Intro to thermoplastics

Thermoplastics — what we’ll focus on in this article — are any plastics that can undergo heating/melting, molding, cooling, and then be reheated or remelted with little to no change to their properties. Thanks to this, scrap pieces left over from thermoforming production can easily be recycled and reused for new parts. 

Thermoplastics’ versatility means that there are many different ways to process them and many types of plastic forming methods, from vacuum, pressure, injection, and rotational molding to extrusion, ultrasonic welding, and more. Just as we’ll see with material options, each of these forming methods has its pros and cons, meaning it’s important to choose both the right method and material for your application.

Thermoplastics vs. thermosets

As the name suggests, thermosets can typically only be used once. Unlike thermoplastics, these materials must “cure” or “set,” meaning they can’t be reused later. Thermosets are usually composed of a two-part mixture of resin and curing agent. While thermoplastics have wide applicability, thermosets are not suitable for many common forming techniques and are generally more expensive to mold.

While we won’t explore thermosets in this article, some common types you might encounter include polyester, polyurethane, silicone, and epoxy.

#1: ABS

ABS, or acrylonitrile butadiene styrene, is easily one of the most common material choices for heavy gauge thermoforming.

Pros of ABS

• Economical
• Stiff, with excellent impact strength
• Can be made flame retardant
• Well-suited to outdoor and marine applications with a high-gloss acrylic cap
• Easy to fabricate, machine, paint, fasten with hardware, bond with adhesive, and ultrasonically weld
• Can enhance other thermoformable materials (e.g,. PVC and polycarbonate) when “alloyed” with them
• Available pre-colored
• Can be patterned in a variety of ways, including wood grain, carbon fiber, mirror finish, and custom artwork

Cons of ABS

• Can perform poorly in high heat

#2: Acrylic 

Polymethyl methacrylate, better known as acrylic or plexiglass and one of several clear thermoplastics, is another economical option but has some important drawbacks for certain applications.

#3: HDPE 

High-density polyethylene is a high-molecular weight (HMW) version of polyethylene and is one of the most common plastics in use today.

Pros of HDPE

• Tough with good impact resistance
• Excellent chemical resistant and electrical properties
• Low coefficient of friction
• Near-zero moisture absorption
• Lightweight
• Easy to process, cut, machine, and bend
• Performs well in cold temperatures
• Can be fastened with hardware

Cons of HDPE

• Not as dimensionally stable as other thermoforming materials
• Not well-suited to adhesive bonding

#4: HIPS 

High Impact Polystyrene is a modification of standard polystyrene — another, more familiar, modification is expandable polystyrene (EPS or “styrofoam”). HIPS is a hard plastic rather than a foam, which makes its applications quite different from styrofoam.

Pros of HIPS

• Low cost
• Excellent impact resistance
• Lightweight
• Easy to form
• Provides high level of detail
• Available in many colors
• Good choice for trays, displays, and models

Cons of HIPS

• Not suited for painting/coating
• Performs poorly under UV exposure

#5: PC 

Polycarbonate is typically found in sheet form and is often used for signs, shields, clear retail enclosures, and windows. 

Pros of polycarbonate

• High impact strength
• Excellent clarity (superior to acrylic)
• UV and heat-resistant
• Easy to machine and cut without shattering
• Easy to join with adhesive, ultrasonic welding, and fasteners

Cons of polycarbonate

• Can be damaged by solvents
• Requires drying prior to forming
• Manufacturing requires higher temperatures and longer cycles
• More expensive than most thermoforming materials

#6: PETG 

PETs are polyethylene terephthalates; PETG adds glycol to the molecular mix to result in a stronger, more durable plastic material. PETGs are common in both thermoforming and 3D printing.

Pros of PETG

• Cheaper than polycarbonate
• Clear, brilliant appearance
• Flexible
• Excellent impact strength
• Great durability
• Easy to form
• Capable of intricate shapes and deep draw depths
• Performs well in low temperatures
• Moderately inexpensive (less than polycarbonate)

Cons of PETG

PETG is a highly versatile option with few disadvantages; however, its performance can suffer under higher temperatures.

#7: PP 

Polypropylene is one of the world’s most common thermoplastics, used in everyday household items like cleaning product packaging and medical devices.

Pros of polypropylene

• Rigid
• Good impact strength
• Chemically inert — has excellent chemical resistance
• Flexible but tough

Cons of polypropylene

• Difficult to form
• Not as dimensionally stable as other thermoformable materials

#8: PVC 

Polyvinyl chloride is a versatile thermoplastic capable of forming both thin sheets and rigid tubing.

Pros of PVC

• Inexpensive
• Hard and rigid
• Good impact strength
• Good weatherability
• Unique chemical resistance range (including some solvents)
• Well-suited to outdoor use
• Easy to machine and fabricate

Cons of PVC

• Difficult to mold

#9: TPO 

Thermoplastic polyolefins are essentially a blend of thermoplastic (polypropylene), rubber, and fillers and combine the best characteristics of each. They are often used to form components of automotive or recreational vehicles, such as bumpers and dashboards.

Pros of TPO

• Outstanding impact properties
• Unusual toughness
• Well-suited for outdoor applications
• Moderately prices

Cons of TPO

• Difficult to form
• Not well-suited to deep or intricate parts

#10: KYDEX^® 

KYDEX^® is a specialty thermoplastic line manufactured by Sekisui SPI. It is an acrylic-polyvinyl chloride alloy, meaning it is essentially a blend of polymethyl methacrylate (PMMA) and PVC that draws on the benefits of both materials. 

Pros of KYDEX^®

• Good general-purpose material
• Excellent impact strength
• Excellent chemical resistance
• Highly cosmetic
• Most grades flame retardant to UL94-V0
• Comes in a variety of colors and textures

Cons of KYDEX^®

• Requires high temperatures to mold
• May be more expensive than other options

Matching materials to applications

As we’ve seen, thermoplastic materials represent a range of price points and properties, from ease of forming and price point to machinability and resistance to various elements. Knowing exactly how your plastic component will be used and what it needs to stand up against is critical information for choosing the right material.

Your material options can also be impacted if you need to use a particular type of forming method:

• Thermoforming/vacuum-forming requires materials be in sheet form
• Injection molding materials must flow well when melted
• Rotational molding requires powered or liquid material
• Blow molding materials must be able to support a parison extrusion

To make matters more complicated, many materials have specific grades within them that may limit your choice of methods or applications.

Let’s find the right material for your project

While it’s good to know your options and approach any custom manufacturing process from an informed perspective, you shouldn’t have to be a thermoforming expert to get plastic components. If you choose to work with us at SSI, our engineers will make sure they understand your project inside and out and work with you to consider all the implications of choosing one material over another. We’re experienced with all the plastic examples mentioned above and can help you from the earliest stages of your design process to ensure your finished component works as needed.