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.

Learn more

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.

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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.

Material Selection (PDF)

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.

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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.

Hot Runner Terminology (PDF)

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.

Learn more

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.

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.

Learn more

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.


Molding Materials (PDF)

3.3 Gate Type Selection

Restricted Flow

Open Flow

Valve Flow

Edge Gate

Gating Options (PDF)

Hot Runner Gate Selection (PDF)

Hot Runner Tip Styles (PDF)

Nozzle Sizes (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.

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6.2 Medical

Syringes, IV components, surgical tools

High-precision, contamination-free molding.

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5.3 Consumer Goods

Electronics, devices

High-volume, fast-production applications.

Learn more

5.4 Packaging

Thin-walled containers, closures, food trays

Requires high-speed molding with minimal waste.

Learn more

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.

Learn more

6.2 Sustainable Injection Molding

Recyclable plastics and bio-based materials require specialized hot runners.

Energy-efficient heaters reduce electricity consumption.

iVG Energy Savings (PDF)

6.3 Runnerless Micro-Injection Molding

Specialized equipment is required for injection molding for small, direct gated plastic parts.

Learn more