Solidification Process(2) - Polymer Processing (Chapter 8,...

Department of Mechanical EngineeringChung-Ang University

Solidification Process(2)- Polymer Processing

(Chapter 8, 12)

Seok-min [email protected]

Department of Mechanical EngineeringChung-Ang University- 2 -

Plastic Products

Plastics can be shaped into a wide variety of products: Molded parts Extruded sections Films Sheets Insulation coatings on electrical wires Fibers for textiles Paints and varnishes Adhesives Various polymer matrix composites


Trends in Polymer Processing

Applications of plastics have increased at a much faster rate than either metals or ceramics during the last 50 years Many parts previously made of metals are now

being made of plastics Plastic containers have been largely substituted

for glass bottles and jars

Total volume of polymers (plastics and rubbers) now exceeds that of metals


Polymer

A compound consisting of long-chain molecules, each molecule made up of repeating units connected together

There may be thousands, even millions of units in a single polymer molecule

The word polymer is derived from the Greek words poly, meaning many, and meros (reduced to mer), meaning part

Most polymers are based on carbon and are therefore considered organic chemicals

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Synthesis of Polymers (폴리머 합성) Nearly all polymers used in engineering are synthetic They are made by chemical processing Polymers are synthesized by joining many small

molecules together into very large molecules, called macromolecules, that possess a chain-like structure

The small units, called monomers, are generally simple unsaturated organic molecules such as ethylene C2H4

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Linear, Branched, and Cross-linked Polymers

Linear structure – chain-like structure (Characteristic of thermoplastic polymers )

Branched structure – chain-like but with side branches (Also found in thermoplastic polymers)

Cross-linked structure- Loosely cross-linked, as in an elastomer- Tightly cross-linked, as in a thermoset

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Thermoplastic Polymers (TP) A thermoplastic polymer can be heated from a solid

state to a viscous liquid state and then cooled back down to solid

This heating and cooling cycle can be repeated multiple times without degrading the polymer

The reason is that TP polymers consist of linear (and/or branched) macromolecules that do not cross-link upon heating

By contrast, thermosets and elastomers change chemically when heated, which cross-links their molecules and permanently sets these polymers

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Mechanical Properties of Thermoplastics

Low modulus of elasticity (stiffness) E is two or three orders of magnitude

lower than metals and ceramics Low tensile strength TS is about 10% of the metal Much lower hardness than metals or ceramics Greater ductility on average Tremendous range of values, from 1% elongation for

polystyrene to 500% or more for polypropylene

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Thermosetting Polymers (TS) TS polymers are distinguished by their highly

cross-linked three-dimensional, covalently-bondedstructure within the molecule

Chemical reactions associated with cross-linking are called curing or setting

In effect, the formed part (e.g., pot handle, electrical switch cover, etc.) becomes one large macromolecule

Always amorphous and exhibits no glass transition temperature

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General Properties and Characteristics of Thermosets

Rigid - modulus of elasticity is two to three times greater than TP

Brittle, virtually no ductility

Less soluble than TP in common solvents

Capable of higher service temperatures than TP

Cannot be remelted - instead they degrade or burn

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Elastomers

Some can be extended 500% or more and still return to their original shape

Two categories: - Natural rubber - derived from biological plants - Synthetic polymers - produced by polymerization processes similar to those used for thermoplastic and thermosetting polymers

Polymers capable of large elastic deformation when subjected to relatively low stresses

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Classification of Shaping Processes

Extruded products with constant cross-section Continuous sheets and films Continuous filaments (fibers) Molded parts that are mostly solid Hollow molded parts with relatively thin walls Discrete parts made of formed sheets and films Castings Foamed products


Compression process in which material is forced to flow through a die orifice to provide long continuous product whose cross-sectional shape is determined by the shape of the orifice Widely used for thermoplastics and elastomers to mass produce

items such as tubing, pipes, hose, structural shapes, sheet and film, continuous filaments, and coated electrical wire

Carried out as a continuous process; extrudate is then cut into desired lengths

Extrusion


Extruder Screw Divided into sections to serve several functions: Feed section - feedstock is moved from hopper and

preheated Compression section - polymer is transformed into

fluid, air mixed with pellets is extracted from melt, and material is compressed

Metering section - melt is homogenized and sufficient pressure developed to pump it through die opening


Die End of Extruder Before reaching die, the melt passes through a screen pack -

series of wire meshes supported by a stiff plate containing small axial holes

Functions of screen pack: Filter out contaminants and hard lumps Build pressure in metering section Straighten flow of polymer melt and remove its "memory"

of circular motion from screw


Extrusion Die(1)

Solid Cross Section

Hollow Shapes


Extrusion Die for Coating Wire

Figure 13.11 Side view cross-section of die for coating of electrical wire by extrusion.


Slit Die ExtrusionProduction of sheet and film by conventional extrusion, Slit may be up to 3 m wide and as narrow as around 0.4 mm A problem is uniformity of thickness throughout width of stock, due

to drastic shape change of polymer melt as it flows through die Edges of film usually must be trimmed because of thickening at

edges

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CalenderingFeedstock is passed through a series of rolls to reduce thickness to desired gage Expensive equipment, high production rates Process is noted for good

surface finish and high gage accuracy

Typical materials: rubber or rubbery thermoplasticssuch as plasticized PVC

Products: PVC floor covering, shower curtains, vinyl table cloths, pool liners, and inflatable boats and toys

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Injection Molding

Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from cavity

Produces discrete components almost always to net shape

Typical cycle time 10 to 30 sec, but cycles of one minute or more are not uncommon

Mold may contain multiple cavities, so multiple moldings are produced each cycle

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Injection molding process• Polymer를 Hopper로부터 Barrel내로 투입,

Heater와 screw 회전에 의한 마찰열로 용융

• Screw 회전으로 용융수지를 screw 전방으로 이동

• Hydraulic unit force 로 용융수지를 delivery system을 통해cavity 내로 충진

• 용융수지가 충분히 고화,냉각될 때까지 냉각 후취출


Injection molding process


1. 가소화 과정 (Plasticating Phase)


2. 충진과정 (Filling Phase)


3. 보압 과정 (Packing Phase)


4. 냉각 과정(Cooling Phase)


Injection machine Cycle

Mold is closed Melt is injected into cavity

Screw is retracted Mold opens

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Injection Molded Parts

Complex and intricate shapes are possible Shape limitations: Capability to fabricate a mold whose cavity is the

same geometry as part Shape must allow for part removal from mold

Injection molding is economical only for large production quantities due to high cost of mold

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Injection Molding MachineTwo principal components:1. Injection unit Melts and delivers polymer melt Operates much like an extruder

2. Clamping unit Opens and closes mold each injection cycle

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Injection Unit of Molding Machine Consists of barrel fed from one end by a hopper

containing supply of plastic pellets Inside the barrel is a screw which:

1. Rotates for mixing and heating polymer2. Acts as a ram (i.e., plunger) to inject molten plastic into mold

Non-return valve near tip of screw prevents melt flowing backward along screw threads

Later in molding cycle ram retractsto its former position

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Clamping Unit of Molding Machine Functions:

1. Holds two halves of mold in proper alignment with each other2. Keeps mold closed during injection by applying a clamping

force sufficient to resist injection force3. Opens and closes mold at the appropriate times in molding

cycle

Types: 1. Toggle2. Hydraulic

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The Mold

The special tool in injection molding Custom-designed and fabricated for the part to

be produced When production run is finished, the mold is

replaced with a new mold for the next part Various types of mold for injection molding: Two-plate mold Three-plate mold Hot-runner mold

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유동 시스템

1)스푸루(sprue) system

금형의 입구에 해당하는 것으로 용융된 수지를 러너에 보내는 역할

2)러너 (Runner)①러너의 기능 : 러너의 기능은 성형기 노즐에서 밀려나온 수지를 캐비티 까지

안내하는 길 역할

②러너의 설계 시 고려할 사항

(a)캐비티의 개수 및 배열에 따라 배치방법을 결정한다.

(b)수지의 온도저하가 최소가 되도록 한다.

(c) 러너에서 압력 손실이 최소가 될 수 있는 구조로 한다.

(d)러너의 체적은 될 수 있는 한 작게 되도록 한다.

③콜드 슬러그 웰 : 고화된 수지가 캐비티 안으로 유입되는 것을 막음

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3)게이트

① 게이트의 역할

- 충진되는 용융수지의 흐름 방향과 유량을 제어 러너 측으로 역류를 방지한다.

- 스프루, 러너를 통과 냉각된 수지는 좁은 게이트를 통과하는 동안 유동 속도가빨라져 마찰열이 발생,

- 러너가 성형품에서 용이하게 절단되도록 한다.

② 게이트의 위치

- 게이트는 성형품의 가장 두꺼운 부분에 설치하는 것을 원칙으로 한다.

- 성형품은 외관상 눈에 띄지 않는 곳, 성형 후 게이트의 끝손질이 용이한 부분에 설치한다.

- 웰드라인이 생기지 않는 곳이나 안 보이는 위치에 설치한다.

③ 게이트의 크기

- 충진시간은 게이트가 클수록 유리

- 잔류 응력에 의한 변형, 휨에 관해서는 게이트가 작은쪽이 유리

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Two-Plate Mold

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Two-Plate Mold Features

Cavity – geometry of part but slightly oversized to allow for shrinkage Created by machining of mating surfaces of two

mold halves Distribution channel through which polymer melt

flows from nozzle into mold cavity Sprue - leads from nozzle into mold Runners - lead from sprue to cavity (or cavities) Gates - constrict flow of plastic into cavity

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More Two-Plate Mold Features

Ejection system – to eject molded part from cavity at end of molding cycle Ejector pins built into moving half of mold usually

accomplish this function Cooling system - consists of external pump connected

to passageways in mold, through which water is circulated to remove heat from the hot plastic

Air vents – to permit evacuation of air from cavity as polymer melt rushes in

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Three-Plate Mold


Hot-Runner Mold

Eliminates solidification of sprue and runner by locating heaters around the corresponding runner channels

While plastic in mold cavity solidifies, material in sprue and runner channels remains molten, ready to be injected into cavity in next cycle

Advantage: Saves material that otherwise would be scrap in

the unit operation

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Shrinkage

Reduction in linear size during cooling from molding to room temperature

Polymers have high thermal expansion coefficients, so significant shrinkage occurs during solidification and cooling in mold

Typical shrinkage values: Plastic Shrinkage, mm/mm (in/in)Nylon-6,6 0.020Polyethylene 0.025Polystyrene 0.004PVC 0.005

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Shrinkage Factors

Fillers in the plastic tend to reduce shrinkage Injection pressure – higher pressures force more

material into mold cavity to reduce shrinkage Compaction time - similar effect – longer time

forces more material into cavity to reduce shrinkage

Molding temperature - higher temperatures lower polymer melt viscosity, allowing more material to be packed into mold to reduce shrinkage

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Transfer Molding

TS charge is loaded into a chamber immediately ahead of mold cavity, where it is heated; pressure is then applied to force soft polymer to flow into heated mold where it cures

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Bonding and Packing

Schematic illustrations of IC packing

Chip-on-board attachment

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Transfer-molding press schematic Runner design

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Blow Molding Molding process in which air pressure is used to inflate soft plastic into

a mold cavity Important for making one-piece hollow plastic parts with thin walls,

such as bottles Because these items are used for consumer beverages in mass

markets, production is typically organized for very high quantities

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Thermoforming

Flat thermoplastic sheet or film is heated and deformed into desired shape using a mold

Heating usually accomplished by radiant electric heaters located on one or both sides of starting plastic sheet or film

Widely used in packaging of products and to fabricate large items such as bathtubs, contoured skylights, and internal door liners for refrigerators

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Thermoforming


Product Design Guidelines: Moldings

Wall thickness Thick cross sections are wasteful of

material, more likely to cause warping due to shrinkage, and take longer to harden

Reinforcing ribs Achieves increased stiffness without

excessive wall thickness Ribs should be made thinner than the walls

they reinforce to minimize sink marks on outside wall

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Product Design Guidelines: Moldings

Corner radii and fillets Sharp corners, both external and internal,

are undesirable in molded parts They interrupt smooth flow of the melt, tend

to create surface defects, and cause stress concentrations in the part

Holes Holes are quite feasible in plastic moldings,

but they complicate mold design and part removal

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Solidification Process(2) - Polymer Processing (Chapter 8,...

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