Hot Runner Basics
Hot runner systems play a crucial role in modern injection molding by increasing efficiency, reducing waste, and improving part quality. While they may require higher upfront costs and maintenance, their benefits make them essential for high-volume production. By understanding hot runner design, material selection, and process control, manufacturers can optimize their molding processes for better performance, lower costs, and improved sustainability.
1. Introduction to Injection Molding
Injection molding is a widely used manufacturing process for producing plastic parts by injecting molten material into a mold. It is commonly used for mass production due to its efficiency, repeatability, and ability to create complex shapes.
1.1 Basic Process of Injection Molding
A standard injection molding machine works by melting plastic and injecting it into a mold to form a solid part. The process consists of four main stages: clamping, injection, cooling, and ejection.
Clamping
The machine holds the mold tightly shut using a clamping unit. The mold consists of two halves: the cavity (outer shape) and the core (inner shape).
Injection
Plastic pellets are fed into a heated barrel, where a rotating screw melts the material through friction and external heaters. Once molten, the screw pushes the plastic through a nozzle into the mold cavity under high pressure.
Cooling
The molten plastic solidifies as it cools inside the mold. Cooling channels circulate water around the mold to speed up this process.
Ejection
Once the plastic part has cooled and hardened, the mold opens, and ejector pins push the part out. The mold then closes again for the next cycle.
The entire process happens in seconds, depending on the part size and material.
1.2 Types of Injection Molding Machines
Hydraulic
Uses hydraulic power to control the clamping and injection process.
Electric
More precise, energy-efficient, and faster.
Hybrid
Combines the advantages of hydraulic and electric machines.
Micro Molding
Micro-injection molding machines are relatively new. Technology like the M3 series, is designed specifically for producing ultra-small plastic parts with extreme precision. Unlike traditional injection molding machines, the M3 system uses a valve-gated hot runner and direct part gating, eliminating cold runners to reduce waste and improve efficiency.
How It Works:
1. Material Preparation and Injection
Plastic pellets are fed into a heated barrel, where they are melted. Instead of using a standard screw, the M3 system employs a precision-controlled hot runner system to inject molten plastic directly into the cavities.
2. Micro-Cavity Filling - Compressibility used as a melt velocity booster.
Plastic melt is compressible, expanding and contracting depending on temperature, pressure and time. This behavior is due to the free space between their macromolecules. When compressed, molten plastic can store visco-elastic energy, which is released as flow velocity when it relaxes. The energy within the condensed melt effectively fills the cavity without the need for precise metering. In fact, metering a micro-volume is not accurate and presents too much room for error, particularly in conventional micro-molding methods.
Read the article ‘What really happens inside a micro mold’ (Moldmaking Technology)
3. Cooling and Solidification
The mold rapidly cools the plastic to maintain tight tolerances. The M3 machine’s efficient thermal management ensures fast cycle times and high repeatability.
4. Ejection and Part Handling
Once the part solidifies, precision ejector pins remove the tiny components without damage. M3 machines use automated handling systems to carefully collect micro parts.
Key Advantages of ISOKOR technology:
- Zero Waste - Eliminates cold runners, reducing material costs.
- High Precision - Ideal for medical, electronics, and microfluidic applications.
- Scalability - Designed for multi-cavity molding, increasing production efficiency.
Explore videos (YouTube)
1.3 Common Materials Used in Injection Molding
Material Selection
The success of a plastic part starts with the right materials selection. Application Engineers must address the complex challenges of each part, with a deep understanding of the mechanical and flow properties of the polymer used.
Thermopastics
ABS (Acrylonitrile Butadiene Styrene)
ABS/PC alloys
Acetal/POM
COC (Cyclic Olefin Copolymer)
COP (Cyclo Olefin Polymer)
ETFE (Polyethylenetetrafluoroethylene)
LCP (Liquid Crystal Polymer)
PEEK (Polyetheretherketone)
PEI (Polyetherimide)
PE (Polyethylene)
PBT (Polybutylene Terephthalate), includes elastomeric grades
PC Polycarbonate)
PEKK (Polyetherketoneketone)
PET (Polyethylene Terephthalate), includes elastomeric grades
PMMA Copolymers (Polymethyl Methacrylate)
Polyamide (Nylon), includes elastomeric grades
PP (Polypropylene)
PS (Polystyrene)
PSU (Polysulfone)
PU (Polyurethane), includes elastomeric grades
SAN (Styrene Acrylonitrile)
TPE (Thermoplastic Elastomers)
Other (Customer Proprietary Materials)
Bio-Materials
PCL (Poly caprolactone)
PGA (Polyglycolic acid)
PLA (Poly lactic acid)
PDS (Polydioxanone)
Copolymers
Additives and Fillers
Color additives (colourants)
Flame retardants (FR)
Glass Fiber (GF)
Carbon Fiber (CF)
Mineral Filler (MF)
Glass Beads
Lubricants
Active pharmaceuticals
TCP (Tricalcium phosphate)
Advanced Polymers
Crystalline thermoplastics such as PEEK, PA4.6, LCP, and PPS are known for their distinct, sharp crystallite melting point. This thermal characteristic demands extremely accurate temperature control of the hot runner along the entire melt channel, from the machine nozzle to the mold cavity.
High Temperature
High temperature plastics are processed at melt temperatures ranging from 300 - 450°C (570 - 840°F). These materials present certain molding challenges due their unique processing characteristics. High mold temperatures require special hot runner systems, designed to deliver reliable performance under extreme conditions.
Bioabsorbable
Bioabsorbable plastics, also known as biodegradable or biocompatible plastics, are designed to break down naturally within the body or the environment. These materials are often used in medical applications, such as sutures, stents, and drug delivery systems, where they provide temporary support or function and then degrade without the need for removal. Made from polymers like polylactic acid (PLA) and polycaprolactone (PCL), bioabsorbable plastics reduce long-term waste and minimize environmental impact. Their development represents a significant advancement in both medical technology and sustainable materials science.
2. Hot Runner Systems in Injection Molding
2.1 What is a Hot Runner System?
A hot runner system is a heated manifold and nozzle system that delivers molten plastic directly into the mold cavities without solidifying in the runners. It eliminates the need for cold runners, reducing material waste and improving cycle time.
2.2 Components of a Hot Runner System
Manifold – Distributes molten plastic to various nozzles.
Nozzles – Deliver molten plastic to mold cavities.
Heaters and Sensors – Maintain consistent temperature for proper flow.
Valve Gates – Control the flow of material into the mold cavities.
2.3 Types of Hot Runner Systems
Open Hot Runner (Thermal Gate)
Plastic continuously flows into the mold cavity.
Simpler design but may cause stringing or drooling.
Valve-Gated Hot Runner
Uses mechanical pins to control plastic flow.
Provides better control, reduced defects, and higher-quality parts.
2.4 Advantages of Hot Runner Systems
Reduced Material Waste – Eliminates cold runners, reducing scrap.
Faster Cycle Times – No need to reheat solidified plastic.
Improved Part Quality – Better consistency, fewer weld lines.
Less Post-Processing – No need to trim runners.
2.5 Advantages of Cooling-Free Valve Gate Hot Runner Systems
New Black Box™ and iVG™ technologies are characterized by their durability and maintenance-free operation over millions of cycles. No more production outages to replace seals or cooling lines. This feature makes Black Box™ cylinders especially attractive for continuous operation in high endurance tools running automated mass production. By contrast, cylinder maintenance for conventional valve gate actuators is complicated in high temperature environments, especially inside stack molds and tandem molds.
Read white paper (PDF)
3. Hot Runner Design Considerations
To ensure a successful hot runner system, engineers must consider several factors:
3.1 Temperature Control
Maintaining uniform heat is critical to avoid hot spots or cold areas that lead to defects.
Heaters and thermocouples must be strategically placed.
3.2 Material Selection
Some materials degrade if exposed to heat for too long, requiring precise temperature control.
Engineering plastics like PPS, PEEK, and LCP require special hot runner designs.
3.3 Gate Type Selection
Restricted Flow
Open Flow
Valve Flow
Edge Gate
Hot Runner Gate Selection (PDF)
Seated vs Threaded Nozzles (PDF)
3.4 Manifold Balancing
Ensuring even flow to all cavities is crucial for part consistency.
Simulations and flow analysis are used to optimize manifold design.
4. Common Issues in Hot Runner Systems and Their Solutions
Burn Marks
Cause: Overheating, trapped gases
Solution: Reduce temperature, improve venting
Flow Imbalance
Cause: Poor manifold design
Solution: Optimize runner layout, use balanced nozzles
Stringing/Drooling
Cause: Excessive heat, poor gate design.
Solution: Lower temperature, use valve gates
Material Degradation
Cause: Long residence time in the manifold
Solution: Optimize cycle time, use heat-resistant materials
Hot Runner Best Practices (PDF)
5. Applications of Hot Runner Systems
Hot runner technology is widely used in various industries, including:
5.1 Automotive
Dashboards, bumpers, lighting components
Large, high-quality parts require minimal defects and fast cycle times.
6.2 Medical
Syringes, IV components, surgical tools
High-precision, contamination-free molding.
5.3 Consumer Goods
Electronics, devices
High-volume, fast-production applications.
5.4 Packaging
Thin-walled containers, closures, food trays
Requires high-speed molding with minimal waste.
6. Future Trends in Hot Runner Technology
6.1 Industry 4.0 and Smart Hot Runners
Sensors & IoT to monitor and adjust temperatures in real time.
AI-based process optimization for cycle time improvements.
6.2 Sustainable Injection Molding
Recyclable plastics and bio-based materials require specialized hot runners.
Energy-efficient heaters reduce electricity consumption.
6.3 Runnerless Micro-Injection Molding
Specialized equipment is required for injection molding for small, direct gated plastic parts.
7. Defects in Injection Molded Plastic Parts: Causes and Solutions
Injection molding is the art of persuading molten polymers to behave. When they misbehave, the evidence appears as cosmetic flaws, dimensional variations, or structural weaknesses. Each defect traces back to thermodynamics, rheology, tooling design, or process discipline. Below is an in-depth breakdown of the most critical issues—and how to tame them.
7.1. Splay (Silver Streaks)
How It Arises
Splay appears as silvery, feathery streaks along the flow direction. It is almost always the signature of moisture or volatiles flashing into steam or gas during injection. Hygroscopic materials (PA, PET, PC, TPU, etc.) are most vulnerable.
Secondary causes include:
• Excessive shear at the gate or screw
• Material degradation (overheating, long residence time)
• Air entrapment near the flow front
Solutions
• Dry material to specification; verify dew point (< –30 °C for many engineering resins).
• Lower melt temperature or screw speed to reduce shear.
• Enlarge the gate or add a more progressive gate type (e.g., tab, fan).
• Purge degraded material; ensure no dead spots in the barrel or manifold.
• Improve venting at end-of-fill.
7.2. Flow Lines
How They Arise
Flow lines appear as streaks, wavy lines, or color variations that trace the path of the melt. They form when the melt cools prematurely or flows non-uniformly.
Causal triggers include:
• Low injection speed
• Cold mold walls
• Thin sections or abrupt wall thickness changes
• Poor flow channel design
Solutions
• Increase injection speed to create fountain flow and uniform shear.
• Raise melt and mold temperature.
• Increase gate size or reposition it to promote uniform flow.
• Add flow leaders or modify sections to maintain consistent wall thickness.
7.3. Weld Lines (Knit Lines)
How They Arise
When two flow fronts meet, they cool and solidify before fully bonding, creating a visible line and a structurally weak region. Contributing conditions:
• Low melt temperature
• Low injection speed
• Poor venting
• Part geometry forcing opposing flow fronts
Solutions
• Increase melt temperature and injection velocity for better molecular entanglement.
• Move the gate to eliminate converging flow fronts.
• Add overflow wells or vents at the weld location.
• Switch to valve gate sequencing to control the fill pattern.
7.4. Flash
How It Arises
Flash occurs when polymer escapes the shutoff surfaces—typically parting lines, ejector pins, slides, or around gates. Its root causes:
• Excess injection pressure
• Worn or damaged mold components
• Insufficient clamping force
• Poor vent design (counterintuitively, low vent depth can cause flash by blocking air and forcing the melt elsewhere)
Solutions
• Reduce injection pressure or switch to a multi-stage velocity/pressure profile.
• Repair shutoffs; harden or resurface wear areas.
• Increase clamp tonnage or reduce the projected part area.
• Improve venting so the melt is not forced out of the parting line.
7.5. Jetting
How It Arises
Jetting occurs when the melt “shoots” into a cavity like a high-speed rope, cooling on the surface before the rest of the melt merges behind it. It’s caused by:
• High injection velocity through a small gate
• Melt not hitting a cavity wall to slow and spread it
• High melt viscosity
Solutions
• Reduce initial injection speed (use a staged fill profile).
• Use a gate design that forces impingement (sub-gate, fan gate, 3-plate).
• Increase melt temperature to improve flow.
• Modify gate orientation so melt flows against steel, not free-air.
7.6. Sink Marks
How They Arise
Sinks occur over ribs, bosses, and thick sections when the surface solidifies faster than the interior. As the interior shrinks, it pulls the surface inward.
Contributing factors:
• Localized thick walls
• Short pack/hold time
• Low packing pressure
• Inadequate cooling near thick features
Solutions
• Maintain uniform wall thickness; hollow out bosses; core ribs.
• Increase pack/hold pressure and time.
• Improve cooling with baffles, bubblers, or conformal channels.
• Add structural support (gas-assist or foam-core where applicable).
7.7. Vacuum Voids
How They Arise
Voids form when shrinkage pulls the solidifying skin outward, leaving a pocket of air inside the part. They thrive in thick sections and low-pressure regions.
Solutions
• Increase pack/hold time and pressure.
• Design parts with uniform walls and avoid thick masses.
• Gate into the thickest section to pack it properly.
• Ensure melt is dry and free of volatiles (to avoid gaseous voids).
7.8. Short Shots
How They Arise
A short shot is an incomplete filling of the mold cavity. It happens when:
• Melt freezes before reaching the ends
• Insufficient injection pressure or speed
• Restricted gates or runners
• Poor venting (air blocks the melt front)
• Material viscosity too high
Solutions
• Increase injection speed, pressure, or material temperature.
• Resize or relocate gates and runners.
• Improve venting at end-of-fill.
• Dry material fully.
• Add flow leaders or change geometry to promote balanced fill.
7.9. Warping (Distortion)
How It Arises
Warp is the part’s poetic attempt to return to its stress-free state. It results from differential shrinkage—cooling faster on one side, uneven packing, or flow-induced molecular orientation.
Common causes:
• Non-uniform wall thickness
• Uneven mold temperature
• Imbalanced fill pattern
• Residual stresses trapped during cooling
Solutions
• Improve cooling uniformity; add channels or balance temperatures.
• Design with consistent wall thickness and gradual transitions.
• Balance runner and gate layout.
• Reduce packing pressure to avoid high frozen-in stresses.
• Slow the cooling rate when possible.
7.10. Burn Marks
How They Arise
Dark, soot-like marks form when air is trapped and diesel combustion occurs as the melt compresses it. Burn marks also arise from degraded material due to high shear or long residence time.
Solutions
• Increase venting at the burn location.
• Reduce injection speed (to avoid air compression).
• Lower melt temperature or screw RPM.
• Reduce hold pressure if material is being over-packed.
7.11. Surface Delamination
How It Arises
Delamination presents as thin, flaky layers separating from the surface. It’s typically caused by:
• Contamination with incompatible polymers
• Over-lubricated regrind or excessive mold release
• Moisture or gas layer forming during filling
• Material degradation
Solutions
• Purge thoroughly and prevent cross-contamination.
• Verify material purity; avoid mixing grades or suppliers.
• Reduce mold release agents or adjust surface finish.
• Increase melt and mold temperature to improve adhesion.
7.12. Preventing Voids, Bubbles, and Internal Defects
Although voids and bubbles manifest differently, they share root causes related to shrinkage, moisture, gases, or poor packing.
How They Arise
• Moisture vaporizes during injection (true bubbles).
• Volatile additives or degradation gases form pockets.
• Trapped air prevents complete filling.
• Internal shrinkage collapses into voids.
Solutions
• Dry the resin to manufacturer specs (especially PA, PC, PET, PBT, TPU, TPE-E).
• Use higher pack pressure and longer hold time for dense interior consolidation.
• Gate into the thickest region to pack it properly.
• Increase mold temperature to delay surface freeze-off.
• Improve venting to evacuate trapped air.
• Reduce screw decompression to avoid sucking air into the barrel.
• Use vacuum-assist molding on highly cosmetic or thick parts.