March 2010 Archives

In order to control the temperature of the mold, it is common to use water as the coolant if the temperature is less than 100°C. The cooling water whose temperature is controlled by the re-circulating pump of the temperature controller, circulates inside the cooling water holes provided inside the mold, and stabilizes the temperature of the mold by heat transfer and radiation.

Normally, it is not possible to see from the outside of the mold the way that the cooling water is circulating (it can be seen if the mold is transparent, though). In actuality there are two patterns for the flow of cooling water in terms of fluid mechanics, namely, "laminar flow" and "turbulent flow".

In order to efficiently carry out temperature control of the mold, the desirable condition for the flow of the cooling water is a "turbulent flow". The condition to make the flow turbulent can be roughly calculated by the indices called the dynamic coefficient of the viscosity of the fluid, the hole diameter, and the Reynolds number that is determined by the flow speed. Limiting the discussion only to water, it is possible to realize a turbulent flow of the cooling water by making the water flow at more than a certain speed. In other words, if the diameter of the cooling water hole of the mold is determined, in order to make the flow turbulent, it is sufficient to make the supply flow rate from the circulating pump higher than a certain value. In this case, since the dynamic coefficient of viscosity changes with the water temperature, it is necessary to change the flow rate in proportion to the water temperature.

The data that are indices for a turbulent flow are listed in the following Table.

Table: Turbulent flow region limiting flow rate of cooling water (L/min)

In plastic injection molding, the molten plastic material flows through the runners, passes through the gates, and then reaches the cavity. The pressure gradually decreases in the process of this sequence of flow. Molten plastic is a viscous fluid having a certain amount of viscosity. In addition, it also has the feature that the viscosity changes depending on the temperature of the plastic, and when the plastic temperature falls below a certain range the plastic can no longer flow and starts to solidify.

Further, whenever a viscous fluid flows through a flow path, there is always a loss in pressure. This is the same as with the flow of water or oil. Now, under what situations is the pressure loss high? Pressure loss is known to occur under the following conditions.

1.In the vicinity of the inlet to the flow path

Whirls are generated in the vicinity of the inlet to the flow path into which the fluid enters thereby causing a pressure loss. It is possible to reduce the pressure loss by making the rounding diameter R large for the corners of the inlet.

2. Where the flow path bends

A pressure loss occurs in the parts where the flow path bends at an angle because there is a change in which the fluid first gets compressed and then expands.

3. Where flexures and bends are present

Whirls are generated in parts where there are flexures and bends thereby causing a loss in pressure.

4. Locations where the flow path expands or contracts

Whirls are generated in parts where the cross section of the flow path becomes wider or narrower thereby causing a loss in pressure. In particular, since a large pressure loss occurs with very sudden expansions or contractions of the flow path, it is necessary that they be strictly avoided.

Although it is possible to know the trends in pressure loss due to the predictions in pressure loss using CAE tools, in order to optimize the detailed flow conditions that become necessary in actual injection molding, it is very important to understand the reasoning according to theory and to utilize the know how obtained by the trial and error of fine adjustments.

Since the textured surface finishing of molds for plastic injection molding is done after completing all of the mechanical forming operations and polishing work, if the appearance is not equal to the desired quality, it is necessary to correct it, or if the damage is so high as to make correction impossible, it is necessary to prepare the mold again. In order to complete the mold within the budget according to the schedule that was planned, it should be understood that textured finishing is a final process that has a high risk. In order to reduce the risks caused by defects in textured surface finishing, it is very important to pay attention to the following aspects.

	（1）	The cavity surface which has to be texture finished is polished carefully using sand paper or abrasive powder thereby removing micro cracks or deformed surface layers caused by machining.
	（2）	For the cavity material, select a steel that has a low probability of having material defects such as voids (air bubbles), inclusion of impurities, inclusion of carbides, etc.
	（3）	Do not carry out texture finishing on parts that were repaired by welding (because it will cause striations).
	（4）	Make the heat treatment of the steel, the cutting direction, and the rolling direction as identical as possible.
	（5）	When processing the side surface of the cavity, set the draft as large as possible.
	（6）	When carrying out processing on the side surface of the cavity, make the wall thickness of the molded product thick and intentionally make the shrinkage large.
	（7）	Adopt a cooling structure that makes it easy to carry out temperature control of the cavity surface, and use a cartridge heater structure.
	（8）	In some cases, a better finish is obtained if the processes are changed so that machining operations are made after texture finishing.
	（9）	Since the visual appearance of the textured finish even changes depending on the type of the molded material, coloring, and amount of glass fibers mixed in, etc., select the type and depth of a textured finish taking into account the importance of past data.

When the mass production of plastic injection molded products is being carried out, there will always be some wearing out or breakage of parts of the mold for plastic injection molding. In such situations, it is necessary to carry out maintenance by replacing parts or making repairs. The common items that require maintenance are the following.

	-	Depressions, scratches and wear of the parting surface
	-	Chipping or depressions on the corners of the cavity
	-	Wear and scraping of the locking block
	-	Wear and cracks of the anguled pin
	-	Wear and scraping of the guide post
	-	Wear and scraping of the guide bush
	-	Scraping of the rail guide by the slide core
	-	Scraping of the center rail
	-	Wear and deformation of the gate
	-	Wear of the internal surface of the sprue bush
	-	Depressions and deformation of the nozzle touching part of the sprue bush
	-	Weakening of the spring elasticity
	-	Wear of the ejector pin
	-	Wear of the hole of the ejector pin
	-	Wear and scraping of the positioning block
	-	Wear and scraping of the return pin
	-	Scraping of the stripper plate
	-	Scraping of the runner stripper plate
	-	Elongation of the bolt
	-	Wear to the bolt threads
	-	Wear of the coupler for the cooling water
	-	Punctures and open circuits in the cartridge heater
	-	Rust and clogging inside the hole for the cooling water
	-	Rust and mold in the periphery of the mold base
	-	Open circuits in the electrical wires and cracks in the cable covering
	-	Fault in the contacts of the limit switches
	-	Clogging due to soot in the air vents
	-	Deformation due to temporal changes in the frame block