June 2010 Archives

When a part of a component of a mold for plastic injection molding has become worn out or when a corner part has been chipped off due to hitting it against some hard object, etc., welding can be used as a method for repairing such parts. Although welding is not used in the case of precision molds because welding can result in big damage to the mold component due to high temperatures or formation of oxide layers during welding, welding is an important repairing technology in the case of large molds, or when the external appearance quality of molded products is not very severe, or for carrying out emergency repairs. In view of this, the outlines of welding are described below.

1. Types of welding

The following are the three major types of welding used for repairing molds.
(1)Brazing
(2)Gas welding
(3)Electrical arc welding

2. Features of the different welding methods

(1)Brazing
Brazing is that of affixing to the part to be repaired a material such as solder or silver by heating it in a flame and melting it. Since the repair will be made using a metallic material that is different from the base material which is carbon steel, the strength of the joint is not very good. However, since this type of welding work is simple, it is suitable for making emergency repairs.

(2)Gas welding
In general, this refers to oxy-acetylene welding. Oxygen and acetylene gases are supplied from two gas cylinders, the two are ignited and their flow rates are adjusted to obtain a very hot pointed jet of flame and the welding is carried out by feeding a welding rod between the part to be repaired and the flame. Although the strength of the welded joint is lower than that of electric arc welding, it can meet sufficiently the purpose of repairs. Compared to electric arc welding, since the thermal effect on the base material is small, this method is suitable for molds that are sensitive to deformations.

(3)Electric arc welding
This is the method of creating heat by causing an arc using an AC current or a DC current. The following three methods are used in this type of welding.

1)Coated electrode arc welding
　This arc welding is done using coated welding rods. This is used very frequently for ordinary mechanical parts.

2)Carbon dioxide gas arc welding
　In this method, the welding is done in a carbon dioxide gas shield. It is possible to prevent oxidization of the welding parts.

3)Inert gas arc welding
　In this method, the welding is done in an inert gas shield such as argon gas, etc. This is the most suitable method for repairing molds.

　Since it is possible to prevent definitely any oxidization of the welded parts and to suppress the generation of oxides, and generation of blow holes or air bubbles is difficult even if grinding is done. However, the cost of the work becomes high, and there is the limitation that this work cannot be made unless the company has the required equipment.

The work of assembling a plastic injection mold is a process step that is always necessary whenever a new mold is prepared. It is very common to carry out the assembly of a mold using the following work procedure.

(1)Confirming the component count of the mold parts
(2)Confirming the procedure of assembling the mold
(3)Provisional assembly of the mold parts
(4)Adjusting the operations
(5)Final assembly of the mold
(6)Confirming the operation in a molding machine
(7)Adjusting the defective locations (if necessary, dismantle and reassemble the mold)

In the process step of assembling a mold, since the results of all operations such as design, machining, finishing, surface treatment, etc., all are collected together, this is also a process step in which any human mistakes or insufficiencies in the respective processes such as design mistakes, machining mistakes, part ordering mistakes, etc., all come out. It is necessary to detect speedily such problematic locations and to arrange for design modifications or machining of the mold. Human mental work or machine processing time are physically required for design modifications or part machining of molds. Whether appropriate and accurate judgment and speedy follow up actions can be done smoothly or not determines the trial schedule of the mold and the amount of time taken to a large extent.

Here, since the parts or data pass through a large number persons and processes until the mold is completed, there will be some inadvertent mistakes in information transmission or misunderstandings by individuals. Although it is important to take actions so that these do not occur, and there are some persons who manage saying "this should never happen according to psychological theory", but that is not an advisable approach. It is very important to have an organization and a command chain system for quickly meeting any problems that have occurred, and this is a realistic approach for such problems.

Let us give you some individual advices for the assembly process.

Verifying the mold assembly procedure

-It is very important to start the assembly work only after thoroughly understanding the assembly procedure. When some work can be carried out simultaneously in parallel, the time taken can be shortened if the work is divided among two or more persons.

Provisional assembly:

-When tightening parts using a hexagonal socket head bolt, etc., all the screws should not be tightened in the beginning, but about four bolts should be selected that take up the maximum force, and these bolts should be provisionally tightened in a balanced manner and in the sequence of opposite bolts first, and these bolts should be tightened fully in the end when the overall position of the part has been finely adjusted. After that, by tightening the remaining bolts, it is possible to stabilize the tightening of the entire mold.

-It is possible to improve the efficiency of the assembly work by the use of small appliances such as steel blocks or pins, sponge, woolen cloth, rubber plates, etc.

Adjusting the operation of the mold:

- In order to confirm the operations of the slide core, ejector pin, return pin, etc., fine adjustments of different parts should be made after thoroughly checking the entire mold. If the work is done by concentrating only on one location right from the start, the assembly will have been done "looking at a tree but not looking at the forest", and hence care should be taken because this can make the mold not to function properly.

Scraping work is that of scraping off very fine depressions on the polished surface using a tool called a "scraper". Such scraping operation is done on the sliding surface of the bed of a machine or on the surfaces of moving machine parts.

The purposes of scraping operations are the following.
(1)To make the lubricating oil stay evenly on the surface.
(2)To reduce the friction resistance by making the contact area small.
(3)To reduce the generation of friction heat by making the contact area small thereby reducing the friction resistance.
(4)To suppress thermal expansion by reducing the generation of friction heat.
(5)To suppress the increase in friction resistance due to thermal expansion.

There are the following types of scraping work,
(1)Flat scraping
(2)Hook scraping
(3)Cant scraping

When carrying out scraping work, it is very important to maintain a clean environment in the work place. It is essential to remove all the time any dust, cutting shreds, sand, etc., using a vacuum cleaner, etc. In addition, when carrying out scraping work, an appropriate cutting oil should be selected, and the work should be carried out while applying it in suitable quantities.

Red lead is used for carrying out scraping work. This is a technique for carrying out scraping work uniformly while checking the scraped surface.

As explained above, scraping work is a precision finishing operation that is to be made mandatorily on the sliding surface, etc., of a precision metal working machine. The reason why Japanese metal working machines are of high precision and with very little deviation is the presence of scraping work.

Although there are very limited examples of carrying out scraping work in forming the parts of molds for plastic molding, depending on the usage this some times has very useful functions. This is the ideal method for preventing the biting of slide cores or moving parts.

The history of the design and preparation of molds for plastic molding is still quite shallow being only about 50 years. It is possible to obtain useful effects by observing well and applying the know how used in other precision machine technologies.

The industry of plastic mold in Japan has been affected by the world-wide economic depression, and is, on the whole, in a stagnant state. However, there are some types of industries that are very robust, depending on the field in which they are active. Further, although Japan has a population of only 130 million people, but there are about 7 billion persons living in this world. If our eyes are turned towards the entire globe, we come to know that the demand for plastic products is extraordinarily huge. Considering that about 40% of the amount spent in the world on mold production is spent on molds made in Japan, it is easy to say that the potential capacity of Japan in molds is very strong in the world.

When we think of the industry of the plastic injection mold from now on, in order to acquire high value orders in the world market, the molds need to be the following:

(1)Molds that can guarantee the quality of molded articles
(2)Molds that offer a high production capacity
(3)Molds that are easy to maintain
(4)Molds that can manufacture special molded products.

The molds that meet the needs such as the above are very highly evaluated in the world market, and even their prices are at appropriately high levels.

However, there will also be practical problems about business matching. While Japan is a country where only Japanese is used for all purposes and it is common for students to study foreign languages such as English for three years in junior high school, three years in senior high school, (and possibly 4 years in the university). In the case of most Japanese persons, in spite of having six to ten years of English language education, Japanese people are not competent in carrying on business communication in English. It is very important from now on that all Japanese persons in the mold industry who will be the back bone of the industry increase their ability in practical English.

Even if it is a single sheet of mold drawing, it takes quite a bit of time to translate that into English. The business chance will be lost if time is consumed for translation. Also, if the translation is entrusted to a translation company, it will be necessary to pay translation charges that are almost equal to the design charges. In order to convert the pinch into an opportunity one has to have the braveness and quickness of wit as if to take out chestnuts from the fire. On this event of reaching the 400th course in this series, one more thing that the author would like to recommend to all readers is that it is important to spend one's time in improving one's English ability.

In order to obtain orders from a good foreign company, it is necessary to converse in English or other languages with the mold designers or managers of the foreign customer company, and to exchange opinions smoothly with them. At present in Japan, the number of mold technicians who can use English for business sufficiently is very small indeed. Don't you think this is an opportunity? If you utilize your free time, it is possible to spend about 200 hours in a year. It is necessary to know that such a method of making efforts is there for opening up new vistas.