4 Design Chapter 1. Manufacturing Consideration

Manufacturing Consideration

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


•Injection molding is a series of

sequential process steps, each of which

has an influence on the properties of

the resultant partoMold filling

oPacking

oCooling

oEjection


•Gating

•Orientation

•Pressure losses

•Frozen in stress

•Shrinkage and Warpage

•Weld/Meld lines

•Flow leaders/restrictors

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

qualityoDetermines the mold filling pattern

oInduces shear and shear heating

oAffects shrinkage and warpage

Gating

•Gating determines the type and cost of

the moldoEdge or sub gated parts can be produced

with a standard cold runner two plate mold

oTop center gating or multiple top gating

required a three plate mold

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

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

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 partoShrinkageoStrengthoResidual stresses

Orientation

•Mold filling related orientation can be affected through process variables that affect mold filling pressure requirementsoFlow direction and speedoChannel dimensionsoTemperatures

•Residual Orientation = Orientation due to flow - relaxation

How Molecular Orientation

Occurs•Molecular orientation develops during

mold filling as the plastic is injected

through the nozzle, runner, gate and

cavity

•The polymer chains become stretched

out due to velocity gradients

•The orientation tends to be in the

direction of flow


Occurs•The blunted shape of most polymer melt

velocity profiles cause 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


Occurs

Effects of Molecular

Orientation•Orientation creates different directional

propertiesoStronger in the flow direction

oWeaker in the transverse direction

Effects of Molecular

Orientation•Typical directional property of an

injected molded part

Orientation

•The degree of orientation caused by mold filling is influenced by processing conditions, material properties, mold design and part designoLarge diameter runners, sprues, gates

along with shorter flow lengths will reduce orientation

oFaster fill rates and higher melt temperatures tend to promote molecular relaxation

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


•Assuming isothermal, laminar, Newtonian fluid (ok for

engineering estimate) the equations for pressure drop

and shear rate are:

oCylindrical Rectangular

r

L

W

H

L


• is the shear viscosityoPa-sec, lb-sec/in2

• is the apparent wall shear rateoSec-1

•Q is the volumetric flow rateoM3/s, ft3/s

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

Apparent vs Corrected Shear

Viscosity

•The corrected shear viscosity isoCylinder Rectangle

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


Example•High impact polystyrene ruler

oSprue 0.313”diameter by 2” lengthoRunner 0.25”diameter by 2.25” lengthoEdge Gate 0.08”deep by 0.4”wide by 0.12” lengthoCavity 0.1”deep by 1.5”wide by 6.03” length

Single cavity

•200 degree centigrade

•1.5 seconds fill time

•n=1


Example•Determine part volume

oCylinder

oRectangle V = L*W*H

•Sprue 0.154in3

•Runner 0.110in3

•Edge Gate 0.004in3

•Cavity 0.905in3


Example•Determine volumetric flow rateoFor single cavity mold

oQT=Qs=QR=QEG=QC

oQT=VT/tF

VT is total volume = 1.173in3

tF is fill time = 1.5 seconds

•QT=0.782in3/sec


Example•Determine apparent shear rate

oCylinder Rectangular

oSprue 259/sec

oRunner 510/sec

oEdge Gate 1830/sec

oCavity 312/sec


Example•Determine apparent shear viscosityoFrom figureoConversion 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


Example


Example•Determine true shear viscosity

oCylinder Rectangle

-n=1

•Sprue 0.046lb*sec/in2

•Runner 0.039lb*sec/in2

•Gate 0.026lb*sec/in2

•Cavity 0.044lb*sec/in2


Example•Determine pressure drop

•Cylinder Rectangular

•Sprue 305 psi

•Runner 716 psi

•Gate 149 psi

•Cavity 1650 psi

•Total 2820 psi

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 oChemical ResistanceoDimensional stabilityoImpact and tensile strength

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

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 shrinkageoWhen the part is ejected the thicker hotter

sections will continue to cool and shrink

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

PVT Behavior

Linear Mold Shrinkage

•Volumetric shrinkage can be predicted

theoretically if PVT characteristics and the

processing conditions

•We need linear shrinkage for cavity designoLinear Shrinkage = 1-(1-volumetric shrinkage)1/3

oCavity dimension=Part dimension/(1-linear

shrinkage)

oExpressed in in/in or mm/mm or %

Uneven Shrinkage and

Warpage•Uneven shrinkage is undesirable because it

can lead to not hitting dimensions, internal

stresses and warpage

•Main causesoDifferential shrinkage due to orientation

oDifferential cooling due to differences in cooling

rate from cavity to core

oCavity pressure differences due to too much

pressure drop through the cavity

Mold Shrinkage Data

Mold Shrinkage Sample

Problem•The material that a part is made from

has a volumetric shrinkage of 0.1in3/in3.

•What must the cavity dimensions be to

make a parto3.02 inches wide

o5.67 inches long

o0.1 inches thick

Mold Shrinkage Sample

Problem

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

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 balancedoOverpacking/underpackingoVariable shrinkageoResidual StressoTendency to warp

Flow Leaders and Restrictors

Weld and Meld Lines

•Formed during filling when melt flow front separates and recombines

•Caused byoMultiple gatesoCores/Holes

•Looks like a crack on the surface of the part

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

appearanceoVary part geometry, part wall thickness and

gating scheme

Weld and Meld Lines

•Processing conditions affect the weld

strengthoMolecular diffusion and entanglement are

necessary to improve weld strength Increase the temperature

Increase the pressure

4 Design Chapter 1. Manufacturing Consideration

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