December 2011 Archives

Thermoplastic resins melt when its temperature becomes higher than its melting point and becomes liquid, but as it is gradually cooled from the liquid state its viscosity gradually increases, and it goes into a rubbery state and it finally solidifies. The temperature at the border from the rubbery state to the solid state (called the glass state) is called its "glass transition temperature". The glass transition temperature is generally indicated as Tg.

Tg is a physical quantity that is determined by the type of plastic. By knowing the value of Tg, it is possible to estimate the time required for the molded item to cool inside the cavity and the molded item temperature at which it can be ejected from the mold.

The values of Tg are given below for the major plastics.

1. Screw Rotation Speed

The screw rotation speed in plastic injection molding is the speed of rotations of the screw for mixing the pellets. Its unit is rpm (revolutions per minute).

If the screw rotation speed is too high, air gets mixed inside the molten plastic which can make gas generation to occur easily. Also, if the screw rotational speed is too low, sufficient kneading will not be made and the material quality can fluctuate.

2. Screw Back Pressure

The screw back pressure is the pressure at the time that the screw moves back for measuring the material. Its unit is MPa or kgf/cm2. The kneading state of the material changes due to changes in the back pressure.

3. Mold Opening and Closing Speed

The mold opening and closing speed is the speed at which the mold is opened and the speed at which the mold is closed. The unit is mm/s. Although the time required for opening and closing the mold being small is effective in shortening the cycle time, if the opening and closing speed is too high, the danger becomes high of the molds colliding with each other, and hence it is necessary to put a brake just before the molds meet.

Also, if the mold opening speed is too high, the mold releasing state fluctuates which can cause fluctuations in the quality.

4. Mold Clamping Force

The mold clamping force is the force for closing the parting surface, and its unit is kN (kilo Newtons) of tf (tons force). Although this unit is usually expressed merely as "tons" while speaking, in mechanics it is correct to append the weight unit "f".

5. Ejecting Speed

The ejecting speed is the speed of forward movement by the ejector rod, and its unit is mm/s. If the ejecting speed is too high, the product may get deformed or the surface may become white. If the ejecting speed is too low, the molding cycle time becomes long.

6. Ejection Count

The ejection count or the number of times of ejection is very often set in the range of 1 to 3. Although ejecting once is enough if the mold is one with good mole release property; however, if it is bad, it may not be possible to take out the molded product from the mold unless ejection is made several times.

1. Cooling Time

The cooling time in plastic injection molding is the period of time over which the molded article is left in the mold to solidify. The cooling time starts when the dwell time ends. The unit is s (seconds).

If the cooling time is too short, the shrinkage becomes high making the dimensions small, and the molded article may also get deformed during its ejection from the mold.

On the other had, if the cooling time is too long, the molding cycle becomes longer reducing the productivity. In other words, the cost of producing a molded article increases. Since the cooling time is a factor having the highest effect among all the factors affecting the molding cycle time, shortening the cooling time becomes a very important key to shortening the molding cycle time.

2. Measuring Position

The measuring position is the position of the screw or plunger for measuring the volume of plastic necessary for injection. The unit is mm.

Fill defects will occur if the measuring position is too short. If the measuring position is too long, excess plastic will remain in the cylinder causing burning, or generating gas. The measuring position should be set to the appropriate value considering the shot volume required for the molded item so that it is neither too small nor too large.

3. Speed - Dwell Swictchover Position

This is the screw position for switching between the injection speed and dwell, and its unit is mm. When this position is changed, there will be fine changes in the external appearance and the dimensions of the molded article.

4. Screw Forward Position

This is the parameter that controls the position to which the screw advances to the most forward position after injection is completed. Its unit is mm.

This is also a factor that controls the amount of cushion. The amount of cushion is the molten plastic remaining inside the cylinder at the tip of the screw after the screw has moved to the most forward position. If the amount of cushion is small, the dwell pressure will not act properly.

If the amount of cushion is too much, the material remains and gets the effect of heat thereby deteriorating.