July 2011 Archives

After a plastic injection mold has been machined using milling, electric discharge machining, or wire cut machining, the surface of its cavity has to be finished mainly by human hands or by polishing using a machine so that the surface is made extremely smooth.
If the condition of polishing is not proper, the glossiness decreases of the transferred surface of the molded item, and the quality of the surface becomes poor. In addition, at the time that the molded item is separated from the cavity, the undulations in the surface become undercuts which may cause defects in separating the molded item from the mold.

The polishing of the cavity surface is done using a hard grinding stone or abrasive powder. Grinding stones or abrasive powders are used successively starting from those with a large grain size and ending with those a small grain size. The polishing work is done while applying cutting oil so that there is no clogging or biting.
The direction of polishing is not only one fixed direction, but the polishing direction is changed frequently such as in the left to right direction, top to bottom direction, or circular direction. Considerations are given so that the polished surface is uniform.

The following are the main polishing materials used.

The granularity of polishing materials can be in the range of #10 to #3000. As the number becomes smaller, the grain becomes rougher.
(Example: The finish will be rough when polished with a grain size of #10 to #30, and the finish will be fine and smooth when polished with a grain size of #1000 to #2000.)

When using a fine grinding stone, the polishing is done by mixing olive oil or a vegetable oil to a wooden material such as maple, pine, willow, balsa, etc.

Pellets of plastic resin generally absorb the moisture in the air to some extent.
If the amount of moisture absorbed is high, the plastic resin can undergo hydrolysis in the process of being melted and kneaded inside the cylinder of the injection molding machine (there are also plastics that undergo chemical dissociation with water as the trigger), or silver-colored lines (silver streaks) appear on the surface of the molded item at the time of injection molding, or defects such as air bubbles, insufficient glossiness, transfer defects, etc. can occur.
In view of this, it is necessary that moisture in the pellets of the molding material is removed beforehand by putting them through a drying unit.
If this preliminary drying is not done properly, it can cause variations in the fluidity or reduction in the material properties, and molding defects.

The following are the main types of drying units used for this purpose.

(1) Hot air dryer
The hopper dryer and the box type drying oven are typical of this type dryer. This is a method in which the moisture content is evaporated by blowing hot air on the pellets.
Although this is very common and simple drying method, it is not suitable for thoroughly removing the moisture content.

(2) Dehumidified hot air dryer
This is the method in which the moisture content in the pellets is evaporated by blowing a hot air the moisture content in which has been removed in advance.
Since the hot air used for drying can be used again after dehumidifying it again, heat loss is low, and it is possible to carry out rational drying.
This method is suitable for drying PBT, etc.

(3) Reduced pressure heat transfer type dryer
This is the method of evaporating the moisture content in the pellets by heat transfer in a reduced pressure environment. Drying becomes possible at a low temperature, and it is possible to prevent oxidization of the plastic and to reduce the effect of additives inside the pellet.
In addition, this is also a method in which heat loss is small.
This type of dryer is attracting a lot of attention as the drying unit of the future.

* Reference: "An Encyclopedia of Injection Molding", 'Peripheral and Auxiliary Equipment', p. 214, (Hideki Kubo, Industrial Research Center of Japan, (2002)) (in Japanese)

A number of plastics are used for food containers. PET bottles, food cups, and food packaging are mostly made of plastics.
Quality control of food containers is necessary to ensure that they are hygienic and that they do not have any flash that can hurt fingers or lips so that people can feel safe about consuming food from the containers.
In addition, even quality defects such as holes or cracks are not allowed because bacteria can penetrate through them and cause the food to rot.

Some typical food containers are the following.

- Margarine container: PP
- Ice cream container: HIPS, HDPE
- Lactobacillus beverage (fermented milk drink) container: HIPS
- Pudding container: PS, PP
- Snacks container: HIPS, PP, HDPE
- Soft drinks bottle: PET

Depending on the usage of the food item, the container will have to have heat resistance (for foods that are heated and consumed), low temperature resistance (for foods consumed after refrigerating or freezing), or gas barrier ability (for food items that are to be stored so that they do not come into contact with oxygen).

Since the sales of food containers is influenced greatly by the design (treatment for beautiful appearance) of the molded item, three dimensional curved shapes and designs are given importance. Therefore, three dimensional solid data becomes important in the design of the molded item and in the mold design.
In addition, since there is the risk of flash being generated at the parting surface of the mold, it is necessary to give sufficient considerations to the parting surface.
Similarly, even the position and method of providing gates are also important.

It is necessary that the cavity and the core are made of a steel material having very good corrosion resistance. Also, since it is desirable that the mold is not coated with grease or oil for promoting sliding, the recommended mold structure is one which can operate safely without any lubrication.
Electrical motor driven type injection molding machine is preferable, and it is still better that the molding and machining is done in a clean room.
Valve gates and hot runners can be used. If large volume production is expected to grow, the effect of eliminating scrap and high cycle speeds are extremely advantageous.

Plastics are used a lot in medical parts. Injection molding is used widely as the method of their manufacture.
Therefore, it can also be said that a substantial part of the medical parts are being manufactured using molds.

Some typical medical parts made of plastic are the following.

1. Cylinders of syringes (injection tube): PP, PE
2. Pistons of syringes: PP
3. Pipette tips: PP
4. Catheters: PVC, PC
5. Blood collection test tubes: PC
6. Petri dishes for culture growth: PS

The plastic materials used for medical parts, depending on the application, can only be used if they satisfy the quality standards of laws related to pharmaceuticals and drugs or of regulations of related governmental ministries or organizations. The use of these materials is limited to plastics that can withstand sterilization processing by irradiation with ultraviolet rays or gamma rays, and to plastics that have passed the clinical approval tests such as blood coagulation reactions or allergy reactions, etc., on human body.
In addition, since these devices are basically of a disposable type, the materials should also be those that do not destroy the environment when disposed off by burning.

It is recommended that molds are manufactured using materials that are resistant to rusting. Therefore, structures preventing corrosion of the cavity and core are used widely such as using stainless steel or ion plating coatings, hard chromium electroplating, etc.
Further, valve gate molds are also used widely for molded products that are manufactured in very large quantities.

Since molded products become quality defects if burrs are present on the periphery of the molded product, precision products are used for the positioning guides on the movable side and fixed side, and also various techniques are adopted for preventing abnormal wear of core pins of various structures.

In order to stabilize the quality of molded products, since it is necessary to design delicately even the mechanisms for the temperature control of cavities and cores, it is necessary to pay attention to the structures of the cooling circuits and heat pipes. The cooling structure on the interior of the core becomes extremely important in the case of the tubes of syringes, etc.

In terms of quality control, removing any dirt or soot adhered to the surfaces of molds is also a very important point. Setting up of gas vents and forced suction structures are also being used.

Plastic magnets are molding materials produced by mixing magnetic powders in a plastic resin or an elastomer. Using injection molding, it is possible to manufacture magnetic molded products with a high degree of freedom in the shape.
Compared to sintered magnets, it is possible to produce products that are lightweight and have shapes with thin walls. However, the magnetic force is larger in sintered magnets.

The following types of magnetic powders are used.

(1) Ferrite type
- Barium ferrites
- Strontium ferrites

(2) Rare earths type - Samarium - cobalt
- Samarium - iron - nitrogen
- Neodymium - iron - Boron

Further polyamides (nylon) such as PA6, PA66, PA12, etc. are used as binders.
In some cases, PPS (polyphenylene sulfide) and PVC (polyvinyl chloride) are also used as binders.

In the case of plastic magnets, in some cases the devices for generating magnetic fields are incorporated in molds for giving polarities to the molded product inside the mold.
There are many methods for this such as having a structure in which permanent magnets are embedded inside the cavity, strong magnetic fields are generated using coils, or receiving the supply of magnetic fields from the molding machine, etc.
Therefore, for all components of molds, it is necessary to separate the use of magnetic materials (steel, nickel, etc.) and non-magnetic materials (stainless steel, copper alloys, etc.).
In addition, since the mold gets worn strongly due to magnetic powders, it is necessary to take measures to enhance the resistance to wear.
Since the molded products taken out stick to each other due to magnetic force, care should be taken in the handling of molded products after they are taken out (making suitable trays, etc.).