
Ask The Moldflow Experts
Appearance Issues in Moldflow
Ask The Moldflow Experts Webinar Topic:
Appearance Issues in Moldflow
November 30th, 2021 | 12:30 PM CDT
One of the most time-consuming and frustrating things about injection molding can be chasing appearance issues after all of the efforts that go into engineering the part and mold. Join our webinar to see how CAE Services’ Experts solve appearance problems and plan to avoid them in the first place.
- Learn different approaches to solving/avoiding knit lines in your part;
- Learn how part design and gate locations can affect the formation of air traps;
- Learn how to size gates properly to avoid gate blush;
- Discover the causes of sink marks and how to avoid them;
- In-depth topic discussion with questions fielded during the presentation + Q&A at the end of the webinar

ATME - Take Away

Knit Lines
The Problem

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Blemish that occurs from the collision of two flow fronts
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Reduces strength
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Unsightly appearance
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Often unavoidable due to geometry features
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Holes
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Tall standing cores
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The Solution

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Parts with holes will have knit lines
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Mold part with hole filled and punch/machine the hole
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Place knit line in acceptable area
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Move gate?
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If you’re lucky
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Re-sequence the flow
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If you have valve gates
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Modify the part design
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Flow leaders/restrictors
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Air Traps
The Problem

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Air that occupies the mold must get out as plastic flows
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Supposed to be through vents
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Part design/gate position can produce backfilling/racetracking
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Non-fills or burned material
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Compressed air pushes back on melt front
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Compressed air = temp increase
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The Solution
Existing tool - Faster filling


Existing tool - vacuum venting

Existing tool - Part Design Change
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Air trap in optical area was unacceptable
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Increased wall thickness in optical area
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Air trap moved to acceptable area

Sink Marks
The Problem
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Depression at the surface where back side features intersect
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Sometimes in very thick sections
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Caused by excessive shrinkage
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Thicker section
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Farther from gate
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Higher temp
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Higher shrink materials
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Beware of corners!
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The Solution
Existing tool - improving sink

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Do a gate freeze study
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Reduce rib/wall ratio
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Welding involved
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Increase pack pressure
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Increases clamp force requirement
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Move gate(s)
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May not be possible
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May cause unintended consequences
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new tool - use simulation results

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Reduce rib/wall ratio
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No welding involved
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Use sink savers?
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Beware of short shots
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Move gate(s)
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Closer to areas of sink
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Sink Depth Results
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Keep below 0.1 mm
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Below 0.025 mm for glossy finishes
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Blush
The Problem
gate blush
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Blemish extending from the gate
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Caused by material degradation
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Excessive shearing breaks molecular chains
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Thin gates
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Too few gates
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Fill rate too fast
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The Solution
Existing tool - improving gate blush
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Slow down fill speed
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May cause unintended consequences
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Increase gate size
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May be undesirable
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May cause sink near gate area
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new tool - use simulation results
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Ensure proper number of gates
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Increase gate size
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Use ram-speed profiling
Make sure part is "feased"
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Preliminary Feasibility Study Assures:
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Part is good for tooling
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Sufficient side wall draft
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Rib-to-wall ratio
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Proper fit/function
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Basic die draw, die pull directions identified
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Some Parting Thoughts
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Use simulation upfront before mold is built
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Try to hardwire solutions into part or mold design
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Use Expert Moldflow engineers
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Expertise helps us know when results are reasonable
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Material Data
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Mesh
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Know how to:
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Identify causes
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Solve Problems
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Mold/Part Design
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Process to fix aesthetics
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