Key Elements In Molding--Die Cooling
Injection molding is formed in the plastic mold and solidification, because each kind of resin requirements of forming temperature and glass transition temperature is different, so there must be a plastic mold temperature control system, can be suitable for forming each resin, highly efficient production.
In injection molding, the melt temperature is usually between 150~350 and 40~120 in the mold, but the temperature of the mold is generally at the temperature, so the heat of the forming material will gradually increase the temperature of the mold. On the other hand, since the nozzle of the heating cylinder is in direct contact with the sprue bushing of the mold, the nozzle temperature is higher than the mold temperature, and the mold temperature will also rise. If the excess heat is not taken away, the mold temperature will continue to rise, thereby affecting the cooling and solidification of the molding. Conversely, if too much heat is removed from the mold and the die temperature drops, the quality of the molded product is also affected.
For plastic with low viscosity and good fluidity (such as polyethylene, polypropylene, polystyrene, ABS, etc.), plastic mould is generally required to be stable (usually not less than 80 degrees Celsius);
For high viscosity and poor fluidity of plastic (such as polycarbonate and polysulfone, polyphenylene ether etc.), in order to improve the filling performance, or large plastic mold, cooling area wide, not only need to set the plastic mold cooling system, also need to set the heating system for heating the die in the mold.
Some plastics are shaped and require molds, cooling systems, and heating systems. When the mold temperature reaches the plastic forming process requirements, you can turn off the heating system, if the injection time, mold temperature is higher than that of the plastic forming process, the cooling system is to open the mold, the mold temperature at the required temperature.
For small thin-wall plastic parts, and the forming process requires that the mold temperature is not too high, the cooling device can be installed without natural cooling.
Influence of mold temperature on products
(1) formability and forming efficiency
When the mold temperature is high, the fluidity of the molten material in the forming space is improved, and the filling can be promoted. But for the forming efficiency (forming cycle), the die temperature decreases moderately, which can shorten the time of cooling and solidification and improve the forming efficiency.
(2) the physical properties of molded products
Usually the molten material filling forming space when the mould temperature is low, the material will rapidly solidified, at this time in order to fill the need of great pressure, therefore, curing the occasion, applied to the molding part of the residual internal pressure, a so-called residual stress.
For hard materials such as PC or denatured PPO, the residual stress will rise to a certain extent, stress cracking will occur, or deformation of molded products will occur. The crystallization degree and crystallization state of crystalline plastics such as PA or POM depend significantly on their cooling rate, and the slower the cooling rate, the better the result. As can be seen, the mold temperature is high, although it is not conducive to forming efficiency, but it is often conducive to the quality of molded products.
(3) the shape of molded articles
The molding flesh, when, if the cooling is not sufficient, the surface subsidence will shrink, even if the flesh is appropriate, the cooling method of poor molding cooling rate of each part is different, also due to thermal shrinkage caused by the warpage and twist deformation, so that each part of the die uniform cooling.
Cooling and heating of dies
The general mold is usually cooled by normal temperature water, and its temperature control, water flow regulation and low fluidity material are mostly formed by this method. But sometimes in order to shorten the forming cycle, water must be cooled again.
Injection molding of small time, holding time is short, the forming period depends on the cooling time, the process in order to improve the efficiency, but also often to the cooling water, cooling with cold water, moisture in the air is condensed in the forming space surface, resulting in molding defects, and to pay attention to.
Forming a high melt point material or a molded product with a thicker meat and a long flowing distance, sometimes to the water pipe to prevent excessive filling or strain. When forming a material with low melting point, the forming area is large or the large mold will also heat the mold. At this time, the mold temperature is controlled by hot water or hot oil or by a heater. When the mold temperature is high, the gap between the sliding parts of the die should be taken into consideration, so as not to make the mold run out of action due to thermal expansion.
In general forming materials, sometimes because of product quality or liquidity and the use of heating methods to control the mold temperature, to solidify the material for the final temperature uniformity, and the use of part of heating, to prevent residual strain.
As mentioned above, the temperature control of the mold is adjusted by cooling or heating.
Design principles of cooling systems
(1) the caliber, the spacing of the cooling pipe and the distance from the forming space surface have a significant influence on the temperature control of the die, and the maximum value of these relations is as follows:
When the diameter of the cooling pipe is , the maximum distance between the tube and the tube is , and the maximum distance between the tube and the surface of the forming space is . Furthermore, the molding thickness thicker than the thickness of the thinner, the cooling pipe must be reduced and is close to the surface forming space interval.
(2) in order to keep the mold temperature distribution evenly, the cooling water should first enter from the higher temperature of the mold, and then recycle to lower temperature and then export. Usually, injection molding material of high temperature near the gate, so cold water, outer part of low temperature, the circulation heat exchange of warm water, the pipeline circulating system is connected, is processed in the mold through hole, hole and hole in mould connection.
(3) when forming PE and other big shrinkage material, the cooling pipe should not shrink along the direction of shrinkage because of its great shrinkage.