How to avoid blistering/whitening of PC solid sheets after hot bending and bending?
Everyone knows that the plastic shape of PC is widely used. Suitable for lighting facilities in high-rise buildings, schools, hospitals, residential areas, banks, and places where shatter resistant glass must be used, widely used for large-area lighting roofs and stair guardrails.
PC solid sheets hot bending, also known as hot pressing, is a process of heating PC solid sheets to a certain temperature, softening it, and then undergoing plastic deformation based on its thermoplastic properties.Solid sheets can be hot bent or cold bent, but because cold bending can only perform simple processing such as straight bending, it is powerless for complex processing requirements such as curvature. Hot bending forming is a relatively simple forming method, but it is also the most commonly used method to obtain parts bent along an axis, which are often used for machine protective sheets, etc. For shets with higher requirements and hot bending of 3mm or more, double-sided heating has a better effect.
However, if not careful during hot bending, it is easy to experience foaming and whitening. How can we avoid this?
The thermal deformation temperature of PC solid sheet is about 130 ℃. The glass transition temperature is about 150 ℃, above which the sheet can undergo hot forming. The minimum bending radius is three times the thickness of the sheet, and the width of the heating area can be adjusted to obtain different bending radii. For the production of high-precision or (and) large parts, it is recommended to use a bending device with temperature controllers on both sides. A simple shaping bracket can be made to allow the sheet to cool in place to reduce deflection. Localized heating can cause internal stress in the product, and special attention should be paid to the use of chemicals for hot bent sheets. In any case, it is recommended to first try making a sample to determine the feasibility of the bending operation and suitable process conditions.
There are generally two methods for preparing heating plates for the company.
1、 Electric heating wire - Electric heating wire can heat the PC solid sheets along a certain straight line (for the line), suspend the part of the PC solid sheets that needs to be bent above the electric heating wire, heat it to soften, and then bend it along this heating softening straight line position.
2、 Oven - Heating and bending the oven is to cause a curved surface change (opposite to the needle) on the PC solid sheets. First, place the PC solid sheets into the oven and heat it as a whole for a period of time. After it softens, take out the softened whole PC solid sheets and place it on the pre made mother mold. Then press it down with the male mold and wait for the plate to cool before taking it out, completing the entire shaping process.
Whether using electric heating wire or an oven to process PC solid sheets, there are often phenomena such as bubbling and whitening at the bending parts, which can affect the appearance or result in high loss rates.
There are usually two reasons that cause bubbling on the sheet:
1、 If the PC solid sheet is heated for too long/at too high a temperature, the board will bubble (the temperature will be too high, the interior will start to melt, and external gas will enter the interior of the sheet). However, unlike in sheet metal production where temperature and heating time are precisely controlled by equipment, post-processing usually relies on manual judgment, so bending generally requires experienced professional workers to complete.
2、 PC (polycarbonate) sheet itself will absorb moisture (at standard atmospheric pressure, 23 ℃, relative humidity of 50%, the water absorption rate is 0.15%). Therefore, if the finished solid sheet is stored for a long time, it often absorbs moisture from the air. If the moisture is not removed before molding, bubbles and misty micro pore groups will appear in the formed product, which will affect the appearance.
To avoid abnormal situations caused by moisture, the sheet should be pre dried at a lower temperature for a period of time before heating and forming. Usually, the moisture can be removed at a temperature setting of 110 ℃~120 ℃, and the dehydration temperature should not exceed 130 ℃ to prevent the board from softening. The duration of removing moisture depends on the moisture content of the sheet, the thickness of the sheet, and the drying temperature adopted. The sheet that has been dehydrated can be safely heated to 180-190 ℃ and can be easily deformed.
PC solid sheet bending is an essential process in solid sheet processing and production. As a production and processing factory, we should comprehensively consider which process to choose based on the specific requirements of the product, and control the key points that are prone to problems, in order to produce PC solid sheet products without bubbles and with standardized dimensions!