How polycarbonate molding works
Polycarbonate is an extremely versatile plastic material, which is used in dozens of different fields, including the automotive and aeronautical industries.
In fact, it can be used to create extremely robust objects, capable of withstanding sunlight and temperature changes, without losing its mechanical peculiarities.
Polycarbonate is also highly appreciated because it allows you to create large objects, given that it is capable of supporting quite considerable weights without deforming.
This plastic material can be subjected to numerous processes, always obtaining excellent results; among them: different molding techniques, laser cutting, turning and polishing, gluing and chemical welding operations.
We at Bellelli Ufficio have chosen polycarbonate as the main material for the creation of our products, precisely because of its notable characteristics!
It is in fact a carbonic acid polyester which is obtained from a compound of organic origin, called bisphenol, which has a very high resistance to UV rays and excellent transparency to light.
It should also be underlined that polycarbonate is non-toxic and has a high degree of printability, with very few limitations in thickness and curvature.
Plastic molding techniques
Plastic materials, from the Latin plasticus (to shape - to model), owe their name to their ability to undergo deformations if subjected to certain thermal or chemical stresses.
Polycarbonate, for example, is processed at high temperatures (between 170°C and 225°C). In this way, the risk of modifying the mechanical properties of the material is practically zero, therefore customized shapes can be obtained, with a very high degree of precision.
To mold an object made of polycarbonate, opaque or transparent, we start from the raw material, which for the majority of applications is made up of sheets of predetermined thickness, very thin or thicker.
Polycarbonate is also available in blocks, however reserved for particular applications, in which a considerable amount of material removal operations must be carried out after moulding.
The optimal results are obtained starting from sheets with a rather limited thickness, usually between 4 mm and 1 cm, already colored.
Even if it is possible to subsequently add a surface pigment, the result will never be comparable to that of a volumetric distribution, in particular due to the exceptional refractive index of light which makes polycarbonate more transparent than glass.
To carry out the printing, hot presses are used, which require the use of particular shapes, which trace the inside and outside of the profile you want to obtain and whose shape depends considerably on the final result.
Usually, the product is ready after molding (which can also take place in series on large numbers), but for some types of objects it is better to carry out a finishing step to eliminate burrs and other slight imperfections, such as the connecting edges.
Molding defects in polycarbonate
Although technologically advanced, polycarbonate molding may present some small inaccuracies.
However, these are problems that are unlikely to escape quality control and which, for a high profile product, are not visible to the end user.
In some cases these are design errors that can undermine structural robustness, while in others they are merely aesthetic defects.
It is obvious that for both one and the other, it is possible to implement preventive or corrective adjustments.
• The most common polycarbonate molding defects are bubbles, or voids, that form inside the pieces. Depending on their extent, they may or may not affect the aesthetics or the mechanical properties of the material.
• Depression, or volumetric contraction, typically occurs when the average thickness is very high and there are notable differences between parts. This phenomenon manifests itself as a slight sag or hollow in some points, due to internal tensions during the thermal stabilization of the piece.
• Burns occur when a gas used for molding is not vented properly to the outside.
• Deformation is linked to pieces with walls that are too thin; it usually occurs when the cooling rate is not uniform and some parts harden earlier than expected, tending to curl.
• Flashing is a phenomenon that occurs at the junction points between two pieces of a molding. It essentially consists of a small amount of residual material that infiltrates the leading edges, which can be easily removed with an abrasive tool.
• if the polycarbonate sheets are colored carelessly it is possible that the pigments used are not distributed perfectly, forming vortices.
• In very large joints, weld seams can be created, i.e. lines and thickenings where the polycarbonate blocks are joined.
This is not a simple aesthetic defect, the weld seam can be problematic for safety. In fact, where it is necessary to pass screws or other elements, it is preferable to slightly increase the thickness to avoid the formation of tension cracks.
• Surface imperfections on polycarbonate products are usually linked to worn and unmaintained tools, which leave circular or spiral marks during post-molding polishing.
This defect can be prevented by keeping the machinery fleet updated and periodically checking the quality of the instruments installed.
The presence of imperfections of this type is also linked to the quantity of finishing steps and is resolved quite easily by carrying out embossing by sandblasting.
• Print marks and demoulding residues can also be more or less visible defects. They are due to the removal of pieces from the molds with special tools, such as pliers or blades, which can leave marks (scratches, lines, dots).
• Superficial problems, such as jetting and orange peels, instead derive from temperature changes and flow variations.
The control methods used by our company to avoid the formation of defects in polycarbonate pieces include visual inspection, destructive testing of the component and density control.
Injection molding parameters
Injection molding is an advanced process that requires specific equipment and rigorous compliance with operating parameters.
Many of the defects found in molded polycarbonate pieces, in fact, derive from failure to comply with the rules.
Let's see the most important ones.
• The operating temperatures are very important to preserve all the initial characteristics and not to generate serious structural problems, such as: presence of cracks in the curves, loss of transparency and color, surface deformations, such as orange peel and depression.
• Thickness is closely linked to temperature because the greater the height of the sheet to be treated, the more difficult it will be to keep it uniform and, consequently, the force to be exerted to change the shape of the polycarbonate varies.
It is very difficult to carry out numerous deformations with a single step, because the temperature can change considerably even a few centimeters apart in the same mold.
• Molding times are very important; they must be short enough to guarantee a uniform temperature and avoid the formation of defects.
Times are even more important when the polycarbonate object to be modeled must take on shapes with cusps, because in this case the thickness becomes very thin.
In fact, polycarbonate in its thinner parts, if particular attention is not paid, risks losing its mechanical properties already inside the mold.
• Pressure is another important element because it must be commensurate with the thickness and viscosity of the piece.
Many injection moldings require increasing pressure on only one of the foil surfaces to adhere the other to the shape, thus significantly reducing manufacturing costs and the risk of cosmetic defects.
Pressure is fundamental because the high temperature air must be pushed with intensity and uniformity in the shortest possible time, to adhere to the cast without local thinning phenomena which could make the finished object unusable.
Every single step must be carried out with the utmost attention and diligence, even the final finishing and intermediate processes: cuts with a hot blade, with traditional saws, laser cuts, drilling and finishing operations.
If a single piece is defective, once assembled with others, the strength and quality of the final piece could be compromised.
ABS injection molding
Injection molding with ABS plastic offers excellent results thanks to single and double injection presses.
It follows the techniques used for polycarbonate, even on small objects, with thermoplastic deformation and resin injection.
ABS plastic is slightly softer to the touch than polycarbonate; it is very widespread due to its excellent mechanical properties and impact resistance.
Furthermore, it is a plastic material that does not release contaminants and toxic substances, even during hot processing, a requirement that makes it ideal for the creation of customized products.
The resistance to impacts, cuts and chemical stress make ABS plastic suitable for making commonly used objects.
Transparent polycarbonate molding
Polycarbonate is available both opaque, translucent and transparent.
This plastic material has a better refractive index than glass, without the risk of yellowing or changing color and transparency in the event of prolonged exposure to sunlight.
Polycarbonate is often used to make outdoor canopies and similar objects, intended to be exposed to the elements.
Due to its polymer structure, it has a high tolerance; it is very shock resistant and is very little affected by extreme temperatures.
The injection molding of transparent polycarbonate allows you to obtain numerous geometric shapes and different sizes, with perfect repeatability.
Polycarbonate injection molding occurs with very rapid temperature heating and cooling times.
This allows for maximum malleability, a very low margin of error and crystal-clear transparency.