Despite the various advances in plastic injection mold technology, there are still challenges to produce high gloss parts with high aesthetic requirements, free from the stresses generated in molding and with high dimensional stability, as well as reliably filling thin walls with great tours.
However, hot/cold molding is gaining rapid acceptance in automotive interiors and many other consumer goods applications that require high surface aesthetics.
Let’s look at the technological options developed in making quality injection mold for high gloss parts thus;
In conventional plastic injection molding, the molten polymer enters the mold cavity and comes in contact with the coldest surface of the cavity. This instantly produces a frozen skin layer. This rapid transition slightly changes the molecular structure of the polymer, resulting in the loss of a part of the natural brightness of the material. Additionally, the polymer adjacent to the frozen skin layer cools rapidly and flows with a higher viscosity with respect to the hotter core of the molten material front, giving rise to various types of visible imperfections.
The principle of hot/cold molding technology is to raise the temperature of the surface of the mold cavity before the polymer enters the cavity, and then to cool the cavity of the mold once it is full, just as in the molding process.
Preventing the polymer surface from freezing instantly during filling allows the material to maintain its natural brightness. In addition, the melt flows with an almost uniform viscosity throughout the filling process, avoiding many of the commonly recognized surface defects associated with conventional plastic injection molding
While cold water is the common cooling medium in hot/cold molding, the manufacturer has several options in selecting the appropriate heating medium. The choice may be based on the personal preferences and financial budgets available, but the main criterion remains the temperature of the mold surface that must be reached for a given polymer in order to obtain the benefits of hot/cold molding.
Hot water pressurized as a heating medium typically requires the least investment of equipment and, in most cases, no modifications are required in the design of the mold or in the mold. The cooling channels existing in the mold are frequently used to circulate hot water in order to achieve that the temperature of the cavity reaches the desired level. The use of hot water as a heating medium makes this technology a very simple and low-cost option to perform mold tests during the development of parts and the update of existing injection molding applications in the process with heat/cold.
The main benefit of hot oil as a heating medium is its high heat transfer capacity. Also, the oil can be heated to 320 ° C or 608 ° F, making it suitable for specialty polymers such as PEEK or high carbon polymers. It is relatively easy to implement and equipment costs are comparatively low. Although the existing cooling channels in the mold could be used to circulate the oil in the development stage or prototype, a production tooling needs dedicated heating and cooling channels, so some modification of the mold is necessary.
Of all the available technologies, steam heating has been practiced for longer and offers the greatest economic benefit for most applications, especially in the production of large volume parts. The steam transports more than six times the thermal energy of the water and 18 times that of the oil, thus allowing total cycle times significantly faster than with the other liquid media. The achievable temperatures are slightly higher than those reached with water
Electromagnetic induction as a heating medium is the most recent development in hot/cold molding technology. This technology allows precise control of temperature, temperature variations in different places of the mold cavity, very fast temperature cycles and relatively high maximum temperatures (> 200 ° C, 400 ° F).