August 2009 Archives

"Elastic modulus" is a material property that indicates the strength or elasticity of the steel materials used for making mold parts. The elastic modulus is also called the "Young's modulus" usually. The elastic modulus is the coefficient of proportionality between the "strain" and the "tensile stress" when the steel material is pulled. This relationship can be expressed by the following equation.

　σ＝EXε

ε: Epsilon
σ: Sigma

In other words, "stress is proportional to strain".

The physical value of the elastic modulus is determined by the type of the metallic material. In general, a material with a larger value for the elastic modulus has a higher tensile stress or rigidity.

The data of the elastic modulus is shown in Table 1 for some typical metallic materials.

In this course, we explain the basic structure of a mold when a hole is to be prepared in a molded product. In order to prepare a hole in the molded product, it is necessary to form a part in to which plastic does not flow using the cavity or a core pin. An example of the basic structure for this is shown in [Fig. 1].

A. Touching structure

This is the most basic structure. A hole is formed by providing a projection from one side making it touch against the plane surface on the other side. A know-how, regarding the surface on the projection side that is touched, is, while preparing the core pins, to make them longer than the reference height by about 0.005 to 0.03 mm as the "compression margin". If this is done, it will be difficult for flash to form on the touching surface, and sharp edges can be obtained in the periphery of the hole in the molded product. A drawback to this structure is that, when the core pin is thin and long, the core pin gets deformed due to the filling pressure of the plastic thereby causing the likelihood of shifts in the hole position, or a bend in the hole. In addition, in some cases the core pin may bend and break due to the filling pressure.

B. Socket structure

In this structure, the tip of the projection provided from one side is made to engage in a hole provided on the other side thereby forming a hole. If this structure is used, since the core pin will have the structure of a beam that is supported at both ends, the capacity to resist the pressure of the plastic becomes better than that of the touching structure and has the effect of preventing bending and breaking of the core pin or shift in the hole position. A taper is provided on the side periphery of the tip of the projection and the mating hole so that they can mate smoothly. A drawback to this structure is that the cost of preparing the mold will be higher than that of the touching structure.

C. Structure for butting in the middle

In this structure, projections are provided from both sides so that they butt against each other in the middle. In this structure, since the total length of the core pins can be made shorter, it is possible to reduce the likelihood of breaking the core pin. A drawback to this structure is that a parting line is generated in the middle of the hole in the molded product.

D. Shut off structure

In this structure, projections are provided from both sides, and their angles are matched in the middle.

E. Structure for socket in the middle

This structure is intermediate between the structures of B and C.

In order to fill the plastic inside the cavity, it is necessary to push it inside the cavity while applying pressure from the injection cylinder. When the plastic is in the heated and molten state, it is a fluid with some viscosity. However, the viscosity starts decreasing as the plastic reaches the cavity while flowing through the sprue and the runner because it loses heat to the surface of the mold. If the viscosity decreases beyond a certain limit, the leading part solidifies due to cooling, and flowing becomes impossible thereafter.

Up to what distance the leading part can flow without cooling and solidifying? By knowing this it is possible to consider at the time of designing the mold the number and placement of gates, the placement of the runner, etc.

An index that becomes a guideline for that is the flow ratio (L/t). The flow ratio is an experimental index indicating the distance to which the leading edge of the flow can reach when a specific plastic is made to flow inside a cavity with a fixed plate thickness and at a fixed pressure.

The flow ratio is expressed, for example, as "the flow ratio (L/t) is 450 to 530 mm when POM plastic is made to flow inside a cavity with a wall thickness of 1 mm and with an injection pressure of 900 kgf/cm2".

In general, the following trends are shown by the flow ratio.

(1) The flow ratio value increases as the injection pressure increases.
(2) The value decreases as the cavity plate thickness decreases.
(3) The value increases as the cavity surface temperature increases.
(4) The value shows some fluctuation depending on the condition of the molding machine and of the mold.
(5) The flow state of a partially thin plate thickness part cannot be the target of prediction.

The flow ratios of major plastics are given below.

When a injection mold is fixed in a molding machine and molten plastic is injected into the interior of the cavity from the injection nozzle, a high filling pressure acts on the inside of the cavity. Since the parting surfaces of the mold try to expand outward due to this pressure, it is necessary to clamp the mold so that it does not open instantaneously.

It is easy to imagine that flash will be generated if the parting surfaces open even very slightly. The force of keeping the mold closed tightly is called the "required mold clamping force". The unit for the required mold clamping force is N (Newtons), or kfg, of tf.

At the time of designing a new mold, it is necessary to obtain by theoretical calculations what is the optimum required mold clamping force that the injection molding machine has to have for the mold to be installed in it. For example, if a required mold clamping force of 100 tf was obtained by calculations, if this mold is installed in an injection molding machine with a 75 tf capacity, the molded product will be full of flash thereby making it impossible to carry out the molding operation. Further, if the mold is installed in a molding machine with a 300 tf capacity, even if the molding operation is possible, since usually the hourly cost of a 300 tf machine is higher than that of a 100 tf machine, the molding operation becomes high in cost.

The required mold clamping force of a mold can be calculated using the following equation.

Here, p will have a value in the range of 300 to 500 kgf/cm2. The value of p varies depending on the type of plastic, molded item wall thickness, cavity surface temperature, molding conditions, etc. To be more accurate, it is recommended to incorporate a pressure sensor inside the cavity, and to collect guideline data from actual measured values. Also, A is the total projection area of the cavity and the runner with respect to the parting surface. Therefore, the value of A varies depending on the number of items molded and on the placement of the runner.

Example of a Calculation

Consider calculating the required mold clamping force when four molded items are obtained using PBT plastic with 30% glass fibers added.
Let us assume that the assumptions for calculation are that the pressure inside the cavity is P = 300 kgf/cm2, the projection area of one cavity is A1 = 15.3 cm2, and the projection area of the runner is A2 = 5.5 cm2.

F = p×A/ 1000
= 300×(15.3×4+5.5)/1000
= 20.01(tf)

Therefore, an injection molding machine that has a required mold clamping force of about 20 tf is required. Giving some margin, it is considered optimum to select an injection molding machine with a 25 to 30 tf rating.