Influence of Mold Temperature on Injection Molded Products

Posted on : Aug 10 , 2022 By  GREFEE

The mold temperature is an important variable in the injection molding. No matter what kind of plastic injection molding, It must be ensured that the surface of the forming mold is basically wet. A hot mold surface render the plastic surface in a liquid status for a long period of time, and is fully enough to produce pressure inside the cavity. If the cavity is filled before before the frozen epidermis hardens, the cavity pressure can place the soft plastic onto the metals to produce a high cavity surface. On the other hand, if the plastic stops in the halfway after entering into the cavity under a low pressure, slight contact with metal will cause stains, sometimes called gate stains, no matter how short the time is.

For each kind of plastic and plastic injection mould part, there is an upper limit for the mold surface temperature. It will cause one or multiple negative influence if exceeds this limit (for example: burrs on the assembly parts). Higher mold temperature indicates lower flow resistance. In many kinds of injection molding machines, this means the plastic flows through the gates and cavities faster. Since the used injection flow control valve cannot correct this change, the faster filling will cause higher effective pressure inside the sprue and cavity, and possibly lead to material overflow and burrs. Since the hotter model does not freeze the plastic that enters the overflow edge area before the high pressure is formed, the molten material can overflow the burr around the ejector rod and overflow into the gap of the dividing line, which indicates a better injection rate control. However, some modern flow control programmer can also achieve it and is widely used in injection mould manufactures different places in the world, such as America, German, China, etc. Generally, the increase of the mold tempera will reduce the condensing layer of plastic on the cavity surface, increasing the flowability of the melts inside the cavity to obtain a larger weight of part and better surface quality. Meanwhile, the increase of mold temperature will also improves the tensile strength of the parts.

Mold insulation methods

For many kind of molds, especially for the engineering thermoplastics, work under the extreme high temperature condition, such as 80 ° C or 176 ° f. Without insulation, the heat lost to the air and the injection molding machine can easily be as much as that lost by the injection cylinder. Therefore, the mold should be insulated from the machine plate. If possible, insulate the mold surface. If it is the hot runner mold, try to reduce the heat exchange between the hot channel part and the cooled injection molded parts, which can reduce the heat loss and pre-heat time.

Temperature control of plastic mold

1.The necessity of the temperature control

(1).  The temperature control is of great significance to the molding, and molding appearance, physical properties of material, molding circulation, etc.  The effect of mold core temperature is quite significant. The general molding condition, the core temperature should be low because it can increase the times of injection. However, the forming cycle is associated with the shape of the product (mold core structure) and the type of finished material also depends on the need to see if there is a need to increase the mold core filling temperature.

(2). To in case the temperature control of stress is the problem of forming material, the only requirement for this is the cooling speed. Short cooling time that is some are hardened and some are soft, which can also avoid stress due to the uneven shrinkage, which is proper temperature control can improve the properties of cooling stress.

(3). Adjustment of crystallization degree of formed materials indicates the by the temperature control of the polysulfide (nylon), polyacetate, polypropylene, and the crystallization degree of other crystalline materials, as well as mechanical properties improvement, requiring a higher mold core temperature.

2. Technological problem

(1).  The heat transfer area required for temperature control is calculated as: t1: melting temperature of the formed material

T0: temperature when taking out the formed part

Cp: specific heat of forming materials

Sh: number of forming times per hour

Moving heat：Q=shx*cp*(t1-t0)kacl

Hrhw: surface film heat transfer coefficient on the cooling pipeline side

d: Diameter of cooling hole (m)

u: Viscosity (kg/mses)

μ : Flow rate (m/ses)

λ : Thermal conductivity of refrigerant (kcal/m2 HRC)

Δ T: average temperature difference between the model and the cooling (heating) medium is Hw: λ d(dug/ μ) (cp u / λ) (kcal/ hr℃)

The required heat transfer area can be obtained from the following formula, a=q/hw x T (M2)

At this time, the heat release of the outside air, the heat transfer of the template, nozzle can be ignored.

(2). Distribution of cooling pipes

Although there are various factors to shorten the cycle time of forming, the prototype manufacturing with excellent cooling effect is a major problem. Uneven cooling and rapid cooling will cause stress, deformation and cracking in the formed parts. Therefore, it is necessary to consider the mold kernel structure according to the hole shape and meat thickness, so as to achieve uniform and efficient cooling performance. Moreover, the number and size of the pipeline are based on the comprehensive consideration of the processing occasions of the model pipeline.

Shown as figure 1, occasions with the same area of formed products, mold core (a) has 5 large pipes, and mold (b) has 2 small pipes. Comparing with each other, according to the drawing of the heat transfer path, the cavity surface of mold (a) has almost equal heat transfer and uniform cooling effect, which is more applicable. Figure 2 shows the approximate isothermal curve of the temperature change between the surface of V-shaped cavity tube and pipeline. The temperature change of large pipes in Figure (a) shows the temperature of circulating water in the pipe is 59.83 ℃, and the temperature of cavity surface operation circulation is 60.5 ℃ of 60 ℃. While in pipe (b), the temperature of circulating water is 45℃, and the temperature change in the cavity surface is from 53.33℃ to 60℃. Even though the large temperature change associated with the surface of the mold core can also be used as a sufficient forming condition, but the temperature control of the mold core is improper. Moreover, the high heat conduction rate indicates that a good efficiency of the thermal conduction system. A good control of the heat transfer indicates that the temperature change on the core surface is small, or low conduction rate will lead to a big temperature change.

Generally, consideration for the thermal cooling system are:

(1).  several through water holes are designed in the heavy mold. The cold water first enters area that around the injection channel, and then the warm water circulates to the outside (Figure 3)

(2).  When using polyethylene, due to its large forming shrinkage, so the cooling pipeline should not be set along the shrinkage direction, resulting in deformation.

(3). Heart-shaped cooling (pic 4) set the pipeline along the heart-shaped profile as much as possible.

Introduction to the physical properties of plastic materials

(1).  It is light, with a specific gravity of 0.9 ~ 2.3, and can be reduced to 0.01 if foamed. (much smaller than aluminum 2.7 and iron 7.8)

(2). The material can be melt under 30 degrees in flow status without post-treatment (the melting point is much smaller than aluminum 600 degrees and iron 1530 degrees).

(3). Properties can be changed according to needs. Different monomers can synthesize non graphic plastics. Different plastics have composite properties, of which some properties can be improved by adding fillers or reinforcements.

(4). Good chemical stability and corrosion resistance, and is not corroded by acid, alkali, oil, salt, water, gas and steam.

(5). Good light characteristics and the PS, PMMA, PC transparent are easy coloring. It can replace fragile glass that is not easy to form. Plastic color particles can get beautiful colors。

(6). Good insulation with adjustable resistance to electricity of 5 ~ 50 kV / mm, and can conduct magnetism by adding conductive filler.

(7). Good friction performance and wear resistance, which is a natural bearing material.

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