Hello Readers, welcome to your own website to understand each and every topic related to the manufacturing process where we transform complex content into simpler ones. In this article, we are focused to cover thermoforming, process capabilities, vacuum thermoforming, pressure thermoforming, mechanical thermoforming, advantages, disadvantages, and applications of thermoforming.
WHAT IS THERMOFORMING?
Thermoforming is a process of forming thermoplastic sheets or films over a mold
through the application of heat and pressure.
In this process,
(A) By making use of radiant heating the sheet is clamped and heated to the sag point (above the glass-transition temperature, of the polymer).
(B) Forced against the mold surfaces through the application of a vacuum or air pressure.
The sheets which are utilized in thermoforming are available as a coiled strip or as lengths and widths of different sizes.
They also are always filled with different materials for creating parts with unique applications.
The mold is typically at room temperature so because of that, the shape created becomes
set upon contact with the mold.
Due to lower strength of the materials formed, the pressure difference caused by a vacuum usually is sufficient for forming.
However, thicker and huge complex parts need air pressure, which may range
from about 100 to 2000 kPa (15 to 300 psi), depending on the type of material and
thickness of the sheet.
Mechanical means, such as the use of plugs, also may be employed to help form the parts.
Average parts are created by thermoforming are packaging, trays for cookies and candy, publicizing signs, coller liners, appliance housings, and panels for shower stalls.
Parts with openings or holes cannot be formed by this process as the pressure difference cannot be maintained during forming.
Since thermoforming is a mix of drawing and stretching operations (similar in some sheet-metal forming), the material should exhibit high, uniform elongation; otherwise, it will neck and tear.
Thermoplastics have huge capacities for uniform elongation by virtue of their high strain-rate sensitivity exponent.
Molds for thermoforming normally are made of aluminum since high strength is not needed hence, tooling is moderately cheap.
Thermoforming molds have small through holes in order to aid vacuum forming.
These holes typically are less than 0.5 mm (0.02 in.) in diameter; otherwise, they would leave marks on the parts formed.
Defects encountered in thermoforming include:
(a) Tearing of the sheet during forming.
(b) Non uniform wall thickness.
(c) Improperly filled molds.
(d) Poor part definition.
(e) Lack of surface details.
Thermoforming include two important steps:
Heating and forming.
Heating is generally achieved by radiant electric heaters, which is found on one or both sides of the beginning plastic sheet at a distance of roughly 125 mm (5in).
The period of the heating process require to sufficiently soften the sheet depends on the polymer its thickness and color.
Techniques by which forming is achieved, can be classified into three basic categories:
(1) Vacuum thermoforming.
(2) Pressure thermoforming.
(3) Mechanical thermoforming.
In our conversation of these techniques, we explaine the forming of sheet stock, however in the packaging industry most thermoforming operations are performed on thin films.
This was the first thermoforming process (just called vacuum forming when it was created in the 1950s).
Negative pressure is utilized to draw a preheated sheet into a mold cavity.
The process is as per follows:
(1) A flat plastic sheet is softened by heating.
(2) The soft ened sheet is placed over a concave mold cavity.
(3) A vacuum draws the sheet into the cavity.
(4) The plastic hardens on contact with the cold mold surface, and the part is taken out
and subsequently trimmed from the web.
A choice to vacuum forming include positive pressure to force the heated plastic into the mold cavity.
This is called pressure thermoforming or on other hand blow forming; its benefits over vacuum forming is that higher pressures can be created due to the latter is limited to a theoretical maximum of 1 atm.
Blow-forming pressures of 3 to 4 atm are normal.
The cycle sequence is same to the past one, the only variation is that the sheet is pressurized from above into the mold cavity.
Vent holes are given in the mold to exhaust the caught air.
At this point it is valuable to recognize negative and positive molds.
The molds are negative molds since they have concave cavities.
A positive mold has a convex shape.
Both types are utilized in thermoforming.
In the instance of the positive mold, the heated sheet is draped over convex form and negative or positive pressure is utilized to force the plastic against the mold surface.
In any case if the part is drawn into the negative mold, then its outside surface will have the specific surface contour of the mold cavity.
Within surface will be an estimation of the form.
Conversely if the sheet is hung over a positive mold, then its inside surface will be identical to that of the convex mold; and its outside surface will follow approximately.
This distinction might be important depending on the requirements of the
The third strategy, called mechanical thermoforming, utilizes matching positive and negative molds that are brought together opposite to the heated plastic sheet, forcing it to assume their shape.
Air pressure is never used in pure mechanical forming.
Its benefits are better dimensional control and the chance for surface enumerating on the two sides of the part.
The disadvantage is that two mold halves are needed so the molds for the other two methods are inexpensive.
APPLICATIONS OF MECHANICAL THERMOFORMING:
Thermoforming is an optional shaping process, the essential cycle being that which delivers the sheet or film.
No one but only thermoplastics can be thermo formed, due to the extruded sheets of thermosetting or elastomeric polymers have already been cross-linked and can’t be softened by by heating again
Normal thermoforming plastics are polystyrene, cellulose acetate and cellulose acetate butyrate, ABS, PVC, acrylic (polymethylmethacrylate), polyethylene, and polypropylene.
Mass production thermoforming operations are run in the packaging industry.
The beginning sheet or film is quickly fed through a heating chamber and after that mechanically formed into the desired shape.
The operations are often designed to create multiple parts with each stroke of the press with the help using molds with multiple cavities.
Sometimes, the extrusion machine that create the sheet or film is found straight forwardly upstream from the thermoforming process, subsequently taking out need to reheat the plastic.
For best effectiveness, the filling cycle to place the consumable food item into the container is placed immediately down stream from thermoforming.
Slight thin film packaging items that are mass created by thermoforming include blister packs and skin packs.
They offer an attractive way to show certain commodity products like as cosmetics, toiletries, little tools, and fasteners.
Thermoforming applications include huge parts that can be delivered from thicker sheet stock.
Models incorporate covers for business machines, boat structures, shower slows down, diffusers for lights, promoting showcases and signs, bathtubs, and certain toys.
Contoured skylights and interior entryways liners for coolers are made, respectively, out of acrylic (due to of its transparency) and ABS (because of its simplicity in forming and protection from oils and fats found in refrigerators.
ADVANTAGES OF THERMOFORMING:
In this low tooling costs due to usually only needing one mould half.
But the quality of injection moulded products is faster.
The thermoforming is high speed which is suitable for JIT (just-in-time) production strategy.
By use of thermoforming it becomes easy to make larger parts with a wider design scope.
The process and materials optimised for cost effectiveness.
Weight-related cost savings.
DISADVANTAGES OF THERMOFORMING:
The process is restricted to thin-walled designs.
The thickness of the part may be uneven in spots, causing weak points.
It’s a bit more expensive when compared to other plastic molding methods like injection molding.
It may need huge plastics than other methods of plastic molding.
It is not a versatile material process, as it’s limited to plastic sheets.
It is use in retail clamshell packaging.
For packaging blisters.
Also use for pick and place trays.
For the purpose of material and handling trays and covers.
We have covered all the important concepts related to thermoforming. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!