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Types of plastics used in stampaggio a iniezione can be divided into two categories: thermoplastics and thermosetting plastics. Common thermoplastics include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS); common thermosetting plastics include phenolic resins, epoxy resins, and unsaturated polyester resins. Thermoplastics are the most commonly used material type in injection molding, so this article primarily covers thermoplastics.
Why are thermoplastics the most widely used materials in injection molding?
Thermoplastics melt into a liquid when heated and solidify into a solid state when cooled. This process can be repeated, making them recyclable and reusable. This property makes thermoplastics easy to process in injection molding, making them suitable for large-scale production. Furthermore, thermoplastics have good plasticity, enabling the creation of complex shapes and easy processing and molding.
In contrast, thermosetting plastics undergo chemical cross-linking upon heating, forming an irreversible three-dimensional network structure. Once solidified, they cannot be softened or reshaped by heating, making them unsuitable for repeated processing. While thermosetting plastics offer high heat resistance and chemical stability, their processing is complex, costly, and non-recyclable, making them less commonly used in injection molding than thermoplastics.
Types Of Plastics Used In Injection Molding: By Features & Applications (The most common form of classification）
| Category (by Application/Performance) | Definition | Example Materials |
| Commodity Thermoplastics | This type of injection-molded plastic is often used in high-volume production where performance requirements are less stringent and cost control is crucial. | PP, PE (HDPE, LDPE, LLDPE), PS, HIPS, SAN, PVC, PET |
| Engineering Thermoplastics | These injection molding plastic types generally exhibit increased strength and toughness and are designed for durable parts requiring superior mechanical, thermal, or chemical properties compared to commodity plastics. | ABS, PMMA, Nylon (PA), PC, POM |
| High-Performance Thermoplastics | These injection-molded plastic types normally can maintain their strength, stability, and resistance even under extreme heat, stress, or chemical environments. | PEI, PSU |
| Thermoplastic Elastomers (Soft & Flexible) | Similar to rubber, these injection molding plastic types can be melted and reprocessed, exhibiting flexibility, softness, and elasticity while remaining as malleable as other thermoplastics. | TPE, TPU, TPR |
Polipropilene (PP)
PP is a thermoplastic made from the polymerization of propylene monomers and widely used in injection molding. It is a semi-crystalline thermoplastic material with excellent moisture and alkali resistance, chemical stability, and mechanical properties.
Applicazioni: Car bumpers, washing machine drums, lunch boxes, chairs, hinges
Pros:
- Low cost, easy processing, suitable for large-scale production
- Good processability, suitable for automated production
- Low density (approximately 0.90 g/cm³)
- High tensile strength, impact strength, and fatigue strength
- Good heat resistance (continuous use temperature up to 110-120°C) and electrical insulation
- Good recyclability
Cons:
- Sensitive to UV rays and oxidants
- Tends to become brittle at low temperatures
- Slippery surface, difficult to bond to other materials, and poor colorability
- Somewhat flammable
- High shrinkage during injection molding, which may lead to dimensional instability
Polystyrene (PS)
PS is a colorless, transparent thermoplastic synthesized from styrene monomers via free radical addition polymerization.
Applicazioni: Food packaging containers, disposable tableware, CD cases, hangers
Pros:
- The addition of rubber or butadiene copolymer significantly improves the material’s toughness and impact resistance.
- Excellent flowability and processability make it suitable for producing complex shapes.
- Low shrinkage (less than 0.5%) helps maintain dimensional accuracy.
- Excellent electrical insulation properties.
- Relatively affordable, easy to obtain and process.
- High transparency (light transmittance can reach 88%-92%).
Cons:
- Brittle and prone to fracture under stress or impact.
- High melting point (210°C-250°C), requiring high processing temperature control.
- Poor thermal stability; prolonged exposure to high temperatures may cause degradation or oxidation.
- Poor resistance to various chemicals (such as acids, alkalis, and solvents) and UV rays.
- Requires high temperature and pressure control, resulting in long injection molding cycles.
- Non-biodegradable.
- Somewhat flammable.
High Impact Polystyrene (HIPS)
HIPS is a modified polystyrene that is made by adding a rubber component (such as polybutadiene) to polystyrene to improve its impact resistance and mechanical properties.
Applicazioni: Refrigerator liners, TV housings, office machine housings, Petri dishes, seeds
Pros:
- High impact strength and toughness, resists breakage upon impact
- Cost advantage over other materials such as ABS
- Easy to dye and surface treat
- Recyclable, meeting environmental standards
Cons:
- Poor heat resistance, low maximum operating temperature (approximately 50°C)
- Poor resistance to UV rays, chemicals, and oxygen
- Low transparency
- Poor electrical insulation performance compared to polystyrene
- Some hygroscopicity
Polyethylene (PE)
PE is primarily made from the polymerization of ethylene. Based on its density and molecular structure, it can be categorized into high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE).
Applicazioni: Daily necessities (buckets, washbasins), toys, bottle caps, and food containers.
LDPE has a high degree of molecular chain branching, a nonlinear structure, and a low degree of crystallinity (approximately 55%-65%). This results in good flexibility and average transparency, but poor mechanical strength, heat resistance, and environmental stress cracking resistance.
Applicazioni: Flexible containers, squeeze caps, packaging films, food packaging
LLDPE has a linear structure with short chain branches and a narrow molecular weight distribution, placing it somewhere between LDPE and HDPE. It offers excellent tear resistance, puncture resistance, and environmental stress cracking resistance, while also boasting superior strength and toughness compared to LDPE.
Applicazioni: Film packaging, barrels, cans, and caps
Styrene Acrylonitrile (SAN)
SAN, a colorless, transparent polymer made from a copolymer of styrene and acrylonitrile, is commonly used in the manufacture of various engineering components and consumer products.
Applicazioni: Kitchenware, dashboard lenses, cosmetic containers
Pros:
- High clarity and good optical properties
- Good chemical stability, resistant to a wide range of acids, bases, solvents, and oils
- High stability, with a high glass transition temperature (approximately 100°C)
- High hardness, rigidity, and strength, with some toughness
- Good dimensional stability
Cons:
- Impact strength is inferior to other engineering plastics (such as ABS or polycarbonate)
- Poor resistance to solvents such as aromatic hydrocarbons, esters, ketones, and strong acids
- SAN may lose clarity and develop a yellow tint as the acrylonitrile content increases
- Degradation or discoloration may occur with prolonged exposure to UV light or high temperatures
Polyethylene Terephthalate (PET)
PET is synthesized from terephthalic acid and ethylene glycol through a polycondensation reaction and exhibits excellent heat resistance, transparency, chemical resistance, and mechanical properties.
Applicazioni: Beverage bottles, food packaging
Pros:
- High strength and lightweight properties enable the production of lightweight and durable components
- Excellent chemical stability, solvent resistance, electrical insulation, and gas barrier properties
- Recyclable, environmentally friendly, and safe for food contact
- Excellent transparency and gloss, making it suitable for transparent packaging and optical applications
Cons:
- Relatively poor heat resistance, a low glass transition temperature (approximately 67-81°C), and a melting point of 260°C, making it susceptible to thermal degradation at high temperatures.
- Not readily biodegradable and susceptible to oxidation under certain conditions
- Moisture content must be controlled during processing to avoid hydrolysis and degradation
- Relatively low impact resistance, inferior to some engineering plastics (such as PBT)
Cloruro di polivinile (PVC)
PVC is a thermoplastic resin formed by the polymerization of vinyl chloride monomer. It has excellent processability and a wide range of applications, suitable for various molding processes, including injection molding.
Applicazioni: Various molds (for automotive, medical, and electrical appliances), building materials, pipes, and electrical housings.
Pros:
- Excellent moisture and weather resistance.
- High hardness and impact resistance, able to withstand physical impact and retain its shape.
- Can be made into rigid or flexible materials as needed.
Cons:
- Decomposes or releases harmful gases at high temperatures, requiring the addition of stabilizers to improve heat resistance.
- Disposal is difficult, and incineration may release harmful gases.
Acrylic (PMMA)
PMMA, or Polymethyl Methacrylate, often referred to as “organic glass” or “acrylic,” is a material made from the polymerization of methyl methacrylate (MMA) monomers. PMMA excels in injection molding, allowing for complex shapes while maintaining high transparency and strength.
Applicazioni: Commonly used as a glass substitute (such as headlight lenses and signal light covers), optical devices (lenses, display screens), signage, toys, and decorative items.
Pros:
- High transparency, with a transmittance of up to 92%, surpassing PS and PC.
- Effective UV filtering and resistance to yellowing after long-term outdoor use.
- Easy to process, allowing for easy modification of injection-molded parts.
- Low density (approximately 1.19 g/cm³) and lightweight.
Cons:
- The surface scratches easily, affecting aesthetics.
- Sensitive to certain solvents (such as alcohols and ketones), easily corroded.
- Continuous operating temperature is relatively low, approximately 80-100°C, making it unsuitable for high-temperature environments.
- Impact resistance is average, with a brittle texture and low load-bearing capacity.
Acrilonitrile Butadiene Stirene (ABS)
ABS is a terpolymer composed of acrylonitrile, butadiene, and styrene. In its molecular structure, acrylonitrile provides hardness and heat resistance, butadiene provides toughness, and styrene provides processability and surface gloss.
Applicazioni: Automotive parts (interior panels), electronic housings (computers, TVs), appliance components, toys (such as Lego).
Pros:
- High impact strength, toughness, and rigidity, with superior impact resistance to materials like polystyrene.
- Low melting point and good processability make it suitable for rapid prototyping and small-batch production.
- Good acid and alkali resistance.
Cons:
- Poor weather resistance and prone to degradation from long-term UV exposure.
- Poor resistance to polar solvents and susceptible to corrosion by certain chemicals.
- Poor high temperature tolerance and low continuous operating temperature (approximately 70°C).
- Somewhat flammable.
Nylon/Polyamide (PA)
PA is a polymer material composed of monomers containing carboxyl and amino groups polymerized through amide bonds. It exhibits excellent mechanical properties, wear resistance, chemical resistance, and self-lubrication.
Applicazioni: Gears, bearings, engine covers, power tool housings, sports equipment
Pros:
- High mechanical strength, good wear resistance, and fatigue resistance
- Self-lubricating properties and a low coefficient of friction
- Good electrical insulation
- Low density, light weight, and easy dyeing
- Excellent heat resistance, reaching temperatures exceeding 100°C
Cons:
- High water absorption requires thorough drying before injection molding, otherwise dimensional changes and performance degradation may occur
- Poor tolerance to acids
- High requirements for molds and equipment during injection molding
- Sensitive to UV rays and high temperatures
Policarbonato (PC)
PC is a thermoplastic polymer with carbonate (-O-CO-O-) chain segments as its primary chain. It is an amorphous thermoplastic material with excellent light transmittance, high mechanical strength, heat resistance, and dimensional stability, making it suitable for injection molding.
Applicazioni: Electronic device housings, automotive headlights, explosion-proof shields, safety mirrors, and large lamp covers.
Pros:
- Like PMMA, it is suitable for high-transparency applications, with a light transmittance of 80-90%.
- It has extremely high impact strength, 250 times stronger than ordinary glass.
- It has good heat resistance, with an HDT of approximately 130°C.
- It has good dimensional stability and weather resistance.
- It has good electrical insulation properties.
Cons:
- It has a high melting point (processing temperature approximately 260-340°C), making processing difficult and costly.
- It is susceptible to hydrolysis in environments with both high temperature and high humidity.
- It is more expensive than other engineering plastics (such as ABS and PVC).
- It scratches easily.
Polyoxymethylene (POM)
POM is a semi-crystalline thermoplastic material, also known as polyacetaldehyde or polyoxymethylene, used to manufacture precision components requiring high strength, low friction, and dimensional stability. Its strength and rigidity approach those of metal, earning it the nickname “plastic steel.”
Applicazioni: Precision gears, bearings, automotive fuel system parts, faucet components, zippers
Pros:
- Its high mechanical strength, high rigidity, and excellent wear resistance make it suitable for precision components and high-load environments.
- Its excellent dimensional stability maintains its shape even under temperature and humidity fluctuations.
- Its low coefficient of friction and self-lubricating properties make it suitable for sliding parts and precision moving parts.
- Its excellent chemical resistance and electrical insulation properties make it suitable for high-pressure applications.
Cons:
- Its high mold shrinkage (up to 1.2% to 3.5%) makes dimensional control difficult during injection molding.
- It is susceptible to thermal decomposition at high temperatures and brittle at low temperatures.
- It is susceptible to aging due to UV rays and oxidation, resulting in performance degradation and surface powdering and cracking.
- Processing is complex and relatively costly.
- It is difficult to bond and incorporate flame retardants.
Polyetherimide (PEI)
PEI is a high-performance engineering plastic, an amorphous thermoplastic polymer. It is formed by the polycondensation of aromatic diphenols and dianhydrides in a solvent and exhibits excellent heat resistance, mechanical properties, electrical insulation, chemical stability, and flame retardancy.
Applicazioni: Medical sterilization trays, electronic and electrical connectors
Pros:
- Excellent thermal stability, with a glass transition temperature (Tg) as high as 215°C and a continuous operating temperature of up to 180°C.
- High mechanical strength, high rigidity, and good impact strength make it suitable for structural components and high-precision applications.
- Excellent electrical insulation properties.
- Intrinsically flame retardant, achieving a UL94-V-0 rating without the addition of flame retardants.
- Excellent resistance to a variety of chemicals (such as hydrocarbons, alcohols, and halogenated solvents).
Cons:
- Relatively high production and processing costs.
- High melt viscosity requires high temperatures and pressures during processing, placing high demands on processing equipment and tools.
- Poor material utilization during processing.
The complex processing process requires high process control, making it unsuitable for large-scale continuous production.
Polysulfone (PSU)
PSU is a high-performance thermoplastic polymer material whose structure contains aromatic groups (typically multiple benzene rings) connected by sulfonyl groups.
Applicazioni: Medical surgical instruments, food processing equipment
Pros:
- High heat resistance, suitable for long-term use within the temperature range of -100°C to 150°C, and short-term use temperatures up to 190°C.
- Provides high strength, high rigidity, good impact resistance, and creep resistance.
- Suitable for injection molding of complex shapes.
- Excellent chemical stability, resistant to acids, alkalis, salts, ketones, esters, and other solvents, hydrolysis resistance, radiation resistance, flame resistance, and extinguishing properties.
Cons:
- Poor flowability, difficult processing, requires pre-drying to a moisture content of less than 0.05%.
- Poor fatigue resistance, poor weather resistance, and prone to stress cracking.
- Relatively high cost, difficult to recycle, and derived from non-renewable resources.
Thermoplastic Rubber (TPR)
TPR is typically based on SBS (styrene-butadiene-styrene) or SEBS (styrene-ethylene-butylene-styrene). It is modified by adding resins, fillers, plasticizers, and other additives to form granular materials. It combines the elastic properties of rubber with those of thermoplastics.
Applicazioni: Tool handles, shoe soles, wheels, seals, medical devices, food contact materials
Pros:
- Compared to traditional rubber, it offers a simpler processing process, requiring no vulcanization.
- It is recyclable, meets environmental standards, and is reusable.
- It exhibits good elasticity, wear resistance, fatigue resistance, and weather resistance.
- It offers a wide range of hardness and good adjustability.
- It has relatively low production costs and can be compounded with a variety of plastics (such as PP, ABS, and PC).
- It is non-toxic and harmless, and complies with multiple environmental standards (such as REACH and EN71).
Cons:
- It may experience a decrease in physical properties due to thermal degradation at high temperatures.
- It lacks chemical resistance (such as oil and solvent resistance) compared to some specialty polymers (such as TPU).
- It performs poorly in extremely low temperature environments.
- It has relatively poor adhesion to some materials, and its bonding performance is inferior to that of rubber or silicone.
- It may experience performance degradation due to creep or aging over time, especially under continuous load or in extreme environments.
Thermoplastic Elastomer (TPE)
TPE’s backbone consists of a hard resin phase and a soft rubber phase, firmly bonded together. This gives it the elasticity of rubber at room temperature and the ability to be plasticized and molded at high temperatures.
Applicazioni: Mobile phone cases, automotive seals, sports equipment grips, children’s play equipment, toothbrush handles
Pros:
- Short processing cycle and high production efficiency
- Non-toxic and odorless, harmless to the environment and personnel
- Meets environmental requirements, and waste can be recycled
- Excellent flexibility, elasticity, and wear resistance
Cons:
- Performance is significantly affected by temperature and environmental factors, and heat resistance is inferior to traditional rubber. Compression set, resilience, and durability are inferior to those of rubber.
- Price is generally higher than similar rubber materials.
- High material cost
Thermoplastic Polyurethane (TPU)
TPU’s molecular structure consists of alternating hard and soft segments. It is a thermoplastic polyurethane elastomer formed by the reaction of a diisocyanate (such as MDI or TDI) with a polyol.
Applicazioni: Mobile phone cases, shoe soles, cable jacketing, industrial casters
Pros:
- Excellent tensile strength, abrasion resistance, tear resistance, and impact resistance
- Excellent flexibility and elasticity, maintaining good flexibility even at low temperatures
- Good resistance to oils, solvents, chemicals, and UV rays
- Recyclable and can be remelted and reshaped after processing
- Can be customized by adding various ingredients (such as flame retardants and pigments)
Cons:
- May degrade with long-term use in extreme environments (such as high temperatures and UV exposure)
- Requires drying before processing
- Relatively narrow hardness range
- Relatively high production cost
- Performance depends to a certain extent on its chemical composition and additives
Types Of Plastics Used In Injection Molding: By Molecular Structure (What Engineers Must Know)
According to different molecular structures, there are two injection molding plastic types: Amorphous Thermoplastics and Semi-Crystalline Thermoplastics. This classification is usually used by injection molding engineers and mold designers.
| Aspect | Amorphous Thermoplastics | Semi-Crystalline Thermoplastics |
| Definition | Plastics with molecules arranged randomly, without a long-range repeating order (similar to glass) | Plastics with molecules arranged in both ordered crystalline regions and disordered amorphous regions |
| Example | ABS, PC, PMMA, PS, SAN, PVC, PEI, PSU | PP, PE (HDPE, LDPE, LLDPE), PET, Nylon (PA), POM, TPU, TPE, TPR |
| Molecular Structure | Random, disordered chains | Mix of ordered crystalline and amorphous regions |
| Appearance | Often transparent(e.g., PC, PMMA) | Usually opaque(e.g., PP, PE, Nylon) |
| Melting Behavior | No sharp melting point; soften gradually | Distinct sharp melting point (Tm) |
| Thermal Transition | Defined by glass transition (Tg) The softening process occurs over a wide temperature range Example: ABS Tg ≈ 105 °C | Defined by Tg + Tm Melting and solidification occur within a narrow temperature range Example: Nylon (PA6) Tg ≈ 50 °C, Tm ≈ 220 °C |
| Dimensional Stability | Less shrinkage (0.2 – 0.8%), better precision Easier to predict and control | More shrinkage (1.0 – 3.0%), higher risk of warpage The mold cavity size must be precisely enlarged to compensate for shrinkage |
| Chemical Resistance | Generally weaker against chemicals (stress cracking possible) | Much better chemical resistance due to tight crystalline packing |
| Mechanical Strength | Good toughness but lower wear resistance | Higher wear resistance, toughness, and fatigue strength |
| Injection Molding |
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Why is it crucial for injection molding engineers to understand this classification?
Primarily, molecular structure directly determines a material’s processing behavior and final performance, thus influencing mold design, process settings, and product quality control. For example:
- Shrinkage has the greatest impact on mold design;
- Thermal behavior and processing temperature influence process window settings;
- Chemical and mechanical properties influence product applications;
- Stress and stress cracking impact product quality.
Therefore, for injection molding engineers, determining whether a material is semi-crystalline or amorphous is fundamental to all subsequent planning. This directly determines: mold design (size magnification, gates, and cooling systems); process settings (barrel temperature, mold temperature, holding pressure, and profile); and potential product risks (warpage, dimensional inaccuracy, stress cracking).
Types Of Plastics Used In Injection Molding: By Mechanical Behavior (Not Common, But Engineers Must Know)
If you’re familiar with resins, you’ll probably be familiar with this classification. This classification is based on the mechanical behavior of a material, specifically how it responds to external forces. It’s a very intuitive and application-oriented classification method that engineers and product designers sometimes use to initially screen materials based on the functional requirements of a product.
Rigid Plastics: Materials with a high elastic modulus and high rigidity. When subjected to external forces, they deform very little and tend to fracture directly into brittle or ductile form after reaching the yield point rather than recovering to their original shape. Examples: PS (Polystyrene), HIPS (High Impact PS), ABS (Acrylonitrile Butadiene Styrene), PMMA (Acrylic), PC (Polycarbonate), SAN (Styrene Acrylonitrile), PVC (rigid form), POM (Polyoxymethylene), PA (Nylon, stiffer grades), PEI (Polyetherimide), PSU (Polysulfone)
Applicazioni comuni: Components requiring shape retention and structural support, such as housings, containers, gears, bearings, lenses, instrument panels, and toy building blocks (e.g., Lego).
Flexible Plastics: Materials with a low elastic modulus and a certain degree of toughness and flexibility. They can undergo significant, reversible deformation under external force, but gradually or partially recover after the force is removed, typically with some permanent deformation. Examples: LDPE (Low-Density PE), LLDPE (Linear Low-Density PE), HDPE (High-Density PE, semi-rigid but still flexible), PP (Polypropylene), PET (Polyethylene Terephthalate, flexible in thin sections), PA (Nylon, flexible grades)
Applicazioni comuni: Flexible packaging films, straws, hoses, hinged containers (such as squeeze bottles for condiments), cards, and other components that require a certain degree of deformability but do not require high resilience.
Elastomers (Soft & Rubber-like): Materials that can undergo significant deformation (elongation several times) when subjected to significant external forces and quickly and almost completely return to their original shape and size after the force is removed. Their core properties are high elasticity and low permanent deformation. Examples: TPR (Thermoplastic Rubber), TPE (Thermoplastic Elastomer), TPU (Thermoplastic Polyurethane)
Common applications: Components requiring extremely high resilience and sealing properties, such as seals, shock absorbers, soft grips, tires, extension tubes, and wearable device straps.
Why is this classification important to engineers?
Function-Based Material Selection: When designing a part, the first question to ask is, “What does it need to do?”
- If it needs to bear load or maintain a precise shape, consider rigid plastics.
- If it needs to flex or absorb small amounts of energy, consider flexible plastics.
- If it needs to seal, absorb shock, provide grip, or expand and contract significantly, consider elastomers.
Process and Design Considerations: Different material categories have different shrinkage rates, rheological properties, and mold release methods. For example, the draft angle and ejection method for an elastomeric seal may differ significantly from those for a rigid housing.
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