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How to Identify and Solve the Injection Molding Bubble Defect?
Created on 01.13
In manufacturing, it's common to see visual or structural issues in finished parts. One annoying problem is internal air pockets, or bubbles. Ever find blisters or hollow spots inside a clear part after it comes out of a mold? This really hurts how many parts pass inspection and how strong they are. This guide will explain the injection molding bubble defects. We'll go over why they show up, how to tell the different types apart, and what you can do to fix them so you can keep things running smoothly.
Identifying the Culprit: Bubbles vs. Vacuum Voids
Before you put a fix in place, you need to figure out what's wrong. When it comes to injection molding, not every bubble is the same. Most bubble-like problems are either gas bubbles or vacuum voids.
The Visual Difference
- Gas Bubbles: Usually, these come from trapped air, water, or plastic that's gone bad. They often look like small, round, or stretched pockets. If you spot a bunch of tiny clouds or the bubble is yellowish or brownish, it's likely gas or heat damage.
- Vacuum Voids: These occur strictly during the cooling process. As the plastic cools, it shrinks. If the outer skin of the part hardens too quickly while the center remains molten, the shrinking material pulls away from itself, creating a vacuum-filled hole. These are typically found in the thickest sections of a part and are usually clear and crisp in appearance.
The "Heat Test" Diagnostic
A simple field test to distinguish them is the flame test. If you carefully heat the affected area with a torch, a gas bubble will often expand or "pop" because the trapped gas increases in pressure. A vacuum void, however, will usually collapse inward as the plastic softens, because there is no air inside to push back—only a vacuum.
Root Causes and Targeted Solutions: A Technical Deep Dive
Based on industry standards and advanced troubleshooting from top-tier molding experts, solving bubble defects requires looking at the "four pillars" of molding: material, machine, mold, and process.
1. Entrapped Air and Improper Venting
One frequent cause of gas bubbles is simply trapped air, sometimes called the diesel engine effect. When melted plastic goes into the mold, it has to push out the air that's already there. If the air can't get out, it gets squeezed and pushed into the plastic.
The Root Cause: Not enough venting at the end of the flow path or in deep areas.
The Targeted Solution:
- Better Venting: Make sure your vents are clean and the correct size (usually .0005 to .0015 inches deep for most plastics).
- Slower Injection: Slowing down the injection speed as the mold fills gives the air more time to escape.
- Gate Placement: If the way the plastic enters the mold causes turbulence, air can get trapped. Check your gate location.
2. Moisture and Volatiles (The Drying Issue)
Many engineering-grade resins are hygroscopic, meaning they absorb moisture from the air. When these pellets enter the heated barrel, the moisture turns into steam, creating "silver streaks" or internal bubbles.
The Root Cause: Insufficient drying time, a malfunctioning desiccant dryer, or exceeding the material's "ambient time" in the hopper.
The Targeted Solution:
- Verify Dew Point: Check that your dryer is hitting a dew point of -40°C (-40°F).
- Proper Pre-drying: Follow the material data sheet (TDS) strictly. For example, polycarbonate (PC) usually requires 3–4 hours at 120°C.
- Reduce Melt Temperature: High heat can exacerbate the turning of moisture into steam.
3. Vacuum Voids and Excessive Shrinkage
Vacuum voids are purely a result of physical physics—the material shrinking more than the machine can compensate for.
The Root Cause: The gate freezes (solidifies) before the part's interior has fully cooled. Once the gate is frozen, no more plastic can be "packed" into the cavity to fill the space left by shrinking molecules.
The Targeted Solution:
- Increase Hold Pressure and Time: This is the most effective fix for voids. Maintain high pressure until the gate has completely solidified.
- Decrease Melt Temperature: Cooler plastic shrinks less than extremely hot plastic.
- Enlarge the Gate: A larger gate stays open longer, allowing for a longer packing phase.
4. Screw and Barrel Dynamics (Back Pressure)
Sometimes, bubbles pop up even before the plastic gets into the mold.
The Root Cause: If the back pressure is too low, air can get sucked into the barrel when the screw is recovering and prepping for the next shot.
The Targeted Solution:
- Increase the Back Pressure: Bump up the back pressure to somewhere between 50 and 150 psi. This will compress the melted plastic and push any trapped air back out.
- Inspect the Check Valve: If the check valve is leaking, it can mess with the pressure and make air pockets. It can also cause issues with getting a consistent cushion of plastic at the end of the injection.
5. Thermal Degradation and Residence Time
If plastic stays in the barrel too long at high temperatures, the polymer chains "crack," releasing gas as a byproduct.
The Root Cause: The barrel is too large for the shot size (long residence time), or the heater bands are malfunctioning.
The Targeted Solution:
- Figure out how long the plastic stays in the barrel: Make sure the plastic isn't sitting in the barrel for more than 2–5 minutes (it depends on the plastic type).
- Look for spots that are too hot: Use a pyrometer to check if the plastic's actual temp matches what the machine is set to.
Proactive Prevention By Optimizing the Injection Molding Workflow
Waiting around to see what happens can cost money. To avoid issues, it's best to improve your process when designing and setting things up.
Keep Wall Thickness Even
It's important to design parts with the same wall thickness all over. If thick and thin sections are next to each other, the thin parts harden first. This stops the flow of material to the thicker part, which causes empty spaces. If you need thick sections, use methods to keep the thickness the same across the part.
Strategic Gate Placement and Sizing
Gates should be positioned at the thickest section of the part. This allows the highest pressure to be applied to the areas most prone to shrinkage. Furthermore, the gate must be large enough to remain open until the thickest part of the cavity has been sufficiently packed.
Mold Flow Simulation
Using software to copy the injection process helps predict air pockets and shrinkage before making the mold. This way, you can change where vents and gates are based on the info. This ensures a more stable process from the start.
Why Does Prevention Matter in the Injection Molding Process?
It's easy to just toss out parts with bubbles, but that's not a real fix. Getting to the root of bubble problems in injection molding is vital because:
- Structural Integrity: A bubble isn't just ugly; it weakens the part. For parts that carry weight, bubbles can cause them to break sooner or fail.
- Cost Efficiency: Lots of scrap parts cut into your profits. The energy, materials, and machine time wasted on bad parts can't be regained.
- Brand Reputation: For industries like medical devices or high-end electronics, clarity and perfection are non-negotiable. Delivering parts with internal defects can damage long-term client relationships.
By mastering the balance between melt temperature, injection pressure, and mold design, you transition from reactive troubleshooting to proactive quality assurance.
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