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Manufacturing Consideration

Manufacturing Consideration

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Manufacturing Consideration

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  1. Manufacturing Consideration

  2. 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

  3. 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

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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. How Molecular Orientation Occurs

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

  15. Effects of Molecular Orientation • Typical directional property of an injected molded part

  16. 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

  17. 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

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

  19. Mold Filling Pressure Loses •  is the shear viscosity • Pa-sec, lb-sec/in2 •  is the apparent wall shear rate • Sec-1 • Q is the volumetric flow rate • M3/s, ft3/s

  20. 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

  21. Apparent vs Corrected Shear Viscosity • The corrected shear viscosity is • Cylinder Rectangle

  22. 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

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

  24. Estimating Pressure Drop Example • Determine part volume • Cylinder • V = *r2 *L • Rectangle • V = L*W*H • Sprue 0.154in3 • Runner 0.110in3 • Edge Gate 0.004in3 • Cavity 0.905in3

  25. Estimating Pressure Drop Example • Determine volumetric flow rate • For single cavity mold • QT=Qs=QR=QEG=QC • QT=VT/tF • VT is total volume = 1.173in3 • tF is fill time = 1.5 seconds • QT=0.782in3/sec

  26. Estimating Pressure Drop Example • Determine apparent shear rate • Cylinder Rectangular • Sprue 259/sec • Runner 510/sec • Edge Gate 1830/sec • Cavity 312/sec

  27. Estimating Pressure Drop Example • Determine apparent shear viscosity • From figure • Conversion factor • Lb*sec/in2 = 6894.7 Pa*sec • Sprue 320 Pa*sec 0.046lb*sec/in2 • Runner 270 Pa*sec 0.039lb*sec/in2 • Gate 180 Pa*sec 0.026lb*sec/in2 • Cavity 305 Pa*sec 0.044lb*sec/in2

  28. Estimating Pressure Drop Example

  29. Estimating Pressure Drop Example • Determine true shear viscosity • Cylinder Rectangle • n=1 • Sprue 0.046lb*sec/in2 • Runner 0.039lb*sec/in2 • Gate 0.026lb*sec/in2 • Cavity 0.044lb*sec/in2

  30. Estimating Pressure Drop Example • Determine pressure drop • Cylinder Rectangular • Sprue 305 psi • Runner 716 psi • Gate 149 psi • Cavity 1650 psi • Total 2820 psi

  31. 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

  32. 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

  33. 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

  34. 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

  35. PVT Behavior

  36. PVT Behavior

  37. 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 %

  38. 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

  39. Mold Shrinkage Data

  40. Mold Shrinkage Sample Problem • The material that a part is made from has a volumetric shrinkage of 0.1in3/in3. • What must be the cavity dimensions be to make a part • 3.02 inches wide • 5.67 inches long • 0.1 inches thick

  41. Mold Shrinkage Sample Problem

  42. 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

  43. 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

  44. Flow Leaders and Restrictors

  45. 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

  46. 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

  47. 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