Plastics: Different Types

A complete guide on the different types of plastic and how to use them with the Sustainable Design Studio's recycling machines.

Colorful plastic toys and debris scattered on sandy ground, including toy pills, a toothbrush, small plastic figurine, and bits of string.

What Are Plastics?

Plastics are synthetic materials made from polymers, which are long molecular chains built from repeating units called monomers. This structure makes them durable, versatile, and cheap to produce. That combination means plastics end up in almost everything, from food packaging to medical devices to car dashboards.

The problem is that durability works both ways. Plastics can take hundreds of years to break down, and every piece of plastic ever produced still exists in some form today. That is exactly why recycling matters, and why understanding the different types of plastic is the first step toward doing it properly.

Most plastics also contain additives that change how they behave, things like UV stabilisers, colourants, or flame retardants. The type and quantity of additives vary depending on the product. This is important to know because additives affect how a plastic performs when you shred, melt, or mould it.

Different Types of Plastic

Thermoset

Thermosetting plastics form permanent chemical bonds when they cure. Once shaped, they cannot be remelted. Think of bread: reheating it does not turn it back into dough. It just burns. The same principle applies to thermosets.
This means thermoset plastics cannot be recycled through melting and remoulding. Common thermoset products include pan handles, toilet seats, shoe soles, and tyres. If you encounter these in your plastic waste stream, set them aside. They will not work with any of the SDS machines.
Thermoplastics

Thermoplastics soften when heated and harden when cooled, and you can repeat this cycle many times. Think of butter: it melts, solidifies, and melts again without changing what it is.
This is what makes thermoplastics recyclable. You can shred them, melt them, and mould them into new products. The SDS Shredder Mini V2, Desktop Shredder V2, and Injection Mini V2 are built for exactly this. Most of the plastics you will work with, including PP, HDPE, and PS, are thermoplastics.

Simply put, we can only recycle thermoplastics.

Recycling symbols with numbered arrows and abbreviations for plastic types, including PETE, HDPE, V, LDPE, PP, PS, and Other.

Resin Identification Code

The resin identification code (RIC) is a classification system used to identify the type of plastic resin used in a product. It consists of a number from 1 to 7 inside a triangle of arrows.

However, these codes can sometimes be very small, and not all plastic types have a number.

When the code is missing or hard to read, the SDS Plastic Scanner app can identify the plastic type from a photo. It is free to download and works on any smartphone.

Types of Thermoplastic

PET - Polyethylene terephthalate

Clear Plastic Bottles, Clear Coffee Cups, Fruit Containers.
PET has a high melting point and works best with the SDS Filament Maker for turning bottles into 3D printer filament. Not suitable for injection moulding at desktop scale.
HDPE - High Density Polyethylene

Milk Bottles, Shampoo Bottles, Bottle Caps, Food Containers.
HDPE is one of the easiest plastics to recycle with the Injection Mini V2. Widely available and produces strong, durable parts.
PVC - Polyvinyl Chloride

Cosmetic Containers, Building Pipe, Commercial Cling Wrap.
PVC releases harmful fumes when heated and should not be processed with any SDS machine. Always separate PVC from your recycling stream.
LDPE - Low Density Polyethylene

Plastic Bags, Cling Wrap, Squeeze Bottles.
LDPE is soft and stretchy, which makes it difficult to shred cleanly. It can be injection moulded but is not recommended for beginners.
PP - Polypropylene

Food Containers, Yogurt Pots, Plastic Chairs, DVD Cases.
PP is the most popular plastic for desktop injection moulding. It flows well, is easy to source, and produces excellent results with the Injection Mini V2 and Premium Moulds.
PS - Polystyrene

Food Containers, CD Cases, Party Cups, Packing Foam.
PS is easy to melt and moulds well, but expanded polystyrene (EPS foam) needs to be compressed before shredding. Solid PS works well with the Injection Mini V2.

3D printers printing blue objects on a workbench in a laboratory or workshop environment.

Type 7 - Other

Type 7 covers all plastics not classified under types 1 to 6. This includes ABS, PLA, Nylon, and many others. Most of these are thermoplastics, which means they can be recycled. ABS and PLA are especially common in 3D printing, and both work well with the Injection Mini V2, making them a great way to recycle failed prints and support material.

Material Grades

Injection Grade

Injection grade plastics are formulated to flow easily into moulds under pressure, filling every detail of the cavity before cooling into a solid part. They have a higher melt flow index, which means they move more freely when melted. This is the grade you will use most with the Injection Mini V2.
Blow Moulding Grade

Blow moulding grade plastics are designed to be inflated into a mould while in a semi-molten state. They have a lower melt flow index and are more viscous, which helps them hold their shape during inflation. Bottles are the most common blow moulded product. These plastics can sometimes be used for injection moulding, but they may not flow as easily into fine details.
Extrusion Grade

Extrusion grade plastics are formulated for continuous processes where molten plastic is pushed through a die to form a consistent cross-section, such as pipes, sheets, or filament. These plastics need to hold their shape as they cool, without collapsing or stretching unevenly. Common extrusion grade materials include HDPE, PVC, and PP.