Bubbles—also called voids, air traps, or gas pockets—are one of the most common defects in injection molding, and one of the easiest to misdiagnose. Unless the design intentionally calls for it, transparent parts are not allowed to contain any bubbles at all. Bubbles also reduce the part's mechanical strength and alter the part weight specified by the customer.

The reason bubbles are so often misdiagnosed is that "a bubble" is not one defect—it is three different defects that happen to look similar. A bubble can come from air, from moisture, or from vacuum shrinkage inside a thick wall. Each has a different cause and a different fix. Applying the wrong fix wastes time and never solves the problem.

This guide breaks down the three causes, shows you how to identify which one you're dealing with, and provides targeted solutions for each.

Bubbles vs. Voids: What's the Difference?

In production terminology, these defects are often grouped under "bubbles", but they split into two broad families:

• Gas bubbles — formed by moisture vaporizing or by air trapped inside the melt. These are genuine pockets of gas.
• Vacuum voids— formed in thick-wall parts by uneven volume shrinkage. These contain no air and no moisture—only vacuum.

The distinction matters because the fixes are completely different. Gas bubbles need drying and venting; vacuum voids need packing and cooling optimization.

Bubbles most often appear at the fill end or in poorly vented areas. When a part has fine internal bubbles, the surface often also shows silver streaks (splay). In transparent parts, bubbles affect appearance directly and indicate poor material quality, while reducing part strength.

The 3 Types of Bubbles: Air, Moisture, and Vacuum

1. Air

Air in the barrel: Plastic pellets contain air between them. As pellets are fed from the hopper into the barrel, this air is carried in with them. A proper back pressure compresses the melt at the screw front, collapsing these bubbles before injection.

Air in the cavity: Whether the part is thick-wall or thin-wall, the cavity contains air. If it is not vented out, it mixes with the incoming melt and forms bubbles.

Injection speed: At very high injection speeds—for example, nitrogen-assisted injection—cavity air may not have time to escape, becoming trapped and forming bubbles. If thin-wall molding demands extremely high injection speed just to fill the cavity, the solution must come from venting, lower clamp force, or vacuum evacuation.

2. Moisture

Plastic pellets absorb moisture from the air. If this moisture is not thoroughly removed, it is released as vapor when the material is heated above 100°C, and it ends up inside the part.

Each plastic has its own required drying temperature and time. Standard conditions for the drying data below are based on 20°C ambient temperature, 65% relative humidity, using a high-efficiency turbo blower—resulting in a moisture content below 0.02%.

In humid climates, standard drying performance degrades. Solutions include extending drying time, enlarging the drying hopper, or switching to a desiccant dryer.

3. Vacuum

When molding thick-wall parts, surface dents are common. These dents form as the plastic shrinks from melt to solid. With proper packing parameters and runner design, they can be avoided.

If the surface of a thick-wall part has already cooled and solidified while the interior is still fluid, the interior can only shrink inward—creating a void. This "bubble" contains no air and no moisture, only vacuum. Its elimination method is the same as for sink marks.

If the cold runner diameter is comparable to the maximum wall thickness, packing pressure can continue to feed material through the still-molten runner and fill the void.

How to Identify Which Bubble Type You Have

If the plastic is transparent or semi-transparent, you can distinguish the cause of the bubbles using these four characteristics:

Practical note: For the heating test, place the part under a graduated optical instrument and observe before and after heating.

How to Fix Bubbles: Solutions by Root Cause

Fixing Air Bubbles

Vent channels: Molds are cut with vent channels on the parting surface, extending from the cavity to the mold perimeter. Vent channels have three parameters: width, depth, and spacing.

Key principle: The vent depth must allow air to escape but not allow the more viscous melt to leak out (which would cause flash).

• Vent depth: no more than 0.03 mm
• Vent width: generally no less than 6 mm
• Spacing: one vent every 25–50 mm

Important: Vent depth is affected by clamp force. Operators should set the lowest clamp force that is still sufficient (i.e., does not produce flash)—not the maximum available. This not only keeps vents from being crushed but also extends the life of the mold and the machine's clamping mechanism (toggle, toggle pins, toggle bushings, tie bars, and platens), and shortens clamp time.

Porous steel: If the part surface does not need to be glossy, the mold can be made from porous steel, using the micro-pores within the steel to vent air.

Vacuum evacuation: In enclosed areas or on cold runners, a vacuum port can be connected to a vacuum pump to evacuate cavity air during injection.

Vacuum evacuation is mutually exclusive with vent channels and porous steel. They cannot be used at the same time—otherwise the vacuum cannot be established.

Fixing Moisture Bubbles

Focus entirely on drying. See the drying section below for material-specific parameters, hopper sizing, and desiccant dryers.

Fixing Vacuum Voids

Vacuum voids are treated exactly like sink marks:

• Optimize packing parameters (higher packing pressure, longer packing time)
• Ensure the cold runner diameter is comparable to the maximum wall thickness so packing can feed through
• Balance wall thickness to avoid abrupt thick sections

Material Drying: Temperatures and Times to Prevent Moisture Bubbles

There is no substitute for correct drying. The table below lists standard drying parameters by material (standard conditions: 20°C ambient, 65% RH, turbo blower; target moisture below 0.02%):

Caution on drying temperature: While raising drying temperature improves drying at a fixed time, excessive drying temperature volatilizes certain material components, affecting color, transparency, and mechanical properties. Never exceed the material supplier's recommended maximum.

The Drying Hopper Capacity Formula

This is where many bubble problems hide. A drying hopper that is too small cannot hold material long enough to dry it properly.

H = 3.6 × s × t / c

Where:

• H = hopper capacity (kg)
• s = weight per shot (per mold cycle), including runner (g)
• c = cycle time (seconds)
• t = drying time (hours)

Worked example — 32-cavity PET preform:

• Shot weight: 32 × 20 g = 640 g
• Cycle time: 24 seconds
• Required drying time (from the table): 4–5 hours

H = 3.6 × 32 × 20 × 5 / 24 = 480 kg

The calculated result is 480 kg—so a 500 kg drying hopper is selected.

Why this matters: A typical supplier hopper-selection table (based on the assumption that injection uses at most 80% of the machine's shot capacity) would recommend only a 100 kg hopper for this application. The gap between 100 kg and 480 kg is enormous—and an undersized hopper means the material near the outlet never gets enough drying time before it is fed into the barrel. This is one of the hidden causes of bubble defects.

Note: The hopper must be paired with a loader that continuously tops it up, keeping a constant material level, so drying is consistent. If material is only added after the hopper runs empty, the material near the outlet enters the barrel before it has dried long enough.

Desiccant Dryers

Enlarging the hopper alone cannot guarantee dryness, because atmospheric humidity changes daily, and drying longer than necessary wastes energy. A desiccant (honeycomb) dryer guarantees dryness independent of atmospheric humidity.

• The desiccant dryer works together with the drying hopper: moist air from the hopper passes into the dryer, is filtered and cooled, and its moisture is absorbed by a molecular sieve in a rotating honeycomb, then returned to the hopper inlet. This creates a closed system, unaffected by atmospheric humidity.
• The molecular sieve is regenerated by a separate air stream that exhausts to atmosphere.
• Honeycomb desiccant dryers achieve a dew point of −40°C, equivalent to 0.60% relative humidity, or 0.013% moisture (128 ppm).
• A desiccant dryer's capacity is rated by how many kg of a given plastic it can dry per hour—this is the basis for selecting a model.

Two-stage drying: Because honeycomb desiccant dryers are not cheap, some processors use two-stage drying hoppers to achieve better drying than a single hopper alone.

Product Design Rules to Prevent Bubbles and Voids

Design decisions made early prevent bubble defects that no amount of process tuning can fix later.

Wall thickness change:

• ❌ Abrupt change (e.g., 3 mm → 1 mm) → bubbles are almost guaranteed, as the thick-wall melt "wraps around" the trapped gas
• ✅ Keep wall thickness change within 30% (e.g., 3 mm → 2.1 mm)
• ✅ Use a gradual transition zone with at least a 30° slope

Ribs:

• ❌ Ribs that are too thick → the rib root cools slowly and forms vacuum voids
• ✅ Keep rib thickness at or below 60% of the main wall thickness
• ✅ Add vent inserts (Φ0.3 mm) at rib locations

Process Settings to Eliminate Bubbles

Injection Speed

• Reduce speed for thick-wall parts to about 60% of the thin-wall setting, using staged control
• High injection speed causes melt shear decomposition that generates gas
• For thin-wall parts that require high speed simply to fill, address the problem through venting rather than overspeeding

Mold Temperature

• Mold temperature that is too low solidifies the surface quickly, trapping gas inside
• Raise mold temperature so the surface stays molten longer and gas can escape

Back Pressure

• Insufficient back pressure draws air in when the screw retracts—back pressure below 5 MPa is essentially self-defeating production
• Set back pressure at about 10 MPa (raise it if you hear venting noise)

Packing and Switch Point

• Set the fill-to-hold switch at about 95% fill
• Use secondary packing to ensure the part is fully packed
• Adequate packing collapses gas bubbles and fills vacuum voids

Mold Flow Analysis

• Predict gas trap locations before cutting steel (investigate the red zones first)
• Check the fill-end pressure—if it is below 5 MPa, add venting

Bubble & Void Troubleshooting Checklist

Related Injection Molding Defects

Bubbles share root causes with several other defects:

• Silver Streaks — internal bubbles often produce visible surface splay; both point to moisture and venting issues. Read: What Causes Silver Streaks in Injection Molding and How to Prevent Them
• Sink Marks — vacuum voids and sink marks share the same root cause (thick-section shrinkage) and the same fix. Read: What Causes Sink Marks in Injection Molding and How to Fix Them
• Burn Marks — trapped gas that causes bubbles can also ignite into burn marks under compression. Read: What Causes Burn Marks in Injection Molding and How to Prevent Them

FAQ: Bubbles in Injection Molding

Q: Why do my transparent parts show bubbles even after drying the material?

A: If drying is verified (moisture below 0.02%, hopper capacity adequate), the bubbles may not be moisture-type. Check whether they are air bubbles (from venting) or vacuum voids (from thick-section shrinkage). Use the four characteristics—count, position, heating test, and shape—to identify the type before adjusting parameters.

Q: Can vacuum voids be removed by increasing packing pressure?

A: Often, yes—but only if the runner is still molten enough to transmit packing pressure. A common related fix is to ensure the cold runner diameter is comparable to the maximum wall thickness, so packing can feed material through to the thick section. If the gate freezes before packing completes, raising packing pressure alone will not help.

Q: What is the correct vent depth for injection molding?

A: For general materials, keep vent depth at ≤0.03 mm so air escapes but melt does not leak (which would cause flash). For higher-viscosity materials like glass-filled resins, a slightly deeper vent may be needed to avoid clogging. Vent width should be at least 6 mm, spaced every 25–50 mm, and clamp force should be kept as low as possible without causing flash—so the vents are not crushed.

Need Expert Help with Your Injection Molding Project?

Bubbles and voids are more than a cosmetic issue—in transparent parts they cause outright rejects, and in structural parts they reduce strength and alter weight. Solving them starts with the right diagnosis and the right manufacturing discipline.

At JCV Precision Manufacturing, we bring over 26 years of injection molding expertise to every project. We operate 142 injection molding machines with clamping forces from 50T to 1,850T, supported by molds designed for 300,000-shot lifespans and custom multi-cavity configurations. Our monthly production capacity exceeds 1 million units.

We work with a full range of engineering thermoplastics—PP, ABS, PS, PE, PA/PA66, POM, and more—selected to match your product's performance and application requirements. Our in-house surface finishing includes printing, laser engraving, and film overlay, delivering production-ready parts straight from the mold.

As a trusted manufacturer for the home appliance and automotive industries, we provide vertically integrated support across the full product lifecycle: from mold design and prototyping through full-scale mass production.

Contact us today for a free consultation or quote on your next injection molding project.