Vacuum forming is a thermoforming process in which a plastic sheet is heated to the desired temperature, and then moulded to a three-dimensional shape using vacuum technology and compressed air.
The vacuum forming process can be used to make all sorts of plastic products including shower trays, machinery panels and even tractor bonnets. The process is ideal for low to medium production volumes and is particularly advantageous economically for larger components.
There are many other benefits of choosing vacuum forming to make your plastic products, over other processing techniques such as blow, rotational and injection moulding:
- Comparatively low tooling costs
- Moulds can be made from cheaper materials and fabricated quicker due to low pressure on the mould
- Low cost prototyping
- Short lead-times
- Flexibility of trim and hole details
- Wide choice of colours, finishes and materials
Read on to learn more about the 8 key components of the vacuum forming process: clamping, heating, sheet level, pre-stretch, vacuum, plug assist, cooling and release, and finishing and trimming.
A clamp frame is used to ensure the plastic sheet is held firmly in place during the vacuum forming process. It is important that the clamp frame is powerful enough to handle the thickest materials expected to be used with the machine: this tends to be up to 6mm when the machine has a single heater, and 10mm with twin heaters.
Now that the plastic sheet is securely positioned over the opening of the vacuum forming machine it is time to heat it up: this is typically done using radiant heaters.
The heaters are mounted within an aluminium reflector plate and use infra-red elements; they move into position both above and below the sheet when it is time to heat the plastic. There are also a series of zones controlled by energy regulators to ensure that the heat applied to the plastic sheet is uniform, both throughout the depth and over the surface area.
There are a variety of heaters that can be used for different requirements, including single heaters, twin heaters and quartz heaters.
The sheet level stage is one that ensures the sheet isn’t sagging down too much into the machine during the vacuum forming process. A built-in photo-electric beam scans between the bottom heater and the sheet of plastic; if the sheet of plastic sags down, therefore breaking the beam, a small amount of air is injected into the bottom chamber to lift the sheet up.
When the plastic has reached the correct forming temperature through heating it then needs to be pre-stretched to achieve an even wall thickness when the vacuum is applied. Air is introduced to blow an air ‘bubble’ of a controlled height into the plastic sheet. The mould is then raised into this pre-stretched sheet ready for the next stage.
Now that the mould has been raised into the sheet a vacuum needs to be applied to assist in forming. A dry vane vacuum pump uses air to suck the sheet into the mould to eliminate any trapped air and ensure the detail of the mould is properly picked up.
With much larger machines a high volume capacity vacuum pump is used alongside a vacuum reservoir to enable faster moulding of the heated sheet before it drops below the correct forming temperature.
Plug assist forming is used when a mould is particularly complicated or deep draws are required in the plastic, as it ensures that the thickness distribution is even and that there is no webbing. It does this by using a plug to push the plastic sheet fully into these deep draws in the mould. It is through this process that as much material as possible is fed into the cavity before vacuuming to avoid any thinning in that area.
Cooling and release
Now that the plastic has been formed it is critical to let it fully cool before being ejected from the mould. If it hasn’t cooled fully the moulding could become deformed which would result in a reject part.
Often high speed fans are used to speed up cooling, or a fine mist of chilled water which used with the fans can reduce cooling time by up to 30%. Once the material has fully cooled it can then be released from the mould by using a small amount of reverse air pressure.
Finishing and trimming
Once released the part is transferred to the trimming station for removal of excess material, finishing and any other required post-forming process. These could include drilling holes, slots and cut-outs into the part and any required decoration, printing, reinforcing and part assembly.
Want to find out more?
Bridgewood are the UK’s leading vacuum forming specialists based in Hessle near Hull. With over 30 years of technical capability and know-how we can use vacuum forming to make your plastic products more cost effective, durable and attractive.
This post is just a brief overview of the key stages in the vacuum forming process. If you want to find out more please feel free to get in touch with an expert today on 01482 646464.