December 2009 Archives

As the mold for plastic injection molding is continued to be used, the slide core or ejector pin, ejector sleeve, and center pin, etc., can cause what is called "galling". "Galling" is the abnormal wear of sliding surfaces, and the causes of abnormal wear are classified as follows.

1. Abrasive wear

This is a form of abnormal wear that can occur easily if there is a difference in the hardness of the materials of the sliding mold parts. This is the phenomenon of a harder material scratching the softer material and getting fused to it.

2. Adhesion wear

In this condition, the projecting parts of mold parts hit against each other and become adhered to each other at the locations where the contact is very severe, and the adhered part falls off becoming wear dust, and the wear continues.

3. Fatigue wear

Fatigue occurs when a mold part repeatedly moves and stops, and this causes wear in this condition. This is the condition when flaking has taken place (peeling off of scales).

4. Fretting Corrosion

This is a form of wear in which wear of the pitching shape occurs in the regions where parts mate with each other with a relatively small clearance. This occurs very often in square keys and key grooves.

5. Corrosion wear

In a corrosive atmosphere of chemical constituents, moisture, or ions, etc., arising from the plastic, wear of the mold parts occurs due to the generation of a potential difference between the two.

When galling occurs, fatal damage is caused to the cavity and the core, and when there is defective movement of the ejector pin or the slide core, it is highly likely to result in breakage of the mold. In order to prevent galling, it is necessary to carry out the appropriate lubrication control, and to use mold parts that are maintenance free.

Bubbles (or voids) are a phenomenon in which air bubbles are left inside the molded product. In the case of transparent molded products such as lenses or prisms, bubbles become external appearance defects or defects in the optical characteristics. In mechanical parts, these lead to a reduction in strength or ultimately breakage.

The causes for the generation of bubbles can be broadly classified into two types.

One type is the bubbles that are caused by bubbles that got mixed with the molten plastic. These are called bubbles.

The other type is a vacuum void generated when the molded product shrinks. When sufficient dwell pressure did not act on the parts where the wall thickness of the molded product is thick, this phenomenon occurs simultaneously with the generation of sink marks due to abnormal shrinkage.

The following countermeasures can be taken for these types of defects.

Countermeasures for Bubbles:

(1) Countermeasures related to molds

	1.	There are no air vents, or they are insufficient.
	2.	There is no cold slag well, or it is too small.

(2) Countermeasures related to the injection molding conditions

	1.	The screw rotation speed is too high.
	2.	The cylinder temperature is too high.
	3.	The injection speed is too high.

(3) Countermeasures related to the design of the molded product

1.

Insufficient preliminary drying of the molded material.

Countermeasures for Voids:

(1) Countermeasures related to molds

	1.	There are no air vents, or they are insufficient.
	2.	There is no cold slag well, or it is too small.
	3.	The sprue and the runners are too thin.
	4.	The gate is too small.

(2) Countermeasures related to the injection molding conditions

	1.	The cavity surface temperature is too high.
	2.	The dwell pressure is too low.
	3.	The dwell time is insufficient.

(3) Countermeasures related to the design of the molded product

	1.	Insufficient preliminary drying of the molded material.
	2.	The wall thickness of the molded product is too high.

Flow marks are a phenomenon in which a pattern of the flow tracks of the molten plastic remains on the surface of the molded product. Depending on the extent, these can become defects in the case of molded products in which their external appearance is an important aspect of quality, such as in the case of home electrical appliances, containers for cosmetics, etc.

Flow marks are caused because there is a difference in the extent to which the cooling takes place upon contact with the surface of the mold at the front end of the plastic when the molten plastic is flowing inside the cavity of the mold.

The following countermeasures can be considered in order to avoid flow marks.

(1) Countermeasures related to molds

	1.	Set the cavity surface temperature a little higher.
	2.	Widen the gate.
	3.	Widen the runner.
	4.	Acquire sufficient cold slag.

(2) Countermeasures related to the injection molding conditions

	1.	Increase the injection pressure.
	2.	Increase the injection speed.
	3.	Acquire sufficient volume and increase the cushion amount.
	4.	Increase the dwell time.
	5.	Increase the dwell time.
	6.	Increase the plastic temperature.

(3) Countermeasures related to the design of the molded product

1.

Make variations in the wall thickness of the molded product small.

A short shot is a phenomenon in which there is an incomplete filling in a part of the molded product.

There are two types of causes for short shots in terms of their nature. One of these occurs because, in the middle of the flow of the molten plastic, the front end of the flow gets cooled and solidifies. The second is caused, in the flow process of the molten plastic, because air traps are generated in the flow depending on the conditions of the flow.

In order to take countermeasures against short shots, it is necessary to verify which of the above two types the short shot belongs to.

n the case of short shots caused by the solidification of the front end of the flow

(1) Countermeasures related to molds

	1.	Widen the gate
	2.	Widen the runner.
	3.	Make the sprue thicker.
	4.	The cold slag well is small.
	5.	Provide a heat insulating plate on the under surface of the mold plate.
	6.	Increase the number of gates.
	7.	Change the position of the gates.

(2) Countermeasures related to the injection molding conditions

	1.	Make the plastic temperature higher.
	2.	Set the cavity surface temperature a little higher.
	3.	Try making the filling pressure higher.
	4.	Try making the dwell pressure higher.
	5.	Try setting the dwell pressure time longer.
	6.	Try increasing the charge quantity.
	7.	Try increasing the volume of the cushion.
	8.	Try changing the injection molding machine.
	9.	Replace the reverse flow prevention ring of the injection unit.
	10.	Change the tip diameter of the injection nozzle of the injection molding machine to a larger size.

(3) Countermeasures related to the design of the molded product

	1.	Increase the wall thickness of the molded product.
	2.	Provide ribs near the parts in which flow is difficult.

In the case of short shots due to air traps

(1) Countermeasures related to molds

	1.	Provide an effective air vent in the part where air traps are occurring.
	2.	Change the gate position.
	3.	Try changing the runner balance
	4.	Try making a structure in which the part with poor flow can be heated.
	5.	Try making the part with poor flow have a divided nested structure.

(2) Countermeasures related to the injection molding conditions

	1.	Try changing the injection speed and changing the flow pattern
	2.	Try changing the screw speed, and the pressure selection position.
	3.	Try making the injection speed slower
	4.	Try setting the cavity surface temperature higher.
	5.	Try setting the mold clamping force a little lower

(3) Countermeasures related to the design of the molded product

	1.	Investigate making the wall thickness of the molded product non-uniform.
	2.	Increase the wall thickness of the molded product.