As someone who’s spent over a decade in the plastics and injection molding industry, I’ve seen my fair share of manufacturing headaches. One issue that keeps cropping up, especially when working with TPE (Thermoplastic Elastomer), is the frustrating problem of gate voids—those pesky cavities or hollow spots near the gate of injection-molded parts. Whether you’re producing soft-touch grips, seals, or medical device components, gate voids can compromise the quality, aesthetics, and functionality of your product. I remember the first time I encountered this issue on a TPE project; it felt like a puzzle with too many pieces missing.
So, why do TPE injection-molded parts develop gate voids, and more importantly, how can you prevent them? In this article, I’ll break down the causes of this defect, share insights from my experience, and offer practical solutions to tackle it. By the end, you’ll have a clear understanding of how to optimize your TPE molding process and produce flawless parts. Let’s dive in!
What Are Gate Voids in TPE Injection Molding?
Before we get into the “why,” let’s clarify what gate voids are. In injection molding, the gate is the point where molten material enters the mold cavity. Gate voids appear as cavities, bubbles, or hollow areas near or around this gate area in the finished part. In TPE parts, these voids are particularly problematic because TPE is often used for flexible, soft-touch applications where surface quality and structural integrity are critical.
Gate voids can manifest as:
Visible surface imperfections, like dents or sink marks.
Internal bubbles that weaken the part.
Incomplete filling near the gate, leading to functional defects.
These defects not only affect the part’s appearance but can also lead to scrap waste, increased production costs, and customer dissatisfaction. Understanding why they occur is the first step to eliminating them.
Why Do Gate Voids Occur in TPE Injection-Molded Parts?
Gate voids in TPE parts are typically the result of a combination of material properties, mold design, and processing conditions. Based on my experience troubleshooting molding issues, here are the primary culprits:
1. High Viscosity of TPE
TPEs, especially high-durometer or highly filled grades, can have high viscosity (resistance to flow). This makes it harder for the molten material to flow smoothly through the gate and fill the mold completely. When the material cools too quickly near the gate, it can solidify before the cavity is fully packed, trapping air or creating voids.
I’ve seen this issue in projects involving SEBS-based TPEs with high filler content. The material’s sluggish flow led to voids near small, restrictive gates, especially in thin-walled parts.
2. Improper Gate Design
The gate’s size, shape, and location play a huge role in material flow. Common gate-related issues include:
Gate size too small: A restrictive gate increases shear stress and heat, causing the TPE to degrade or cool prematurely, leading to voids.
Poor gate location: If the gate is placed in a thin section or far from thick sections, the material may not pack properly, leaving voids.
Inadequate gate type: Certain gate types, like pinpoint gates, can be problematic for TPE due to their small size and high shear.
For example, a study from ScienceDirect notes that improper gate design is a leading cause of voids in elastomer injection molding, as it disrupts uniform flow and packing.
3. Insufficient Packing Pressure or Time
During injection molding, packing (or holding) pressure is applied after the initial fill to compensate for material shrinkage as it cools. If the packing pressure is too low or the packing time is too short, the material near the gate may not be fully compressed, resulting in voids or sink marks.
This is a common issue with TPEs because they have a high shrinkage rate (typically 1-3%) compared to rigid plastics. I’ve seen cases where increasing packing time by just a few seconds eliminated gate voids entirely.
4. Trapped Air or Moisture
Air or moisture trapped in the mold or material can create voids. This happens when:
Air is trapped in the mold: Poor venting prevents air from escaping as the TPE fills the cavity, forming bubbles near the gate.
Moisture in the material: TPEs, especially TPU, are hygroscopic and absorb moisture. When heated, this moisture turns into steam, creating voids.
I once worked on a TPE medical component where gate voids were traced back to improperly dried material. After implementing a proper drying protocol, the issue disappeared.
5. Inconsistent Melt Temperature
TPEs are sensitive to processing temperatures. If the melt temperature is too low, the material becomes viscous and struggles to fill the mold, leading to voids. Conversely, if it’s too high, the material can degrade, forming gas pockets that cause voids.
Maintaining a consistent melt temperature within the supplier’s recommended range (typically 180-230°C for most TPEs) is critical. I’ve found that barrel temperature profiles with a gradual increase from rear to nozzle help achieve uniform melting.
6. Rapid Cooling Near the Gate
TPE parts cool faster in areas with thin walls or near the gate, where the material first enters the mold. If the mold temperature is too low or the cooling time is too short, the material can solidify before the cavity is fully packed, creating voids.
This is especially problematic for thin-walled TPE parts, where rapid cooling exacerbates shrinkage and void formation.
How to Prevent Gate Voids in TPE Injection Molding
Now that we’ve identified the causes, let’s explore solutions. Over the years, I’ve developed a systematic approach to eliminate gate voids, combining material optimization, mold design tweaks, and process adjustments. Here’s how you can tackle the problem:
1. Optimize Gate Design
A well-designed gate ensures smooth material flow and proper packing. Consider these strategies:
Increase Gate Size: Use larger gates (e.g., edge gates or fan gates) for high-viscosity TPEs to reduce shear and improve flow. For example, a gate diameter of 1-2 mm is often suitable for TPEs.
Choose the Right Gate Type: For TPEs, edge gates or sprue gates are often better than pinpoint gates, as they allow better flow and packing.
Position Gates Strategically: Place gates in thicker sections of the part to ensure even filling and minimize premature cooling.
Add Venting: Incorporate vents (0.01-0.03 mm deep) near the gate and at the end of the flow path to allow trapped air to escape.
In one project, switching from a pinpoint gate to an edge gate for a TPE seal reduced gate voids by 90%, saving thousands in scrap costs.
2. Adjust Processing Parameters
Fine-tuning your injection molding parameters can work wonders. Here’s a table summarizing key adjustments:
|
Parameter |
Issue Causing Voids |
Solution |
Typical Range for TPE |
|---|---|---|---|
|
Injection Speed |
Too fast: traps air; too slow: premature cooling |
Use moderate speed, profile if needed |
20-50 mm/s |
|
Packing Pressure |
Too low: incomplete filling |
Increase to 50-80% of injection pressure |
50-100 MPa |
|
Packing Time |
Too short: shrinkage voids |
Extend until gate freezes |
2-5 seconds |
|
Melt Temperature |
Too low: high viscosity; too high: degradation |
Maintain supplier’s recommended range |
180-230°C |
|
Mold Temperature |
Too low: rapid cooling |
Increase to improve flow and packing |
20-40°C |
I always recommend starting with the TPE supplier’s processing guidelines and adjusting based on trial runs. For instance, increasing packing pressure from 40 MPa to 60 MPa resolved gate voids in a TPE grip project.
3. Ensure Proper Material Preparation
TPE quality directly impacts molding outcomes. Follow these steps:
Dry the Material: For hygroscopic TPEs like TPU, dry at 80-100°C for 2-4 hours before molding to remove moisture. Use a desiccant dryer for best results.
Check Material Quality: Ensure the TPE is free of contaminants and has consistent pellet size to avoid flow inconsistencies.
Use Low-Viscosity Grades: If voids persist, consider a TPE grade with lower viscosity for better flow, especially for thin-walled parts.
A client once struggled with voids in a TPU part. After confirming the material wasn’t dried properly, we implemented a drying step, and the voids vanished.
4. Improve Mold Venting
Effective venting prevents air entrapment. Here’s how:
Add vent channels (0.01-0.03 mm deep) at the gate and cavity extremities.
Use ejector pins or parting lines as additional venting paths.
Clean vents regularly to prevent clogging with TPE residue.
In a TPE overmolding project, adding two extra vents near the gate eliminated bubbles that were causing voids.
5. Balance Cooling and Shrinkage
TPE’s high shrinkage requires careful cooling management:
Increase Mold Temperature: A mold temperature of 20-40°C helps slow cooling near the gate, allowing better packing.
Extend Cooling Time: Add 1-2 seconds to the cooling phase to ensure the gate freezes after packing.
Use Cooling Channels: Ensure uniform mold cooling to avoid hot spots that cause uneven shrinkage.
I’ve found that a mold temperature of 30°C works well for most SEBS-based TPEs, balancing flow and cycle time.
6. Conduct Mold Flow Analysis
For complex parts, use mold flow simulation software (e.g., Autodesk Moldflow) to predict flow behavior and identify potential void areas. This can guide gate placement and process settings before cutting steel. In one project, a mold flow analysis revealed that a gate was too far from a thick section, causing voids. Relocating the gate solved the issue.
7. Test and Iterate
Trial and error are part of the process. Use these tests to validate your fixes:
Short-Shot Testing: Inject partial shots to check flow patterns and gate performance.
Cross-Section Analysis: Cut parts near the gate to inspect for internal voids.
Pressure Loss Testing: Measure pressure drop across the gate to ensure it’s not restrictive.
Document each trial to build a knowledge base for future projects. I keep a log of every TPE molding run, which has saved me countless hours on similar jobs.
Real-World Case Study: Conquering Gate Voids in TPE Seals
A few years ago, I worked with a client producing TPE seals for automotive weatherstrips. The parts had persistent gate voids near the sprue gate, causing leaks and failing quality checks. The initial setup used a small pinpoint gate, low packing pressure, and no material drying.
After analyzing the issue, we made these changes:
Switched to an Edge Gate: A larger gate reduced shear and improved flow.
Increased Packing Pressure: Raised from 30 MPa to 50 MPa to ensure cavity packing.
Added Venting: Incorporated 0.02 mm vents near the gate.
Dried the TPE: Implemented a 3-hour drying cycle at 90°C for the TPU-based TPE.
The result? Voids were eliminated, and the parts passed leak tests with flying colors. The client reduced scrap by 95%, saving significant costs. This project taught me the importance of addressing gate voids holistically—material, mold, and process all need to align.
Common Questions About TPE Gate Voids
To wrap up, here are answers to some frequent questions I’ve encountered about gate voids in TPE molding:
Q1: Can gate voids be fixed after molding?
A: Unfortunately, gate voids are internal defects and can’t be repaired post-molding. The focus should be on prevention through proper design and processing. For surface voids, minor cosmetic fixes like filling may work but aren’t ideal for functional parts.
Q2: Are gate voids more common in TPE than other plastics?
A: Yes, TPEs are more prone to gate voids due to their high viscosity, high shrinkage, and sensitivity to moisture. Rigid plastics like ABS or PP have lower shrinkage and viscosity, making voids less frequent.
Q3: How do I know if my gate is too small?
A: Signs include high injection pressure, burn marks, or incomplete filling. Measure the pressure drop across the gate—if it’s above 20 MPa, consider enlarging it. Mold flow analysis can also confirm gate size issues.
Q4: Does mold temperature affect gate voids?
A: Absolutely. A mold that’s too cold (below 20°C) causes rapid cooling near the gate, leading to voids. Aim for 20-40°C, depending on the TPE grade.
Q5: Can I reuse TPE with gate voids?
A: Regrinding and reusing TPE is possible, but voids may indicate degraded material. Test the reground material’s properties (e.g., viscosity, tensile strength) to ensure it meets specs before reuse.
Final Thoughts
Gate voids in TPE injection-molded parts can feel like a relentless foe, but they’re not unbeatable. By addressing high viscosity, gate design flaws, insufficient packing, and trapped air or moisture, you can produce pristine parts that meet the highest standards. My years in the field have taught me that success in TPE molding comes down to precision, testing, and a willingness to iterate. Every void-free part is a small victory, and those wins add up to better products and happier customers.
If you’re battling gate voids or planning a new TPE project, don’t hesitate to experiment and consult with your material supplier or a molding expert. Have a specific challenge? Share it in the comments—I’d love to help troubleshoot. Here’s to molding TPE parts that are as flawless as your vision!
