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#102 Molding Conditions of an Injection Molding Machine (1)

Category : Technical Information
November18, 2011
1. Molding Conditions of an Injection Molding Machine

In order to form a molded article by actually carrying out injection molding using a plastic injection molding machine, it is necessary to carry it out in a condition in which the mold and the molded material are made to match with the specifications of the molded article. If merely a molten plastic is injected into the mold, solidified, and taken out of the mold, there will be large variations in the molded article dimensions, fluctuations in glossiness, and generation of external appearance transfer striations, and it is not possible to obtain products with a uniform quality.

In order to obtain injection molded articles with the desired quality, it is necessary to adjust various parameters called the "molding conditions" of the injection molding machine by carrying out the work of adjusting the conditions so that the specifications are satisfied while controlling the dimensions, external appearance, and quality of the molded article.

In other words, the quality of the produced injection molded article depend on whether the adjustment of the molding conditions is good or bad. If the range of this parameter adjustment of this adjustment of molding conditions is wide, the range of changing the quality specifications becomes wide, and making the adjustments becomes easy. When the usual comment that "the mold has been prepared well" is made, very often the width of such condition adjustment is wide. To put it in another way, during the design of the mold, it is necessary to give considerations so that it is possible to set wide the range of adjustment of the molding conditions. For example, the design is made so that sufficient flow rate of the cooling circuit is acquired, providing gas vents appropriately thereby making it difficult for burns to occur even when high speed filling is made, etc.

The typical molding condition parameters are explained below.

(1) Barrel temperature

The barrel temperature can be adjusted by the temperature setting of the injection barrel. The appropriate range of the barrel temperature can be set while referring to the recommended temperature range that varies with the grade of the resin and which is provided by each resin manufacturer.

It is possible to measure the actual resin temperature by inserting the thermocouple of a thermocouple type thermometer into the resin that has been injected in a purging injection.

(2) Mold temperature

The mold temperature is a very important parameter that affects the shrinkage of the molded article and the transfer of the cavity surface. It is very common to use a water cooled type temperature controller when the mold temperature is in the range of 15 to 90°C. When the mold temperature exceeds 90°C, either a pressurized water type or an oil type temperature controller is used, or the temperature control is made using a cartridge heater.

Although what is being controlled is the mold temperature, what is really important is the cavity surface temperature. In order to control this accurately, the common method is to adjust the mold temperature based on the actual value measured using a contact type surface temperature measuring instrument.

As for the method of incorporating a non-contacting type infrared temperature sensor inside the cavity, although it is very effective because it is possible to measure accurately the mold temperature in real time, it requires equipment and know how.

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