Shanghai Jiao Tong University 1 Polymers ME 250: Design & Manufacturing I School of Mechanical Engineering

Shanghai Jiao Tong University 1

Polymers

ME 250: Design & Manufacturing I

School of Mechanical Engineering


Outline Introduction Polymer （聚合体） Processing Summary


Introduction

Learning Objectives Polymers as Engineering Materials Chemical Structure of Polymers Commonly Employed Polymer Processing Techniques Applications Polymer Composites


Polymers - Thermoplastics vs Thermosets

Thermoplastics （热塑形塑料） – Plastics that can be repeated

softened by heating and repeatedly hardened by cooling through

a temperature range that is characteristic of the plastic. In the

softened state, the plastic can be shaped into articles.

Thermosets （热固性塑料）– Plastics which, after having been cured by heat or other means are mostly infusible or insoluble.

Thermosets cannot be reprocessed.


Polymers - Thermoplastics vs ThermosetsThermoplastics molecules are linear or branched, and as a rule are

soluble in specific organic solvents.

In thermosets, the molecules are chemically cross-linked, and

cannot be dissolved by organic solvents


Polymer Processing

Injection Molding( 注射成型） Compression Molding （压塑成型） Transfer Molding （传递模塑法） Blow Molding （吹塑成型） Extrusion （挤压） Thermoforming （热力塑型） Casting （铸造） Rotational Molding （转动成型）


Injection Molding

In the injection molding process, thermoplastic polymer granules are fed via a hopper into a heated barrel. A screw or ram forces the molten polymer into a mold cavity. Pressure is maintained until the part has

hardened and then the part is ejected out of the mold.


Injection Molding

Injection Molding-products


Injection Molding - AnalysisThermoplastics shrink during solidification.

Cavity must be larger than part size.

Dc=Dp+DpS+DpS2

Dc: dimension of cavity [mm]

Ds: dimension of part [mm]

S: shrinkage value [mm/mm]

DpS2: accounts for shrinkage


Injection Molding Shrinkage of typical thermoplastics


Injection Molding – Examples

One part is made of polyethylene （聚乙烯） . Its length is 80mm. What is the part cavity length?

shrinkage of polyethylene = 0.025

Dc= Dp+DpS+DpS2 =80+80x0.025+80x(0.025)

2

=82.05mm Shrinkage

Injection pressure Compaction time Molding temperature Part thickness


Compression MoldingCompression molding is the most widely used process for thermosetting polymers. In this

process, a pre-measured amount of material, preheat to some temperature is placed in

an open mold cavity. A plug or cover closes the most, and then heat and pressure are

applied to cause the polymer to soften, flow to fill out the mold and cure. Curing of

thermosetting polymer involves a time dependent chemical reaction.


Transfer Molding

Transfer molding is a variant of compression molding applied to thermosetting polymers.

The polymer powder or preform is loaded into a heated cylinder where the polymer is

melted. Upon melting, a piston or ram forces the molten polymer under pressure via a

runner into a mold cavity.


Blow MoldingBlow molding is the process used to make plastic bottles and containers out of

thermoplastic materials. The process works by extruding a tube of semi-molten heated

polymer down the center of a closed mold. Air is then injected to expand the polymer

(like a balloon). The heated polymer expands in a uniform thickness to form the desired

shape.


Blow Molding


ExtrusionExtrusion is the process of forming continuous shapes of constant cross-section out of

thermoplastic polymers. The process works by forcing molten polymer under pressure

through a metal die. Structural forms such as channels, bars, rounds, angles, tracks,

hoses and tubing are made by this process.


Extrusion


ExtrusionExtrusion is also used to make plastic bags and films


ThermoformingThermoforming is a method of forming polymers sheets or films into shaped objects. The

process works by claming a sheet at edges and heating the sheet. On heating, the sheet

softens and slumps into a mold cavity. Vacuum suction pulls the sheet so as to conform

to the mold shape. The sheet is then allowed to cool. Thermoforming is well suited to low

volume production of containers.


Thermoforming


CastingCasting is the process of forming solid or hollow shapes by pouring molten polymer into a

metal mold. The polymer flows into the mold and undergoes solidification or curing. This

process is widely used to make silicone elastomers （硅橡胶） , and epoxy and polyester resins （环氧树脂） .


Casting


Reaction Injection MoldingIn the Reaction Injection Molding process, polymer reactants are pumped at high

pressure into a mixing chamber before flowing into a mold at atmospheric pressure. The

chemical reaction is exothermic. The gases evolved due to chemical reaction produce

sufficient pressure to cause the mixture to fill out the cavity. The heat evolved by the

chemical reaction speeds the curing of the polymer. This process is most often applied to

large parts (typically made of polyurethane) such as dashboards and fenders.


Rotational MoldingRotational molding is used to make hollow parts at low costs. The process works by

charging a metal mold with thermoplastic polymer pellets. The mold is then heated to

soften and melt the polymer, and rotated about two axes. The plastic pellets melt and

stick to the mold walls to form a uniform shell that conforms to the inside surface of the

mold. This process is useful for making large containers such as garbage cans in an

inexpensive manner without large tooling costs.


Polymer CompositesA composite is a material composed of two ormore different materials with propertiessuperior to those of the individualcomponents. A polymer composite is apolymeric material reinforced with a secondmaterial to obtain specific properties. Thereinforcement can be in the form of laminates,particles, woven fibers, chopped fibers, flakesor a regular rigid reinforcement. These typesof reinforcement are added to enhancemechanical properties. Discontinuousreinforcements such as flakes, particles andchopped fibers are good at enhancing shortterm mechanical properties but are not aseffective as continuous reinforcements inincreasing creep strength and other long termmechanical properties.


Polymer CompositesA variety of reinforcements can be used in polymer composites. These vary from

glass, metals, asbestos, boron fibers, carbon, to polymers and fabrics


Glass Fiber Reinforced Polymer Composites

Glass fibers are the most commonly

used reinforcement in polymer

composites. The most commonly used

forms of glass reinforcements chopped

strands, woven strands or mat cloths.

Woven glass reinforcments can be

obtained ina wide variety of weaves.

Mat cloths are made from randomly

intertwined discontinuous fibers.

The glasses are often chemically

treated to provide better bonding to the

polymer (resin) material. Essentially,

molecules are incorporated such that

one end bonds well to the glass while

the other end of the molecule chemically

bonds to the organic polymer.



Hand lay-up involves coating a mold or form with resin. A layer of glass reinforcement is

applied and the reinforcement is saturated with resin. Additional such layers are applied,

alternating with resin and fiber to make up the desired thickness. This techniques is

particularly useful for very large objects (e.g. boats) and/or curved surfaces where robotic

application is difficult.

Filament winding uses a special machine to wind a continuous glass fiber dipped in resin

around a rotating mandrel. This process is most useful for making pipes and tanks for

handling chemicals.


Filament Winding



Compression molding of fiber reinforced composites uses a pre-manufactured sheet molding compound

(SMC) or bulk molding compound (BMC) . The SMC is a sheet composed of resin and glass fiber

reinforcement. The sheet molding compound is cut and placed in a heated mold. The BMC is a kneaded

dough like mixture of resin and fiber. The heated mold halves compress the SMC or BMC to flow and fill

the mold cavity. An example part made by this process is automotive fenders.

Continuous pultrusion is a process similar to extrusion. It is used for making glass reinforced shapes that

can be made by pulling resin impregnated fiber strands through a heated die. The glass fiber strands are

pulled through a resin bath into a heated die and finally through a curing oven where the resin cross-links

and hardens. Pipes, channels, I-beams and similar shapes can be manufactured. Example parts are

structural members around corrosive chemical tanks.


Pultrusion - Examples



Chopped fiber spraying is similar to hand lay-up but is more automated and is faster. The two component

resins are mixed in the hand-held gun and sprayed at the mold surface. Continuous strands of fiber are

chopped and incorporated into the spray. This process can be used to make large structures such as boats,

shower stalls and bath tubs.

In the resin transfer molding process, glass fibers are cut and shaped to the desired thickness in an open

mold, The mold is then closed and evacuated. Resin is pumped into the mold. When the mold is filled, resin

transfer is stopped and the part is allowed to cure. This is an important process for the production of large

boats, replacing the hand lay-up process.


Resin Transfer Molding


Resin Transfer Molding – Example


Summary

Processing Techniques Application

Injection molding: open shape with complex features

Blowing molding: thin-walled, seamless, bottle-like shape

Rotational molding: large, thin-walled, seamless, hollow shape

Thermoforming: large, flat, thin-walled shape

Documents

Shanghai Jiao Tong University 1 Polymers ME 250: Design & Manufacturing I School of Mechanical Engineering