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


If you are experiencing appearance issues:

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Knit Lines

The Problem

  • Blemish that occurs from the collision of two flow fronts

  • Reduces strength

  • Unsightly appearance

  • Often unavoidable due to geometry features

    • Holes

    • Tall standing cores

The Solution

  • Parts with holes will have knit lines

    • Mold part with hole filled and punch/machine the hole

    • Place knit line in acceptable area

  • Move gate?

    • If you’re lucky

  • Re-sequence the flow

    • If you have valve gates

  • Modify the part design

    • Flow leaders/restrictors

Air Traps

The Problem

  • Air that occupies the mold must get out as plastic flows

  • Supposed to be through vents

  • Part design/gate position can produce backfilling/racetracking

  • Non-fills or burned material

    • Compressed air pushes back on melt front

    • Compressed air = temp increase

The Solution

Existing tool - Faster filling

Existing tool - vacuum venting


Existing tool - Part Design Change

  • Air trap in optical area was unacceptable

  • Increased wall thickness in optical area

  • Air trap moved to acceptable area


Sink Marks

The Problem

  • Depression at the surface where back side features intersect

  • Sometimes in very thick sections

  • Caused by excessive shrinkage

    • Thicker section

    • Farther from gate

    • Higher temp

    • Higher shrink materials

    • Beware of corners!


The Solution

Existing tool - improving sink

  • Do a gate freeze study

  • Reduce rib/wall ratio

    • Welding involved

  • Increase pack pressure

    • Increases clamp force requirement

  • Move gate(s)

    • May not be possible

    • May cause unintended consequences

new tool - use simulation results

  • Reduce rib/wall ratio

    • No welding involved

    • Use sink savers?

    • Beware of short shots

  • Move gate(s)

    • Closer to areas of sink

  • Sink Depth Results

    • Keep below 0.1 mm

    • Below 0.025 mm for glossy finishes


The Problem

gate blush

  • Blemish extending from the gate

  • Caused by material degradation

    • Excessive shearing breaks molecular chains

    • Thin gates

    • Too few gates

    • Fill rate too fast


The Solution

Existing tool - improving gate blush

  • Slow down fill speed

    • May cause unintended consequences

  • Increase gate size

    • May be undesirable

    • May cause sink near gate area


new tool - use simulation results

  • Ensure proper number of gates

  • Increase gate size

  • Use ram-speed profiling

Make sure part is "feased"

  • Preliminary Feasibility Study Assures:

    • Part is good for tooling

    • Sufficient side wall draft

    • Rib-to-wall ratio

    • Proper fit/function

    • Basic die draw, die pull directions identified


Some Parting Thoughts

  • Use simulation upfront before mold is built

  • Try to hardwire solutions into part or mold design

  • Use Expert Moldflow engineers

  • Expertise helps us know when results are reasonable

    • Material Data

    • Mesh

    • Know how to:

      • Identify causes

      • Solve Problems

        • Mold/Part Design

        • Process to fix aesthetics