Voids in Injection Molding

Introduction

Voids are internal cavities or empty spaces that form inside an injection molded part.

Unlike sink marks, which appear as visible depressions on the surface, voids may remain hidden inside the component. This makes them particularly important for parts that rely on structural strength, pressure resistance, sealing, dimensional stability, or consistent mechanical performance.

Voids commonly develop in thick sections where the outer material solidifies first while the interior continues to cool and shrink. If the shrinking core cannot receive enough additional material during packing, an internal cavity may form.

Void formation can be influenced by:

• Excessive wall thickness
• Local material buildup
• Insufficient packing pressure
• Insufficient holding time
• Premature gate freeze
• Poor gate location
• Uneven cooling
• Material condition
• Part geometry

Because voids are often internal, troubleshooting requires engineers to evaluate not only the visible part but also the filling, packing, cooling, and shrinkage behavior of the entire molded section.

What Are Voids in Injection Molding?

A void is an internal cavity formed when material shrinks away from itself during solidification.

During injection molding, the outside surfaces of a plastic part usually cool first because they are in direct contact with the mold.

The interior of a thick section remains molten for longer.

As this inner material cools, it contracts. If sufficient molten plastic can still flow through the gate and runner system during the holding stage, packing pressure can compensate for some of this shrinkage.

If additional material can no longer reach the shrinking region, the center of the section may separate internally and create a void.

Voids are commonly found in:

• Thick walls
• Thick bosses
• Heavy rib intersections
• Large structural sections
• Corners with excess material
• Areas far from the gate

The location and shape of a void can provide useful information about the underlying cause.

Voids vs. Sink Marks

Voids and sink marks are closely related because both can result from volumetric shrinkage in thick sections.

The difference is where the deformation occurs.

Sink Mark

A sink mark forms when the outer surface is pulled inward as the interior shrinks.

The result is a visible surface depression.

Void

A void forms when the outer surface has already become rigid enough to resist inward movement.

Instead of pulling the surface inward, the shrinking interior separates internally and creates an empty cavity.

A thick section may therefore develop either:

Internal shrinkage → Surface moves inward → Sink Mark

or:

Internal shrinkage → Surface remains rigid → Internal Void

The final defect depends on part geometry, material behavior, mold temperature, cooling rate, and packing conditions.

What Causes Voids in Injection Molding?

1. Excessive Wall Thickness

Excessive wall thickness is one of the most common design-related causes of internal voids.

Thick sections cool from the outside inward.

The surface may already be solid while the center remains hot and molten. As the core eventually cools, it contracts more significantly.

If this shrinkage is not compensated during packing, a cavity may develop inside the section.

Maintaining a more uniform wall thickness can help reduce this thermal imbalance and improve solidification behavior. AccuMolds' wall-thickness guidance similarly identifies voids among the defects associated with improper wall-thickness design.

2. Material Buildup Around Bosses and Ribs

Bosses, ribs, gussets, and other structural features can create unexpectedly thick intersections.

A boss that joins directly to a thick wall, for example, may create a large local mass even if the surrounding nominal wall thickness is acceptable.

Potential high-risk areas include:

• Solid bosses
• Thick rib roots
• Multiple ribs meeting at one point
• Heavy corner intersections
• Large attachment features

These regions retain heat longer and can develop localized shrinkage.

3. Insufficient Packing Pressure

After the cavity fills, holding pressure pushes additional material into the cavity as the plastic begins to shrink.

If packing pressure is too low, the molding machine may not supply enough material to compensate for volumetric contraction.

This can increase the likelihood of:

• Voids
• Sink marks
• Dimensional variation
• Reduced part weight

However, packing pressure should be adjusted systematically. Excessive pressure can create other issues such as residual stress or flash.

4. Insufficient Holding Time

Holding pressure can only compensate for shrinkage while the gate remains open.

If holding time ends too early, pressure may be removed while the interior of the part is still shrinking.

Additional material can no longer enter the cavity, increasing the probability of an internal void.

Process development should therefore evaluate both:

• Holding pressure
• Holding time

rather than adjusting one independently.

5. Premature Gate Freeze

Even with adequate machine settings, packing becomes ineffective once the gate freezes.

A gate that is too small may solidify before the thickest areas of the part have completed enough of their shrinkage.

This prevents pressure from reaching the remaining molten core.

Gate design should therefore consider:

• Gate size
• Gate thickness
• Gate location
• Flow length
• Part thickness
• Material behavior

6. Poor Gate Location

Gate location affects how effectively holding pressure reaches different regions of the cavity.

If a thick section is far from the gate, pressure losses through the flow path may reduce the amount of compensation available during shrinkage.

In some cases, moving the gate closer to the heavy section or changing the flow strategy can improve packing effectiveness.

7. Uneven Cooling

Cooling conditions can also influence whether internal shrinkage develops into a void.

A thick region that remains hot significantly longer than surrounding sections may continue contracting after neighboring areas have already solidified.

Potential contributors include:

• Poor cooling-channel placement
• Deep cores
• Heavy bosses
• Local hot spots
• Asymmetric cooling

Cooling design should therefore be evaluated together with wall thickness and packing.

How Part Design Influences Void Formation

Part design has a major impact on the likelihood of internal voids.

Thick Sections

The thicker the section, the longer the center remains molten.

Where possible, designers should avoid large solid masses.

Solid Bosses

A solid boss may create significant material concentration.

Coring the boss can reduce section thickness while maintaining the required functional geometry.

Heavy Rib Intersections

Several ribs meeting at one location can create a thick internal node.

Rib thickness, spacing, and connection geometry should be reviewed as an integrated system.

Thick Corners

Sharp or heavy corners may contain more material than the adjacent walls.

Using appropriate radii and more gradual transitions can help maintain a more consistent section.

How Mold Design Influences Voids

Mold design determines how material reaches the cavity and how heat leaves the part.

Gate Design

The gate must remain open long enough to support effective packing.

For heavy sections, engineers may need to review:

• Gate cross-section
• Gate location
• Runner pressure loss
• Number of gates

Cooling Layout

Cooling channels should provide controlled heat removal around thick regions.

Hot spots around large bosses, deep cores, or heavy intersections may require additional cooling consideration.

Pressure Transfer

A mold may fill completely but still pack poorly.

The important question is not only whether material reaches the region during filling, but whether pressure can still reach that region during solidification.

How to Troubleshoot Voids

Troubleshooting should begin with the void location and the local part geometry.

Void in a Thick Section

Check:

• Local wall thickness
• Material buildup
• Holding pressure
• Holding time
• Gate freeze time
• Local cooling

A recurring void in the center of a heavy section usually indicates that shrinkage compensation is insufficient.

Void Near a Boss or Rib Intersection

Check:

• Boss coring
• Rib thickness
• Rib-to-wall connection
• Heavy intersections
• Local cooling

Reducing the local material mass may be more effective than simply increasing process pressure.

Voids Far from the Gate

Check:

• Gate location
• Pressure loss
• Flow length
• Packing effectiveness
• Gate size

If the defect occurs consistently in a remote region, the pressure path may be limiting compensation.

Voids Appear After a Process Change

Review:

• Holding pressure
• Holding time
• Melt temperature
• Mold temperature
• Cooling time
• Material lot or drying condition

Some engineering plastics require controlled drying before molding, and poor material preparation can contribute to internal defects and inconsistent performance. AccuMolds' injection molding process guide notes that improper drying can contribute to internal voids in moisture-sensitive materials.

How to Prevent Voids

Maintain More Uniform Wall Thickness

Avoid unnecessarily thick sections and abrupt material buildup.

Where additional stiffness is needed, use properly designed ribs rather than simply increasing wall thickness.

Core Out Heavy Features

Bosses and other large features can often be hollowed to reduce internal mass.

This improves:

• Cooling consistency
• Material efficiency
• Shrinkage behavior
• Cycle time

Optimize Packing Conditions

Holding pressure and holding time should provide sufficient shrinkage compensation while the gate remains open.

Part weight studies can help determine when additional holding time no longer produces meaningful material compensation.

Review Gate Size and Location

Gate design should support both cavity filling and effective packing.

A gate that fills the cavity successfully may still freeze too early for thick regions.

Improve Cooling Around Hot Spots

Local cooling should be evaluated around:

• Thick walls
• Bosses
• Core regions
• Heavy intersections
• Large structural features

Control Material Preparation

Moisture-sensitive resins should be dried according to appropriate material requirements before processing.

Material condition should be treated as part of process control rather than only as a machine-setting issue.

Preventing Voids Through DFM

Many void risks can be identified before tooling begins.

During DFM review, engineers should evaluate:

• Nominal wall thickness
• Thick sections
• Boss geometry
• Rib intersections
• Corner transitions
• Gate size
• Gate location
• Expected pressure path
• Cooling-channel layout
• Material shrinkage
• Material drying requirements

A useful engineering relationship is:

Section Thickness → Cooling → Shrinkage → Packing Demand → Void Risk

The goal is to create geometry that cools predictably while allowing sufficient packing pressure to compensate for normal material shrinkage.

Correcting thick sections and poor pressure paths during CAD and mold design is generally more efficient than attempting to compensate for them later through increasingly aggressive process settings.

Need Engineering Support?

Internal voids can result from the interaction between part geometry, material shrinkage, packing conditions, gate design, cooling performance, and material preparation.

AccuMolds supports injection molding projects from early DFM review through tooling and scalable production. The company's current custom molding workflow includes engineering feedback, mold design and manufacturing, injection molding optimization, and production support.

Whether you are developing a new molded component or troubleshooting recurring internal voids, our engineering team can help review wall thickness, structural features, gate strategy, cooling, material requirements, and process conditions.

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