Plastic injection molding is one of the most popular as well as versatile manufacturing techniques. It has multiple advantages like the flexibility of design, the ability to work with multiple plastic resins simultaneously, cost-effectiveness, precision and accuracy of the products, etc.
The technique hence reigns supreme in the world of plastics.
However, the number of plastic polymers invented till date are numerous. With such a wide list to choose from, it can often be overwhelming for manufacturers and designers to choose the best one for a product.
The article here will take you through the common material selection factors as well as the most widely used plastic resins and their benefits. Read on to know more.
Material selection factors
Each plastic resin has its own unique properties. It is important to consider the defining properties of each to choose the right one for your product. It is also noteworthy that resin properties are also often enhanced by the use of fillers like silica, calcium carbonate, mica, alumina trihydrate, glass, clay, etc. Following are the key properties that need to be considered for each plastic resin before making a selection for the plastic injection molding process:
Hardness, also popularly referred to as Shore hardness is used to measure the hardness/stiffness of the plastic resin. Hardness is basically the measure of resistance to indentation for a particular plastic resin. It is measured on the Durometer scale.
Various products have different levels of hardness requirements when it comes to plastics. Before moving on to plastic injection molding, it is important for the manufacturer to choose the appropriate type of plastic for a particular product and design.
Picking the right material means considering its usage environment, abrasion levels, as well as the purpose of the product. Plastic resins like HDPE (High-Density Polyethylene), PVC are on the softer side of the scale and are suitable for products that need soft feel. Others like PP (Polypropylene), Nylon are of moderate softness/hardness whereas those like ABS(Acrylonitrile Butadiene Styrene) and PC(Polycarbonate) are considered for hard applications.
Material flexibility is another very important parameter that needs to be considered for product applications. Flexibility is defined as the ability of a material to bend without breaking.
It is also noteworthy to mention that flexibility is not necessarily similar to softness. Various plastic resins that are ranked as soft (like Nylon) on the Durometer scale are not flexible and break easily when stress/load is applied to it. ABS, on the other hand, is a hard plastic but is considerably flexible.
Multiple industrial applications have requirements for plastics that are sufficiently hard/durable and yet flexible. It is important to consider all the properties of a plastic resin before making a final choice for a product.
The weight of a plastic resin material plays a very vital role in its selection for different applications. The weight of plastic resin is determined by the density of the plastic polymer.
Polypropylene (PP), High-Density Polyethylene (HDPE) and Acrylonitrile Butadiene Styrene (ABS) are considered as lightweight plastics. On the other hand, PVC, Polycarbonate (PC) are considered heavy.
This is again a very critical important factor for plastic injection molding. That said, it is more important to consider the other properties of a plastic resin suitable for a particular application before cost.
Cost of different plastic resins used for plastic injection molding varies based on availability, ease of production, polymer properties, etc. Plastic resins like HDPE, PP, and ABS are relatively less costly than resins like Nylon, PC, etc.
– Various other factors that are considered before selection of a plastic resin include the impact strength, tensile strength, elasticity, chemical resistance, heat deflection/tolerance, and water absorption.
Most widely used plastic resins
With hundreds of existing as well as emerging resins in the market, it can often be overwhelming to select a particular plastic resin for plastic injection molding. However, the process is not as daunting as it seems.
Each plastic resin has its unique properties, advantages as well as disadvantages. Following are some of the most commonly used resins for plastic injection molding:
1. Acrylonitrile Butadiene Styrene (ABS)
ABS is a very popular plastic in the world of injection molding. With its superior properties like toughness, impact resistance, heat resistance, durability, strength, the resin finds its applications in multiple industries.
Its lightweight and strength make it a popular choice for the automotive industry. Since ABS has combined strengths of both acrylonitrile and styrene polymers, it is a perfect fit for applications which require toughness of the material.
ABS can also be easily molded into desirable shapes and offers glossy effects with a superior finish quality.
Other properties of ABS like easy adhering to itself and other similar plastics makes it a good choice for co-injection molding. Additionally, the adhering properties of ABS make it great for metal coatings.
ABS plastics find their use in multiple applications like appliances, computer parts, housing equipment, sports and musical instruments, medical devices, helmets, toys, electronics, automobiles, etc.
2. Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE)
LDPE is one of the most flexible types of plastic resins available. It is widely used for its superior moisture resistance properties, chemical resistance, impact strength, as well as cost-effectiveness.
Various applications of LDPE include household goods, medical products, electrical wirings, computer components, food industry (like juice boxes), toys, agricultural products, furniture, compost bins, etc. That said, LDPE has low-temperature resistance and is not suitable for high-temperature environments.
HDPE, on the other hand, has a higher impact resistance, moisture resistance, heat resistance, tensile strength, and ductility. Additionally, HDPE is a food-grade plastic and is hence a preferred resin when it comes to food packaging materials.
These plastics are hence used in applications where more strength, rigidity and heat resistance is required. Applications that make use of HDPE are kitchenware, refrigerator shelves, plastic bags, plastic containers, water bottles, etc.
HDPE is not just inexpensive to start with but also 100% recyclable which makes it a highly cost efficient plastic resin.
3. Polycarbonate (PC)
Polycarbonates are extremely hard and strong plastics and provide extremely high impact resistance. Additionally PC plastic resins are transparent and amorphous and are used in applications where transparency and strength are required together.
PC plastic resins are commonly used to manufacture bulletproof glass. Other uses of PC are in the manufacture of eyeglasses, lenses, medical devices, mobile phones, DVDs and car parts.
4. Polyamide (Nylon)
Polyamide or Nylon finds its usage in a large range of sectors. This plastic resin comes with superior properties like toughness, chemical, and electrical resistance, wear resistance as well as stability.
Nylon polymers offer resistance to rugged environments that bring in abrasion, high impact or chemical contacts.
Polyamide or Nylon plastic resin is used largely in the production of medical parts, automotive industry, sports gear, and other industrial tools and equipment.
5. High Impact Polystyrene (HIPS)
HIPS resin offers good dimensional stability, impact resistance and customizable surfaces which are easy to decorate. Applications which require printing, use of glues and decoration parts, etc use HIPS.
Applications like display boards, garden equipment, packaging material, TV parts, toys, cold drink cups, computer housings, etc utilize HIPS. It is also highly utilized in graphic arts as well as digital printing equipment.
HIPS is also very cost-efficient which makes it a preferred choice for various applications.
6. Polypropylene (PP)
PP is perhaps the most used plastic resins globally. The plastic has good chemical and electrical resistance, hardness, high melting point, stress resistance as well as high elasticity. Further, it is also highly valued for its insulation properties.
Various fillers are added to customize the plastic resin to add more strength to it. The additives lend further strength to the plastic (thereby known as the steel of the plastics)
Its diverse applications include electrical equipment, household appliances, automotive industry, packaging materials, construction purposes, etc.
PP is particularly preferred in the food and beverage industry since the polymer does not leach chemicals and is considered safe for food products.
It is also used in combination with other plastics to create kitchen and tableware, toys, medical components, washing machine buckets, etc.
Similar to acetal, it also offers properties of minimizing friction and is used in point of contact applications.
7. Acrylic (PMMA)
Poly(methyl methacrylate), popularly known as Acrylic or PMMA is a widely used plastic resin, especially in applications where substitutes for glass are needed.
It’s properties like transparency, chemical resistance, hardness, impact resistance, dimensional stability make it a preferred choice for a wide range of applications that are manufactured using plastic injection molding technique.
Additionally, acrylic is a very low-cost plastic resin and is easily available.
Acrylic is a transparent and moldable plastic used in the preparation of windows, car screens, picture frames, beauty products, paints, eye lenses, etc.
8. Polyetheretherketone (PEEK)
PEEK is a very widely used high-performance plastic that offers extreme resistance to even the harshest of chemical attacks. Additionally, the plastic resin is resistant to thermal and aqueous degradation.
The high strength and robust properties of PEEK make it suitable for various applications like piston parts, compressor plates, pumps, etc. Its chemical resistance properties make it a good choice for use in liquid chromatography columns and other parts used in chemical processing industries.
Additionally, it is also being highly used in sectors like the automotive industry, aerospace and even in medical parts (like implants).
9. Acetal/ Polyoxymethylene (POM)
Acetal or polyoxymethylene is popular for its outstanding properties like hardness, wear resistance, chemical resistance, etc. Additionally, the plastic resin has good colorization properties, low moisture absorption as well as good heat distortion.
Acetal is a great choice in applications where dimensional stability, as well as friction resistance, is required. This plastic resin is widely used in the food and beverage industries. Additionally, it is used for other sectors like the automotive industries, household appliances, plastic gears, etc.
Its properties of minimizing friction make it an excellent choice for point of contact applications like guitar picks, buckles, pumps and valve parts, fixtures, etc.
10. Thermoplastic Polyurethane (TPU)
The soft and elastic properties of TPU make is an ideal choice for products that require rubber-like elasticity. Additionally, the high tensile strength, as well as the ability to withstand high temperatures, makes it a good choice for applications like cable insulations, electrical tools, sports gear, etc.
Choosing the right material for creating a part using plastic injection molding technique is absolutely essential for optimal performance. And that’s not the only reason to do so. It saves a good amount of time as well as money spent during the manufacturing process.
The huge range of plastic resins available opens up doors for unlimited plastic molding opportunities and designs. However, it is important to understand that all applications have different material needs when it comes to injection molded plastics.