What are thermoplastics?

Thermoplastics are defined as polymers that can be melted and remelted almost indefinitely. They melt when heated and harden when cooled. However, when frozen, thermoplastics become glassy and break. These characteristics, which give the material its name, are reversible, so the material can be repeatedly reheated, remoulded and frozen. Thermoplastics are therefore mechanically recyclable. Some of the most common types of thermoplastics are polypropylene, polyethylene, polyvinyl chloride, polystyrene, polyethylene terephthalate and polycarbonate.

What is the basic raw material for thermoplastics?

Plastics are made from natural materials such as cellulose, coal, natural gas, salt and crude oil through a process of polymerisation or polycondensation.

Plastics are made from natural and organic materials such as cellulose, coal, natural gas, salt and, of course, crude oil. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used. The production of plastics starts with the distillation of crude oil in an oil refinery. Here the heavy crude is separated into groups of lighter components, called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds consisting of carbon and hydrogen), which differ in the size and structure of their molecules. One of these fractions, naphtha, is the crucial compound for the production of plastics.

Two main processes are used to produce plastics: polymerisation and polycondensation, both of which require specific catalysts. In a polymerisation reactor, monomers such as ethylene and propylene are joined together to form long polymer chains. Each polymer has its own properties, structure and size depending on the different types of base monomers used.

There are many types of plastics, which can be grouped into two broad families of polymers:

Thermoplastics (which soften when heated and harden again when cooled).
Thermosets (which never soften once moulded).

Properties of thermoplastics

Thermoplastics have a simple molecular structure comprising chemically independent macromolecules. When heated, they soften or melt, then mould, form, weld and solidify on cooling. Multiple heating and cooling cycles can be repeated, allowing them to be reprocessed and recycled.

Applications of thermoplastics

Thermoplastics have been around for a long time and are an important part of everyday life today. For example:

Acrylonitrile butadiene styrene (ABS) is a thermoplastic that is used to manufacture.
Sports equipment.
Toys (e.g. LEGO® blocks).
various automotive parts.

Polycarbonate is used to make:
CDs and DVDs.
Drinking bottles
Food storage containers
Eyeglass lenses.

Polyethylene is probably the most common thermoplastic and is used to make

Shampoo bottles
Plastic shopping bags
Bulletproof vests.

Main types of thermoplastics What are they and what are they used for?

PET (polyethylene terephthalate)

Polyethylene terephthalate or PET belongs to the family of polyesters. It is widely used in everyday items and is easily recyclable. It has a semi-crystalline form when stable. Its most common applications include rigid and flexible packaging, as it is very lightweight.

PET is one of the plastics that are part of everyday life. This polymer is used in packaging, fabrics, films and moulded parts for automotive and electronics, among others. For all applications that require a lightweight and impact-resistant material, PET is the material of choice. Moreover, it should not be forgotten that PET is one of the most recycled thermoplastics.

In addition, continuous efforts have been made to tailor the properties of PET to improve its performance with beneficial cost profiles to meet the demands of high-end applications.

At the same time, polypropylene, polyethylene and polyvinyl chloride are equally popular for these advantages, so how do you find the right one for you? In this guide, you’ll find the key insights to find the right grade from the thousands of options available.
HDPE (high density polyethylene)
High density polyethylene (HDPE) is a thermoplastic polymer made from petroleum. As one of the most versatile plastic materials, HDPE is used in a wide variety of applications such as plastic bottles, milk jugs, shampoo bottles, bleach bottles, cutting boards and pipes. Known for its outstanding tensile strength and high strength-to-density ratio, HDPE plastic has a high impact strength and melting point.
In addition to its use for food applications, it can be found in unusual places, such as wood-plastic composites, plastic surgery, specifically in skeletal and facial reconstruction, snowboarding, food and beverage packaging, etc.

LDPE (low density polyethylene)

Low density polyethylene or LDPE belongs to the polyethylene branch of thermoplastics. It is soft, light, strong and flexible by nature. This thermoplastic material is also known for its low temperature compatibility and good corrosion resistance. The polymer also has good chemical properties and impact resistance, making it easy to manufacture or process. It has a melting point of 110°C.

The names themselves can decipher the main difference between LDPE and HDPE. HDPE has a higher density than LDPE, which means that the former has more mass than its volume.

Its structure is more branched rather than in perfect rows, and this is the reason for its low density/volume, so it is often used in applications where strength and structural rigidity are an important requirement.

PVC (polyvinyl chloride)

Polyvinyl chloride (PVC or vinyl) is an inexpensive and versatile thermoplastic polymer that is widely used in the construction industry to manufacture door and window profiles, pipes (potable and waste water), cable and wire insulation, medical devices, etc. It is the third most widely used thermoplastic material in the world. It is the world’s third largest thermoplastic material by volume, after polyethylene and polypropylene.

It is a solid, white, brittle material, available in powder or granules. Due to its versatile properties, such as light weight, durability, low cost and easy processability, PVC is currently replacing traditional building materials such as wood, metal, concrete, rubber, ceramics, etc. in various applications.

PP (polypropylene)

Polypropylene is a type of polyolefin that is slightly harder than polyethylene. It is a basic plastic with low density and high heat resistance. One of its common uses is biaxially oriented polypropylene (BOPP) for making transparent bags.

Polystyrene PS

Polystyrene plastic (PS) is a naturally transparent thermoplastic that is available both as a typical solid plastic and as a rigid foam material. PS plastic is commonly used in a variety of consumer product applications and is also particularly useful for commercial packaging.

The solid plastic form of polystyrene is commonly used in medical device applications, such as test tubes or Petri dishes, or in everyday items, such as smoke detector housings, the case in which CDs were purchased, and often as a container for food, such as yoghurt, or the red “solo” cup in which one drinks at a barbecue.

Nylon

Nylon is a polyamide, i.e. a polymer containing amide chemical functional groups (RCONR’R”), which has very particular physical properties, especially in terms of strength, elasticity and transparency.
When subjected to high temperatures, it melts, greatly reducing its viscosity. Its melting point is about 263°C and it is soluble in phenol and formic acid.
Nylon is currently used in fishing lines and nets, textile zips, synthetic instrument strings, fan blades, gears, screws, automobile fuel tanks, pantyhose, etc.

Injection moulding of thermoplastics

Thermoplastics have seen common use in the injection moulding process, as they produce parts that are flexible, precise and have aesthetically pleasing surface finishes. Thermoplastics are also valued for their recyclability, as products created from them can be remelted and re-melted into different shapes through the injection moulding process. This has led to the popularity of thermoplastics in the toy, furniture and clothing industries, as it allows parts to be recycled and reshaped after damage, wear and tear.

The thermoplastic injection moulding process begins with the creation of a mould, usually made of metal such as steel or aluminium. The thermoplastic resin is then melted into liquid form in a heated barrel before being injected into the mould to cool as a solid. Once the part has set and fully cured, it is removed from the mould and the process is complete.

However, thermoplastic injection moulding requires high heat and pressure to successfully create parts. For this reason, it is not always a cost-effective solution, especially for orders that require the production of a large number of parts. Thermoplastics are also ineffective for parts that are regularly exposed to extreme heat or rapidly varying temperatures due to their ability to melt.

What are thermosets?

Unlike thermoplastics, thermosets (also known as thermoset plastics or thermoset polymers) are materials that remain in a permanent solid state after being cured once. The polymers in the material cross-link during the curing process to form an unbreakable and irreversible bond. This means that thermosets do not melt even when exposed to extremely high temperatures.

Common examples of thermoset plastics and polymers are epoxy, silicone, polyurethane and phenolic. In addition, some materials such as polyester can come in both thermoplastic and thermoset versions. Unlike thermoplastic pellets, thermoset polymer components are stored in liquid form, usually in large tanks or containers.

Different thermosets offer different advantages when used as a production material. For example, epoxies are very elastic, tough and resistant to many chemicals, while phenolics are very flame resistant. For a more in-depth look at the advantages of one of the most popular thermosets, polyurethane, see our article here.

Advantages of thermosets over thermoplastics

Thermoset plastics and polymers offer a number of advantages over other materials, including thermoplastics. The most obvious advantage over thermoplastics is that thermosets do not melt when exposed to heat. They also do not deform, warp or lose their shape at extremely low temperatures. This makes them ideal for any parts or machinery that will be used in extreme climates or in environments that experience regular temperature variations.

However, this is not the only advantage thermosets offer over thermoplastics. Thermosets are low-viscosity and easy to work with because they exist in liquid form at room temperature, which means no heat is required. They also carry a lower health risk than thermoplastics, as no potentially toxic fumes, such as styrene, are released during the moulding process.

Injection moulding with thermoset polymers can be carried out using much less heat and pressure than when using thermoplastics. As a result, injection moulding of thermosets (which includes reaction injection moulding (RIM) and long fibre injection moulding (LFI)) can be carried out at a much lower cost. Moulds for this process are cost-effective and easy to manufacture; they can be made from various materials, such as aluminium, kirksite alloys, nickel, epoxy, silicone and fibreglass.

Reaction injection moulding with thermosets is a time- and cost-saving process. Cycle times for the production of a single part range from one to several minutes. In addition, the ease of mould construction allows prototype development to be completed quickly. A working prototype of a desired part can typically be created within 3 to 15 days.

Structural advantages

Thermosets have excellent “flowability”, which means that they easily and effortlessly fill all the crevices and corners of the mould. This allows the creation of both larger overall parts and much more complex and detailed geometric shapes than anything that can be produced with metal or thermoplastics. This is another way in which choosing thermoset injection moulding helps you save money. Instead of producing each small part individually and then joining them together, as is traditionally done when using metal, the parts can be combined into a larger, more complex whole right in the mould itself.

In addition, thermosets have all the light weight and flexibility that have made thermoplastics so popular, while adding strength, toughness, durability and impact resistance. They are dimensionally stable and structurally sound, and experience little or no shrinkage when demoulded. Thermoset polymers are also easily reinforced by the addition of reinforcing materials such as fibreglass, carbon and Kevlar.

Parts created by thermoset injection moulding can have varying wall thickness in a single part, something that is impossible to do with other materials and processes.

Thermoset polymers have excellent resistance to a range of environmental factors beyond just high and low temperatures. Thermosets can survive exposure to the elements without breaking, warping or scratching. They are also resistant to a range of chemicals, including organic and inorganic acids. Many thermosets, such as polyurethane, are also waterproof, dielectric and radiopaque, allowing them to be used in the creation of boats, insulation, medical equipment and a range of other products.

Insert potting is a simple and uncomplicated process with thermoset injection moulding.

Aesthetic advantages

Choosing thermosets does not mean sacrificing the high-quality finish for which thermoplastics are traditionally known.

In the IMP process, the mould is sprayed directly with gel coat or paint prior to injection of the thermoset. This creates unbreakable bonds between the surface and the paint; this excellent adhesion prevents chipping, flaking and cracking even on paint jobs that are regularly exposed to the elements. This makes thermosets an excellent choice for materials that frequently experience extreme weather or dirty conditions, such as truck cabs and construction machinery.

We can offer you low or high gloss Class A finishes straight from the mould, a result similar to painted metal at a fraction of the price and manufacturing time. Paintable thermosets, such as polyurethane, can also mimic highly detailed textures such as stone, wood and metal.

The extreme detail provided by the “fluidity” of thermosets also allows for the smallest aesthetic touches, such as the addition of branding or logos on the product.

Overall, while thermoplastics are still beneficial in certain situations, thermosets offer a greater number of advantages and are useful for a much wider range of products and parts.