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Thermoforming vs Injection Molding for Plastic Part Production

It’s never easy moving a product concept from design to manufacturing and making the decision to go with thermoforming vs injection molding often only makes it harder. One of the biggest challenges is tooling, especially when the product is made from plastic or incorporates plastic components. Injection molding tools are expensive, especially for larger parts, and they take a long time to make. This leads to high piece costs for prototypes and initial samples while extended lead times can put the project behind schedule.

Thermoforming offers a way around these problems. Thermoforming tools are less complex than those for injection molding and can often be procured in a fraction of the time. They’re also practical for molding large parts and considerably less expensive than those for injection molding. The processes aren’t interchangeable though, so it’s important to consider how plastic parts will be manufactured during the design phase.

Key to designing for manufacture is understanding what thermoforming is capable of, and how it compares to injection molding. Here’s a look at thermoforming vs injection molding for plastic parts.

A Thermoforming Primer

As the name of the process implies, thermoforming entails using heat to form plastic to the shape required. The plastic starts out as flat sheet which is heated to its glass transition temperature, (Tg), typically 285 – 375⁰F (140 – 190⁰C). Once soft and pliable, the sheet is pulled, pushed or both, over or into a mold and held in place as it cools.

When the plastic has cooled enough to become rigid again it’s taken from the mold and the excess material trimmed away. If needed, the part can then go for painting or other finishing operations, after which it’s ready to be shipped or used.

plastic part being thermoformed

With that background, it’s now time for a deeper dive. The following sections will address:

  • Plastics for thermoforming
  • Heating methods
  • Vacuum, pressure, and twin sheet forming
  • Tooling
  • Quality issues

Plastics for Thermoforming

Almost any thermoplastic can be thermoformed because they become soft and pliable when heated to their Tg. The plastics most commonly formed are:

  • ABS – Acrylonitrile styrene butadiene, stiff and strong with good resistance to impacts and abrasion
  • HIPS – High impact polystyrene
  • HDPE – High-density polyethylene, inexpensive and water-resistant
  • PVC – Polyvinyl chloride, inexpensive, high density, good chemical resistance, electrical insulator
  • PET – Polyethylene terephthalate, high strength, and stiffness, low density
  • PETG – Polyethylene terephthalate glycol-modified, good impact resistance, transparent,, often used with food

For thermoforming these are first extruded as sheet material. Thickness is typically between 0.080” and 0.800”. Any thicker and it becomes difficult to soften and deform, any thinner and it will tear as it’s stretched.

Heating Methods

Thermoforming machines use radiant heaters above the sheet. These throw out infrared light that the plastic absorbs. Plastic type and color does affect how quickly the sheet reaches the required temperature. As the plastic softens it starts to sag which provides a visible indication that it’s ready to be formed.

Convection heating is sometimes used for pre-heating and to ensure the material is dry.

Vacuum, Pressure, and Twin Sheet Forming

There are two approaches to forming: either the sheet is draped over a male mold, (one that stands up above the base,) or it’s pushed into a female or cavity mold. Making the sheet conform to the mold is achieved with vacuum, pressure, or both.

Vacuum pulls the sheet down against the surface of the mold. Pressure pushes it against the mold. Vacuum is faster, cheaper, and best for making large parts that don’t have sharp corners, tight radii, or a lot of detail. Pressure is slower but achieves a higher level of detail.

Notice that thermoforming is a one-side technique. This means textures or patterns are only formed on the side of the sheet that contacts the tool.

Some modern thermoforming machines use a combination of vacuum and pressure. They lift the tool up against the sagging sheet, then push and pull selectively to make the sheet conform tightly to the surface underneath.

Twin sheet forming is a way to make parts with hollow sections. Typically, this is done by blowing air in between two sheets that are clamped together around their edges. This pushes the material against two opposing tools to create the required shape.

Tooling

Most thermoforming tools are machined from aluminum. As they are one-sided and don’t need high strength this is usually a relatively quick process. To save time and money, as when making prototypes or samples, a thermoformer may use wood or a 3D-printed pattern.

Quality issues

The biggest challenge is maintaining the required sheet thickness as it stretches over the tool. This requires careful design of tooling and a detailed understanding of the forming process. Sheet orientation is a factor as the material stretches differently along each axis. Uneven heating can also control rates of stretch.

An Injection Molding Primer

Injection molding entails forcing liquid plastic into a cavity under pressure. The plastic cools and solidifies, after which the cavity opens to eject the part or parts. Parts are trimmed to remove sprue, (plastic that has solidified in the delivery channels,) and then sent for finishing.

injection molding machine

Three aspects of injection molding needing a more detailed explanation are:

  • Tooling
  • Machines and process
  • Quality

Tooling

Injection molding tools comprise at least two parts that close to form the cavity and open so the part can be ejected. To minimize cycle times tools may be both heated, to stop or slow solidification, and cooled, to accelerate solidification.

Mold tools are usually machined from tool steel. This withstands high loads without deflection and resists wear, which ensures long life. It’s also very difficult to machine and often needs electro-discharge machining, (EDM) as well as extensive grinding and polishing. Geometric precision must be maintained to ensure the two parts close accurately without leaving gaps for the plastic to extrude through. These factors mean injection molding tools are expensive and take a long time to make.

Machines and Process

In an injection molding machine, pelletized plastic material is fed into a screw barrel. Here’s it’s heated as the screw rotates, becoming liquid in the process. When the tool is closed the machine forces plastic into the cavity under high pressure and at high velocity.

The high forces involved tend to open the tool, so the machine must counter these with very high clamping loads. The larger the part being made, the higher these forces are. As a result injection molding anything other than the smallest parts requires robust tools and a massive machine.

To maximize output rates from expensive tooling and machinery it’s common to mold multiple parts simultaneously. This further increases the complexity, and thus the cost, of the tooling.

Quality

Two challenges in injection molding are knit lines and non-fills. Knit lines occur when separate flows of plastic within the cavity meet and rather than merging, a distinct boundary forms between them. This results in a weak point.

Non-fills are when the plastic failed to completely fill the cavity, which results in an area of missing material. This can be caused by trapped air.

Comparing Thermoforming vs Injection Molding

Key characteristics of injection molding are:

  • A more integrated process – a single machine turns plastic pellets into (almost) finished parts
  • Capable of short cycle times and high output rates
  • Can produce intricate, highly detailed parts
  • Maintains a high level of part-to-part consistency
  • Requires large, powerful machinery
  • Tooling must be robust, which also makes it very expensive
  • A high level of process control is needed to avoid making defective parts

In contrast, thermoforming is notable for the following reasons:

  • Tooling is less complex than that needed for injection molding and can be produced in less time
  • Low tooling costs make the process economic for short runs and small quantities
  • Avoiding high forces on the tool allows the creation of much larger tools and therefore parts

Thermoforming vs Injection Molding: The verdict

In summary, these two processes occupy different places within plastic part manufacturing. Injection molding is best for high-volume production of small, ideally complex, plastic parts. Thermorforming is for producing larger plastic parts in small to medium volumes, (250 – 3,000 pieces,) and where expectations for geometric precision are lower.

In addition, the simplicity of the tooling gives thermoforming a substantial lead time advantage. While it can take months to go from part design to delivery of the first injection molded parts, thermoforming gets parts in the hand in just a few weeks, and sometimes faster.

Applications of Thermoforming

Any large plastic part with a relatively low level of feature complexity is a candidate for thermoforming. To clarify this, here’s a list of industries that can benefit, along with some typical parts.

  • Logistics – returnable dunnage
  • Automotive – air ducts, seat components
  • Aerospace – ducts, window shades, seat components
  • Medical – hospital diagnostic and medical carts, medical device outer skins
  • Power tools – housings, cases
  • Buildings and construction – access panels and doors, wall and ceiling panels, drainage components
  • Office and commercial – furniture, housings for printers and copiers

Applications of Injection Molding

Injection molding is best for high-volume production of small parts. Many medical devices and components come under this description, as do countless consumer and industrial products from housings, bottles, and toys to bird feeders, sporting goods, and plumbing components. If it’s plastic, it can probably be injection molded, though economics may make thermoforming a less expensive alternative.

Before Selecting a Process – Consult an Expert

With some part designs, it’s obvious which manufacturing process is the right one. However, in many cases, the decision is far less straightforward. Plastics Design & Manufacturing can bring over 45 years of experience to bear on your challenge. Let us tell you if thermoforming or injection molding would be right for you. Contact us to start that conversation.

 

 

Sources

Large Plastic Parts Thermoformed in Georgia – C&K Plastics https://youtu.be/xuv2O-bD-tI

https://www.plasticingenuity.com/blog/common-plastics-for-thermoforming

https://advancedplastiform.com/common-plastics-used-in-thermoforming/

Thick Sheet Thermoforming – Lesson 1 Introduction: Plastics and the Thermoforming Process https://youtu.be/alq3RDZN4jo

Thick Sheet Thermoforming – Lesson 2 Properties of Extruded Sheet https://youtu.be/S3R90PVqFCs

Thick Sheet Thermoforming – Lesson 3 Effective Heating of the Plastic Sheet https://youtu.be/8eEZ7ft505Y

Thick Sheet Thermoforming – Lesson 4 Temperature Control, Vacuum and Pressure Systems https://youtu.be/GgWG6L-99V4

Thick Sheet Thermoforming – Lesson 5 Molds –Venting, Mold Cooling and Finished Part Properties https://youtu.be/xxluV7epsa8

Thermoforming Lesson 2: – Plastic Materials and Properties https://youtu.be/k074QPKTuSo

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