Addressing Sink Marks and Voids in Injection Molding

Causes:

• Insufficient Injection Pressure or Holding Pressure:Low pressure can lead to inadequate material compression and compensation for shrinkage.
• Short Injection or Holding Time:Not allowing enough time for the molten plastic to pack the mold cavity sufficiently.
• Slow Injection Rate:Can result in premature cooling of the melt front, leading to incomplete filling and pressure loss.
• High Melt or Mold Temperature:Can exacerbate shrinkage by increasing the temperature differential during cooling.
• Inadequate Cooling:If the part is ejected before sufficient cooling, it can deform and develop sink marks.
• Low Insert Temperature:If inserts are too cold, the surrounding plastic may shrink excessively.
• Small Nozzle Hole or Partial Blockage:Can cause significant pressure drops, hindering proper filling.
• Insufficient Material Supply:Directly leads to underfilling and subsequent shrinkage.

Solutions:

• Increase Injection Pressure and Speed:Enhance melt compression density and ensure complete filling.
• Extend Injection and Holding Time:Allow adequate time for melt compensation and reduce shrinkage. However, avoid excessive holding pressure, which can cause flash.
• Optimize Cooling:Adjust barrel temperature to lower melt temperature. Improve mold cooling system by lowering cooling water temperature or enhancing cooling in affected areas, ensuring uniform cooling.
• Increase Insert Temperature:If defects occur around inserts, raise their temperature.
• Inspect and Clean Nozzle:Replace or clean nozzles if they are too small or blocked.
• Increase Material Feed:Ensure sufficient material is supplied to the mold.

Causes:

• Small Runner and Gate Cross-Sections:Restrict melt flow, causing excessive pressure loss and hindering material compensation.
• Asymmetrical Gate Placement or Uneven Filling:Leads to inconsistent pressure distribution and localized shrinkage.
• Unreasonable Feed Port Position:If gates are not placed at thick sections, material may not adequately pack these areas.
• Poor Mold Venting:Trapped air can prevent complete filling and proper packing.
• Mold Wear or Pressure Release:Worn molds can lose pressure, leading to insufficient packing.

Solutions:

• Enlarge Runner and Gate Cross-Sections:Reduce flow resistance and improve pressure transmission.
• Optimize Gate Placement:Position gates symmetrically and, ideally, at the thickest sections of the part.
• Improve Gating System:For defects far from the gate, enlarge the gating system, especially at "bottleneck" areas, to ensure smooth melt flow and pressure transmission.
• Utilize Wing Gates for Thick-Walled Parts:For thick-walled parts where direct gating is problematic, a wing gate can be used. This involves attaching a small wing to the part, placing the gate (e.g., reverse gate or pin gate) on the wing, and then removing the wing after molding. This transfers the sink mark defect to the disposable wing.
• Regular Mold Maintenance:Regularly check for mold wear and poor venting. Replace worn parts and improve mold venting conditions.

Causes:

• High Material Shrinkage Rate or Poor Flowability:Materials with high shrinkage or low flow can exacerbate sink marks and voids.
• Insufficient Lubricant in Material:Can hinder melt flow and packing.
• Moisture in Raw Material:Can lead to gas formation and voids.

Solutions:

• Select Low-Shrinkage Resins:For parts with high surface quality requirements, choose resin grades with lower shrinkage rates.
• Add Lubricants:If poor melt flow causes underfilling and sink marks, add an appropriate amount of lubricant to improve flowability, or increase the size of the gating system.
• Pre-dry Raw Materials:If moisture is the cause, pre-dry the raw materials thoroughly.

Causes:

• Significant Wall Thickness Variations:Thick sections are prone to sink marks due to insufficient pressure and localized shrinkage.

Solutions:

• Maintain Uniform Wall Thickness:Design parts with consistent wall thickness as much as possible. If variations are unavoidable, adjust the gating system parameters to compensate.