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Manufacturing Consideration

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Manufacturing Considerations Injection Molding is a high speed, automated process that can be used to produce simple to very complex parts The part designer must recognize that the design of the part determines the ease of molding, the tooling requirements and the cost Also the designer must recognize that the properties of the part are greatly affected by the mold design and processing conditions

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Manufacturing Consideration Injection molding is a series of sequential process steps, each of which has an influence on the properties of the resultant part –Mold filling –Packing –Cooling –Ejection

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Manufacturing Consideration Gating Orientation Pressure losses Frozen in stress Shrinkage and Warpage Weld/Meld lines Flow leaders/restrictors

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Gating The gate is the melted plastics entry into the mold cavity Usually the thinnest cross section in the system The gate type, number of gates and gate location has a dramatic effect on overall part quality –Determines the mold filling pattern –Induces shear and shear heating –Affects shrinkage and warpage

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Gating Gating determines the type and cost of the mold –Edge or sub gated parts can be produced with a standard cold runner two plate mold –Top center gating or multiple top gating required a three plate mold

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Gate Design Rules Gate centrally to provide equal flow length Gate symmetrically to avoid warpage Gate into thicker sections for better filling and packing Gate long, narrow parts from an end for uniform flow

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Gate Design Rules Position the gate away from load-bearing areas Hide the gate scar Gate for proper weld-line location and strong weld lines Multiple gates shorten flow lengths Locate gates on either side of a weak core or insert

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Orientation Almost all injection molded parts have some degree of frozen-in molecular orientation The degree is determined by the molecular weight, relaxation characteristics, and processing conditions Orientation greatly affects the properties of the part –Shrinkage –Strength –Residual stresses

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Orientation Mold filling related orientation can be affected through process variables that affect mold filling pressure requirements –Flow direction and speed –Channel dimensions –Temperatures Residual Orientation = Orientation due to flow - relaxation

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How Molecular Orientation Occurs Molecular orientation develops during mold filling as the plastic is injected through the nozzles, runner, gate and cavity The polymer chains become stretched out due to velocity gradients The orientation tends to be in the direction of flow

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How Molecular Orientation Occurs The blunted shape of most polymer melt velocity profile causes most of the orientation to occur toward the surface. The molecules at the core remain random Extreme in injection molding where the melt adjacent to the cold mold will freeze first, leading to high interfacial shear stresses and not allowing for relaxation Problems are most significant for higher molecular weight plastics and fiber reinforced plastics

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How Molecular Orientation Occurs

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Effects of Molecular Orientation Orientation creates different directional properties –Stronger is the flow direction –Weaker in the transverse direction

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Effects of Molecular Orientation Typical directional property of an injected molded part

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Orientation The degree of orientation caused by mold filling is influenced by processing conditions, material properties, mold design and part design –Large diameter runners, sprues, gates along with shorter flow lengths will reduce orientation –Faster fill rates and higher melt temperatures tend to promote molecular relaxation

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Mold Filling Pressure Loses When selecting a gate location, it should be such that the mold fills uniformly, the pressure drop is not excessive and the shear rate does not exceed the limit of the polymer The designer must obtain an estimate of the pressure drop to evaluate the moldability of the part with respect to a proposed gating scheme The pressure drop depends on the material, mold and processing conditions

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Mold Filling Pressure Loses Assuming isothermal, laminar, Newtonian fluid (ok for engineering estimate) the equations for pressure drop and shear rate are: –Cylindrical Rectangular r L W H L

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Mold Filling Pressure Loses is the shear viscosity –Pa-sec, lb-sec/in 2 is the apparent wall shear rate –Sec -1 Q is the volumetric flow rate –M 3 /s, ft 3 /s

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Apparent vs Corrected Shear Viscosity Most viscosity data is of the form apparent shear viscosity at the wall as a function of wall shear rate and temperature If shear viscosity is described as apparent, it is not corrected for pseudo-plastic behavior

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Apparent vs Corrected Shear Viscosity The corrected shear viscosity is –Cylinder Rectangle

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Estimating Pressure Drop Determine part volume Determine volumetric flow rate Determine apparent shear rate Determine apparent shear viscosity Determine true shear viscosity Determine pressure drop

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Estimating Pressure Drop Example High impact polystyrene ruler –Sprue 0.313diameter by 2 length –Runner 0.25diameter by 2.25 length –Edge Gate 0.08deep by 0.4wide by 0.12 length –Cavity 0.1deep by 1.5wide by 6.03 length Single cavity 200 degree centigrade 1.5 seconds fill time n=1

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Estimating Pressure Drop Example Determine part volume –Cylinder V = *r 2 *L –Rectangle V = L*W*H Sprue0.154in 3 Runner0.110in 3 Edge Gate0.004in 3 Cavity0.905in 3

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Estimating Pressure Drop Example Determine volumetric flow rate –For single cavity mold –Q T =Q s =Q R =Q EG =Q C –Q T =V T/ t F V T is total volume = 1.173in 3 t F is fill time = 1.5 seconds Q T =0.782in 3 /sec

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Estimating Pressure Drop Example Determine apparent shear rate –CylinderRectangular –Sprue259/sec –Runner510/sec –Edge Gate1830/sec –Cavity312/sec

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Estimating Pressure Drop Example Determine apparent shear viscosity –From figure –Conversion factor Lb*sec/in 2 = 6894.7 Pa*sec Sprue320 Pa*sec0.046lb*sec/in 2 Runner270 Pa*sec0.039lb*sec/in 2 Gate180 Pa*sec0.026lb*sec/in 2 Cavity305 Pa*sec0.044lb*sec/in 2

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Estimating Pressure Drop Example

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Determine true shear viscosity –CylinderRectangle –n=1 Sprue 0.046lb*sec/in 2 Runner 0.039lb*sec/in 2 Gate0.026lb*sec/in 2 Cavity 0.044lb*sec/in 2

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Estimating Pressure Drop Example Determine pressure drop CylinderRectangular Sprue305 psi Runner716 psi Gate149 psi Cavity1650 psi Total2820 psi

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Frozen in Stress Molding factors, such as uneven part cooling, differential material shrinkage or frozen in flow stresses cause undesirable residual stress Residual stresses can adversely affect –Chemical Resistance –Dimensional stability –Impact and tensile strength

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Shrinkage and Warpage Injection molding is used to produce parts with fairly tight dimensional tolerances Many plastics exhibit relatively large mold shrinkage values If a plastic exhibits uneven directional shrinkage, warpage will result Shrinkage is affected by the material, the mold, the part geometry and the processing conditions

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Shrinkage and Warpage Parts with thick and thin wall sections can easily warp because the thick sections take longer to pack and cool, resulting in uneven shrinkage –When the part is ejected the thicker hotter sections will continue to cool and shrink

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PVT Behavior of Plastics Plastics have a positive coefficient of thermal expansion and are highly compressible in the molten state Volume of any given mass will change with both temperature and pressure Semi-crystalline plastics shrink more than amorphous because of the ordered crystalline regions

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PVT Behavior

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Linear Mold Shrinkage Volumetric shrinkage can be predicted theoretically if PVT characteristics and the processing conditions We need linear shrinkage for cavity design –Linear Shrinkage = 1-(1-volumetric shrinkage) 1/3 –Cavity dimension=Part dimension/(1-linear shrinkage) –Expressed in in/in or mm/mm or %

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Uneven Shrinkage and Warpage Uneven shrinkage is undesirable because it can lead to not hitting dimensions, internal stresses and warpage Main causes –Differential shrinkage due to orientation –Differential cooling due to differences in cooling rate from cavity to core –Cavity pressure differences due to too much pressure drop through the cavity

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Mold Shrinkage Data

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Mold Shrinkage Sample Problem The material that a part is made from has a volumetric shrinkage of 0.1in 3 /in 3. What must be the cavity dimensions be to make a part –3.02 inches wide –5.67 inches long –0.1 inches thick

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Mold Shrinkage Sample Problem

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Flow Leader and Restrictors Ideally the melt should flow from the gate, reaching the extremities of the cavity all at the same time To achieve balanced fill, the filling pressure drop associated with each and every flow path must be equal Pressure drops can be balanced by making local adjustments in the part wall thickness

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Flow Leader and Restrictors Flow Leader are local increases in wall thickness to promote flow Flow restrictors are local decreases in wall thickness to reduce flow If flow is not balance –Overpacking/underpacking –Variable shrinkage –Residual Stress –Tendency to warp

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Flow Leaders and Restrictors

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Weld and Meld Lines Formed during filling when melt flow front separates and recombines Cause by –Multiple gates –Cores/Holes Looks like a crack on the surface of the part

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Weld and Meld Lines The strength of the weld line can be significantly lower Try to eliminate completely or locate in non critical area in terms of load and appearance –Vary part geometry, part wall thickness and gating scheme

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Weld and Meld Lines Processing conditions affects the weld strength –Molecular diffusion and entanglement are necessary to improve weld strength Increase the temperature Increase the pressure

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