June 2012 Archives

Insert molding is a method of injection molding in which an insert (a metallic part or other inserted item) is installed inside the cavity while the mold is still open, and the mold is then closed and the injection molding is carried out.
This method is used for the injection molding of housings in which brass screws or shafts are insert molded, or for electronic components in which metal terminals or contacts are insert molded.

The simplest method of placing the inserts inside the cavity is to affix to the cavity by hand or by robots the inserts which are machined separately as individual parts (Fig. 1).

In order to manufacture a large quantity of a part efficiently, there has also been proposed a method of progressive molding in which the molding is done successively after automatically feeding into the cavity a metal hoop in which the required shapes have been formed previously by a progressive press forming die (Fig. 2).

Since the insert molding method is a method for manufacturing a molded item by a combination of a metal formed part and plastic, it is necessary to accumulate know-how by making trial and error in order to combine material with different properties such as thermal expansion coefficient, heat transfer coefficient, strength, etc.
Good injection mold design will be difficult if the accuracies and capacities of press forming are also not understood.

Insert molding is a method of molding that offers a high value addition in addition to making it possible to manufacture at a low cost various precision electronic components or excellent quality molded items for automobile parts. On the contrary, without a high level of technical ability, it is not possible to manufacture large quantities in a stable manner.
The precision insert molding technology and mold preparation technology of Japan can be said to be of the top level in the world.

In order to realize insert molding practically, the excellence in the press forming die design and fabrication technology, for example, the technology of positioning or the technology of mold part machining, etc., need to be incorporated into the plastic mold design technology in a flexible manner.

Blow molding is a method of injection molding hollow molded items such as PET (polyethylene terephthalate) bottles for juice, etc.
This method is being used widely in the world for manufacturing shampoo containers, containers for spices such as soy sauce, or containers for detergents, etc.
The plastic resin that can be blow molded are polyethylene terephthalate, polypropylene, polyvinyl chloride, nylon, polycarbonate, etc.

Blow molding is done using a special blow molding machine. And also, the mold used has only a female type cavity and there is no male type core.
Instead of that, there is a nozzle for blowing air into the mold, a pre-formed balloon shaped molded item called a parison is blown and made to contact the cavity closely thereby transferring the shape.

In the case of blow molding, usually a relatively soft metal is used for the cavity. While aluminum alloys, bronze, etc. are commonly used, special steels are also used.
Since very frequently beautiful glossiness is expected in the surface of the molded item, sufficient care has to be taken about polishing the inside surface of the cavity.

Further, multiple cavities in a mold are common in the blow molding of containers for drinks, and since a high cycle rate is expected, techniques will be necessary for the function of cooling and adjusting the temperature of the mold.

Since most of the blow molded items are of the disposable variety, it is extremely important that the product design and planning and the mold design are made considering the recycling of these products.

Special methods of plastic injection molding are explained starting from this lesson. Firstly, in this lesson we describe the two color injection molding method.

The two color injection molding method is a method of molding that is recently being called the "two material injection molding method" or the "different material injection molding method", etc. Two types of thermoplastic resin are successively injected into the mold by respectively different injection cylinders, thereby producing a product with two types of colors.

This is a molding technology that is being used widely in the preparation of key tops for high end desktop PCs, or the illuminated buttons of car navigation units, etc.
In general, it appears that very often two plastic resin of the same type such as PS plastic or ABS plastic are used. This is because there is very good adhesion between the two molded items. Although it is possible to produce molded products from two different types of plastics resin such as ABS and POM, the adhesion between them is not necessarily good. (There are different applications when the adhesion is good and when the adhesion is not good.)
In addition, recently there are some unique combinations that have been realized such as combinations of thermoplastic plastic material with a thermoplastic elastomer (rubber-like plastic resin). (Sports goods, etc.)

In order to practically use the two color injection molding method, usually, a special injection molding machine will be necessary. Such machines are being made by manufacturers of injection molding machines in Japan as well as in countries like Switzerland and Germany. The injection molding machine is equipped with two injection units, which respectively pour the molten material into the interior of the cavity of the mold via their respective sprues.
In the mold, the female part of the cavity is formed on the fixed side of the respective plastic material.
On the other hand, two male cores of the same shape are formed on the moving half, and the space between the male parts can be moved by a rotating mechanism or a sliding mechanism. (There are several types of patterns of this structure.)

In the two color injection molding method, since a beautiful multi-function molded product can be produced in one step, it is possible to produce a molded item with high value addition. It is also possible to have multiple cavities in a single shot in the case of molded items with small sizes.
However, the design of the molds requires knowledge about the design of wall thicknesses and know-how about the bonding between different plastic materials. Some techniques will be necessary regarding the temperature control of molds also.

Although it is common that the specifications of injection molding machines differ depending on the manufacturer, it is important for practical application to compare the size or dimensions and capacity of machines of different manufacturers.

It is good to check the following specifications while comparing the specifications of different injection molding machines.

• Machine name
• Screw diameter
• Injection volume
• Plasticizing capacity
• Injection pressure
• Injection ratio
• Mold clamping force
• Mold clamping method
• Mold clamping stroke
• Tie bar distance
• Daylight
• Die plate dimensions
• Machine dimensions
• Machine weight
• Drive method
• Nozzle diameter
• Nozzle tip SR
• Ejector rod diameter
• Type of associated take out machine
• Year, month, and date of purchase
• Cumulative number of shots
• Special specifications

If these specifications are entered in a table using a spreadsheet software such as Excel, at the time of designing a mold, it is becomes easy to organize the information in one's mind and to make appropriate decisions.

Since there are many items that need to be judged during the design of a mold, saving the time and effort required for "searching for information" and using that saved time for making judgments is a skilful method of using time.

In engineering plastics resin and super engineering plastics resin, the types of plastics resin in which case the surface temperature of the cavity exceeds 100°C is increasing. If the surface temperature of the cavity exceeds about 90°C, normally it becomes difficult to increase the temperature when the temperature control is being made using hot water.
In general, the following methods are used.

(1) Temperature control using oil

In the temperature control using oil, oil ejected from a re-circulating pump is re-circulated via a joint hose through a flow path provided in the mold plate and cavity, thereby maintaining a constant temperature. Once the temperature rises up to the set temperature, it is possible to maintain a relatively stable temperature. However, there is the disadvantage that it takes time for the temperature to rise. Further, there is the danger of people suffering burns while handing the oil, and also there is the problem that later treatment of the oil is cumbersome.

(2) Electric heater

In the temperature control using an electric heater (cartridge heater), the temperature is maintained constant using the heater in combination with a temperature sensor (thermocouple, etc.). Since the thermal capacity is high, there is the advantage that the temperature rise is fast.
However, the temperature near the heater is high and the temperature far from the heater is low and hence it becomes difficult to maintain a uniform temperature distribution.
In addition, since a heater has a limited life, it will be necessary to replace the heater periodically.
Mounting holes and clearances will be very important in mounting the heater. If the clearance is too much, the heater will be in the no load condition and hence its life will be shortened.
In order to carry out more accurate temperature control, it is recommended to place a heat insulating plate between the platen of the injection molding machine and the mold installation plate. Or else, it is also possible to provide heat insulating plates surrounding the mold.

Table Example of materials requiring high cavity surface temperatures

In the case of molds for the injection molding of PPS or liquid crystal polymer, since it is necessary to maintain the cavity surface temperature at more than 100°C, temperature control using oil or temperature control using a cartridge heater will be required. The temperature controlling method used very frequently with the cartridge heaters available in the market is the "ON-OFF Control" method in which the temperature is controlled by turning ON and OFF the power supply to the heater.
Since this "ON-OFF Control" can be constructed with a simple switching device, although the price of the controller is relatively low, the variations in the cavity surface temperature are large, and there is the drawback that it is difficult to stabilize the temperature.
If the cavity surface temperature is unstable, it will cause fluctuations in the condition of shrinkage, the dimensions, and the glossiness of the surface in a precision molded item.

In view of this, the "PID Control" method is recommended for stabilizing the cavity surface temperature as much as possible.
The PID Control is a method of controlling that can shorten the time required for the temperature to become stable by using controls of the following types:
Proportional
Integral
Derivative.

Cartridge heaters can have a maximum load of 10 kW with an AC three phase 200V power supply.
Therefore, as an example of use, it is possible to use two three phase 200V 1 kW heaters and to incorporate them at the movable side mold plate and the fixed side mold plate, and to carry out stable control of the cavity surface temperature using PID control.

Heat insulating plates are very important parts for stabilizing the temperature of a mold for injection molding, or for realizing energy savings while maintaining the temperature. They are used as mandatory items in the injection molding of engineering plastics or super engineering plastics.
Misumi offers heat insulating plates as a line up of products, which can be selected suitably depending on the application.
In general, the method of using a heat insulating plate can be of the following two types.
1.Using by fixing to the platen of the injection molding machine.
2.Using by fixing behind the mounting plate of the mold.

The following items are among the criteria for selecting the heat insulating plates.

1.Withstanding temperature

As a guideline, the following recommended usage temperatures are available.

• Recommended usage temperature: Up to 100°C
• Recommended usage temperature: Up to 180°C
• Recommended usage temperature: Up to 220°C
• Recommended usage temperature: Up to 400°C
• Recommended usage temperature: Up to 500°C

2.Material

The material is the factor related to the recommended usage temperature and the compression strength.
The following materials are available.

• Cotton cloth + phenol resin (Bakelite)
• Craft paper + phenol resin (Bakelite)
• Glass fiber + silicate binder
• Glass fiber + ultra high heat resistant epoxy resin
• Glass fiber + phosphate binder
• Glass fiber + borate binder

Asbestos is not handled by Misumi since it is considered to be hazardous to health.