Follow these guidelines to prevent premature wear, flash and abrasions.

The blade ejector is basically a rectangular ejector pin. It performs the same function as the round thimble, that is, helps eject a part of the part. They are most commonly used on the bottom of deep ribs.

The biggest advantage of the blade ejector is the amount of surface area to push the part during the ejection process. Figure 1 shows a 0.080 inch wide rib with two 1/16 inch ribs. (0.062 inch) round thimble and standard 0.062 × 0.172 inch thimble blade. The surface area of ​​the blade pushed against the part in this example is almost twice that of the combination of two circular pins.

The molded part adheres to the core with a certain force. The ejector pins, sleeves, blades, etc. must overcome this force without leaving marks on the parts or causing deformation. The larger the surface area in contact with the part, the smaller the chance of damage to the part. Commercially available ejector blades come in various widths and thicknesses. Therefore, it is always a good idea to use the thickest and widest practical ejector blades. Compared with thinner and narrower blades, thicker and wider blades are also less likely to bend.

Figure 1 The surface area of ​​the ejector pin and ejector blade.

If the ribs are very thin, such as 1/16 inch or smaller, and do not allow the protruding boss of a larger diameter thimble, the blade ejector is one of your few options. As shown in Figure 2, the small diameter thimble is likely to bend during the ejection process or be embedded in the rib. Despite their obvious advantages of larger surface area, if you don't know it, you should first understand that blade ejectors don't have much experience with them, and you should only use them when absolutely necessary. They are much more expensive than standard ejector pins, and several important guidelines should be followed to prevent catastrophic damage to blades and molds.

Figure 2 Small diameter thimble embedded in the rib.

One of the reasons to avoid the blade ejector is the surface contact area. A round thimble is located in a round hole with a slight gap around it. But the circular pin is never perfectly centered in its hole. It moves to one side, causing thin line contact. The same is true for blade jets. They also turn at least to one side, usually on both sides. But this will result in surface contact, which is much larger than the line contact on a circular thimble. Therefore, the wear rate of the rectangular ejector blade is much faster than that of the circular ejector pin.

If the thimble or thimble is worn out, you only need to replace it. However, if a pin or blade wears out the hole or slot where it is located...that's another matter. It is relatively easy and low cost to increase the diameter of the thimble wear hole and install the standard 0.005 inch oversized thimble. But if the notch of the ejector blade is worn, you need to increase the notch and install a custom blade, or you need to weld and machine the worn notch to the original size. Depending on how the blade is installed, it may not even be repaired by welding.  

Years ago, the width and thickness of OTS (off-the-shelf) ejector blades were not very accurate and had to be reground to the appropriate size. According to the domestic websites I checked, these tolerances have become better, usually +0.0000 inches to -0.0003 or -0.0006 inches. Some offshore mold supply companies provide tolerances as low as +0.0000 to -0.0001 inches. This can be important when molding very low viscosity materials such as nylon. But remember, people make mistakes. It is wise to apply the old Russian proverb “Trust but verify” (No, former President Ronald Reagan did not invent this sentence.) Before blindly installing them into the mold, you should Measure all parts first.

All ejector blades should be locked.

I know that a tool shop intends to grind off the four sides of a new ejector blade by a few thousandths. Their reason is that when the groove or hole wears out, they don't have to order a custom blade after repairing the mold. In fact, the mold maker started at 0.005 inches. Blades that are too small-know that it will be cost-effective in the future.  

An important aspect when choosing a blade ejector is the surface finish—actually the surface roughness—usually specified in Ra units. Ra is the arithmetic average of the surface height, which is measured between the peaks and valleys of the surface using a profiler. Surface finish plays an important role in determining the performance of an object. Objects with high Ra values ​​have irregularities on the surface, which will create nucleation sites for the formation of cracks and corrosion. Rougher surfaces also have a higher coefficient of friction, which causes them to heat up, wear faster and may wear out. When the surface finish of the two parts is very smooth, any lubricant applied will form a thin film between the parts, which helps prevent them from contacting each other—just like the tires of a car seaplane in the rain. This is called elastohydrodynamic lubrication or EHL.

Offshore parts are manufactured in accordance with various standards, such as DIN (German Association for Standardization), JAP (Japanese Standards Association), JIS (Japanese Industrial Standard) or European metric system, and they all have metric dimensions and tolerances. When you evaluate the surface finish level of any component, keep in mind that there is a huge difference between Ra in microinches and micrometers. If the specification of the component is in inches, the surface finish is correspondingly in microinches. On the contrary, for DIN, JIS and other metric specifications, the surface finish of Ra is in micrometers; 1 microinch (μin.) = 0.0254 micrometer (μm). For example, the English level is 4.0 μin. Metric grade with Ra equal to 0.10 μm. radium.  

There are four common methods for installing the ejector blade in the mold, as shown in Figure 3. The worst method is when a part of the blade is installed in multiple mold parts. The possibility of perfectly aligning the machining grooves is slim. This will lead to accelerated wear and underflow. Another method is to install the entire width of the blade in one assembly. Although this is an acceptable method, it is difficult to fix. It cannot be welded, but it can be ground or EDM processed into larger sizes. 

Figure 3 Four common installation methods of ejector blades.

A very common method of installing the ejector blade is in the wire EDM hole. This is a quick and relatively cheap method, but it is also not a simple repair method. One advantage of this method is that the fine EDM surface is sufficient to retain a very thin lubricant film, which helps reduce the wear rate. The best way is to machine a wide and shallow groove in a mold part. If worn, this type of installation is the easiest to repair. This is also the easiest way to adjust the gap to get good ventilation without any downward flashing.  

No matter what you believe, read or learn, the ejector blade should never be flush with the rib or the edge of the part. Figure 4 is an example of a poorly positioned blade. It should retract at least 0.005 inches from any edge or corner. There are three reasons for this. The first is that you want to place a small amount of plastic in the blind corner. This helps prevent downward flashing. The second reason is that if the ejector blade is even slightly bent or deflected to one side during ejection, you don't want the sharp edge of the blade to scratch the side of the core. The third reason is that it is easier for tool manufacturers to start with a small step than with a smooth transition.

Figure 4 An example of poor ejector blade position.

Unless a special contour is machined on the surface of the blade ejector, most mold designers believe that there is no need to key or orient the head. They assume that the blades are automatically aligned. Although this may be true, why do you let the wear-prone blade surface take care of its alignment? The Japanese understand this and provide a variety of profiled head styles as a standard, as shown in Figure 5. In addition, any mold with ejector blades should be guided. Heavy, sagging ejector plates can cause misalignment of pins, sleeves, and blades and premature wear or wear.

Figure 5 Various Japanese head contours to help align the ejector blades.

The shoulder length of commercially available blade ejectors can be as short as 5/8 inches and as long as 5-1/4 inches. The length you choose is very important. Ideally, when the ejector plate is in the retracted position, you want the shoulder of the ejector blade to engage the back of the B fixing plate at least 1/8 inch. This usually requires the B-type fixing plate to be thicker than the ejection stroke. This B-type retainer joining method is the same as you use a small diameter shouldered thimble, for the same reason-to prevent bending and breaking.

About the author: Jim Fattori is a third-generation mold maker with more than 40 years of experience in engineering and project management for custom and proprietary moldmakers. He is the founder of Injection Mold Consulting LLC in Pennsylvania. Contact: jim@injectionmoldconsulting.com; Injection Mold Consulting Network

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