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Glossary: Design Terms
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A
- Acceptable runner/cavity ratio
- runner systems designed for high pressure drops to minimize material usage and increase frictional heating in the runner.
- Amorphous
- non-crystalline. Material assumes more random molecular structure when cooling.
- Artificially balanced runner system
- balancing a runner system by adjusting the pressure drop of a long large diameter runner against
a short small diameter runner. Since pressure drop over the small diameter runner will be much
more affected by heat loss than the large diameter runner, an artificially balanced runner system
will work with a set range of molding conditions. The width of this range of molding conditions
determines the stability of the molding.
B
- Backflow
- the melt flows back out of the mold, returning to the runner(s) thru the gate.
- Balanced runner
- in injection molding, a runner system designed to place all cavities at the same distance from the sprue.
C
- Cavity
- the interior opening in the mold where the part is formed.
- Commonly Used Conversions
-
KPa x 0.145 = psi
MPa x 145 = psi
ºC x 1.8 + 32 = ºF
Liters/min x 0.2642 = Gal/min
Inches x 25.4 = mm
Flow rate = ((# of cavities) x (volume per cavity))/(injection time)
- Constant pressure gradient
- pressure drop per unit length. The constant pressure gradient principle says that the most efficient
filling pattern is when the pressure gradient is constant along the flow path.
- Controlled frictional heating
- runners designed to deliver a higher melt temperature to the cavity. This results in lower stress
levels in the part without causing material degradation due to long exposure to elevated temperatures.
- Cooling time
- the elapsed time required for the melt to reach its ejection temperature.
- Crystalline
- material assuming fixed internal forms. Slow cooling areas have high crystallinity and shrinkage.
- Cycle
- complete, repeating sequence of operations for injection molding a part.
- Cycle time
- elapsed time between a certain point in one cycle and the same point in the next cycle.
D
- Differential cooling
- occurs when one area of the part cools at a different rate or when the mold surfaces are at different
temperatures. Warping results from differential cooling.
- Dimensional stability
- controlled expansion and shrinkage rates.
- Dominant flow
- at the juncture of two confronting flows the dominant flow will reverse the direction of the other.
- Draft
- degree of taper of the cavity/core side walls, allowing the part to be removed easily from the mold.
E
- Edge gate
- entrance to the part from the runner located on the parting line.
- Element
- a triangle defined by three nodes, creating the basis for the finite element analysis.
- End of flow
- the melt is just touching the last part of the mold to be filled and the pressure at that point
is zero.
- Ejector pin
- part kick-out mechanism.
F
- Family mold
- a mold which produces non-identical parts simultaneously.
- Filler
- material added to the molding material to reduce cost and/or improve mechanical properties.
- Fill pattern
- the contours of the advance of the material as the cavity fills. (See flow pattern)
- Fill pressure
- the pressure required to fill the cavity.
- Finite element analysis
- the solution of simultaneous equations for each element (triangle) with resulting pressure, temperature
and elapsed time at each node and the direction and speed of the material (at the instant of fill).
- Flash
- excess material retained by the part along the parting line.
- Flow balancing
- modifying flow paths, particularly runner sections, so that all flow paths within a mold fill
in equal time with equal pressure.
- Flow deflector
- local reduction in thickness to divert flow.
- Flow leader
- local increase in thickness to encourage flow in a particular direction.
- Flow pattern
- the contour the melt takes sequentially as it fills the cavity. A properly designed mold should fill with a straight
flow front with no changes in direction throughout filling.
- Flow rate
- the volume of material pa/ssing a fixed point per unit time.
- Freeze off
- the temperature of the material is reduced to the point that it blocks an area it would fill if
it were hotter.
- Frictional heating
- heat generated by the friction of the chains of molecules slipping past each other.
G
- Gate
- the narrowed link between the runner and the cavity.
- Glass transition
- change in an amorphous polymer from viscous to hard and relatively brittle.
H
- Hesitation effect
- occurs in parts of varied thicknesses. The flow moves preferentially into a thicker area causing
an adjacent thin area to freeze off while the thicker area fills. Gates should be positioned as
far as po/ssible from where the flow divides into thick and thin flow paths.
- Hygroscopic
- readily absorbs moisture.
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