November 2011 Archives

1. Filling Pressure

Among the plastic injection molding conditions, the filling pressure, also called the primary pressure, is the pressure required for filling the inside of the mold with plastic. The unit of filling pressure is MPa or kgf/cm². That is, it is the unit of pressure. There are also injection molding machines that adjust the condition by changing what percentage of the maximum filling pressure of the machine is to be used as the filling pressure (for example, 35%).

Filling defects (short shot) occurs when the filling pressure is too low. When the filling pressure is too high, the parting surface opens instantaneously due to the pressure, which may cause flashes (burrs) to appear on the periphery of the molded item. Further, there may be defective releasing of the molded item from the mold.

2. Injection Speed

The injection speed is the speed of the molten plastic when it is being injected into the mold. The unit of this speed is mm/s.

This is also indicated as the injection rate which is the injected volume per unit time. The unit in this case will be cm³/s.

Filling defects can occur when the injection speed is too low, and burrs and jetting (the defect in which corrugated pattern appears on the surface of the molded item) can occur when the filling speed is too high.

3. Dwelling Pressure

The dwelling pressure is the pressure that is applied during the period after the entire cavity has been filled with molten plastic until the gate has been sealed (the gate has solidified) so that the plastic does not flow back from the gate. Its unit is MPa or kgf/cm².

When the dwelling pressure is too low, the plastic flows back from the gate causing the surface of the molded article to get dented (sink), or the shrinkage becomes too large making the dimensions smaller.

When the dwelling pressure is too high, the dimensions may become too large, or burrs can occur, or there may be defective releasing from the mold.

The dwelling time is very important in making the dwelling pressure have the appropriate effect, and it is very important to understand that the dwelling pressure must not be released until the gate has been sealed.

4. Injection Time

Generally, the sum of the fill time and the dwelling time is called the injection time. The dwelling time is adjusted by changing the injection time. The unit is s (seconds).

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.

1. Power Source of an Injection Molding Machine

The following three types are the most frequently used power sources of injection molding machines.

1) Hydraulic type

2) Electric motor driven type

3) Hybrid type

The hydraulic type is the one that has been used most often so far ever since the injection molding machine was developed. The mold is opened and closed by the hydraulic pressure of the working fluid supplied by driving a hydraulic pump by an electric motor, thereby operating the mechanism during injection. While this method has advantages such as it is possible to achieve high mold clamping forces relatively easily, it also has drawbacks such as it is difficult to carry out precision control, it is difficult to operate several actuators in parallel at the same time, etc.

The electric motor driven type uses a method of driving using an AC servomotor under computer numerical control (CNC). In this method, it is possible to realize accurate operation with no use of operating oil, and also, it is possible to carry out clean and silent operation. There is also the advantage over the hydraulic type that the running cost is low because the electric power consumption is about 40 to 50% of the hydraulic type. Recently, electric motor driven types with mold clamping forces up to 2000 tons haven been developed, and this method will become the major one from now on at mold clamping forces of 350 tons or less.

The hybrid type is one in which both the hydraulic method and the electric motor driven method driving the machine are used, with the intention of obtaining the advantages of both methods. The general pattern is to make either the injection side or the movable side hydraulic type.

4) Electric Control Device

A control computer is used for carrying out process control for controlling the operation of the injection molding machine and the molding conditions, and for carrying out feedback control using sensor detection, and a control device is used for controlling the screen display, etc. The molding conditions, etc. can be recorded in a computer, and it is also possible to record them in an external computer connected via a LAN, or in an external memory.

1. Injecting Device
   The injecting units of a plastic injection molding machine is a unit where an injection plunger or a screw moves forward inside a cylinder to inject molten plastic into the mold.
   The performance of an injecting unit is expressed by the injection rate (the volume of molten plastic that is injected in one second, cm3/s) or the injection speed (the speed of forward movement of the plunger, mm/s).
   
   In the case of super engineering plastics, there are some materials that cause loss of molded article quality unless an injecting device with a high injection rate is used, or on the other hand, there are also plastics for which the suitable injecting devices are those that can inject at a low speed in a stable manner.
   
   It can be said that the type of injecting unit used is closely related to the molding material to be used.
   
   The tip of an injecting unit is provided with a nozzle, and normally the tip of the nozzle is machined to a spherical shape so that it can contact closely with the spherical seat of the sprue bush of the mold, and considerations have been given so that there is no leaking of the plastic during injection.  There are many types of nozzles, such as the open nozzle, the shut off nozzle, or the nozzle with a needle valve, etc.
   
   
2. Mold Clamping Unit
   The mold clamping unit is a unit in charge of installing the mold in the injection machine, and of opening and closing the mold.  It is necessary to clamp the mold under a high pressure so that the mold does not open when the molten plastic is being injected into the mold.  Ordinarily, the size of an injection molding machine is expressed in terms of the claming force.  For example, "a molding machine with a mold clamping force of 160 tons", etc.
   
   The methods of clamping a mold can be classified into the following two types.
   
   1) Direct pressure type
   2) Toggle type
   
   The direct pressure type is one which the platen on the moving half is operated through a hydraulic cylinder for mold clamping.  In this method, since it is possible to clamp the mold with a constant pressure applied over a wide area, this method is suitable for the molds of molded articles having a large projected area.  In addition, the adjustment of the clamping force is easy, and there is the advantage that the work of installing the mold is not cumbersome even if the thickness of the mold changes.
   
   The toggle type is one which uses the clamping force when a toggle link mechanism extends or contracts.  This type has a high speed of opening and closing, and while it has the effect of shortening the cycle time, it takes time for set up because the work of adjusting the clamping force has to be done for each mold.
   
   A mold protecting device is provided in recent mold clamping units.  There are also units in which mold clamping is done at a low pressure up to a certain mold clamping process, and a protective device is provided that has a sensor that stops high pressure mold clamping if, by chance, any foreign matter or a runner, etc., has entered between the molds.
   
   Normally, a mold clamping unit is also provided with a molded item ejecting unit which has a structure in which a knock-out bar moves forward from the moving half and causes the ejector plate of the mold to move forward thereby ejecting the molded article.