March 2011 Archives

Molding shrinkage ratio is one of the most important factors in the design of molds for plastic injection molding.

The molding shrinkage ratio α can be expressed by the following equation.

α=（L₀-L）/L₀, where,
α：Molding shrinkage ratio (no units)
L₀：Mold dimensions (mm)
L：Product dimensions（mm）

However, in actuality, since the volume shrinks when the molten plastic cools, if the concept of volume shrinkage ratio is used, it can be expressed by the following equation.

αv=（V₀-V₁）/V₀, where,
αv：Volume shrinkage ratio (no units)
V₀：Volume of the mold cavity（mm³）
V₁：Volume of the molded product（mm³）

Here, if the volumes V₀ and V₁ are considered as three dimensional objects, it is possible to express them using the following equations.

L₀=3√V0
L₁=3√V1

Therefore,

αv=（V₀-V₁）/V₀
=1-（V₁/V₀）1/3
=1-（v₁/v₀）1/3 , where,
v₁：Specific volume at room temperature （mm³/g）
v₀：Specific volume under molding conditions （mm³/g）

In addition, it is known that the state equation of the molten plastic can be expressed as follows.

（P+πi）（v-ω）=R'T, where,
v：Specific volume of the molten plastic（mm³/g）
πi：Internal pressure (atm)
ω：Specific volume at absolute zero degrees（mm³/g）
R'：R/M（Corrected gas constant atm mm³/g・mol）
M：Molecular weight in units of molecular activity

If αv =1-(v₁/v₀)1/3 is substituted in the above equation, we get:

α=1-3√y
=（1-y）/3+（1-y）2/9

y=v₀（P+πi）/R'T+ω（P+πi）

From this, it is also clear theoretically that the Pressure P and the temperature T of the molten plastic have a large influence on the molding shrinkage ratio α.

When molds for plastic injection molding are used for molding in mass production, the performance of molds decreases gradually due to wear, rust, etc. Maintenance (maintenance management) is necessary in order to repair such decreases in performance.

It is recommended to pay attention to the following points as the key points in the maintenance of molds for plastic injection molding.

1.Scratches in the pinch off surface of the core pin
2.Wear in the pinch off surface of the core pin
3.Fusing of the pinch off surface of the core pin
4.Depression in the butting surface of the core pin
5.Scratches and corrosion in the mirror finished parts
6.Peeling off of electroplating
7.Wear and deformation of the submarine gate hole
8.Wear and deformation of the pin point gate hole
9.Depression in the periphery of the parting surface
10.Wear of the ejector pin hole
11.Biting of the ejector sleeve hole
12.Soot collected in the air vent parts
13.Biting of the sliding surface of the slide core
14.Weakening of the coil springs
15.Warping and deformation f the cavity frame
16.Cracks in the corner parts of the cavity
17.Biting of the ejector guide pin and bush
18.Wear in the nozzle touching surface of the sprue bush
19.Adhesion of scale and rust in the cooling water hole
20.Water leakage in the cooling water hole

After the molds are used actually for injection molding for some time, the mold components that use carbon steels and alloy tool steels and that have been heat-treated may generate warps, deformations, or their dimensions may have increased slightly.
The phenomenon of changes occurring in the dimensions after the passage of time in this manner is called "aging".

The main cause of changes with time occurring in carbon steels of alloy tool steels is known to be the expansion in volume caused when the austenite structures remaining during quenching changes to martensite structures.

The process of quenching is that of changing the austenite structure to the martensite structure by suddenly cooling from the quenching temperature (about 800°C, but this varies depending on the type of steel).
While the sudden cooling is done using water, oil, or a salt bath, although the conversion to martensite is promoted when the temperature is less than 0C, if the temperature is not that low, a small quantity of austenite structure remains within the martensite structure. This remaining part is called "remained austenite".
It has been known that this remained austenite gradually changes to martensite structure with the passage of time as mentioned above, and at that time the volume also expands. As a consequence, if the quantity of remained austenite is large, it can be assumed that the trend is that of a large change with time.

In order to reduce the remained austenite, it is effective to carry out deep cooling treatment (subzero treatment) by creating a low temperature environment of about -80°C using a coolant such as Freon (the user of Freon cannot be recommended due to environmental protection), and to carry out sudden cooling in that environment.
If liquid oxygen or liquid nitrogen is used, although it is possible to cool down to -180°C to -190°C, since the cost becomes high this is only done under special cases.

Therefore, in the case of components that should not undergo changes with time, it is recommended to carry out subzero treatment.
However, although the remained austenite would have become small in a subzero treated steel, at the same time, since even the hardness would have increased and the internal strain would have become large, appropriate annealing processing will be required. (There are recommended values for the conditions of annealing depending on the type of steel.)