January 2011 Archives

There are more and more engineering plastics and super engineering plastics for which the cavity surface temperatures exceeds 100°C.
When the surface temperature of the cavity exceeds around 90°C, normally, raising the temperature becomes difficult when the temperature control is done using water (hot water).
Generally, the following methods are used.

(1) Oil temperature control

In temperature control using oil, the temperature is maintained constant by re-circulating oil from a circulating pump by passing it through joints and hoses and a flow path provided in the mold plate or cavity. Once the temperature rises up to the preset temperature, it is possible to maintain a relatively stable temperature.
However, there is the drawback that raising the temperature takes time.
In addition, there is the danger of burns in handling the oil, and also the after-treatment of oil is cumbersome.

（2）Electric heater control

In temperature control using an electric heater (cartridge heater), the temperature is maintained constant by using the heater in combination with a temperature sensor (thermo couple, etc.). Since the thermal capacity is high, there is the advantage that raising the temperature is fast.
However, the temperature in the vicinity of the heater is high and the temperature becomes low at locations distant from the heater, and it is difficult to maintain a uniform temperature distribution.
In addition, the life of the heater is limited and it needs to be replaced periodically.
The mounting holes and the clearance are very important in installing a heater. If the clearance is large, the heater gets heated excessively and hence its life gets reduced. The Misumi M-HTM3021 (Mold temperature controller for cartridge heater) can control the heater temperature using the PID control method, and extremely stable temperature control becomes possible more than an ON-OFF control type controller,
In order to carry out temperature control more accurately, it is recommended to install a thermal insulating plate between the platen of the injection molding machine and the installation plate of the mold. Or else, it is also possible to install thermal insulating plates in the periphery of the mold plate.

Table: Examples of molding materials with high cavity surface temperatures

Electric injection molding machines are coming into wide use in the case of injection molding machines with mold clamping forces of 1800t or less. In order to bring out to the maximum extent the excellent features possessed by fully electric injection molding machines, it is very important to carry out mold design giving attention to the following factors.

(1) Accurate positioning

In a fully electric driven injection molding machine, although it is possible to carry out at high speeds the opening and closing of molds, in order to prevent breakage or abnormal wear at the time of contact between core pins while carrying out accurate positioning of the cavities on the movable side and on the fixed side, it is recommended to install precision positioning guides.

The recommended Misumi standard components are:

	-	Tapered tweezers TPN, etc.
	-	Tapered block sets TBS, etc.
	-	Side straight block set TSSB, etc.

(2) Smooth operation of the ejector plate

In a fully electric motor driven injection molding machine, although it is possible to carry out operations of ejector plates such as high speed operation and multiple stage ejection, if the construction is not designed so that the operation of the ejector plate is smooth, biting of the ejector pin or biting of the return pin are likely to occur.
Therefore, it is recommended to use an ejector guide system.

The following Misumi standard components are being used widely:

	-	Precision ejector guide pin EGH, etc.
	-	Ejector guide bush EGBH (Plain type) 　　　　　　　　　　　　EGBZS (Copper alloy non-lubricated type) 　　　　　　　　　　　　EGBL (Linear type) 　　　　　　　　　　　　EMBS (Ball slider), etc.

Further, it is recommended to install a switch for confirming the returning of the ejector plate.

For this purpose, the following are recommended:

	-	V-156-1A5-T (Micro switch)
	-	MGXL-8F (Contactless type)

(3) Smooth operation of the slide core

Smooth operation of the slide core is also important. In order to prevent biting, it is recommended to provide a lubricating groove in the guide rail, or to use a non-lubricating type material (oil free metal).

The following Misumi standard components are recommended:

	-	GR5SCM (Guide rail with groove)
	-	SRTL (Non-lubricating guide rail)
	-	CGZ (Non-lubricating center rail)
	-	STW (Non-lubricating slide plate).

(4) Stabilization of the cavity surface temperature

In order to stabilize the molding conditions, it is very important to stabilize the mold cavity surface temperature. In order to achieve this, it is recommended to use the following standard components:

	-	HIP (Thermal insulation plate), etc.
	-	M-HTM3021 (For cartridge heaters)
	-	Mold temperature controller: PID control type