Mold injection molding technology for hot and cold circulation

- Jan 16, 2018-

In the process of injection molding, mold temperature is an important factor affecting the quality of parts. Generally speaking, higher mold temperature can usually achieve higher surface quality. Cold/hot circulating mold injection molding technology is a method to control mold temperature in the process of injection molding. The technology requires that the heating temperature on the mold surface should be above the glass transition temperature (Tg) of the plastic, so as to facilitate injection molding, and then rapidly cooling the mold temperature, so that the parts are cold and solid so that they can be ejected.

Hot/cold cycle mold injection molding process can greatly enhance the surface quality of injection molding parts, also reduced by painting, grinding to eliminate surface defects such as secondary processing process, thereby reducing the manufacturing cost. In some cases, painting or powder coating can be eliminated. Hot/cold mold injection can also improve the surface finish of fiberglass reinforced structural materials, and used in occasions where high gloss is required on the surface of the product. Other advantages that can be obtained by using this technology include: reducing the molding stress, reducing or eliminating the defects such as the swirl and weld seams, and improving the flow length of the melt, producing the thin wall products.

1. Working principle

The traditional injection molding machine can also use the hot/cold circulating mold injection molding process. First, a special auxiliary system installation is needed to realize rapid heating and cooling on the surface of the mold. There is a need for hot water and steam. Some systems require a boiler to produce steam, while others form steam in the control unit.

In order to control the process effectively, several thermocouples must be assembled on the outer surface of the mould to monitor the temperature. The mould, injection molding machine, hot/cold control device must require intelligent connection, so as to realize the stability of machining process.

In the early stage of the injection molding cycle steam or superheated water cycle flow, heating surface of mould, mould temperature higher than the glass transition temperature of the resin around 10 ℃ ~ 30 ℃. When the setting temperature is reached, the injection molding machine obtains the signal, and the plastic melt is injected into the cavity. After the cavity is filled with the injection, the cooling water circulates into the mold, and the plastic parts are quickly cooled and finalized, and then the products are pushed out. There is a valve switch that turns into steam or over hot water, or cooling water. After the parts are cooled, open the mold and push out the parts, and the system switch is turned into the mold heating stage.

2. Key points of mold design.

The factors that determine the success of the hot/cold circulation mold injection molding process in the whole cycle are not only related to the processing materials, but also the design style and structure of the mold. The time required to heat and cool the mold is determined by the thickness of the steel. For the heat exchange cycle, it is best to minimize the thickness of the steel. The mold cavity and core can be assembled by inlays, which is better than cutting the excavation in the template so that the template thickness can be reduced. In order to reduce heat loss and increase thermal efficiency, these embedded parts can be assembled with air gap and isolation material as far as possible in the shape cavity and core.

In addition to the need to reduce the mold steel consumption, but also need to consider the effect of mould heating, mould manufacturing material should be used in the high thermal conductivity, such as copper beryllium alloys or other have higher thermal conductivity of the alloy material, to shorten the time needed for heating and cooling mould surface. In addition, the cooling water channel is designed to be close to the surface of the mold to speed up the response time. In most cases, however, this will be constrained by the geometry of the plastic products. The design adopts the method of preserving cooling, which is very reasonable and effective, that is, the layout of the cooling water pipe is designed according to the surface shape of the plastic parts.

3. Technical advantages

Hot/cold cycle mold injection molding technology can greatly improve the appearance of the injection molding parts, the most prominent effect is to use the amorphous resin processing components, including the types of resin are: PC resin, PC/ABS blends, PC/PBT, mixing. When the surface temperature of the mold is higher than the Tg of some amorphous resin, the resin melt will not form a layer in the injection stage, and the melt can move freely. The result is that when the melt touches the mold surface, it does not freeze, which is different from the traditional injection molding process.

When filling the mold, a thin layer of polymer melts away, leaving the outer surface of the mat template, thereby increasing the gloss of the parts and reducing the surface roughness. Research shows that if the parts gloss increased by 50% ~ 90%, and for glass fiber reinforced material, the surface roughness index - Rmax can be increased by 70%, compared to not fill material index increased by 20%.

Hot/cold mould injection molding process has a positive effect on improving the width and visibility of the weld mark. Once in the same mould for three kinds of different materials processing contrast tests, the results show that the traditional injection molding process of the products, the surface of the weld mark width between 6 to 13 micron, hot/cold mold injection molding have been the products completely invisible weld mark, and its width cannot be detected. This huge advantage can be saved by painting and other secondary processes, especially for special occasions.

The internal stresses in the product can cause the parts warping and even shorten the service life of the parts. The internal stress of traditional injection molded parts is very high. Carbon tetrachloride is a known solvent that causes stress cracking in plastic parts. Hot/cold cycle of mould processing plastic parts with low internal stress, stress cracking, using this solution will not cause components to save parts require annealing treatment process before use.