March 2012 Archives

Question,

Calculate the thermal stress σ acting on the manifold in the following example of settings.
However, your calculations should be based on the assumptions given below.

Overall length of the manifold L = 150 mm
Room temperature of a molding factory in Japan in December = 18°C
Plastic used = ABS plastic
Manifold steel material = S55C

Sample Answer

Firstly, we obtain the change in the dimensions Δl due to temperature rise as follows.

Where,
α (alpha): Linear thermal expansion coefficient of the manifold steel material (mm/°C)
In the case of S55C, we assume that α is equal to 12×10-6 (mm/°C)).
Δt: Temperature change from room temperature up to the set temperature of the manifold (°C)

Therefore, the thermal stress σ is given by:

Where,
ε (epsilon): strain (%), and
E: Young's modulus of the manifold steel material (MPa or kgf/mm2).
In the case of S55C, E = 21,000 kgf/mm2.
Δl (delta) = Amount of thermal expansion of length (mm)
L: Overall length of the manifold (mm)

In a hot runner mold, usually a manifold is used for diverting the molten plastic resin injected from the nozzle of the injection molding machine to the hot runner. A manifold is a plate for branching to the runner, has a heater incorporated in it, and hence the plastic resin inside the manifold is maintained in the molten state and is maintained in a state in which it can flow.

A large amount of heat will be accumulated in the manifold in order to maintain the plastic in the molten state, and as a result, the steel material itself of the manifold undergoes thermal expansion.

Since the fact that the steel material undergoes thermal expansion implies that its external dimensions expand, this in turn generates compression stresses. Such stress generated due to heat is called "thermal stress".

Unless we know the size of the thermal stress, a large compression stress will act on the mold, particularly on the mold plate on the fixed side and on the mold base components, which can lead to deformations of the cavity and core, burrs, and can also cause breakage of the cavity in extreme cases.

In view of this, in this lesson we learn the method of calculating the thermal stress acting on the manifold. The thermal stress σ (sigma) acting on the manifold can be calculated using the following equation.

Where,
ε (epsilon): strain (%), and
E: Young's modulus of the manifold steel material (MPa or kgf/mm²).
In the case of carbon steel, E = 20,000 to 21,000 kgf/mm².
Δl (delta) = Amount of thermal expansion of length (mm)
L: Overall length of the manifold (mm)

Here, Δl can be calculated as follows.

Where, α (alpha): Linear thermal expansion coefficient of the manifold steel material (mm/°C)
In the case of carbon steel, α is about 11.5 to 12.8×10-6 (mm/°C)).
Δt: Temperature change from room temperature up to the set temperature of the manifold (°C)
The thermal stress can be obtained from these equations.

In the next lesson, we will practice some actual examples of calculations.

In a company manufacturing and selling plastic molded articles, it is necessary to carry out a careful analysis of what expenses are actually being incurred where and for what item and to be of use to the management. This kind of analysis is called cost analysis. If a cost analysis is made, it can be understood that the following items will have to be accounted as the production expenses of the molded artilcles.

Cost analysis of plastic molded items:

Material expenses
Direct overhead (workers)
Indirect overhead (office workers, managers)
Insurance
Mold depreciation
Machine depreciation (molding machine)
Machine depreciation (peripheral equipment)
Maintenance expenses
Building depreciation
Electricity expenses
Water expenses
Defect countermeasure expenses
Inspection expenses
Packaging expenses
Freight expenses

In addition, the following expenses are accounted in some cases.

Secondary processing expenses
Assembly expenses
Technology license fees (patent utilization royalties, etc.)

Further, the sales price will be the profit added to these expenses.
The ratio (%) of profits to the sales price is called the gross profit, and the ratio obtained after deducting taxes from the gross profit is called the net profit ratio (%).
Being able to produce a large number of high profit ratio molded items is the path to good management.

Polylactic Acid (PLA) is a plastic resin that can be manufactured using only plant-origin raw materials without using any petroleum based or chemical based raw materials, and in addition, it is an ideal environment-friendly plastic that is decomposed completely into water and carbon dioxide by the actions of naturally occurring bacteria after it has been disposed of. However, the following doubts come to mind. Since appropriate information is difficult to obtain regarding these doubts, some people may come to misunderstandings about them. The points of view that are supported by a large number of concerned persons at present are described below.

Q1. Why is that polylactic acid resin is not in widespread use in spite of being an excellent material?

A1. While many reasons can be thought of, the most important reason is that the cost of the raw material is still high compared to the raw materials of polyolefin plastics (polypropylene, polyethylene, etc.) or styrene based plastics (PS, ABS, etc.). It was inevitable that the cost of raw materials was still high because the production volume is still low. However, the cost difference is gradually becoming smaller because of big increases in the cost of petroleum and because of the sudden increase in the production volume of polylactic acid plastics. As a result, the trend is towards the problem of the cost of raw materials becoming smaller.

Q2. Is it not possible to burn off polylactic acid ?

A2. It can also be burnt off similar to fossil fuel based plastics. The amount of carbon dioxide and heat generated at that time is about half that of fossil fuel based plastics, and it has been confirmed that these plastics are superior even when burnt in terms of the effect on global warming.

Q3. Is it not possible to reuse the runner, etc. in the case of polylactic acid?

A3. They can be reused.

Q4. What time period is required for biodegradation?

A4. For biodegradation to start, it is necessary that the environment is suitable, such as the quantity of microorganisms, temperature, humidity, pH, etc. Biodegradation does not start under the normal environment of use in our daily life (such as stationery, container, etc.) just because of touching by hand or by keeping water at room temperature in a container. Biodegradation starts in soil or in compost if the conditions are satisfied. In soil, about one month is required for biodegradation to start. In compost, biodegradation starts after about three days if the temperature inside the compost increases to near 60°C. A film shaped molded item put in compost gets completely decomposed in about two weeks. If a molded item with a wall thickness of about 1 mm is buried in soil, it takes about four to five years for it to get completely decomposed.

Q5. Is there no danger of causing a food shortage problem if corn is used as the raw material?

A5. The corn that is being used at present for the manufacture of polylactic acid plastics is millet that is being produced for use as animal fodder for cows, horses, etc. It is not the sweet corn eaten by people. About 99% of the corn being produced in the world is for fodder, and the corn consumed directly as food by people is estimated to be about 1%. At present, the production volume of polylactic acid is about 0.01% of the world's plastic production including those based on petroleum, and even if it increases to about 10%, it is almost impossible to think of this corn usage ever becoming a problem of food for human consumption considering the production capacity of millet.

Q6. Can polylactic acid plastics be eaten?

A6. Polylactic acid plastic is not edible. However, it is harmless even if consumed accidentally. When using for food containers, it is necessary to use a grade that conforms to safety standards of the industry and laws and regulations.

Q7. Is it possible to add coloring agents to polylactic acid plastics?

A7. Coloring is possible in master batches, etc. Coloring agents that are biodegradable are also being developed.

References: Various newspaper reports and material catalogs

Polylactic Acid (PLA) is thermoplastic plastic using 100% plant-origin raw materials, and lends itself easily to injection molding, extrusion molding, and blow molding. In addition, it is a plastic resin that can be synthesized without using any petroleum based or chemical based raw materials. Furthermore, it has the excellent environmental characteristic that when this plastic is disposed of in soil, it is completely decomposed into water (H₂O) and carbon dioxide (CO₂) by the enzymes produced by the bacteria in the soil.

Although the presence of this plastic was confirmed more than ten years ago, the method of manufacturing it industrially on a large scale had not been established, and it was almost never specified for use in mass produced items. However, Cargill Dow LLC, which is a joint venture between Cargill Inc. and Dow Chemicals Inc. of USA, succeeded in developing an industrial method of mass manufacturing lactic acid, and it started to be sold in the market. At present, this company has changed its name to NatureWorks LLC and is expanding its business.

The raw material for the polylactic acid plastic is of vegetable origin as mentioned above. At present, starch or sugar (glucose) is being used. In other words, the raw materials will be corn or various tubers, sugarcane, sugar beets, etc. When using starch as the raw material, a reaction called hydrolysis occurs when water is added to starch, it chemically changes to glucose. This is the same reaction as that which occurs when we chew rice and it gradually tastes sweet.

Next, lactic acid bacteria (lactobacillus) are added to the glucose. These microorganisms called lactic acid bacteria have the ability to chemically convert glucose into lactic acid. Lactic acid is a material that is synthesized naturally inside the human body, and it has been known to cause muscle pain such as neck pain or shoulder pain when it accumulates in the muscles.

Next, by carrying out a treatment called dehydration reaction of this lactic acid, it is possible to change it into a material called lactide. There are two types of lactide, namely, L-lactide and D-lactide, but mostly it will be L-lactide. Although whether the result of reaction is L-lactide or D-lactide depends on the type of lactic acid bacteria, not many bacteria have been found that can be used for manufacturing D-lactide.

Next, by carrying out processing called ring opening polymerization of the above lactide, it is possible to obtain polylactic acid plastic.

Polylactic acid is a polymer and is thermoplastic. Therefore, it melts when heated to a high temperature and becomes a solid when cooled. The glass transition temperature is around 57°C. In other words, it is possible to mass produce items by injection molding using molds.

Injection molding of normal polylactic acid plastic can be made by setting the mold temperature to about 30°C to 40°C. Even the design of molds can be made once some experience has been gained. However, since the withstanding temperature of the molded items will be around 60°C, and since even their mechanical strength is also not high, the applications of the molded items will be limited to those satisfying these conditions. Transparent grades are also available. As a result of research in various companies, polylactic acid plastics are now being sold that can withstand temperatures of around 120°C. In the case of these grades, care should be taken during the mold design because the fluidity is bad, mold releasing is extremely bad, and also the cooling time becomes long. The scope of applications of polylactic acid plastics has become considerably wide due to the advent of the high heat resistant grade. Studies are now under way in various fields about the application of these plastics.