Glossary: Design Terms
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- Racetrack effect
- thicker area in flow cross section speeds up the flow.
- Regrind
- previously molded material re-entering the proce/ssing cycle.
- Reinforced plastics
- high strength fillers imbedded in the part.
- Release agent
- the mold, coated on mold or formulated in material which facilitates removing the part.
- Rib
- a reinforcing section of the part.
- Runner design
- using the runner as a flow control device (positioning the gate and using the size of the runner
to control the filling pattern within the cavity) in addition to getting the melt into the cavity.
- Runner balancing
- developing a runner system which delivers the required amount of melt to each cavity with the
correct pressure to finish filling all the cavities simultaneously at the correct temperature
for the part.
- Runners
- channels cut in the mold which transport the melt from the entrance to the mold to the
entrance to the cavity.
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- Scrap
- any output of a mold that is not usable as the primary product as defined by part quality standard.
- Shear rate
- the rate at which a layer of melt slides over the layer below. Shear rate is velocity-related
rather than force- related.
- Shear stress
- the shearing force divided by the area. It is always a maximum at the outside of the flow channel.
As it is force-related, it depends on the viscosity of the material, which in turn depends on
the material and molding conditions. The maximum allowable stress level is usually taken as 1%
of the tensile strength of the material. High shear stress is unimportant at gates, and in sprues
and runners.
- Short shot
- incomplete filling of part.
- Short shot progre/ssion
- sequential views of the flow front through the cavity.
- Shrinkage
- contraction upon cooling of all or areas of the part. Shrinkage occurs less is disorientated material
and more across chains of molecules than along their lengths. Lower pack area have lower areas
of orientation and shrinkage.
- Single cavity mold
- produces one part with each cycle.
- Sink marks
- an indentation on the surface of the part as a result of significant local change in wall section.
The mark will occur in the thicker area.
- Skin
- a relatively dense layer at the surface of the part.
- Slide
- projection in the mold used to form the geometry of the part, which is not in the direction of
the closing of the mold and must be withdrawn before the part can be ejected.
- Splay
- silver streaks like marks caused by moisture in material, material degradation, or wiping of the entrapped air on surface of mold.
- Sprue
- the entrance to the mold interior into which the melt is injected.
- Stress Concentrators
- abrupt changes in geometry of the part serve as the focus of high stresses. Various means can
be devised to relieve the abruptness of the geometric changes and thus the stresses.
- Structural strength
- integrity of the molded part. Factors that influence structural strength are material selection, part design, proce/ssing conditions and application environment.
- Subgate
- entrance to the part from the runner located below the parting line. On ejection the part breaks
away from the subgate.
- Submolding
- break up of a full molding into component areas.
- Surface appearance
- the esthetics surface of molded part. Mold construction, part design, material selection and molding conditions can affect the part surface appearance.
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- Tabgate
- edge-gating or sub-gating into a tab added to the part for gating purposes only.
- Thermal conductivity
- ability of a material to conduct heat.
- Thermal degradation
- deterioration of the material by heat or time subjected to heat, characterized by molecular sci/ssion.
- Thermal shut-off
- material freezes causing blockage.
- Thermoplastic
- material that will repeatedly soften when heated and harden when cooled.
- Thermoset
- material that has undergone a chemical reaction so that it remains hardened, even when reheated, characterized by molecular cross linking.
- Thickness
- depth of the material and contributory to pressure requirements; thickening reduces the pressure
required to fill the part.
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- Undercut
- a protuberance or indentation that impedes withdrawal from a mold.
- Underflow
- the dominant flow of two confronting flows, over the other. The lesser flow reverses direction
giving poor surface appearance and structural strength. Underflow should be avoided by positioning
gates so that the flow fronts meet at the end of filling.
- Unidirectional flow pattern
- plastic flowing in one direction with a straight flow front throughout filling.
- Uniform cooling time
- cooling time the same throughout the part to avoid warping.
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- Vicat softening point
- the temperature at which a flat ended needle will penetrate a specimen under a specified load
using a uniform rate of temperature rise.
- Virgin material
- never processed other than that required for initial pellet manufacture.
- Viscosity
- resistance to flow of a liquid.
- Void
- an unfilled space within a solid material.
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- Warping
- acquired contortion of the part due to differential shrinking of the material.
- Weld line
- the intersection of two confronting flow fronts creating areas of local weakness. Weld lines should
be positioned in the least sensitive areas.
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