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Mold injection molding technology for hot and cold cycles

Release time:

2017-10-12

In the production process, mold temperature is an important factor affecting the quality of parts. In general, setting a higher mold temperature usually results in a higher part surface quality. Cold/hot cycle mold injection molding technology is a method of controlling the temperature of the mold by using the hot cycle during the injection cycle. This technology requires that the heating temperature of the mold surface be above the glass transition temperature (Tg) of the plastic to facilitate injection molding, and then quickly cool the mold temperature to make the part cold set for easy ejection.

The hot/cold cycle mold injection molding process can greatly improve the surface quality of the injection molded parts, and reduce the overall processing cost by reducing the secondary processing process such as painting and grinding to eliminate surface defects. In some cases, both lacquering or powder coating can be dispensed with. Hot/cold mold injection molding can also improve the surface finish of glass fiber reinforced structural materials, which is used in applications where high gloss is required for the surface of the product. Other advantages that can be obtained with this technology include reduced molding stress, reduced or eliminated defects such as swirls and weld joints, and increased melt flow length, resulting in thin-walled products.

SABIC Innovative Plastics has been working on this technology in Japan for several years, and its first application example is the processing of feet for a car roof rack rail, which used to be metal. When using Xenoy 1760 PC/PBT resin with an 11% glass fiber content for experimental processing, the surface quality was not ideal due to the appearance of swirls and weld seams. Due to the presence of fiberglass, the surface of the part is very rough and needs to be polished and polished instead of lacquered. The use of hot/cold mold injection molding eliminates surface defects in these products and eliminates the need for sanding.

For polycarbonate, PC/ABS mixtures,

and PC/PBT mixtures, injection processing using hot/cold cycle molds is very successful, and the surface quality problems are minimized in processing TV shells, condensers, car audio parts, and laptop covers.

1 How it works

Traditional injection molding machines can also utilize the hot/cold cycle mold injection molding process, firstly, a special auxiliary system device needs to be installed to achieve rapid heating and cooling of the mold surface. Superheated water and steam are required, some systems require an external boiler to produce steam, while others generate steam within the control unit. SAB's R&D center in the Asia-Pacific region uses steam, while its polymer processing R&D center in Massachusetts, USA, utilizes a superheated water system, and a unit made by the German company Single Temperiertechnik can provide superheated water of up to 200°C.

In order to effectively control the machining process, multiple thermocouples must be assembled close to the outer surface of the mold to monitor the temperature. Molds, injection molding machines, and hot/cold controls must be intelligently connected to achieve process stability. SABO has developed its own intelligent control device for each component.

In the initial stage of the injection cycle, steam or superheated water circulates and heats the surface of the mold, so that the mold temperature is about 10 °C ~ 30 °C higher than the glass transition temperature of the resin. When this set temperature is reached, the injection molding machine receives a signal to inject the plastic melt into the mold cavity. After the mold cavity is full, the injection is completed, and the cooling water circulates into the mold, and the plastic parts are quickly cooled and shaped, and then the products are ejected. There is a valve switch that alternates between steam or superheated water, or cooling water. After the part has cooled, the mold is opened and ejected, and the system transfer switch is switched back to the mold heating phase.

2 points of mold design

The success of the

hot/cold cycle mold injection molding process throughout the cycle is not only related to the processing material, but also to 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, and for heat exchange cycles, it is best to minimize the thickness of the steel. The cavity and core can be inlaid and assembled, which is better than cutting and excavating the formwork, which can reduce the thickness of the formwork. To reduce heat loss and improve thermal efficiency, these inserts can be assembled using air gaps and isolation materials as much as possible on the cavity and core support platen.

In addition to reducing the amount of mold steel, it is also necessary to consider the influence of alternating hot and cold molds, and the materials used in mold manufacturing need to have high thermal conductivity, such as copper-beryllium alloy or other alloy materials with high thermal conductivity, to shorten the time required to heat and cool the mold surface. In addition, the cooling water channel is designed close to the mold surface to speed up response time. In most cases, however, this is limited by the geometry of the plastic product. The design adopts conformal cooling method is very reasonable and effective, that is, the layout of the cooling water channel is designed according to the surface shape of the plastic parts.

There are several different process technologies for conformal cooling, such as laser sintering, direct metal plating, and so on. SABOR partnered with Fast4M Moulding to develop a layer pressing molding process called Fast-Form. This mold manufacturing technique is made by laser cutting and bonding copper with a stack of steel sheets. This process has the characteristics of low cost, convenient comprehensive use of conformal technology and through the cooling water pipeline, and design of a large exhaust area.

3. Technical advantages

Hot/cold cycle mold injection molding technology can greatly improve the appearance and texture of injection molded parts, the most prominent effect is the use of amorphous resin processing parts, including the types of resin: PC resin, PC/ABS mixture, PC/PBT mixture. When the surface temperature of the mold is higher than the Tg of an amorphous resin, the resin melt does not form a layer of skin during the injection stage, and the melt can move freely. The result: the melt does not freeze when it hits the mold surface, unlike conventional injection molding processes.

During mold filling, a thin layer of polymer melt escapes, leaving on the outer surface of the pad template, thereby increasing the gloss of the part and reducing the surface roughness. The results show that if the gloss of the part is increased by 50%~90%, for glass fiber reinforced materials, the surface roughness index-Rmax can be increased by 70%, and the index of unfilled materials is increased by 20%.

The hot/cold mold injection molding process has a positive effect on improving the width and visibility of the weld line. The results show that the width of the surface of the welding line of the products processed by the traditional injection molding process is about 6~13 microns, and the products obtained by injection molding on the hot / cold mold are completely invisible to the welding marks, and their width cannot be detected. This great advantage can eliminate secondary processing steps such as lacquering, especially suitable for special occasions.

The

residual internal stress in the injection molding can cause the part to warp and even shorten the service life of the part. Conventional injection-molded parts present with high internal stresses. Carbon tetrachloride is a solvent known to cause stress cracking in plastic parts. Plastic parts machined with hot/cold cycle molds have low internal stress and do not cause stress cracking with this solvent, eliminating the need for annealing before the part is used.