December 2012 Archives

In order to calculate the cooling time of plastic injection molded items, it is necessary to acquire the thermal diffusion coefficient of the plastic　resin. Table 1 gives the thermal diffusion coefficients of some major plastic resins.

: The thermal diffusion coefficient of the plastic at the cavity surface temperature (mm2/sec)
　α＝λ／（c・ρ）
　　　λ：Thermal conductivity of the plastic resin(kcal/m• h• °C)
　　　c：Specific heat of the plastic resin(kcal/kg• °C)
　　　ρ：Density of the plastic resin(kg/m3)

Table 1 Thermal diffusion coefficient of major plastic resins

Extrusion molding is a method of continuously molding plastic molded articles having the same cross-sectional shape over its length such as rainwater guttering, gray colored drain water pipe, etc.
A dedicated extrusion molding machine is required for carrying out extrusion molding. In addition, dies are also required for obtaining the shape of the molded item. A die for extrusion molding is generally called an extrusion die.

There are various types of extrusion dies such as the following.

	-	Straight die
	-	Cross-head die
	-	Flat die
	-	Die for solids
	-	Die for films
	-	Die for pipes
	-	Die for coverings

Extrusion dies are prepared from stainless steel, nickel-chromium steel, or alloy tool steel. The dies are hard-chrome plated or surface treated so as to withstand corrosion and wear.
The holder part of the die is made of a soft steel such as S50C, etc.

The shape and design of the die are very important factors determining the quality of the molded item. The processing conditions such as the speed of extrusion are also important factors.

Extrusion molding is the second most frequently used method of molding plastic molded articles next only to injection molding. Unlike injection molding, once a die is prepared, a relatively large quantity can be produced from the die. Therefore, this can be said to be a molding method that finds a lot of applications in the equipment manufacturing industry.

The vacuum molding method is a method used for molding articles such as egg packages, pudding cups, packaging containers, etc. that are made of thin plastic sheets.

In the vacuum molding method, melting of raw material pellets such as in extrusion molding is not made, but a plastic plate formed in advance in the shape of a sheet is deformed by vacuum suction on to a heated mold (see the figure).

The mold is provided with several holes for vacuum suction. In order to support the deformation of the plastic sheet, a plug is also sometimes lowered to press down the sheet.
On the other hand, the compressed air molding is a molding method in which the plastic sheet is deformed by compressed air.

The edges of the items molded using these methods are cut by a press in a succeeding step or else, they are cut by scissors if such parts are small in quantity.

The plastics that can be molded in this manner are:

	-	Polystyrene foam
	-	Hard PVC
	-	Polypropylene
	-	Polyethylene terephthalate
	-	Methyl ester methacrylate (acrylic plastic), etc.

It is only necessary to prepare either the male mold or the female mold.

The materials used for the mold are mainly nonferrous metals such as an aluminum alloy, duralumin, zinc alloy, brass, etc.