Vacuum flow is critical to the process of vacuum forming. After heating the sheet to its forming temperature, vacuum must follow immediately as the tool is raised. In addition to the vacuum pump and hoses, the seals on the table and the clamp frame all play a pivotal role in creating effective vacuum to achieve well defined formed parts.
Vacuum Pumps
There are three different types of vacuum pumps:
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- Diaphram Pumps — Often used in smaller desktop machines, these are robust pumps that require very little maintenance. They typically have a 25” Hg – 5.52 m3.hr flow rate.
- Dry Rotary Vane Pumps — Often used in smaller desktop machines, these are robust, low maintenance pumps that cope with dirt well. They have a flow rate of 2-3 times greater than of the diaphragm pump.
- Oil Filled Rotary Vane Pumps — These are used in larger floor standing machines with vacuum tanks, where the machine is used constantly. They are higher maintenance and oil level needs monitoring. They usually have a 28” Hg – flow rate.
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Vacuum Tanks
Vacuum tanks allow you to create instant vacuum, stored from the vacuum pump, instead of waiting for the pump to generate the vacuum. You can use either rectangular or cylindrical tanks.
Tanks can be useful with more demanding materials — like polycarbonate or APET — where there is an ideal forming temperature and instantaneous vacuum is necessary.
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Vacuum Holes and Their Positions
Vacuum holes or slots should be positioned at the lowest point in cavities or at changes in sections. Every corner requires a hole and they should generally be spaced 25mm (1”) apart.
The thinner the material, the smaller diameter the vacuum hole should be. You can read more about which material thickness corresponds to the diameter of a hole and how to keep your vacuum holes as small as possible here.
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Vacuum Flow
When using multiple tools, try to create an even vacuum flow from the main central vacuum hole on the table to the underside of each of the tools. You should use spacers for vacuum flow to raise each tool off the baseboard. For short runs — which is usually what most of our customers are looking at — the spacers can be wire mesh, washers, thin plywood strips (1mm) or double sided foam tape (1mm). Foam tape can compress with heat, so additional harder spacer strips can be used to prevent this.
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Air Pockets
High sided tools will create large pockets of air that the vacuum pump or tank will need to remove. Any voids that contain air on the underside of
the tool will also have to be removed by the vacuum system. Voids on the underside of tools can be reduced by using blocks of wood or any solid material to fill the space.
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To read more about vacuum holes, including alternatives to vacuum holes, head over to the Formech blog here.
