Vacuum Forming Homework Sheet

Vacuum forming is a simplified version of thermoforming where the extruded sheet of plastic is heated, stretched onto or into a single-surface female mold, and held against the mold by vacuum, which results in the replication of the mold surface.

This process is typically restricted to forming plastic parts that are rather shallow in depth. Relatively deep parts can be vacuum formed if the sheet is mechanically or pneumatically stretched prior to contact with the mold surface and before the vacuum is applied.

Vacuum formed parts are thinner in the bottom and corners than at the top or rim, and have a relatively uneven wall thickness. The most common applications for vacuum forming focus primarily on shallow-draw parts or where wall thickness is not critical to the function of the part.

Advantages of Vacuum Forming Compared to Thermoforming

  • Cheaper plastic costs – Cost per parts to the customer is lower.
  • Cheaper machine costs – Overall machine cost to operate is lower.
  • Easier production – Because of the lack of detail in the package, the operation is less complex to execute.

Disadvantages of Vacuum Forming Compared to Thermoforming

  • Material distribution is difficult to control.
  • Cannot produce as many parts.
  • Highly detailed parts are difficult to achieve.
  • Absorbed moisture can expand, forming bubbles within the plastic.
  • Webs tend to develop around the mold from overheating.

Thermoforming (Pressure Forming)

Thermoforming is an economical process and more versatile than vacuum forming in regards to manufacturing returnable packaging.

Technically, all thermoforming methods utilize some type of pressure to stretch the extruded sheet against the mold surface.

The defining characteristics between vacuum forming and thermoforming occur when the differential pressure across the sheet thickness exceeds 15 lbs./square inch. When thermoforming, this process is traditionally performed with air pressure up to 150 lbs./square inch on the free side of the sheet and vacuum on the sheet surface closest to the mold. The air pressure is contained in a pressure box that clamps the sheet against the mold surface, resulting in high quality surface textures.

Pressure forming is used in thin-gauge applications to improve cooling cycle times by rapidly stripping the sheet from the plug and driving it against the cold mold. This improved cooling time is a significant process improvement between thermoforming and vacuum forming.

The extruded sheet of plastic is heated and then stretched over or onto a mold. Vacuum pulls the plastic into the female mold, while a plug is driven into the plastic accompanied by air pressure to force the plastic into each area of the female mold. This gives thermoformed parts much greater detail than parts utilizing the vacuum forming process.

There are two general thermoforming process categories. Sheet thickness less than 0.060 inches is usually delivered to the thermoforming machine from rolls or from a sheet extruder. Thin gauge roll-fed or inline extruded thermoforming applications are dominated by rigid or semi-rigid disposable packaging. Sheet gauge greater than 0.120 inches is typically considered heavy-gauge thermoforming, and this process is primarily reserved for permanent structural components.

Advantages of Thermoforming Compared to Vacuum Forming

  • Better material distribution.
  • Faster cooling time because of air pressure.
  • Allows for high-volume production of parts.
  • Allows for greater part detail

Disadvantages of Thermoforming Compared to Vacuum Forming

Process for making enclosures

There are several processes that can be used to make enclosures for electronic products.

Line bending

Line bending is used to make simple bends in a sheet of thermoplastic polymer. A simple enclosure can be made from two U-shaped parts, attached together using a suitable adhesive.

Line bending involves heating the material along a line, using a heating element. The plastic softens as it heats, allowing it to be bent. As the plastic cools it will retain its shape. A wooden former is often used to ensure accurate bending.

Vacuum forming

Vacuum forming is used to make many different products from thermoplastic sheets. These include enclosures, packaging, helmets and baths.

The sheet is heated to make it flexible, formed over a mould, and then cooled to become hard again.

Step 1 - Plastic placed over mould

Step 2 - Plastic heated

Step 3 - Air in mould removed

The moulds used in vacuum forming have the following characteristics:

  • They are often made from wood or medium density fibreboard (MDF).
  • They can be shaped by hand or using computer-controlled machines.
  • The sides of the mould must slope to allow the plastic product to be lifted off or pulled out. This slope is called the draft angle. It should be between five and 10°. If there was no angle, the plastic product might stick into the mould.
  • The corners of the mould should have a small radius.
  • Any recesses must have small vent holes drilled in them to prevent trapped air stopping the plastic sheet forming.

Vacuum forming can only be used to make shapes of simple profiles, as any overlaps would cause the plastic to be stuck on to the mould.

Injection moulding

Injection moulding is used to make a wide range of plastic products, from enclosures to model construction kits, chairs and toys. The process is very fast and complicated shapes can be made from both thermoplastic and thermosetting polymers.

Injection moulding involves heating the polymer until it is liquid and then using pressure to force it into a mould. Injection-moulded parts can often be identified by their sprue point, a slight protrusion where the plastic was injected into the mould. They may also have a split line visible if the sides of the mould did not fit together perfectly.

Laser cutting

Although laser cutting is a relatively new process, it is already widely used in all levels of production. A computer-controlled laser beam burns through a sheet of material, such as thin sheets of plastic, MDF or aluminium. It can cut simple 2D shapes that can be joined together to make an enclosure.

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