TECHNICAL INFO

Transcription

1 Background Thermoforming is carried out by heating the ArmaFORM AC grade foam core to its softening point and forcing it against the contour of a female or male mould The AC grade thermoforms very well as it is a pure thermoplastic core material to both single and for thinner sheets also double curved surfaces, see picture 1 Picture 1 Thermoformed AC grade sheets in different thicknesses ArmaFORM AC grade PET core material exhibits a glass transition temperature (Tg) close to 75 C, normally foam cores cannot be used at much higher temperatures than their Tg However crystallization in ArmaFORM PET cores produces a crystalline structure that will act as a static, non-movable system until melting begins in the crystalline phase, at about C (melting point Tm) It will take hours to melt all crystalline structures at temperatures of 180 C while it will melt rapidly at 240 C This allows for a wide range of processing temperatures for thermoforming PET cores but normally it takes place around 200 C There are at least a dozen thermoforming methods: vacuum assisted, pressure, drape, sweep, match mould and free forming, to name just a few All figures in this section are derived from previous production experience and testing Adjustments normally have to be made depending on individual production conditions such as core density and thickness used and the radius in the mould Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom

2 Moulds If the production moulds are not available thermoforming moulds can be made of most common materials If a series is small, or if you are working on a prototype, a wooden mould out of plywood is easy to build and give acceptable performance, see picture 2 and 3 Picture 2 Simple thermoforming mould out of MDF The disadvantage of wood is the heat build-up in the mould, the cooling time is long and productivity is therefore relatively low for continuous production Steel or aluminium moulds are preferable for series production due to their high thermal conductivity and their stability Plastic/composite moulds can be used but they also accumulate heat Single curved products with a radius larger than 400 mm are best formed on a male mould This could either be sweep forming with a thin steel foil or vacuum bagging Vacuum bagging is a simple operation with low tooling costs, but is has some disadvantages It takes time to apply the vacuum bag, and the sheet might cool down too much This can be avoided if the vacuum bag is assembled on a cold sheet and mould The mould is then placed in a hot air oven The temperature inside the sheet is measured with a thermal gauge When the right temperature is reached the vacuum is applied With vacuum bagging care has to be taken so that the bag does not compress edges or corners The mould is then taken out of the oven and allowed to cool with the vacuum being maintained Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom

3 Consideration has to be given to time, temperature and vacuum to avoid creep effects If the radius is below 400 mm, a female mould should be used It could either be vacuum bagged or match mould formed The latter should be used if the radius is small, the thickness or density is high and a high load is required Fixed stops must be used to avoid compression of the core Heating Several different ways can be used to heat up the AC grade core material, a heated platen press with fixed stops or a circulating hot air oven is often utilised Another option is to use infrared heaters that have the advantage of heating through the thickness at ones The IR-waves will penetrate deep enough on thicknesses up to at least 40 mm Shorter wavelength decreases the temperature in the surface without significant difference in core temperature so short IR heating should then be preferred It is crucial to reach the same temperatures everywhere in the board The best compromise found here is ARMA6 (36kW-short wavelength IR) The temperature in all three cases should be kept within ±5 C If the temperature is too high the dimension stability will be affected and if it is too low the stiffness will be to great and can result in spring back of the sheet An uneven temperature distribution can make the ArmaFORM core sheets twist or warp so it is important to have an even temperature on both sides of the sheet Picture 3 Simple thermoforming mould out of MDF, core thickness 75 mm, 300 mm radius Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom

4 Temperature and times The following oven temperatures should be used for the ArmaFORM AC grade, independent of radius Thickness <20 mm >20 mm Temperature ( C) +185 to to +210 The following times should be used for the different thicknesses, independent of radius and grade Thickness (mm) Time (min) minute per C The temperature and time are dependent on the local conditions and should be calibrated prior to start of production Start with the lowest time and temperature The following temperatures in the centre of the core should be used for the different qualities when a cold sheet is vacuum bagged in a hot air oven Temperature ( C) The time from removal from the heating unit until the pressure is applied must not exceed 05 minutes to avoid the ArmaFORM AC grade sheet cooling down Dimensional stability ArmaFORM AC grade will change its dimensions when heated in accordance with temperatures and times mentioned above The following maximum figures as percentages of the original dimension are valid if heated without any restrictions like a vacuum bag Note that this will mean a decrease of the density of the core sheet and this has to be taken into consideration during the design phase - Length/width = % - Thickness = -2-0 % Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom

5 The spring back of ArmaFORM AC grade is normally neglect able It should also be noted that the edges of thermoformed pieces have a tendency to straighten out If the sheets are not to be edge trimmed after the thermoforming process care has to be taken to how the weld lines are orientated In this case the weld lines has to oriented along the line of curvature, otherwise there is a risk that the edge will be uneven after thermoforming, see picture 4 Picture 4: Thermoformed sheets with different orientation of the weld lines EFFECT ON PHYSICAL PROPERTIES The ArmaFORM AC grade is affected in two ways during thermoforming 1) Decrease in density during heating due to the expansion of the sheet 2) Stretching of the outer radius Both will decrease the physical properties slightly Typical decrease is 0-5 % but from design standpoint 10 % should be used This has to be verified for the actual structure and heating method used Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom

6 Storage and long term properties The thermoformed core parts have excellent long term stability with no relaxation and spring back even after 6 months storage However initial results from storage at higher temperature indicate that it has to be kept below 50 C to avoid relaxation and spring back If the thermoformed core parts have to be transported with a risk of exceeding 50 C it is recommended to strap the pieces in a fixture in such a way that spring back is avoided Status : Technical Info 5 - June 2011 Armacell provides this information as a technical service To the extent the information is derived from sources other than Armacell, Armacell is substantially, if not wholly, relying upon the other source(s) to provide accurate information Information provided as a result of Armacell s own technical analysis and testing is accurate to the extent of our knowledge and ability, using effective standardized methods and procedures Each user of these products, or information, should perform their own tests to determine the safety, fitness and suitability of the products, or combination of products, for any foreseeable purposes, applications and uses by the user and by any third party to which the user may convey the products Since Armacell cannot control the end of this product, Armacell does not guarantee that the user will obtain the same results as published in this document The data and information is provided as technical service, and the data and information are subject to change without notice Armacell Benelux SA Rue des Trois Entités, 9 B-4890 Thimister-Clermont Phone: +32 (0) infoarmaform@armacellcom wwwarmacell-core-foamscom