Injection molds and precision molding - guide 2025

Introduction to injection molds

Injection molds are a key tool in the plastic injection molding process. They give shape to the product, determine its quality, dimensional accuracy and production capabilities. Investment in a well-designed and manufactured injection mold is the foundation of success in plastic component production.

Precision plastic molding is a particularly demanding area, where dimensional tolerances are minimal and quality requirements are maximal. It is used in medical, precision electronics, optics and other fields requiring highest quality.

In this comprehensive guide, we will present everything about injection molds - from construction and types, through design and materials, to advanced precision molding technologies.

Construction and elements of injection mold

An injection mold is a complex tool consisting of many elements working together. Understanding mold construction is crucial for understanding the injection process.

Main elements of injection mold:

1. Forming plates

• Mold cavity - creates the outer surface of the molded part
• Core - creates the inner part of the molded part
• Made of high-grade tool steel
• Require precise machining (CNC milling, EDM, polishing)

2. Injection system

• Sprue bushing - connects with injection molding machine nozzle
• Runners - guide molten plastic to cavities
• Gate - point of plastic entry to mold cavity
• Gate types: point, edge, tunnel, submarine

3. Cooling system

• Cooling channels in mold plates
• Cooling water or oil circuits
• Crucial for cycle time and molded part quality
• Requires uniform temperature distribution

4. Ejection system (ejectors)

• Ejector pins - push molded part from mold
• Ejector plate - synchronizes all pin movement
• Return pins - ensure system return to initial position
• Types: cylindrical, blade, sleeve, air

5. Guidance and centering system

• Guide pillars - ensure precise guidance of mold halves
• Guide bushings - cooperate with pillars
• Centering - ensures accurate mold positioning

6. Moving cores and slides

• Enable forming undercuts
• Controlled hydraulically or mechanically
• Require precise adjustment and synchronization

Types of injection molds

Injection molds are classified according to various criteria. The most important division is by number of plates and method of plastic feeding.

Two-plate molds

Two-plate molds are the simplest and most popular type of molds. They consist of two main plates - fixed and moving.

Advantages of two-plate molds:

• Simple construction and lower production cost
• Ease of maintenance and repairs
• Faster replacement and mounting
• Lower mold weight

Application:

• Mass production of simple elements
• Molded parts without complex undercuts
• Packaging, lids, containers

Three-plate molds

Three-plate molds have an additional plate separating the runner from the mold cavity.

Advantages of three-plate molds:

• Automatic separation of runner
• Possibility of multi-point gating
• Better molded part aesthetics (no gate marks)
• Optimization of multi-cavity mold filling

Application:

• Production of elements with high aesthetic requirements
• Molded parts requiring uniform filling
• Automatic production lines

Multi-cavity molds

Multi-cavity molds have multiple cavities in one mold, allowing production of several or dozens of molded parts in one cycle.

Advantages of multi-cavity molds:

• High production efficiency
• Lower unit cost per molded part
• Optimal machine utilization
• Shortened order fulfillment time

Challenges:

• Higher initial mold cost
• Need for precise runner balancing
• More difficult maintenance
• Required higher clamping force of injection molding machine

Hot runner systems

Hot runner systems keep plastic in runners in liquid state through electric heating.

Advantages of hot runner:

• No waste - no solid runner to recycle
• Shorter cycle time - no need to cool runner
• Better quality - fresh plastic in each cycle
• Material savings - up to 30% consumption reduction
• Possibility of sequential injection - filling control

Disadvantages of hot runner:

• Significantly higher mold cost (30-50% more expensive)
• More complicated maintenance
• Required additional temperature control
• Potential leakage problems

Materials for injection molds

Choice of material for injection molds is crucial for tool durability and production quality.

Tool steels for molds:

1. Carbon steels (P20, 1.2311)

• Good machinability
• Medium hardness (28-32 HRC in as-delivered state)
• Used for molds for medium series (up to 500k cycles)
• Relatively low cost

2. Carburizing steels (1.2312, S50C)

• Possibility of surface hardness increase by carburizing
• Good wear resistance
• Series up to 1M cycles

3. Hardening steels (1.2343, H13)

• High hardness after heat treatment (48-52 HRC)
• Excellent wear and abrasion resistance
• Series above 1M cycles
• Ideal for abrasive plastics (PA + GF)

4. Stainless steels (1.2316, 420SS)

• Corrosion resistance
• Used for PVC and corrosive plastics
• Do not require protective coatings
• Higher cost

5. Premium alloy steels (1.2379, NAK80)

• Very high surface quality
• Minimal distortion after heat treatment
• Ideal for optics and transparent elements
• High cost

Other materials:

• Aluminum (7075, QC-10) - prototype molds, short series, fast cooling
• Beryllium-copper - inserts for intensive cooling
• Sintered carbides - points most exposed to abrasion

Injection mold design

Designing injection molds is a process requiring knowledge in mechanical design, plastics technology and numerical simulations.

Mold design stages:

1. Molded part analysis

• Geometry and dimensions
• Functional requirements
• Plastic type
• Production series size
• Quality requirements and tolerances

2. Mold division (parting line)

• Determining parting line
• Undercut analysis
• Planning moving cores

3. Gating system design

• Gate location - impact on quality and appearance
• Gate type (point, edge, tunnel)
• Runner balancing in multi-cavity molds
• Decision on hot runner vs. cold runner

4. Cooling system

• Cooling channel placement
• Coolant flow simulation
• Cooling time optimization
• Conformal cooling

5. Ejection system

• Ejector placement
• Ejection force calculation
• Preventing molded part deformation

6. CAE simulations

• Moldflow, Moldex3D - filling simulation
• Weld line analysis
• Shrinkage and warpage prediction
• Injection parameter optimization

CAD/CAM software:

• SolidWorks - 3D design
• CATIA - advanced automotive projects
• NX (Unigraphics) - full CAD/CAM integration
• Cimatron - specialized for injection molds
• PowerMill, Mastercam - CNC machining programming

Precision plastic molding

Precision plastic molding is a specialized field requiring highest quality machines, molds and processes. Dimensional tolerances are on the order of ±0.01mm or less.

Precision molding requirements:

1. Electric injection molding machines

• High repeatability (±0.1%)
• Precise positioning (±0.01mm)
• Stable process temperature (±0.5°C)
• Digital control of all parameters

2. Precision molds

• Premium steel (NAK80, 1.2379)
• Precision machining (tolerances ±0.005mm)
• Mirror polishing (Ra < 0.1 µm)
• Precise cooling system

3. Process control

• Cavity pressure monitoring
• Real-time temperature control
• 100% inspection vision system
• SPC (Statistical Process Control)

Precision molding applications:

• Optics - lenses, prisms (tolerance < 0.005mm)
• Medical - diagnostic parts, implants
• Electronics - connectors, housings
• Automotive - sensors, precision connections
• Micro molding - elements < 1g, micrometer tolerances

Advanced molding technologies

Modern injection molds utilize advanced technologies allowing production of increasingly complex elements.

1. Multi-component injection (2K, 3K molding)

• Combining different plastics in one element
• Hard + soft (e.g. housing + seal)
• Different colors without painting
• Requires special injection molding machines and rotating molds

2. Insert molding

• Encapsulating metal or other material inserts
• Elimination of assembly operations
• Better connection strength

3. Gas-assisted injection

• Gas injection into molded part interior
• Weight and material consumption reduction
• Sink mark elimination
• Possibility of thicker walls

4. In-Mold Labeling (IML)

• Label application directly in mold
• High graphics quality
• Durable label-plastic connection

5. MuCell (micro-foaming)

• Nitrogen injection in form of micro-bubbles
• Weight reduction up to 20%
• Lower internal stresses
• Shorter cycle time

Mold maintenance and upkeep

Proper maintenance of injection molds is key to long tool life and stable production quality.

Mold maintenance program:

Daily maintenance (by operator):

• Cleaning mold surface from plastic residue
• Visual damage inspection
• Lubrication of pillars and moving elements
• Cooling system tightness check

Periodic maintenance (every 10-50k cycles):

• Thorough cleaning of vent channels
• Forming surface wear inspection
• Ejection system check
• Cooling channel inspection (calcium, rust)
• Hot runner seal replacement

Major inspection (100-500k cycles):

• Complete mold disassembly
• Critical dimension measurement
• Forming surface polishing
• Worn element replacement
• Welding and damage regeneration
• Calibration and adjustment

Typical problems and solutions:

• Flash - parting surface wear, too low clamping pressure
• Short shots - gate obstruction, poor venting, low temperature
• Sink marks - insufficient packing, too short packing time
• Ejector marks - too high ejection forces, uneven distribution
• Warpage - uneven cooling, internal stresses

Mold production costs

The cost of an injection mold is one of the main investments in injection production. Prices can range from several thousand to several hundred thousand zlotys.

Factors affecting mold cost:

1. Geometry complexity

• Simple geometry: 10,000 - 50,000 PLN
• Medium complexity: 50,000 - 150,000 PLN
• High complexity: 150,000 - 500,000+ PLN

2. Number of cavities

• Single-cavity mold - cost base
• 2 cavities - +40-60% cost
• 4 cavities - +100-150% cost
• 8+ cavities - +200-400% cost

3. Injection system

• Cold runner - standard cost
• Hot runner - +30-50% of mold cost

4. Mold material

• Aluminum - 50-70% of steel cost
• P20 steel - standard pricing
• Hardened steel - +20-40%
• Premium steel (NAK80) - +50-100%

5. Country of production

• China - 30-50% of Europe cost
• Poland/Eastern Europe - 60-80% of Germany cost
• Germany/Austria - reference price

Return on investment calculation:

• Mold cost / (molded part price - material cost - labor cost) = break-even point
• Typically 10,000 - 100,000 pieces to return investment

Quality control and tolerances

In precision plastic molding quality control is crucial. Modern injection molds must ensure repeatability at micrometer level.

Mold and molded part inspection methods:

1. Dimensional inspection

• Digital calipers - accuracy ±0.01mm
• Micrometers - accuracy ±0.001mm
• CMM (Coordinate Measuring Machine) - 3D measurements, accuracy ±0.002mm
• Optical scanners - full 3D geometry
• Profile projectors - contour inspection

2. Surface quality inspection

• Roughness (Ra) - contact and optical profilometers
• Gloss - glossmeters
• Visual inspection - for aesthetic defects

3. Mechanical properties analysis

• Tensile strength
• Impact resistance
• Hardness

Standards and norms:

• ISO 20457 - injection mold terminology
• ISO 12165 - dimensional tolerances
• DIN 16742 - technical mold deliveries
• SPI/SPE - surface finish standards

Trends and future

The injection mold industry is developing dynamically. The most important trends are:

1. 3D printing of molds (Additive Manufacturing)

• DMLS (Direct Metal Laser Sintering) - steel molds from 3D printers
• Conformal cooling - cooling channels impossible to make traditionally
• Mold production time reduction from weeks to days
• Prototype molds from resin or aluminum

2. Digitization and Industry 4.0

• Digital mold twins - simulation before production
• IoT in molds - sensors monitoring mold condition
• Predictive maintenance - AI predicts failures
• Blockchain in mold lifecycle management

3. New materials

• Metal-ceramic composites - higher hardness
• Nanocomposite coatings - ultra-low friction
• High-performance steels - longer life

4. Sustainable development

• Regeneration and modernization of old molds instead of new ones
• Material consumption optimization
• Molds for biopolymers

Summary

Injection molds are the foundation of plastics production. Precision plastic molding requires highest quality tools and processes.

Key conclusions from the guide:

• Mold types - two/three-plate, multi-cavity, hot runner - each has its application
• Materials - from aluminum to premium steels, choice depends on series size
• Design - CAD/CAM and simulations are standard
• Precision - electric injection molding machines and premium molds for tolerances <±0.01mm
• Costs - from 10k to 500k+ PLN, depending on complexity
• Maintenance - regular inspection ensures long life
• Future - 3D printing, digitization, new materials

If you need support in injection mold design or precision molding implementation, contact TEDESolutions. We offer comprehensive technical consulting and Tederic injection molding machines ideal for precision production.

Also see our articles about injection molding machines, plastics processing and injection molding machine brand comparison.