The term “degradation of physical properties of plastics” refers to a problem in which the original physical properties of a material deteriorate through the injection molding process, causing strength to fall below the design value and resulting in damage to the part in question.
This is a problem common to all plastic processing methods, and is a phenomenon not seen in other materials and processing methods. Plastic molding is a process that almost always involves heat, and especially in the case of injection molding, the material remains in a molten state in the cylinder for quite a long time, so careful control of the molding process and shape design that does not force the molding process to extreme limits are key to preventing problems.
Another problem of insufficient strength is what is known as “environmental cracking.
This is a phenomenon often seen in polyethylene. When a molded product is left in chemicals under a constant load for a long period of time, cracks may appear at a stress far lower than the allowable stress of the original material. This is also known as stresscracking, and is discussed here.
This is not the same as material degradation, which is discussed separately.
(1) Due to thermal degradation of plastics
When the molecular weight of a plastic falls below a certain value, its impact value and other physical strength suddenly decrease.
The cause of the decrease in molecular weight is thermal decomposition. The cause of the decrease in molecular weight is pyrolysis. Pyrolysis occurs when plastic materials melt in a cylinder at temperatures far above the melting temperature.
temperature, or if the plastic material is heated to that temperature for a long period of time, even if it is at the proper melting temperature.
Pyrolysis begins when the plastic material is heated to a temperature far above its melting temperature in the cylinder, or when it is heated to that temperature for a long period of time, even at an appropriate melting temperature. This can be a problem in the design of molded products and an important item in quality control of molded products.
This can be a problem in molding product design and an important item in molding product quality control.
Long residence time in the cylinder is a common problem when the injection molding machine is too large for the molded product.
This is a common problem when the injection molding machine is too large for the molded product. Some plastic materials containing flame retardants are also susceptible to thermal decomposition. This is because flame retardants themselves are not heat resistant.
This is because flame retardants themselves are not heat resistant. It is important to keep the cylinder temperature as low as possible and to avoid prolonged residence time at the molding site.
It is important to keep the cylinder temperature as low as possible and to avoid prolonged residence time at the molding site.
(2) Due to hydrolysis
Some plastics are decomposed by moisture when heated with moisture in them.
This tendency is strong in ester-bonded polycarbonate, PBT, PET, and polyamide (nylon), which is highly water absorbent.
This tendency is strong. Sufficient pre-drying is necessary prior to molding.
(3) Due to orientation of plastic and filler
Injection molding is characterized by the orientation of molecules along the flow of the material.
The crystalline plastic tends to be weak in the direction perpendicular to the flow direction, while the crystals are also oriented. This tendency is even more pronounced in crystalline plastics because crystals are also oriented.
This tendency is even more pronounced in crystalline plastics because the crystals are also oriented. If the filler is flat or fibrous, it will also be aligned along the flow direction.
The same tendency is also present when the fillers are flat or fibrous, since they are aligned with the flow.
Molded products with this tendency tend to break in the flow direction under load, so the gate position can be changed or the shape of the molded product can be modified.
Therefore, it may be necessary to change the gate position or devise the shape of the molded product to cover the weak point by adding reinforcement in the weak direction.
It may be necessary to change the position of the gate or to devise the shape of the molded product and add reinforcement in the weak direction to cover the weakness.
(4) Influence of recycled material
In injection molding, except for hot runner molds, sprues and runners are always molded at the same time.
Therefore, the treatment of the sprues and runners is either to be disposed of as waste or to be crushed for recycling.
Injection molding sites sometimes recycle virgin (new) material by mixing a percentage of recycled material with it to reduce the cost of the molded product.
In some cases, virgin (new) material is mixed with a percentage of recycled material for recycling as a cost-saving measure for molded products. There are cases where 100% recycled materials are used in daily sundries.
The problem with recycled materials is that the thermal history is generally unclear and the mixing ratio is often not constant.
Therefore, deteriorated materials with decreased molecular weight may be mixed into the molded product, resulting in a significant decrease in strength.
If high reliability is required for the material of the molded product, recycling of recycled materials should be avoided for the above reasons.
(5) Due to other influences
When mass production molding is started with incomplete material replacement in the screw cylinder of an injection molding machine
The strength of the product may be significantly reduced due to contamination of the used material with a different type of material.
Normally, the presence of different materials can be detected visually, so when materials are changed, it is necessary to check all of the materials in the first few dozen shots immediately after the start of molding.
When materials are changed, it is necessary to check all the shots in the first few dozen immediately after the start of molding.
Weld may cause insufficient strength. Special attention should be paid to welds with fillers.
In this case, the weld is at a right angle to the molten resin flow.
In this case, the fibers are at right angles to the molten resin flow and do not jump into the facing material.
The strength of polyamide (nylon) differs greatly between the dry state (almost no moisture) immediately after molding and the state in which it has absorbed moisture after being left to absorb it naturally.
The strength of polyamide (nylon) differs greatly between the dry state (almost no moisture) immediately after molding and the state in which it has absorbed moisture when left in the natural environment. This material absorbs moisture easily, and the moisture absorption rate can be as high as 1.5%.
Some molded products may become quite brittle in a dry state, resulting in a decrease in impact value.
Therefore, there is a method of allowing the molded product to absorb moisture before use. If the molded product is left in the air, it will absorb moisture, so it will not be in a “dry” state.
However, it is important to note that the molded product is in a “dry” state immediately after molding.
Comments