October 2011 Archives

While molds are precision machines for molding plastic, it is necessary to acquire appropriate strength in order to maintain its functions. In addition, since a mold becomes a heavy load when carrying out the operations of installing it or removing it, sufficient strength has to be acquired so that accidents causing injury to humans is not caused due to sudden problems such as the mold getting deformed or damaged due to its own weight while it is being carried.

In order to do this, it is not sufficient to ensure the strength based merely on experience or intuition, but it is important to carry out mold design using strength calculations based on the mechanics of materials.

In the design of plastic injection mold, it is necessary to utilize the data of the following experiments based on the mechanics of materials.

■ Static strength tests

• Tension test
• Bending test
• Shear test
• Compression test
• Buckling test
• Torsion test

■ Dynamic strength tests

• Impact test
• Charpy impact test
• Izod impact test
• Fatigue test

■ Industrial tests

• Creep test
• Wear test, etc.

Empirical formulae that have been technically established should be used for the calculations related to the strength of materials, and it is necessary to use a safety margin factor regarding the results of calculations.

Further, although it goes without saying, the results of calculations should strictly follow the "ethics of engineers", and it is necessary to take decisions based on good intents.

Since the prevention of mold damage accidents and acquisition of safety depend upon the judgment by the designing engineer, one should always be strict with oneself so that too much weight is not given to designs for cost reduction thereby blunting one's own most important decisions. This is something a mold design engineer should always remember.

The surfaces of the cavity and core are likely to be oxidized (rusted) and corroded by the volatile components from the molded material or from moisture content in the atmosphere.
Considering the mold release of the molded article, it is necessary that the surface of the cavity is carefully polished and is maintained in a shining condition.
Plating the surface of the cavity is used as a measure for preventing oxidization or corrosion. Hard chromium plating is a typical method of plating among the different varieties of plating.

Hard chromium plating has good wear resistance and corrosion resistance.
The hardness of the film on the surface is 700 to 1000 HV, and the surface has a white metallic glossiness.

To carry out the plating, the work to be plated is immersed in a plating solution made of chromic acid　anhydride, sulfuric acid, etc., and an electric current is passed to form an electroplated film.

The plated film is a robust passivated film of a base metal in air. Since the surface energy of the plated film is low, it has the property of being hard to be adhered to by other materials. Therefore, it can also be said that it has the property of making it easy for the plastic molded article to be released from the mold.

In order to carry out plating skillfully, it is recommended to carefully polish the part of the surface that is to be plated, thereby putting it in a state in which the plating can adhere to it easily.

In fields other than molds, this type of plating is used for preventing rust in the parts for food production machines, mill rolls, and for preventing rusting of machine parts.

In general, since the waste plating liquid treatment and management of plating liquid are important from the point of view of pollution prevention, in most cases, the work of hard chromium plating is entrusted to specialized companies.

When molding is being carried out using a mold, sometimes accidents occur in which a part gets damaged or broken due to some reason. Since a mold is a machine, the parts constituting a machine can get damaged due to some reason. The important thing is to find out the cause of such damage, and to take technical measures to prevent such an accident from occurring again.
If repairs are done while the reason for the damage is still vague and no fundamental technical measures have been taken, the same accident may occur again, or can even have the danger of causing harm to the workers.
Analysis of an accident of mold damage starts with detailed investigative analysis.

Types of damages

• Damage
• Wear
• Corrosion
• Fatigue
• Impact (shock)
• Excessive stress
• Fracture
• Breakage, rupture

Items to be investigated in the event of a mold damage accident

(1) Investigation of materials

• Investigation of manufacturing history
• Composition analysis (chemical analysis, impurity analysis, etc.)
• Investigation of mechanical properties (tensile test, compression test, buckling test, impact test, hardness test, etc.)
• Fatigue
• Surface observation and evaluation (macro, micro)
• Investigation of surface conditions, investigation of residual stress
• Investigation by observation of fracture surface

(2) Investigation of design standards

• Verification of the design presumptions

(3) Investigation of usage environment

• Investigation of external operating forces (size, repetition, impact, creep, etc.)
• Investigation of external environment (temperature, humidity, chemical atmosphere)
• Investigation of status of repairs

(4) Simulation

• Investigations of material mechanics
• Verification using simulation software
• Reproduction experiment

(5) Overall investigation

* Note 1)
Reference: "Causes and Countermeasures of Damage to Machines and Equipment" (Shin-ichi Nishida, Nikkan Kogyo Shimbun, Ltd. (in Japanese))

In plastic injection molding, the number of items that can be molded in one shot using one mold is called the number of cavities.
The number of cavities can be from 1 to a reported maximum of 216.

Since the number of items molded in one shot becomes large as the number of cavities becomes large, it is possible to reduce the cost of production per item.
However, as the number of cavities becomes larger, there is a trend of the quality fluctuations becoming large such as the dimensions, weight, external appearance, etc. of the molded item of the different cavities.
It is necessary to determine whether to make the number of cavities large or small upon studying comprehensively the advantages and disadvantage.

In general, the following trend is seen in the number of cavities depending on the type of mold.