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

Shanghai Jiao Tong University 2 Outline Introduction Polymer （聚合体） Processing Summary

Shanghai Jiao Tong University 3 Introduction Learning Objectives Polymers as Engineering Materials Chemical Structure of Polymers Commonly Employed Polymer Processing Techniques Applications Polymer Composites

Shanghai Jiao Tong University 4 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.

Shanghai Jiao Tong University 5 Polymers - Thermoplastics vs Thermosets Thermoplastics 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

Shanghai Jiao Tong University 6 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）

Shanghai Jiao Tong University 7 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.

Shanghai Jiao Tong University 8 Injection Molding Injection Molding-products

Shanghai Jiao Tong University 9 Injection Molding - Analysis Thermoplastics shrink during solidification. Cavity must be larger than part size. Dc=Dp+DpS+DpS 2 Dc: dimension of cavity [mm] Ds: dimension of part [mm] S: shrinkage value [mm/mm] DpS 2 : accounts for shrinkage

Shanghai Jiao Tong University 10 Injection Molding Shrinkage of typical t hermoplastics

Shanghai Jiao Tong University 11 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 = Dc= Dp+DpS+DpS 2 =80+80x x(0.025) 2 =82.05mm Shrinkage Injection pressure Compaction time Molding temperature Part thickness

Shanghai Jiao Tong University 12 Compression Molding Compression 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.

Shanghai Jiao Tong University 13 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.

Shanghai Jiao Tong University 14 Blow Molding Blow 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.

Shanghai Jiao Tong University 15 Blow Molding

Shanghai Jiao Tong University 16 Extrusion Extrusion 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.

Shanghai Jiao Tong University 17 Extrusion

Shanghai Jiao Tong University 18 Extrusion Extrusion is also used to make plastic bags and films

Shanghai Jiao Tong University 19 Thermoforming Thermoforming 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.

Shanghai Jiao Tong University 20 Thermoforming

Shanghai Jiao Tong University 21 Casting Casting 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）.

Shanghai Jiao Tong University 22 Casting

Shanghai Jiao Tong University 23 Reaction Injection Molding In 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.

Shanghai Jiao Tong University 24 Rotational Molding Rotational 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.

Shanghai Jiao Tong University 25 Polymer Composites A composite is a material composed of two or more different materials with properties superior to those of the individual components. A polymer composite is a polymeric material reinforced with a second material to obtain specific properties. The reinforcement can be in the form of laminates, particles, woven fibers, chopped fibers, flakes or a regular rigid reinforcement. These types of reinforcement are added to enhance mechanical properties. Discontinuous reinforcements such as flakes, particles and chopped fibers are good at enhancing short term mechanical properties but are not as effective as continuous reinforcements in increasing creep strength and other long term mechanical properties.

Shanghai Jiao Tong University 26 Polymer Composites A variety of reinforcements can be used in polymer composites. These vary from glass, metals, asbestos, boron fibers, carbon, to polymers and fabrics

Shanghai Jiao Tong University 27 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.

Shanghai Jiao Tong University 28 Glass Fiber Reinforced Polymer Composites 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.

Shanghai Jiao Tong University 29 Filament Winding

Shanghai Jiao Tong University 30 Glass Fiber Reinforced Polymer Composites 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.

Shanghai Jiao Tong University 31 Pultrusion - Examples

Shanghai Jiao Tong University 32 Glass Fiber Reinforced Polymer Composites 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.

Shanghai Jiao Tong University 33 Resin Transfer Molding

Shanghai Jiao Tong University 34 Resin Transfer Molding – Example

Shanghai Jiao Tong University 35 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