January 2012 Archives

We describe here the method of determining the dimensions of a side gate that is used very frequently in the molds for injection molding. A side gate is the simplest gate design. Since the molded article and the gate are joined, the gate is cut off in a secondary operation. A side gate can make the molten plastic flow into the cavity in a stable manner, and also since it is possible to apply the dwell pressure sufficiently, it is possible to prevent flashes in the mold.

The side gate dimensions are determined in the following two steps.

First Step: Determining the gate depth

First check the wall thickness t (mm) of the molded article.
Next, obtain the plastic resin dependent coefficient n from Table 1.

Table 1

Next, obtain the depth h (mm) of the side gate using the following equation.

h = n ∙ t

Second Step: Determining the gate width

First obtain the surface area A (mm²) of the molded item.
Next, obtain the molding material dependent coefficient n from Table 1.
Finally, obtain the gate width W (mm) from the following equation.

W = n ∙ √A/30

In plastic injection molding, it is necessary to take out the molded article certainly from the mold.

Since the biggest advantage of injection molding is that it is possible to carry out mass production without requiring a human operator, if the molded article remains in the mold and it becomes necessary to take it out manually by a human operator, an operator will be required to be present at all times near the molding machine, automated operation cannot be made, and hence the personnel cost becomes very high.

If by chance, a mold is closed with a molded article still remaining in it, in the worst case, the cavity or the core will be damaged causing a big economic accident. If a mold is damaged, in most cases it is not possible to repair it easily, and hence it is necessary to adjust the mold so that the molded article can be taken out 100% from the mold.

Now, why does a molded article remain in the mold? A molded article remaining in the mold is called a mold release defect. A mold release defect occurs due to a combination of the following causes.

1. Contraction of the molded article
2. Wall thickness of the molded article
3. Internal diameter of the molded article
4. Ease of sliding of the plastic resin
5. Surface roughness of the cavity and core
6. The draft angle of the cavity and core
7. The surface temperature of the molded article at the time of opening the mold
8. Ejection speed
9. Number and placement of the ejection pins
10. Corner shape of the molded article
11. The state of vacuum between the molded article and the cavity as well as the core
12. Orientation of the glass fibers, etc.

Since it is very difficult to take countermeasures counter to common perception when a mold release defect has occurred, it is very important to give sufficient considerations at the time of the initial investigations during the design of a mold. In specific terms, the key point is to carry out sufficient investigations of the shape and material of the molded article, gate position, the draft angle, the height of the molded article, placement of the slide core or the inclined pin, the wall thickness of the molded article, etc., and how you can foresee the locations where a mold release defect can occur. Since it is not possible to take countermeasures if it is not possible to foresee such possible occurrences, it is very important to fully understand samples of various previous mold release defects and defect analysis reports.

In other words, it is very important that history reports have been accumulated thoroughly of all the mold release defects that have occurred in the past. Regarding defects that are caused by a combination of reasons such as mold release defects, it is difficult to find the causes from the analyses. Although a veteran designer can foresee such possible causes of mold release defects through tacit knowledge, this is very difficult if your experience is not sufficient. Because of this, it is critical to systematically accumulate data where previous occurrences of mold release defects are appropriately recorded.

In the injection molding of plastics resin, you may all have experienced that the settings of the molding conditions, particularly of the injection pressure or the injection speed, and of the mold temperature are greatly affected by whether or not the fluidity of the molten plastic is good or bad.
While there are several methods for evaluating the fluidity of plastics, the easiest method which is being used as a guideline is the Melt Flow Rate (MFR) method.

The melt flow rate is measured by putting the plastic resin under test in a testing equipment called a melt low indexer, heating the plastic resin, making the molten plastic resin with a prescribed weight to flow, and measuring the weight of the plastic that flows out as an index of the melt flow rate. The method of this test has been stipulated in JIS and ISO standards.
A plastic resin with a high value of melt flow rate is evaluated as one with good fluidity. The flow is poor of a plastic resin whose melt flow rate is low.
However, the melt flow rate evaluates the fluidity in a static condition, whereas in actual injection molding the plastic resin flows in a very short time through a very narrow gate, so it is necessary to understand well the fact that the melt flow rate as it is does not fit in the case of injection molding.

As explained above, the melt flow rate is used as an index of the fluidity of the plastic resin for grasping the properties of the plastic resin in a simple manner.