Clear Plastics: Types, Uses, And Injection Molding Tips

Table of Contents

1. Introduction

2. Classification Based on Light Transmittance

3. Resin Categories

4. Properties and Processing of Key Transparent Plastics

5. Application-Based Material Selection

6. Key Considerations During Injection Molding

7. Conclusion

1. Introduction

Today's industrial products, especially consumer electronics, smart home appliances, medical devices, automotive parts and other fields, the demand for ‘texture’ and ‘sense of technology’ is becoming increasingly high, and clear plastics is one of the ideal materials to balance the optical properties and structural strength. Data show that in 2024, the global clear plastics market size has exceeded 12 billion U.S. dollars, is expected to reach more than 17 billion U.S. dollars in 2029, compound annual growth rate of more than 7%. From PMMA optical grade leading, to impact-resistant PC, to the packaging industry preferred PET, transparent plastics continue to break the traditional ‘fragile impression’, with excellent comprehensive performance, to support the design of multiple scenarios of innovation.

Clear plastics play a vital role in modern product design due to their excellent light transmission, aesthetics, and versatility. Understanding how to choose, process, and apply these materials is essential for engineers, designers, and manufacturers. This article provides a comprehensive overview of clear plastics, including material classifications, performance characteristics, molding considerations, and industry applications.

2. Classification Based on Light Transmittance

Depending on light transmittance in the visible range (400–800 nm), plastics are categorized into three types:

• Transparent materials: Light transmittance above 80%

• Translucent materials: Light transmittance between 50%–80%

• Opaque materials: Light transmittance below 50%

Understanding these differences helps in selecting the right material based on visual clarity, appearance, and functional performance.

3. Resin Categories

3.1 Transparent Resins

• Most clear plastics fall into this group, including: 

  PMMA, PC, PS, PET, PETG, Clear ABS, Clear PP, Clear PA, SAN (AS), K-Resin, MS, MBS, PES, JD Series, CR-39, TPX, HEMA, F4, F3, EFP, PVF, PVDF, EP, PF, UP, Cellulose Acetate, Cellulose Nitrate, EVA, and more.

• The most commonly used ones in industry are:
  PC, PMMA, PS, PET, PETG, AS, BS, MS, MBS, transparent ABS, transparent PP, and transparent PA.

3.2 Translucent Resins

Mainly include PP, PE, PA, and PVB (polyvinyl butyral).

3.3 Opaque Resins

Very few plastics are fully opaque. Common examples: PPO and POM.

Material Selection Tips

To qualify as a transparent plastic, a material must offer:

• High optical clarity

• Good strength and impact resistance

• Scratch and wear resistance

• Heat and chemical resistance

• Low water absorption

While PC is often the most ideal transparent material, it is expensive and more difficult to process. PMMA is frequently chosen as a substitute. PET requires stretching (e.g., through blow molding) to improve its mechanical properties, making it ideal for packaging.

4. Properties and Processing of Key Transparent Plastics

4.1 PMMA (Acrylic)

Characteristics:

• High viscosity and moderate flowability

• Requires high injection temperature and pressure

• Transparency up to 92% for white light

• Excellent weather resistance

• Low birefringence, ideal for optical disks

Disadvantages:

• Poor impact resistance

• Easily scratched

• Brittle, requiring surface hardening to improve wear resistance (can reach hardness of 5–6H)

3.2 PC (Polycarbonate)

Characteristics:

• High viscosity and melting temperature

• Narrow processing window (270–320°C)

• Requires high temperature, not necessarily high pressure

• High shrinkage, but good dimensional stability

• Risk of internal stress and cracking if improperly molded

Advantages:

• Outstanding impact strength

• Excellent creep resistance and dimensional stability

• Good weather resistance, heat resistance, and flame retardancy

• Optical clarity second only to PMMA and PS

Disadvantages:

• Poor fatigue strength

• Easily scratched (can be improved by incorporating siloxane groups into the polymer chain)

• Poor resistance to strong acids, alkalis, and UV

• High viscosity = poor flow

3.3 PET (for Preforms)

Characteristics:

• High molding temperature (260–300°C)

• Good flow once melted, but narrow process range

• Requires stretching (e.g., blow molding) to develop final properties

Advantages:

• Excellent transparency and weather resistance (in amorphous state)

• High dimensional stability and wear resistance

• Good electrical insulation

• Popular for bottles: lightweight, high strength, non-toxic, impermeable

Disadvantages:

• Slow crystallization, difficult to mold

• Long cycle time, high molding temperature

• Poor impact resistance

• Improved by reinforcing with glass fibers or blending with other materials

5. Application-Based Material Selection

5.1 Consumer Products

• Transparent Films: PE, PP, PS, PVC, PET

• Transparent Sheets & Plates: PP, PVC, PET, PMMA, PC

• Transparent Tubes: PVC, PA

• Transparent Bottles: PVC, PET, PP, PS, PC

5.2 Lighting Equipment

• Used in lamp covers: PS, modified PS, AS, PMMA, PC

5.3 Optical Instruments

• Hard lenses: CR-39, JD resin

• Contact lenses: HEMA

5.4 Transparent "Glass" Applications

• Transportation glass substitutes: PMMA, PC

• Architectural panels: PVF, PET

5.5 Solar Energy

• PMMA, PC, GF-UP, FEP, PVF, Silicone-based plastics

5.6 Optical Fiber

• Core layer: PMMA or PC

• Cladding layer: Fluorinated olefin or methacrylate polymers

5.7 Optical Discs

• PC, PMMA

5.8 Transparent Encapsulation

• Surface-hardened PMMA, FEP, EVA, EMA, PVB

6. Key Considerations During Injection Molding

Clear plastics require high surface quality. Defects such as flow lines, air bubbles, haziness, black spots, discoloration, and poor gloss must be avoided.

6.1 Material Preparation and Drying

• Raw materials must be clean and sealed to prevent moisture absorption

• Proper drying is essential; moisture can degrade the plastic

• Use a hopper dryer and ensure air is filtered and dehumidified

6.2 Machine Cleaning

• Residual material can cause contamination

• Clean the screw and accessories before and after molding

• Use PS or PE for screw cleaning if no commercial cleaning resin is available

• If idle for long, lower barrel temperature below 160°C (for PC, PMMA)

6.3 Mold and Product Design Considerations

• Maintain uniform wall thickness; adequate draft angles

• Avoid sharp corners and notches (especially for PC)

• Use wide, short runners and gates to ensure smooth flow

• Mold surfaces should be smooth (Ra < 0.8 μm)

• Provide sufficient vents for air release

• Wall thickness ≥ 1mm for most transparent parts (except PET)

6.4 Injection Molding Process Optimization

• Use injection machines with dedicated screws and independently heated nozzles

• Set temperature high enough for proper melt flow, without degradation

• Use higher injection pressures to overcome viscosity, but avoid excessive internal stress

• Apply multi-stage injection: slow – fast – slow

• Keep packing time and cycle time short to reduce internal stress

• Use low screw speed and back pressure to minimize degradation

• Maintain precise mold temperature control (higher mold temps usually yield better parts)

6.5 Other Notes

• Avoid using mold release agents where possible to protect surface quality

• Recycled material use should not exceed 20%

• Post-treatment is often required (except for PET) to relieve internal stress:

• PMMA: Hot air circulation at 70–80°C for 4 hours

• PC: Heat with clean air, glycerin, or mineral oil at 110–135°C for up to 10 hours

• PET: Must undergo biaxial stretching to develop mechanical properties

7. Conclusion

Clear plastics are critical materials in industries ranging from consumer goods to optics and renewable energy. Understanding the differences between materials like PMMA, PC, and PET—as well as mastering their processing nuances—is key to achieving high-quality, defect-free parts. By optimizing resin choice, mold design, machine settings, and post-treatment processes, manufacturers can ensure optimal transparency, durability, and performance.

At Alpine Mold, we are a professional mold manufacturing and plastic injection molding expert based in China. With decades of experience and advanced production capabilities, we specialize in delivering high-precision transparent plastic parts and custom mold solutions for clients worldwide. Whether you're developing a new product or improving an existing one, Alpine Mold is your reliable partner for transparent plastic manufacturing excellence.