Mold injection molding technology for hot and cold cycle

During the production process, the mold temperature is an important factor affecting the quality of the parts. In general, setting a higher mold temperature generally results in a higher surface quality of the part. Cold/hot cycle mold injection molding is a method of controlling the mold temperature by thermal cycling during the injection molding cycle. This technique requires that the surface of the mold be heated above the glass transition temperature (Tg) of the plastic to facilitate injection molding, and then the mold temperature is rapidly cooled to cool the part to facilitate ejection.

The hot/cold cycle mold injection process can greatly improve the surface quality of the injection molded parts, and reduce the overall processing cost by reducing the secondary processing steps such as painting and polishing to eliminate surface flaws. In some cases, lacquering or powder coating can be omitted. Hot/cold molding can also improve the surface finish of glass reinforced structural materials for applications where high gloss is required on the surface of the article. Other advantages that can be obtained with this technique include: reducing molding stress, reducing or eliminating defects such as swirling, weld seams, and increasing the flow length of the melt to produce thin-walled articles.

SABIC Innovative Plastics has been working on this technology for several years in Japan. Its first application example was the machining of the legs of a car roof rack rail. In the past, metal feet were used. When the Xenoy 1760 PC/PBT resin with a glass content of 11% was used for the experimental processing, the surface quality was not satisfactory because of the occurrence of swirls and weld seams. Due to the presence of glass fibers, the surface of the part is very rough and requires a secondary treatment of sanding and polishing instead of lacquering. The use of hot/cold injection molding eliminates surface defects in these products and eliminates the need for a sanding process.

For polycarbonate, PC/ABS blends, PC/PBT blends, injection processing using hot/cold cycle molds is very successful, for processing TV housings, concentrating panels, car audio components, laptop covers, etc. All have achieved surface quality problems to a minimum.

1 working principle

Conventional injection molding machines can also utilize hot/cold cycle mold injection molding processes. First, a special auxiliary system device is required to achieve rapid heating and cooling of the mold surface. Superheated water and steam are required. Some systems require an external boiler to generate steam, while others form steam in the control unit. SAB's R&D center in the Asia-Pacific region uses steam, while the Polymer Processing R&D Center in Massachusetts, USA, utilizes a superheated water system. The device from Germany's Single Temperiertechnik can supply up to 200 °C. Hot water.

In order to effectively control the process, multiple thermocouples must be placed next to the outer surface of the mold to monitor the temperature. Molds, injection molding machines, and hot/cold control devices must require intelligent connections to achieve process stability. SABIC has developed its own intelligent control unit for each component.

In the initial stage of the injection cycle, steam or superheated water circulates to heat the surface of the mold so that the mold temperature is higher than the glass transition temperature of the resin by about 10 ° C to 30 ° C. When the set temperature is reached, the injection molding machine obtains a signal to inject the plastic melt into the mold cavity. After the cavity is filled, the injection is completed, the cooling water is circulated into the mold, the plastic parts are rapidly cooled and shaped, and then the product is ejected. There is a valve switch that turns to convert steam or superheated water or cooling water. After the part has cooled, the mold is opened and the part is ejected, and the system transfer switch is re-turned to the mold heating stage.

2 points of mold design

The factors that determine the success of the hot/cold cycle mold injection process over the entire cycle cycle are not only related to the material being machined, 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 it is preferable to minimize the thickness of the steel for the heat exchange cycle. The cavity and core can be assembled in a way that is better than cutting and excavating on the template, thus reducing the thickness of the template. In order to reduce heat loss and improve thermal efficiency, these inserts can be assembled using air gaps and insulating materials on the cavity and core support template.

In addition to reducing the amount of mold steel, it is also necessary to consider the influence of the alternating heat of the mold. The material used in the mold manufacturing needs to have a 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 to be close to the mold surface to speed up response time. However, in most cases this will be constrained by the geometry of the plastic product. The design is conservative and effective, that is, the arrangement of the cooling water pipe is designed according to the surface shape of the plastic part.

There are several different process technologies for conformal cooling, such as laser sintering, direct metal plating, and the like. SABIC worked with Fast4M Molding to develop a layer press process called Fast-Form. This mold manufacturing technique uses a stack of steel sheets that are laser cut and bonded to copper. This kind of process has the characteristics of low cost, convenient comprehensive use of shape-preserving technology, continuous cooling water pipeline, and design of a large area of ​​exhaust area.

3 technical advantages

Hot/cold cycle mold injection technology can greatly improve the appearance of injection molded parts. The most outstanding effect is the use of amorphous resin processing parts, including resin types: PC resin, PC / ABS compound, PC / PBT mixed material. 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 phase, and the melt is free to move. The result: it does not freeze when the melt hits the surface of the mold, which is different from the traditional injection molding process.

During filling, a thin layer of polymer melt escapes and remains on the outer surface of the mat 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% to 90%, the surface roughness index -Rmax can be increased by 70% for the glass fiber reinforced material and by 20% with respect to the index of the unfilled material.

Hot/cold mold injection molding processes have a positive impact on improving the width and visibility of weld lines. The processing and comparison tests of three different materials were carried out on the same mold. The results show that the products processed by the traditional injection molding process have a weld line width of about 6 to 13 microns on the hot/cold mold. The products obtained by injection molding were completely invisible to the weld line and could not be detected in width. This huge advantage eliminates the need for secondary processing such as painting, and is especially suitable for certain special occasions.

The residual internal stress of the injection molding in the product can cause the component to warp or even shorten the service life of the component. Conventional injection molded parts have high internal stresses. Carbon tetrachloride is a known solvent that causes stress cracking in plastic parts. The plastic parts processed by the hot/cold cycle mold have low internal stress, and the use of such a solvent does not cause stress cracking of the parts, thereby eliminating the annealing process required before the parts are used.

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