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Synthetic and Cellulosic Fiber Formation Technology

How it's made Most synthetic and cellulosic manufactured fibers are created by extrusion forcing a thick, viscous liquid (about the
consistency of cold honey) through the tiny holes of a device called a spinneret to form continuous filaments of semi-solid polymer.

In their initial state, the fiber-forming polymers are solids and therefore must be first converted into a fluid state for extrusion. This is usually
achieved by melting, if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated), or by dissolving them in a suitable
solvent if they are non-thermoplastic cellulosics. If they cannot be dissolved or melted directly, they must be chemically treated to form
soluble or thermoplastic derivatives. Recent technologies have been developed for some specialty fibers made of polymers that do not melt,
dissolve, or form appropriate derivatives. For these materials, the small fluid molecules are mixed and reacted to form the otherwise
intractable polymers during the extrusion process.

SpinneretsThe Spinneret
The spinnerets used in the production of most manufactured fibers are similar, in principle, to a bathroom shower head. A spinneret may
have from one to several hundred holes. The tiny openings are very sensitive to impurities and corrosion. The liquid feeding them must be
carefully filtered (not an easy task with very viscous materials) and, in some cases, the spinneret must be made from very expensive,
corrosion-resistant metals. Maintenance is also critical, and spinnerets must be removed and cleaned on a regular basis to prevent clogging.

As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a rubbery state and then solidified. This
process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name,
where short pieces of staple fiber are twisted into yarn. There are four methods of spinning filaments of manufactured fibers: wet, dry, melt,
and gel spinning.

Wet Spin 1Wet Spinning

Wet spinning is the oldest process. It is used for fiber-forming substances that have been dissolved in a solvent. The spinnerets are
submerged in a chemical bath and as the filaments emerge they precipitate from solution and solidify.

Because the solution is extruded directly into the precipitating liquid, this process for making fibers is called wet spinning. Acrylic, rayon,
aramid, modacrylic and spandex can be produced by this process.

Dry Spin 1Dry Spinning

Dry spinning is also used for fiber-forming substances in solution. However, instead of precipitating the polymer by dilution or chemical
reaction, solidification is achieved by evaporating the solvent in a stream of air or inert gas.

The filaments do not come in contact with a precipitating liquid, eliminating the need for drying and easing solvent recovery. This process
may be used for the production of acetate, triacetate, acrylic, modacrylic, PBI, spandex, and vinyon.

Melt Spin 1Melt Spinning

In melt spinning, the fiber-forming substance is melted for extrusion through the spinneret and then directly solidified by cooling. Nylon,
olefin, polyester, saran and sulfur are produced in this manner.

Melt Spin 2

Melt spun fibers can be extruded from the spinneret in different cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others).
Trilobal-shaped fibers reflect more light and give an attractive sparkle to textiles.

Pentagonal-shaped and hollow fibers, when used in carpet, show less soil and dirt. Octagonal-shaped fibers offer glitter-free effects. Hollow
fibers trap air, creating insulation and provide loft characteristics equal to, or better than, down.

Detailed production flowcharts:

Acrylic
Nylon (Polyamide)
Polyester

Gel Spinning
Gel spinning is a special process used to obtain high strength or other special fiber properties. The polymer is not in a true liquid state during
extrusion. Not completely separated, as they would be in a true solution, the polymer chains are bound together at various points in liquid
crystal form. This produces strong inter-chain forces in the resulting filaments that can significantly increase the tensile strength of the fibers.
In addition, the liquid crystals are aligned along the fiber axis by the shear forces during extrusion. The filaments emerge with an unusually
high degree of orientation relative to each other, further enhancing strength. The process can also be described as dry-wet spinning, since the
filaments first pass through air and then are cooled further in a liquid bath. Some high-strength polyethylene and aramid fibers are produced
by gel spinning.

Stretching and Orientation

While extruded fibers are solidifying, or in some cases even after they have hardened, the filaments may be drawn to impart strength.
Drawing pulls the molecular chains together and orients them along the fiber axis, creating a considerably stronger yarn.
2.
Common fiber-forming polymers include cellulosics (linen, cotton, rayon, acetate), proteins (wool, silk), polyamides (PA 6 and PA 66),
polyester (PET), polyolefins, vinyl polymers, acrylics, PTFE, polyphenylene sulfide (PPS), aramids, and polyurethanes41. Each of these
materials is unique in chemical structure and potential properties. Polyurethanes are elastomeric materials with high elongation and elastic
recovery, and whose properties nearly match those of elastin tissue fibers. This material—when extruded into fiber, fibrillar, or fabric form—
derives its high elongation and elasticity from alternating patterns of crystalline hard units and non-crystalline soft units. The reactivity of
tissues in contact with fibrous structures varies among materials and is governed by both chemical and physical characteristics. Absorbable
materials typically excite greater tissue reaction, a result of the nature of the absorption process itself. Among the available materials, some
are absorbed faster (e.g., PGA, polyglactin acid) and others more slowly (e.g., polyglyconate). Semi-absorbable materials such as cotton and
silk generally cause less reaction, although the tissue response may continue for an extended time. Nonabsorbable materials (e.g., nylon,
polyester, PP) tend to be inert and to provoke the least reaction.
Polyesters. PET is one of the most widely used fibers in medical textiles. Applications include surgical sutures, arterial and stent grafts,
surgical gowns and drapes, and hospital bedding. The fiber is sometimes blended with cotton or rayon for wear, comfort, and breathability.
Polyamides. The two most commonly used polyamide fibers in medical textiles are polyamide 6 (nylon 6) and polyamide 66 (nylon 66).
Polyamide fibers have excellent mechanical strength. The polar polyamide links make the materials made from polyamides fairly hydrophilic
and wettable. Wholly aromatic polyamide (aramid) fibers have very high strength and excellent mechanical properties. They are used for
high strength applications like orthopedic devices and protective equipment.
Polyolefins. Fibers made from PE and PP are very light weight with specific gravities ranging from 0.91 to 0.96. They have low surface
energies and are hydrophobic. Due to their lower melting points, these materials are used as binder fibers for nonwoven fabrics. Fibers from
UHMWPE with molecular weights >1 million grams/mole have extremely high strength and are also bioinert. Strong, thin, pliable fibers
from UHMWPE are used as implantable sutures within the body.
Elastomers. Fibers from elastomers have high stretchability, elasticity, and flexibility. They can stretch up to 200% and return to its original
shape and size. Applications using elastomeric fibers include bandages and hosiery.
Biodegradable Fibers. Fibers from PGA, PLA, and PCL, have been used where biodegradation or bioresorption is required. The rates of
degradation or resorption differ by chemical structure (see Chapter 8, Other Polymers: Styrenics, Silicones, TPEs, Biopolymers and
Thermosets for Medical Devices). Although these materials were initially developed for use as biodegradable sutures, they are also being
explored in constructing implant devices.
Fluoropolymers. PTFE is one of the more commonly used fluoropolymers in medical textiles. In most medical textile applications, however,
the polymer is used in its expanded form, known as ePTFE. The structure consists of “nodes” and “fibrils.” The nodes are perpendicular to
the direction of stretch and are interconnected by the fibrils. The porosity and the pore size of the structure and textile can be varied by
varying the stretch conditions. Two primary applications of ePTFE in medicine are surgical sutures and arterial and stent grafts.
Table 10.5 lists typical fiber properties of some plastics used in medical textiles.

17.9.2 Antistatic finishing agents by fibre type

Each of the major fibre forming polymers has specific requirements that limit the type of antistatic system that can be used. The following
are general guidelines:
1.
Polyester fibres generally use phosphate based antistatic agents. Polyester filament finishes are based on short chain alcohol
phosphates or ethoxylated alcohol phosphates (low level of ethoxylation) that have good thermal stability and good compatibility
with ester/surfactant based finishes. Cationic antistatic agents are occasionally used for specialized filament applications. Polyester
staple uses solid phosphate technology combined with a non-ionic surfactant based finish. Early finishes were based on C12 alcohol
phosphate, but the demand for finishes with reduced tacking under higher humidity conditions moved the technology to C18 alcohol
phosphate based products and these provide the current finish technology for this large volume market.
2.
Nylon fibres require the use of phosphate esters as antistatic agents. Nylon has the highest tendency to generate charge during
processing, and cationic antistatic agents are not effective because they are absorbed into the fibre and do not provide
significant conductivity. The generation of static charge on nylon is very sensitive to finish composition. Phosphate esters vary
greatly in how they interact with the fibre to impact charge generation. In many nylon finishes, the antistatic package use in the
finish is based on blends of phosphate esters, and amine neutralized phosphates are useful in minimizing static charge generation.
Care must be taken to use higher molecular weight phosphate esters in that lower molecular weight materials absorb into the nylon
fibre over time and this significantly reduces the level of static protection for the fibre.
3.Polyolefin fibre finishes use antistatic packages based on higher molecular weight, such as amine neutralized and phosphate esters
synthesized from ethoxylated alcohols.
4.Acrylic fibre uses cationic antistatic agents exclusively. The antistatic agent level in the finish tends to be high (30–40% of
the finish composition) and the most common used materials used are based on the dimethyl sulfate or diethyl sulfate based
quaternary salts of ethoxylated primary alkyl amines.
Blow molding
From Wikipedia, the free encyclopedia
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The blow molding process

Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other
hollow shapes.
In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding.
The blow molding process begins with softening plastic by heating a preform or parison. The parison is a tube-like piece of plastic with a
hole in one end through which compressed air can enter.
The plastic workpiece is then clamped into a mold and air is blown into it. The air pressure inflates the plastic which conforms to the mold.
Once the plastic has cooled and hardened the mold opens and the part is ejected. Water channels within the mold assist cooling.
Extrusion blow molding[edit]

Extrusion blow molding

In extrusion blow molding, plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a
cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow  bottle, container, or part. After the plastic has
cooled sufficiently, the mold is opened and the part is ejected.[2]
"Straight extrusion blow molding is a way of propelling material forward similar to injection molding whereby an Archimedean screw turns,
then stops and pushes the melt out. With the accumulator method, an accumulator gathers melted plastic and when the previous mold has
cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn
continuously or intermittently.[3] With continuous extrusion the weight of the parison drags the parison and makes calibrating the wall
thickness difficult. The accumulator head or reciprocating screw methods use hydraulic systems to push the parison out quickly reducing the
effect of the weight and allowing precise control over the wall thickness by adjusting the die gap with a parison programming device.
Continuous extrusion equipment includes rotary wheel blow molding systems and shuttle machinery, while intermittent extrusion machinery
includes reciprocating screw machinery and accumulator head machinery.
Spin trimming[edit]
Containers such as jars often have an excess of material due to the molding process. This is trimmed off by spinning a knife around the
container which cuts the material away. This excess plastic is then recycled to create new moldings. Spin Trimmers are used on a number of
materials, such as PVC, HDPE and PE+LDPE. Different types of the materials have their own physical characteristics affecting trimming.
For example, moldings produced from amorphous materials are much more difficult to trim than crystalline materials. Titanium coated
blades are often used rather than standard steel to increase life by a factor of 30 times.
Injection blow molding[edit]

Injection blow molding a plastic bottle

The process of injection blow molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM
process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This
is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is
divided into three steps: injection, blowing and ejection.
The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed
into a hot runner manifold where it is injected through nozzles into a heated cavity and core pin. The cavity mold forms the external shape
and is clamped around a core rod which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a
thick tube of polymer attached, which will form the body. similar in appearance to a test tube with a threaded neck.
The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The end of the core rod opens and
allows compressed air into the preform, which inflates it to the finished article shape.
After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod
and as an option can be leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending
on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and
ejection.
Injection stretch blow molding[edit]
Injection Stretch Blow Molding has two main different methods, namely Single-stage and Double-stage process. The Single-stage process
is then again broken down into 3-station and 4-station machines.
Single-Stage[edit]
In the single-stage process, both preform manufacture and bottle blowing is performed in the same machine. The older 4-station method of
injection, reheat, stretch blow and ejection is more costly than the 3-station machine which eliminates the reheat stage and uses latent heat in
the preform, thus saving costs of energy to reheat and 25% reduction in tooling. The process explained: Imagine the molecules are small
round balls, when together they have large air gaps and small surface contact, by first stretching the molecules vertically then blowing to
stretch horizontally the biaxial stretching makes the molecules a cross shape. These "crosses" fit together leaving little space as more surface
area is contacted thus making the material less porous and increasing barrier strength against permeation. This process also increases the
strength to be ideal for filling with carbonated drinks.
Two-stage[edit]
In the two-stage injection stretch blow molding process, the plastic is first molded into a "preform" using the injection molding process.
These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed
later (after cooling) into a reheat stretch blow molding machine. In the ISBM process, the preforms are heated (typically using infrared
heaters) above their glass transition temperature, then blown using high-pressure air into bottles using metal blow molds. The preform is
always stretched with a core rod as part of the process.
References[edit]
1. ^ Jan Schroers; Thomas M. Hodges; Golden Kumar; Hari Raman; Anthony J. Barnes; Quoc Pham; Theodore A. Waniuk
(February 2011). "Thermoplastic blow molding of metals". Materials Today. 14 (1–2): 14–19. doi:10.1016/S1369-7021(11)70018-
9.
 2. ^ John Vogler (1984). Small Scale Recycling of Plastics. Intermediate Technology Publication. p. 6.
3. ^ Extrusion Blow Molding Technology, Hanser Gardner Publications, ISBN 1-56990-334-4

Calendering is a speciality process for high-volume, high quality plastic film and sheet, mainly used for PVC as well as for certain other
modified thermoplastics.
The melted polymer is subject to heat and pressure in an extruder and formed into sheet or film by calendering rolls. The temperature and
speed of the rolls influences the properties of the film.
Calendering allows speciality surface treatments of the film or sheet such as embossing or enhancing the physical properties or in-line
lamination
Calendering is a finishing process applied to textiles and plastic. During calendering rolls of the material are passed between several pairs of
heated rollers, to give a shiny surface. Extruded PVC sheeting is produced in this manner as well other plastics. Calendering is a final process
in which heat and pressure are applied to a fabric by passing it between heated rollers, imparting a flat, glossy, smooth surface. Lustre
increases when the degree of heat and pressure is increased. Calendering is applied to fabrics in which a smooth, flat surface is desirable,
such as most cotton, many linens and silks, and various man-made fabrics.

The molten material is fed tothe calender rolls from a Banbury mixer and two-roll mill system, or from a large extruder.The major
plastic material that is calendered is PVC. Products range from wall covering andupholstery fabrics to reservoir linings and agricultural
mulching materials.Owing to the large separating forces developed in the calender gap, the rolls tend tobend. This may result in undesirable
thickness variations in the finished product.Compensations for roll deflections are provided by using crowned rolls having a largerdiameter
in the middle than at the ends or by roll bending or roll skewing.
Calender installations require large initial capital investment. Film and sheet extrusion are competitive processes because the capital
investment for an extruder is only afraction of the cost of a calender. However, the high quality and volume capabilities of calendering lines
make them far superior for many products.Calendering in principle is similar to the hot rolling of steel into sheets. It is interesting to note that
strip casting of semi-solid alloys can be modeled with the help of thehydrodynamic lubrication approximation for a power-law viscosity
model, just like plasticscalendering. The process of calendering is also used extensively in the paper industry.
Extrusion of polymers
Dr. Dmitri Kopeliovich

Extrusion is a process of manufacturing long products of constant cross-section (rods, sheets, pipes, films, wire insulation coating) forcing
soften polymer through a die with an opening.
Polymer material in form of pellets is fed into an extruder through a hopper. The material is then conveyed forward by a feeding screw and
forced through a die, converting to continuous polymer product.
Heating elements, placed over the barrel, soften and melt the polymer. The temperature of the material is controlled by thermocouples.
The product going out of the die is cooled by blown air or in water bath.
Extrusion of polymers (in contrast to extrusion of metals) is continuous process lasting as long as raw pellets are supplied.
Extrusion is used mainly for Thermoplastics, but Elastomers and Thermosets are also may be extruded. In this case cross-linking forms
during heating and melting of the material in the extruder.
The thermoplastic extruded products may be further formed by the Thermoforming method.
A principal scheme of an extruder is shown in the picture.

tTransfer molding is similar to compression molding; however, the material is first placed in a transfer chamber prior to entering the mold.
As in compression molding, thermosets that are cross-linked with heat are used for transfer-molding applications. Multiple cavities can be
used within transfer molding since the material is entering the mold after the mold is closed. Since runners and sprues are present, shear is
created. This facilitates heat needed for crosslinking and flow to the cavities. Transfer-molding machines are also generally positioned with
the molds opening vertically.
Since the mold is closed and clamped prior to the material entering the mold, there is no presence of flash with transfer  molded parts. The
dimensions of the final parts are very accurate due to the flow of the polymer being gated. Another advantage is that the cure time is faster
since there is the presence of shear flow, which creates heat. Inserts can also be used to create more complex parts than can be created by
compression molding.
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Processing of plastics
Roy J. Crawford, Peter J. Martin, in Plastics Engineering (Fourth Edition), 2020
4.8 Transfer moulding
Transfer moulding is similar to compression moulding except that instead of the moulding material being pressurized in the cavity, it is
pressurized in a separate chamber and then forced through an opening and into a closed mould. Transfer moulds usually have multi-cavities
as shown in Fig. 4.76. The advantages of transfer moulding are that the preheating of the material and injection through a narrow orifice
improves the temperature distribution in the material and accelerates the crosslinking reaction. As a result the cycle times are reduced and
there is less distortion of the mouldings. The improved flow of the material also means that more intricate shapes can be produced.

The success of transfer moulding prompted further developments in this area and clearly it was only a relatively small step to an  injection
moulding process for thermosets as described in Section 4.3.9.
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Plastics Processing
Anshuman Shrivastava, in Introduction to Plastics Engineering, 2018
5.6 Transfer Molding
Transfer molding is typically used for thermoset materials and is slightly different than traditional compression molding. It is a combination
of injection and compression molding process. In this process the polymer is preheated in a holding chamber called the pot. A plunger is then
used to transfer the polymer from the pot into the closed heated mold, then compressed into desired shape as shown in Fig. 5.13. Preheating
enables lower pressure requirements for the transfer operations and shorten molding cycle. Transfer molding is very useful in producing
intricate composite with various plastics and metal inserts. Common examples are gas valves, electronic circuits, and sparkplugs with
wires [31,35,36].

The process produce less flash as the material is injected into a closed mold. However, the sprue could add to the overall scrap and can
produce more wastage as compared to compression molding. Transfer molding has many benefits, which include high cavity count, design
flexibility with sharper edges, no deflashing necessary as it produces flash free parts, lower cost due to simpler pot and plunger designs, short
production cycle compared to compression molding but slower to IM. Some disadvantages include higher mold maintenance as insert
transfer tool requires higher mold maintenance, and the tool design could get complex and expensive to include inserts. The overall operation
could be cumbersome as each charge is loaded manually.
Commercial Composite Processes: These Commercial Processes Produce Far More Parts than the High-performance Processes
F.C. Campbell, in Manufacturing Processes for Advanced Composites, 2004
Transfer molding
Transfer molding (Fig. 15) is similar to compression molding, except that the charge of molding compound is heated to 300–350 °F in a
separate chamber; transferred by a ram under heat and pressure into a closed die where the shape of the part is determined and cure takes
place in usually 45–90 s. The process is used for small intricate parts requiring tight dimensional tolerances. Although good resin flow is
required, the process is not sensitive to precise cha

Rheology of thermosets: the use of chemorheology to characterise and model thermoset flow behaviour
P.J. Halley, in Thermosets, 2012
Description and examples
Transfer moulding is a variant of compression moulding where the injection of the resin is controlled by a transfer ram. The process consists
of a mould cavity and a transfer cavity. The filled charge is initially placed in the transfer cavity, heated until softened and then pressure is
applied to the ram, causing the charge to flow through the transfer port to the mould cavity. Excess charge is employed to allow a hold
pressure to be applied to the mould cavity and to accommodate sample shrinkage during cure. The pressure is maintained during cure and
released when material is gelled. Typical systems are epoxy resins (typical epoxy–novolac systems) with silica fillers, hardeners, catalyst and
rubber modifiers used in integrated chip packaging. This process is well described by Fig. 4.1.

Here the transfer moulding process is shown where the epoxy-silica charge in transferred to the mould to encapsulate the integrated circuit.
Note in this case the fragile integrated circuit pattern is placed inside the heated mould (typically around 170–180 °C) prior to transfer of the
filled epoxy encapsulant. The filled epoxy is heated (to around 80–90 °C) before being injected into the mould. This injection is conducted in
such a way that the viscosity is at a minimum value (e.g. the temperature is high enough to melt the filled resin and reduce the viscosity due
to thermal effects, but not high enough to induce high viscosities from cure effects) such that minimal damage is done to the integrated
circuitry.
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Encapsulation process technology

Kun Fang, in Encapsulation Technologies for Electronic Applications (Second Edition), 2019
3.2.1.1 Molding equipment
Transfer molding requires four key pieces of capital equipment: a preheater, a press, the die mold, and a cure oven. The transfer molding
press is normally hydraulically operated. Auxiliary ram-type transfer molds are commonly used in transfer molding.
The mold has a built-in transfer pot separate from the mold cavities as shown in Fig. 3.3. The molding compound is placed in the pot. Both
the volume and the size of the molding compound preforms have to be appropriately selected for the press capacity. The mold is then
clamped. The transfer plunger is activated to apply the transfer pressure to the molding compound. The molding compound is driven through
the runners and gates into the cavities. Fig. 3.3A shows the top view of a transfer molding setup design with multiple transfer pots and
plunger where each transfer pot is connected to only two cavities. Fig. 3.3B shows a single transfer pot and plunger connected to multiple
cavities.

Since the 1980s, aperture-plate and multiplunger molds have been the dominant approaches to plastic-encapsulated microelectronics (PEMs)
molding. Tables 3.1 and 3.2 compare the features of these methods. Aperture-plate molds are a patented transfer molding technology (US
Patent No. 4,332,537, June 1, 1982) exclusively developed for PEMs. An aperture-plate mold design is shown in Fig. 3.4 [1]. An aperture-
plate (or cavity plate) mold is constructed by assembling a series of stacked plates. The lead frames form the aperture plates. The top and
bottom of the body are formed by separate plates. The bottom body-forming plate contains the runner system, while the top plate is finished
for either laser or ink marking. The gates are positioned between the runners, parallel to the bottom of the body; the aperture plate cavities
can be formed anywhere along this intersection, and their width can be any fraction of this length of intersection. This flexibility of gate
positioning, along with the much lower pressure drop across the gate in an aperture plate, results in negligible wire sweep and paddle shift
during molding. These molds are also highly adaptable for different package types and pinouts.

Multiplunger molds, also called gang-pot molds, have a number of transfer plungers, typically feeding one to four cavities from each transfer
pot. They are highly automated and can be easily set and optimized for a new molding compound. However, their productivity is much less
than aperture-plate molds due to the number of cavities available (up to 100 in multiplunger vs. 1000 in aperture-plate). Also, the use of low
preform-preheat temperatures in manual tools can lead to some molding-compound temperature-related problems. Fig. 3.5 shows a
multiplunger mold used for simultaneous encapsulation of dual in-line packages and quad flatpacks (QFPs). Fig. 3.6 

The multiplunger mold consists of two halves, referred to as the top and bottom molds. The mating surface of these two halves is called the
“parting line.” Platens are massive blocks of steel used to bolt the two mold halves to the molding press.  Fig. 3.7 shows different parts of a
transfer mold press including platens and cavities. Fig. 3.8 shows a close-up top view of a transfer pot, runners, and cavities. The mold press
also includes guide pins and ejector pins. The guide pins ensure proper alignment and movement of the two halves. The ejector pins aid the
ejection of the component after the mold has opened.

The gates in the molding equipment are located such that they can be easily removed and polished if necessary. Properly designed gates
should allow proper flow of material as it enters the mold cavity. Gates should be located at points away from the functioning parts of the
molded component.
Vents are provided in all transfer molds to facilitate the escape of trapped air. The locations of these vents depend on the part design and
locations of pins and inserts. Vents should be sufficiently small so that they allow the air but not the molding compound to pass through.
Vents are often placed at the far corners of the cavity, near inserts where a knit line will be formed, or at the point where the cavity fills last.
The specific mold design depicted in Fig. 3.8 is used for the encapsulation of quad flat packages (QFPs). In this design, one transfer pot feeds
> 10 cavities. The top part of the figure shows the lead frame with the encapsulated packages. The bottom part of the figure shows the
cavities. As seen, the material flows through the runners and gates.

Processing of Fluoroelastomers
Jiri George Drobny, in Fluoroelastomers Handbook (Second Edition), 2016
7.7.3 Transfer Molding
The transfer molding process, shown in Fig. 7.1817 involves using a piston and cylinder device to force rubber through small holes into the
mold cavity. A piece of uncured compound is put into a part of the mold called the  pot, and a plunger then pushes the stock into the closed
mold through a sprue. The mold is kept closed while the rubber cures. The plunger is then raised, and the  transfer pad material is removed
and discarded. The mold is opened for removal of the part; then the flash and sprue material is trimmed off and discarded.
Compared to compression molding, transfer molding provides better product consistency, shorter cycle times, and better bonding of rubber to
metal inserts.17 However, considerable material is lost as scrap in the transfer pads, sprues, and flash. The stock must have relatively low
viscosity and adequate scorch safety for adequate flow into the mold. 9 The rapid transfer of compound from the pot through small sprues to
the mold cavities imposes high shear and considerable heat generation, so the stock is heated quickly to curing temperature. Sprue size
should be kept as small as is practical, to minimize damage to parts on demolding and tearing from the  molded parts. However, sprues must
be large enough to allow adequate flow of the compound. Somewhat lower mold temperatures may be usable for transfer molding to get cure
times comparable to those for compression molding. The basic three-plate multiple cavity transfer mold is more complex and expensive than
a compression mold, but is better suited to molding intricate parts or securing inserts. 19 Only a single piece of rubber is used to fill all mold
cavities in a heat, so preparation of preforms is much simplified. Since the mold is closed during filling, flash is minimized through gates and
vents. Several transfer molding process variants and mold designs are described by Ebnesajjad. 18
Nicholas P. Cheremisinoff Ph.D., in Condensed Encyclopedia of Polymer Engineering Terms, 2001

TRANSFER MOLDING
In transfer molding, the thermosetting molding powder is placed in a chamber or pot outside the molding cavity and subjected to heat and
pressure to liquefy it. When liquid enough to start flowing, the material is forced by the pressure into the molding cavity, either by a direct
sprue or through a system of runners and gates. The material sets hard to the cavity shape after a certain time (cure time) has elapsed. When
the mold is disassembled, the molded part is pushed out of the mold by ejector pins, which operate automatically.  Figure 1 illustrates the
molding cycle of pot-type transfer molding.

Moving from left to right in this figure, the molding compound is placed in the transfer pot. In the middle sketch, the molding compound is
forced under pressure when hot through an orifice and into a closed mold. When the mold opens (rightmost figure), the sprue remains with
the cull in the pot, and the molded part is lifted out of the cavity by ejector pins. (Source: Chanda, M. and S. K. Roy, Plastics Technology
Handbook, Marcel Dekker, Inc., New York, NY, 1987). See also Plunger-TypeTransfer Molding.

The Transfer Molding Process

The transfer molding process is the most popular integrated circuit encapsulation method for essentially all plastic packages. Transfer
molding is a process of forming components in a closed mold from a thermosetting material that is conveyed under pressure, in a hot, plastic
state, from an auxiliary chamber, called the transfer pot, through runners and gates into the closed cavity or cavities.  Figure 3 depicts a
typical transfer-molding set-up.

One of the main limitations of the transfer molding process is the loss of material. The material left in the pot or well and in the sprue and
runner is completely polymerized and must be discarded. For small parts, this can represent a sizable percentage of the weight of the parts
molded. Consequently, the trend is to utilize fully automatic transfer molding presses, which mold a few parts very efficiently. Another
limitation of today’s transfer molding process is the need to use standard metal lead frames.
In this process, leadframes are loaded (six to twelve in a row) in the bottom half of the mold. Both the moving platen and the transfer plunger
initially close rapidly, but the speed reduces as they close. After the mold is closed and clamping pressure is applied, the preform of molding
material is placed in the pot, and the transfer plunger or ram is activated. Preheating of the  molding compound is done by a high-frequency
electronic method that works on a principle similar to microwave heating. The transfer plunger then applies the transfer pressure, forcing the
molding compound through the runners and gates into the cavities. This pressure is maintained for a certain optimum time, ensuring proper
filling of the cavities. The mold then opens, first slowly. This step is known as the slow breakaway. Sometimes it is desirable to have the
transfer plunger move forward so that it pushes out the cull, or the material remaining in the pot. Finally, the component is ejected using
the ejector system in the mold.
The parameters controlled by the press include the temperatures of the pot and the mold, the transfer pressure, the mold-clamping pressure,
and the transfer time to fill the mold cavities completely. The mold temperature should be high enough to ensure rapid curing of the part.
However, precautions should be taken to control the mold temperature, because too high a mold temperature may result in precure or
solidification of the molding compound before it reaches the cavities. Electric heating is the most commonly used method for heating the
molds. Multiple electric heating cartridges are inserted both in the top and bottom halves of the molds, positioned to supply heat to all the
cavities. The applied pressure ensures the flow of material into all parts of the cavity.
Clamping pressure applied on the mold ensures the closure of the mold during polymerization or cure and against the force of the material
entering the cavities. Packing pressure, usually higher than the transfer pressure, is applied once the mold has been filled. While it is being
transferred into the mold, the molding compound is reacting. Consequently, the viscosity of the material increases, gradually at first. As the
reacting molecules become larger, the viscosity increases more rapidly until gelation occurs, at which point the material is a highly cross-
linked network.
The pressure applied by the transfer plunger is critical. It should be sufficient to force the material through the runners and gates into the
cavity and hold the material until polymerization. For high throughput, it is desirable to transfer and react the material as rapidly as possible,
but decreasing the transfer time requires an increase in the transfer pressure to fill the mold. The high transfer pressures, typically as high as
170 MPa (25 ksi), can cause damage within the package, such as wire-sweep short or, in extreme cases, wirebond lift-off, shear, or fracture.
These problems can be avoided by careful design of the package size and the transfer-gate size to minimize shear rate and flow stresses
during cavity filling.
After 1–3 min at the typical molding temperature of 175 °C, the polymer is cured in the mold. Following curing, the mold is opened, and
ejector pins remove the parts. The molded packages are ready for ejection when the material is resilient and hard enough to withstand
the ejection forces without significant permanent deformation. After ejection, the molded leadframe strips are loaded into magazines, which
are postcured in a batch (4–16 h at 175 °C) to complete the cure of the encapsulant. Postcure normally involves holding the part in an oven at
a temperature somewhat lower than the mold temperature but well above the room temperature for several hours. In some instances,
postcuring is performed after code marking to eliminate an additional heat-cure cycle. The most important consideration in postmold cure
analysis is the development of thermomechanical properties. Other reliability concerns in plastic encapsulated packages can be found
in Condra et al. (1994), 

Compression molding and transfer molding

Compression molding and transfer molding are widely used for processing thermosetting polymers. A schematic diagram of compression
molding is shown as Fig. 5A. Compression molding can be used to fabricate both thermoplastics and thermosets and requires heating up of
raw materials (in the form of powders or pellets) in the mold. For thermoplastics, the temperature should be higher than Tg during the
forming process. A pressure is applied in compression molding and should be maintained during cooling. For compression molding, an
appropriate amount of the polymer and necessary additives are thoroughly mixed and loaded between upper and lower parts of the mold.
Both parts are heated up and only one is movable. After the mold is closed, heat and pressure are applied. The polymer in the mold is melted
and flows into all space of the mold cavity. After a pre-set time, the mold is cooled and opened up to eject the product with desired shape. A
major advantage of compression molding is the low capital cost due to the simplicity of the process.

Transfer molding is a variation of compression molding for processing thermosetting polymers. In this forming process, the solid ingredients
are initially melted in a heating chamber. The melted viscous polymer is then injected into the mold cavity and a pressure is applied. Transfer
molding can process thermosets with complex parts.