Vacuum Forming Process
The clamp frame is used to clamp the sheet around the perimeter and needs to have sufficient strength to hold the sheet firmly in place throughout the moulding process. The clamp frame needs to be suitable to hold the thickness of the material that the machine is producing. In the case of our specialist custom-made machinery, we have machines capable of producing components from 0.38mm up 12mm in thickness. As you can imagine a sheet 12mm thick and a moulding table of 3500mm x 2500mm means that sheets can be of substantial weight. The clamp needs to be strong enough to support the weight and resilient enough to be sent time and time again into a heating oven that can reach temperatures of several hundred degrees.
Heaters are generally of infra-red elements however there are other types of elements which provide flash heating all the way to the other spectrum the older ceramic heating elements. Ceramic elements also have the disadvantage of having a higher thermal mass meaning that they are slower to warm up to temperature and have a longer response time to be adjusted once heated up.
Modern quartz-style heating elements have less thermal mass and are considerably easier to adjust during the moulding process. The modern machines allow for many different zones to be selected from one element through to a group of elements allowing for the sheet to be warmed up using specific acquired knowledge to ensure that the sheet remains uniform during the forming process and does not thin out excessively giving a faulty or rejected part. All our machines are fitted with state-of-the-art digital pyrometers interacting with the temperatures on the heating elements, temperatures of the sheet and temperatures of the oven. An accurate pyrometer enables the operator to adjust the temperature of the different heating functions to provide greater control of the moulding process.
Heating also involves close control of areas of greater heat intensity, heat loss around the perimeter of the machine caused by air currents and absorption of heat from the clamping and frame system. Our quartz-based heating elements provide for an overall cost saving during the production process as there is an overall power drop in the heaters but also allow for a more controlled and higher quality finished product for our customers.
Auto Level sheet
Some of our machines are fitted with a photoelectric beam which scans the distance between the bottom heater and the sheet of plastic. If the sheet begins to sag the sensor will alert the machine and a small burst of air will be injected into the machine’s lower chamber to lift the sheet stopping it from sagging and more importantly stopping the sheet from falling onto the bottom heating elements.
The majority of larger machines we use have the pre-stretch function. Once the plastic has reached the required temperature the machine will inject a stream of air into the bottom chamber of the machine creating a bubble which ensures even wall thickness when the mould is lifted and the vacuum is finally applied. This method is used to ensure that consistent vacuum moulded parts are produced. Combining this process with correct vacuum, and air pressure as well as having the ability to use plug assist will all ensure that consistent and high-quality parts are produced.
Plug assist is used when the straight vacuum is not able to distribute the plastic sheet evenly to all the required areas. For example, there are intricate cavities which require the sheet to be forced into these areas so that the vacuum can take over. To help spread the sheet out more evenly a plug will be used to push the sheet into the mould before the vacuum is applied. This allows for more plastic to reach into the cavities and fill the corners limiting the thinning out of the sheet.
Once the material is draped over the mould a vacuum is then applied to assist in forming the sheet. We commonly use a dry vane vacuum pump to draw all the trapped air from between the sheet and the mould. With larger machines, a vacuum cylinder or reservoir is used in conjunction with a high-volume vacuum pump. This will enable a rapid vacuum to be applied before the sheet temperature drops below the ideal forming temperature creating rejected or faulty parts.
Cooling and release
Once the part is formed the plastic must be allowed to cool before it is realised. If this process is rushed the deformation of the part will occur leading to a rejected part. In order to speed up the cooling process, high-speed fans are fitted to many machines and are activated once the forming has been completed. An option of a spray mist is also fitted to allow for a fine mist of cooled water to be shot over the part as the fans are activated. New advancements in refrigerated water cooling, as well as water-cooled parts, allow for a much faster cooling speed of production parts. Cooling within the mould is generally available to regulate the temperature of the tool when dealing with crystalline and crystallising polymers such as PP, HDPE and PET.
Trimming and finishing
Once the formed part has been removed from the machine the excess material is removed from the mould. The parts are then moved to either a laser or a 3 or 5-axis CNC machine where the part is then precision trimmed. Other post-forming processes may then be applied such as glueing, combining moulded parts, or secondary material and printing, reinforcing or strengthening through welding the part with other moulded or raw plastic materials.
We offer a large range of trimming methods. The most suitable trimming method will be individually assessed based on the type of material, the size of the part, the overall draw, the quantity required as well as ultimately the final requirement of the part.
Our in-house fabrication area will then complete the part through any number of processes ensuring that the final part is suitable for the application and exceeds client expectations.