Alexei Volkov
Plastic is used in almost every consumer product and industry, from electronics to aerospace. With tens of thousands of commercial options for plastic materials, choosing the right one for injection molding is critical. The right material can ensure part longevity, efficiency, and optimal performance, while the wrong choice can lead to part failure, equipment damage, and litigation. Material selection depends on various factors, including desired tensile strength, flexural modulus, toughness, electrical insulation, temperature resistance, chemical resistance, FDA compliance, aesthetics, mechanical requirements, tolerances, environment, assembly, sterilization, regulatory requirements, electrical properties, and cost. There are two main types of thermoplastic materials: amorphous and semicrystalline. Amorphous thermoplastics are highly transparent and formable but have poor chemical resistance, while semicrystalline thermoplastics offer great fatigue and stress-cracking resistance. Common plastics used in injection molding include ABS, polypropylene, polycarbonate, and acrylic.
| Characteristics | Values |
|---|---|
| Tensile strength | High for acetal polymers like TECAFORM and Delrin |
| Flexural modulus (bending stiffness) | |
| Izod impact (toughness) | High for ABS, PC, and ABS/PC blends |
| Electrical insulation | |
| Temperature resistance | High for PC and ABS/PC blends |
| Chemical resistance | High for polyethylene, TECAFINE PP, and TECAFINE PE |
| FDA compliance | Important for medical or consumer products |
| Cost | Polypropylene is cheap |
| Aesthetic requirements | PMMA is transparent and clear; ABS is opaque and compatible with colorants |
| Mechanical requirements | Forces, motion, and interactions with other parts |
| Tolerances | Low shrinkage and good dimensional stability for PC and ABS/PC blends |
| Environment | Chemical, molding temperature, humidity, cycling |
| Assembly | Ultrasonic welding, laser welding, adhesive bonding, pad printing |
| Sterilization | Single-use or sterilized reusable; gamma, e-beam, EtO, or steam |
| Regulatory requirements | Biocompatibility requirements for medical products |
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What You'll Learn
The type of plastic: ABS, polycarbonate, polypropylene, etc
When choosing the type of plastic for injection moulding, it is important to consider the desired properties of the final product, such as tensile strength, flexural modulus, toughness, electrical insulation, temperature resistance, chemical resistance, FDA compliance, and cost. The type of plastic will have a major impact on the performance and quality of the final product.
One of the most commonly used plastics for injection moulding is Acrylonitrile Butadiene Styrene (ABS), an opaque amorphous thermoplastic with a low melting point. ABS is reasonably priced, strong, tough, and has a decent appearance. It is also forgiving if standard design rules are not followed. However, ABS has a relatively low melting point.
Polycarbonate (PC) is another option. It is stronger and more impact-resistant than ABS, with higher heat resistance and the ability to withstand higher temperatures. PC is less forgiving than ABS if standard design rules are not followed.
If toughness is not a priority, polypropylene (PP) is a good choice. It is a versatile, chemical-resistant, lightweight thermoplastic. Polypropylene is often used for automotive bumpers, chemical processing equipment, and medical equipment.
For structural applications, acrylic (PMMA) is a good option. It is an amorphous thermoplastic that is strong, clear, and lightweight, making it a great alternative to glass. Acrylic is also resistant to sunlight and water degradation, making it suitable for outdoor use. However, PMMA can be a little brittle.
For applications requiring the elastic properties of rubber combined with the processing advantages of thermoplastics, Thermoplastic Elastomer (TPE) is a good choice. TPE is commonly used for seals, gaskets, soft-touch grips, and medical tubing.
With so many options available, it is important to carefully consider the desired properties and requirements of the final product to choose the most suitable type of plastic for injection moulding.
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Part quality and performance
When selecting a plastic material, it is important to consider the desired tensile strength, flexural modulus (bending stiffness), Izod impact (toughness), electrical insulation, temperature resistance, chemical resistance, FDA compliance, and the cost of the final product. For instance, if the product will be subjected to high temperatures, polycarbonate (PC) is a better choice than Acrylonitrile Butadiene Styrene (ABS) due to its higher heat resistance.
The type of plastic chosen will depend on the specific requirements of the application. For example, if transparency is a key requirement, acrylic (PMMA) or polycarbonate (PC) may be suitable options. If impact resistance is a priority, ABS, PC, or polypropylene (PP) could be considered. PP is also a good choice when chemical resistance is required, as it is widely used in applications where sterilization is important.
Additionally, the assembly method should be taken into account. For instance, if ultrasonic welding or adhesive bonding will be used, compatible materials must be selected. Regulatory requirements for specific industries, such as medical, automotive, and aerospace, should also be considered. For instance, medical products have specific biocompatibility requirements based on patient contact.
Material selection can be a daunting task due to the vast number of plastic materials available, including specialty plastics, polymer blends, and different grades of the same plastic material. It is recommended to consult experts and utilize online material databases to make an informed decision.
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Aesthetic requirements
When choosing a plastic for injection moulding, one of the factors to consider is aesthetics. Aesthetic requirements such as transparency and gloss are important to think about. For instance, if transparency is a requirement, acrylic (PMMA) is a good option as it is clear, strong, and lightweight. It is also a great alternative to glass and can be used for outdoor windows. However, PMMA can be a little brittle. Another option for transparency is polycarbonate (PC), which is tougher than PMMA but may not be as aesthetically pleasing.
If gloss is an important aesthetic requirement, Acrylonitrile Butadiene Styrene (ABS) is a good choice. ABS is an opaque polymer that is compatible with colorants and offers various textures and surface finishes. It is also strong, tough, reasonably priced, and relatively forgiving in terms of design rules. However, ABS has a relatively low melting point.
Other aesthetic considerations may include the desired colour, finish, and overall appearance of the product. It is also worth noting that some plastics, like polypropylene (PP), are cheaper but may not have the same rigidity and cosmetic appeal as other options.
In addition to aesthetics, it is crucial to evaluate other factors such as mechanical requirements, tolerances, the environment in which the product will be used, assembly methods, sterilization requirements, regulatory requirements, electrical properties, and cost. These factors, along with aesthetic requirements, will help guide the selection of the most suitable plastic for injection moulding.
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Mechanical requirements
When choosing a plastic for injection moulding, there are several mechanical requirements to consider. Firstly, understand the complete application requirements and run a design analysis on the 3D model. This includes the complexity of the mould design and the desired end function of the product.
The first property to consider is the product's desired tensile strength, or resistance to being pulled apart. This is typically measured in PSI (pounds per square inch). Another property to consider is Izod impact (notched) or toughness, which is the impact resistance of the material. This is measured by swinging an arm into the notched sample and recording the energy required to break the part, measured in ft·lb/in2.
Other mechanical requirements include the product's flexural modulus (bending stiffness), electrical insulation, temperature resistance, chemical resistance, and FDA compliance. For instance, if a chemical capable of inducing stress cracking is present in the application environment, the maximum operating stress of the plastic will decline.
The thermal properties of the plastic will determine the maximum and minimum working temperature, the melting temperature, and how the material will react in the mould and as it cools. Injection moulding pressure and flow rate of the material can also cause shrinkage and dimensional instability if not chosen carefully.
The hardness, flexibility, weight, and cost of the plastic are also important factors to consider. For instance, a more flexible resin is not suitable for functions where rigidity and strength are required. The cost of the plastic should be considered, but material decisions should be based on the product application rather than cost alone.
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Chemical resistance
When choosing a plastic for injection moulding, chemical resistance is an important factor to consider. Chemical resistance determines how well a plastic can resist degradation by certain chemicals. For instance, gasoline melts polystyrene but does not affect polyethylene, which is why plastic gas cans are made of polyethylene. Therefore, it is crucial to identify the chemicals that the final product will come into contact with and select a plastic with good chemical resistance.
Polyphenylene Sulfide (PPS) has the broadest resistance to chemicals of any engineered thermoplastic. PPS is highly resistant to flame, acids, ketones, alkalis, and hydrocarbons, making it ideal for challenging corrosive, high-heat, and chemically strenuous environments. Similarly, polyoxymethylene plastics, commonly known as acetal, are highly durable and resistant to wear and abrasion, making them a popular choice for engineering projects with moving parts.
Nylon is another plastic with good chemical resistance. It offers high tensile strength and outstanding fatigue resistance, and it is resistant to many common chemicals that attack other plastics. However, nylon provides poor resistance to strong acids and bases compared to other plastics. Different types of nylon, such as Nylon 11, Nylon 12, and Nylon 46, offer varying levels of chemical resistance and other mechanical properties.
Polyethylene (PE) plastics, including High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE), also exhibit good chemical resistance. They are resistant to moisture and chemicals, with LDPE being softer and more flexible than HDPE. Acrylic, or polymethyl methacrylate (PMMA), is another plastic with good chemical resistance. It is strong, clear, and shatter-resistant, but it cannot resist solvents and is easily stained by greases and oils.
When selecting a plastic for injection moulding, it is essential to consider the specific chemicals the product will be exposed to and choose a plastic with the appropriate chemical resistance properties.
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Frequently asked questions
The choice of plastic will depend on the specific requirements of the product. Some of the key factors to consider include the desired tensile strength, flexural modulus (bending stiffness), toughness, electrical insulation, temperature resistance, chemical resistance, FDA compliance, cost, and aesthetic requirements such as transparency.
Some commonly used plastics include ABS, which is strong, tough, and reasonably priced; Polypropylene (PP), which is cheap and rigid; Polycarbonate (PC), which is tougher than ABS and has higher temperature resistance; and Acrylic (PMMA), which is transparent, strong, and lightweight.
There are hundreds of different plastic resins available, each with unique properties and processing requirements. It is important to consider the specific requirements of your project and seek expert advice to select the most suitable resin. Online material databases, such as Matweb.com, can also help in selecting the right resin by allowing you to search for materials based on specified properties.
Choosing the right plastic can ensure part longevity, efficiency, and optimal performance under various conditions. It can also help improve the form, fit, and function of the final product. Conversely, selecting the wrong material can lead to part failure, equipment damage, and potential litigation issues.
