it cools. A uniform gas pressure is applied throughout the plastic. Gas is exhausted to
atmosphere or for recovery before mold opens. Plastic refill commences after the nozzle valve is
closed or after the plastic feed gate has solidified. The ‘full shot’ method is normally applicable
for components in which there are thick and thin sections. The gas flows into the path of least
resistance in the thicker sections where the plastic interior is still in a molten state [2]. The
pushed melt needs to expel from the cavity to another place. The place is called overflow and
wholly wastes material.
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
Advantages
Material savings (weight, cost) for thick-walled parts up to 40%.The combined benefits
of not packing a molding are less material is used. By not having to pack the material,
and in thicker components the resultant hollow core, can save as much as up to 40% on
the * Reduced Cycle times by 50% or more when compared to standard injection
molding of thick-walled parts Another major benefit is the reduction in machine cycle
times that can be achieved. With no molten core to solidify, the material in the mold
cavity solidifies quicker thus enabling the component to be ejected sooner.
Smooth surface in comparison with structural foam. External gas injection provides an
enhanced surface definition of the component.
Lower clamp forces
Improved holding pressure effect
High flexural stiffness and torsional rigidity
Low internal stress level and low warpage for thick and thin wall combinations (uniform
shrinkage and pressure)
Reduction of sink marks
Design freedom
Fewer weld lines due to fewer injection points
Longer flow lengths or lower number of injection points required for large thin-walled
molded parts because gas channels act as flow leaders[4]
Disadvantages
Special care must be taken in designing parts. High cost of tooling and mold flow analysis [4].
Applications
Most plastic injection molded components can benefit from the use of gas assisted
molding. Applications from consumer goods to automotive parts benefit from the process. The
typical are: Toys, auto parts & anything with thick areas. [4]
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
External Gas Assisted Molding Applications:
Flat panels for office equipment.
Computer enclosures.
Furniture, i.e. tabletops.
Automotive panels.
Domestic appliances - e.g. fridges.
4.2 Fusible Core Injection Molding
Fusible core injection molding, also known as lost core injection molding, is a specialized
plastic injectionmolding process used to mold internal cavities or undercuts that are not possible
to mold with demoldable cores.Strictly speaking the term "fusible core injection molding refers
to the use of a fusible alloy as the core material;when the core material is made from a soluble
plastic the process is known as soluble core injection molding. This process is often used for
automotive parts, such as intake manifolds and brake housings, however it is alsoused for
aerospace parts, plumbing parts, bicycle wheels, and footwear. The most common molding
materials are glass-filled nylon 6 and nylon 66. Other materials include unfilled
nylons,polyphenylene sulfide, glass-filled polyaryletherketone (PAEK), glass-filled
polypropylene (PP), rigid thermoplasticurethane, and elastomeric thermoplastic polyurethane[2].
The process consists of three major steps: casting or molding a core, inserting the core
into the mold and shootingthe mold, and finally removing the molding and melting out the core.
First, a core is molded or die cast in the shape of the cavity specified for the molded component.
It can be madefrom a low melting point metal, such as a tin-bismuth alloy, or a polymer, such as
a soluble acrylate. The polymerhas approximately the same melting temperature as the alloy, 275
°F (135 °C), however the alloy ratios can bemodified to alter the melting point. Another
advantage to using a metal core is that multiple smaller cores can be castwith mating plugs and
holes so they can be assembled into a final large core.One key in casting metal cores is to make
sure they do not contain any porosity as it will induce flaws into themolded part. In order to
minimize porosity the metal may be gravity cast or the molding cavity may be
pressurized.Another system slowly rocks the casting dies as the molding cavity fills to "shake"
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
the air bubbles out. The metal cores can be made from a number of low melting point alloys,
with the most common being a mixture of58% bismuth and 42% tin, which is used for molding
nylon 66. One of the main reasons it’s used is because itexpands as it cools which packs the
mold well. Other alloys include tin-lead-silver alloys and tin-lead-antimonyalloys. Between these
three alloy groups a melting point between 98 and 800 °F (37–425 °C) can be achieved. Polymer
cores are not as common as metal cores and are usually only used for moldings that require
simple internalsurface details. They are usually 0.125 to 0.25 in (3.2 to 6.4 mm) thick hollow
cross-sections that are molded intwo halves and are ultrasonically welded together. Their greatest
advantage is that they can be molded in traditionalinjection molding machines that the company
already has instead of investing into new die casting equipment andlearning how to use it.
Because of this polymer core materials are most adventitious for small production runs
thatcannot justify the added expense of metal cores. Unfortunately it is not as recyclable as the
metal alloys used incores, because 10% new material must be added with the recycled
material[2].
Molding
In the second step, the core is then inserted into the mold. For simple molds this is as
simple as inserting the coreand closing the dies. However, more complex tools require multiple
steps from the programmed robot. Forinstance, some complex tools can have multiple
conventional side pulls that mate with the core to add rigidity to thecore and reduce the core
mass. After the core is loaded and the press closed the plastic is shot.
Melt-out
In the final step, the molded component and core are both demolded and the core is
melted-out from the molding.This is done in a hot bath, via induction heating, or through a
combination of the two. Hot baths usually use a tubfilled with glycol or Lutron, which is a
phenol-based liquid. The bath temperature is slightly higher than that of thecore alloy’s melting
point, but not so high that it damages the molding. In typical commercial applications the
partsare dipped into the hot bath via an overhead conveyor. The advantage to using a hot bath is
that it is simpler thaninduction heating and it helps cure thermoset moldings. The disadvantage is
that it is uneconomically slow at a cycletime of 60 to 90 minutes and it poses environmental
cleanup issues. Typically the hot bath solution needs cleaning orreplacement every year or every
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
half year when used in combination with induction heating. For thermoplastic moldings
induction heating of the core metal is required, otherwise the prolonged heat from a hotbath can
warp it. Induction heating reduces the melt-out time to one to three minutes. The disadvantage is
thatinduction heating does not remove all of the core material so it must then be finished off in a
hot bath or be brushedout. Another disadvantage is that the induction coils must be custom built
for each molding because the coils mustbe 1 to 4 in (25 to 100 mm) from the part. Finally,
induction heating systems cannot be used with moldings thathave brass or steel inserts because
the induction heating process can destroy or oxidize the insert. For complex parts it can be
difficult to get all of the core liquid to drain out in either melt-out process. In order toovercome
this the parts may be rotated for up to an hour. Liquid core metal collects on the bottom of the
heatedovercome this the parts may be rotated for up to an hour. Liquid core metal collects on the
bottom of the heatedbath and is usable for a new core.
Equipment
Traditional horizontal injection molding machines have been used since the mid-1980s,
however loading andunloading 100 to 200 lb. (45 to 91 kg) cores are difficult so two robots are
required. Moreover, the cycle time isquite long, approximately 28 seconds. These problem are
overcome by using rotary or shuttle action injectionmolding machines. These types of machines
only require one robot to load and unload cores and have a 30%shorter cycle time. However,
these types of machines cost approximately 35% more than horizontal machines,require more
space, and require two bottom molds (because one is in the machine during the cycle and the
other isbeing unloaded and loaded with a new core), which adds approximately 40% to the
tooling cost. For small parts,horizontal injection molding machines are still used, because the
core does not weigh enough to justify the use of arotary machine. For four-cylinder manifolds a
500-ton press is required; for a six- to eight-cylinder manifold a 600- to 800-tonpress is
required[1].
Advantages and disadvantages
The greatest advantage of this process is its ability to produce single-piece injection
moldings with highly complex interior geometries without secondary operations.
Similarly shaped objects are usually made from aluminum castings, which can weigh
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
45% to 75% more than a comparable molding. The tooling also lasts longer than metal
casting tooling due to the lack of chemical corrosion and wear. Other advantages include:
Very good surface quality with no weak areas due to joints or welds
High dimensional accuracy and structural integrity
Not labor intensive due to the few secondary operations required
Little waste
Inserts can be incorporated
Two of the major disadvantages of this process are the high cost and long development
time.
Another disadvantage is the need for a large space to house the injection molding
machines, casting machines, melt-out equipment, and robots
Application
The application of the fusible core process is not limited just to the injection of thermoplastics,
but withcorresponding core alloys also to thermosetting plastic molding materials (duroplast).
The fusible core process findsapplication, for example, for injection molded passenger car engine
intake manifolds. By modifying the equipment,small molded parts like valves or pump housings
can be manufactured, as the manufacture of the fusible cores andthe injected parts can be carried
out on an injection molding machine. [3]
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
5.Materials
There are many types of materials that may be used in the injection molding process.
Most polymers may be used, including all thermoplastics, some thermosets, and some
elastomers. When these materials are used in the injection molding process, their raw form is
usually small pellets or a fine powder. Also, colorants may be added in the process to control the
color of the final part. The selection of a material for creating injection molded parts is not solely
based upon the desired characteristics of the final part. While each material has different
properties that will affect the strength and function of the final part, these properties also dictate
the parameters used in processing these materials. Each material requires a different set of
processing parameters in the injection molding process, including the injection temperature,
injection pressure, mold temperature, ejection temperature, and cycle time. A comparison of
some commonly used materials is shown below[5]
Material name Trade names Description Applications
Acetal Celcon, Delrin,
Hostaform,
Lucel
Strong, rigid, excellent
fatigue resistance, excellent
creep resistance, chemical
resistance,moisture
resistance, naturally opaque
white, low/medium cost
Bearings, cams, gears,
handles, plumbing
components, rollers,
rotors, slide guides,
valves
Acrylic Diakon,
Oroglas,
Lucite,
Plexiglas
Rigid, brittle, scratch
resistant, transparent,
optical clarity, low/medium
cost
Display stands, knobs,
lenses, light housings,
panels, reflectors, signs,
shelves, trays
Cellulose Acetate Dexel,
Cellidor,
Setilithe
Tough, transparent, high
cost
Handles, eyeglass
frames
Polyamide 6 (Nylon) Akulon,
Ultramid,
Grilon
High strength, fatigue
resistance, chemical
resistance, low creep, low
Bearings, bushings,
gears, rollers, wheels
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
friction, almost
opaque/white, medium/high
cost
Polycarbonate Calibre,
Lexan,
Makrolon
Very tough, temperature
resistance, dimensional
stability, transparent, high
cost
Automotive (panels,
lenses, consoles),
bottles, containers,
housings, light covers,
reflectors, safety
helmets and shields
Polyether Sulphone Victrex, Udel Tough, very high chemical
resistance, clear, very high
cost
Valves
Polyethylene - Low
Density
Alkathene,
Escorene,
Novex
Lightweight, tough and
flexible, excellent chemical
resistance, natural waxy
appearance, low cost
Kitchenware, housings,
covers, and containers
Polyethylene - High
Density
Eraclene,
Hostalen,
Stamylan
Tough and stiff, excellent
chemical resistance, natural
waxy appearance, low cost
Chair seats, housings,
covers, and containers
Polystyrene -
General purpose
Lacqrene,
Styron,
Solarene
Brittle, transparent, low cost Cosmetics packaging,
pens
Thermoplastic
Elastomer/Rubber
Hytrel,
Santoprene,
Sarlink
Tough, flexible, high cost Bushings, electrical
components, seals,
washers
Table 1: Materials.
6. Conclusion
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
The GAIM and fusible core injection molding were devised to solve the problems of
manufacturing hollow parts and to improve surface quality on a molding produced in the
conventional injection molding.This processes have wide applications in field of automobile
industries and also in aerospace industries. The above process finds application, for example in
injection molded passenger car engine intake manifolds. By modifying the equipment,small
molded parts like valves or pump housings can also be manufactured.
7.References
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T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING
1. Seong-Yeol Hana, Jin-Kwan Kwag b, Cheol-Ju Kimb, Tae-Won Park , Yeong-Deug
Jeong “A new process of gas-assisted injection molding for faster cooling” ELSEVIER
Journal of Materials Processing Technology (2004) page no.155–156,1201–1206.
2. P.K. Bharti “Recent Methods For Optimization Of Plastic Injection Molding Process –A
Retrospective And Literature Review” et. al. / International Journal of Engineering
Science and Technology Vol. 2(9), 2010, page no. 4540-4554.
3. J. Avery “Gas-Assist Injection Molding: Principles and Applications” Hanser
Gardner Publication Inc., Cincinnati, 2001.
4. “Injection Molding” Engineered Materials Handbook Desk Edition, 2005 Michelle M.
Gauthier, Editor, page no. 299-307.
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