Plastics: Mold Filling, Weld Lines, and Gating

In the third training course of this six-part series, ’Mold Filling, Gating and Weld Lines’ provides the learner with information needed to successfully work with plastic part design. Topics explain fatigue and cyclic stress, gating and weld line considerations, and mold filling processes. Gate types, location, and importance is described; as well as an explanation regarding weld line occurrence and strength determination. All training within this series was developed by Dr. Robert Malloy, a respected author and professor at the University of Massachusetts, Lowell.

Any worker who is or who is training to be an injection molding part designer.

• General Filling Process
• Pressure Drop
• Viscosity, Shear Rate, and Polymer Orientation
• Mold Filling Simulations
• Gating Techniques: Manually Degated Gated
• Gating Techniques: Self-Degating Gates
• Gating Techniques: Hot Runner Gating
• Gating Techniques: Gate Location and Multiple Gates
• Weld and Meld Lines
• Improving Weld Line Strength: Flow Length
• Improving Weld Line Strength: Venting
• Improving Weld Line Strength: Process Variables
• Improving Weld Line Strength: Material Variables
• Improving Weld Line Strength: Other Methods
• Computer Simulations for Weld Lines

• Know the four steps that comprise the mold filling process.
  • List the four steps of the mold filling process.
  • Identify when filling stage ends.
• Recognize the pressure required to force the polymer through the cavity.
  • Agree that a great deal of pressure is required to force the polymer to cavity.
  • Explain why a great deal of pressure is required to force the polymer to cavity.
• Know the many methods used to measure a polymer’s viscosity.
  • Define 'viscosity'.
  • Define the 'Shear Rate'.
• Be aware of the information that can be extracted from mold filling simulations.
  • Select both Cooling and shear heating as taking place during filling.
  • Recall that the material close to the wall cools and solidifies.
• Identify the gates that are used for parts that require manual degating.
  • Agree that manually degated gates must be removed from the part.
  • Choose the most common gate.
• Recognize the gates that are removed from the part during opening or ejection.
  • Explain how the gate stays with the runner and is sheared from the part.
• Know how a heated runner can transfer material from the sprue to the mold.
  • Describe how material is transferred to the mold.
  • Select one of the most simplistic Hot Runner Systems.
• Recognize that gate location must be addressed during the part design process.
  • Recall that the gate location must be addressed during the part design process.
  • Agree that a single gate may not provide enough material to fill sufficiently.
• Identify the points at where melt fronts intersect.
  • Describe Weld and Meld Lines.
  • Explain how Meld Lines are formed.
• Recognize how to reduce the Pressure Drop and promote uniform packing.
  • Choose Flow Length as a significant factor in Weld Line strength.
  • Explain that the farther the weld line is from the gate, the cooler the melt front.
• Know how to significantly lower Weld Line strength.
  • Agree that improper venting can also significantly lower Weld Line strength.
  • Explain what happens to gas when a Weld Line forms.
• Recognize that there are many variables that can impact weld line strength.
  • Agree that Melt Temperature can play an important role in Weld Line strength.
  • Recognize that Mold Temperature can play a major role in Weld Line formation.
• Recognize that material aspects can affect weld line strength.
  • Explain what happens to materials with fillers or reinforcements.
  • Recall that other additives also decrease Weld Line strength.
• Identify the methods used for improving weld line strength.
  • Explain the purpose of the SCORIM process.
  • Recall that some molders can manipulate Valve Gates to inject sequentially.
• Recognize the usefulness of filling simulation to aid in weld line detection.
  • Explain that computer simulations for Weld Line detection and strength can be nearly impossible without the aid of a filling simulation.