Use these design methods to prevent wear, burrs, and mold damage. #Best Practices

Last month I discussed several basic methods of forming through holes in molded parts. This month, I will dive into more details and provide more variations that you might consider. You can also consider combining the various functions of several of these methods to suit your specific application.

Figure 1 describes the six more common methods of creating through holes in molded parts. They all use a core pin. Although Figure 1 shows the core pin installed on the B plate, most of these methods can be applied to the core pin installed on the A plate, ejector sleeve, or even the cam. In addition, although the methods discussed here are applicable to cylindrical core pins, they can also be combined into various shapes with the same intent and purpose—for example, to form a key hole in a molded part.

Figure 1 Various methods of forming through holes (GL).

Method G: This describes a core pin designed for a very critical aperture. The diameter of the pin is slightly larger, and then gradually adjusted to the required size of the hole in the part. If the hole in the formed part is too large for the established process parameters, you can grind the steel safety pin slightly smaller. If the hole in the molded part ends up being too small, the pin can be replaced without having to increase the diameter of the through hole.

This method is suitable for blind holes and through holes in parts. The core pin retainer used to hold the pin in place is a cheap and effective method. You can consider using it for non-laminated B-series mold bases, stripper plates, and other designs that do not have a fixed plate.

Method H: Here, two opposite core pins are connected to each other. For this type of application, the use of fully hardened core pins is essential. Spring loading one of the pins will greatly extend the life of both pins. I want to know how many of you realize that this picture is incorrect. (Yes, this is a test.) Since the core pin on the core side of the mold is spring-loaded and the hole is a countersunk hole, the spring-loaded pin will try to push the molded part away from the core-possibly causing it to stick Cavity.

When the hole in the part is quite long and the hole diameter is quite small, two opposing core pins are usually used to reduce the length. If the two pins have the same diameter, one of them should be deliberately larger than the required hole diameter. If one of the pins is deflected due to the flow of molten material, the screw, pin, or other item can pass through the hole unimpeded.

Method I: This is basically a mirror image of Method H. However, in this case, the spring-loaded pin on the cavity side helps prevent the part from sticking in the cavity. It does not matter whether the reduced diameter part is made of pins in the cavity or in the core. The part design will determine which configuration is preferable.

Method J: Method J is the same as Method I, except that the faces of the core pins are interlocked. There are various designs available for interlocking two mating pins. Ideally, every time you open the mold, the mold you use should allow any debris to fall freely from the interlocking genitals. Or you should leave space between the male and female interlocks to keep some debris.

Note how the interlock does not extend all the way to the edge of the pin. The surface of the pin with the male interlock has a small flange or flat part. This is done to prevent side flashing. When concentricity is important, the height of a small-diameter pin is extremely long, or the flow of plastic material is likely to cause the pin to bend, interlock the two mating pins.

The interlocking device should allow debris to fall freely from the female side.

Method K: This describes a small diameter core pin. The through hole of the small pin should start from the parting line of the mold, because when you machine the hole from the back of the cavity or core insert, you face the risk of the drill or end mill "drifting". This is why the through hole of the small core needle should be reduced.

Reamers, rings, and honing can help repair curved or banana-shaped holes, but if the holes are not perpendicular to the parting line, they cannot straighten the holes. That is when fixture grinding or high-speed milling is required. The small core needle has a high risk of bending. They should be guided to the opposing mold assembly by any method you like. (See also last month’s article for various pilot methods.)

Method L: This is an alternative to method K. The pin only enters the cavity by the interlocking amount. As with method J, the pin surface has a small flat surface to prevent side flash. The pin is spring loaded to compensate for any changes in height or injection pressure. Due to the small diameter of the pin, a hardened spacer is used between the back of the pin and the top of the stacked Belleville spring. This allows the use of a spring with sufficient force.  

No matter which method you use to form holes in the molded part, precise alignment between the cavity plate and the core plate is extremely important. When the pin is against a flat surface, a tapered interlock may be acceptable. They are also suitable for interlocking pins, as long as the angle on the pin interlock is greater than the angle on the tapered interlock. For core pins with guides that enter the cavity, straight edge interlocking is required, and the engagement length of the core pin is greater than the engagement length of the core pin in the cavity.

Correct alignment is the key to mold life.

As the molten plastic cools, it shrinks onto the core pin. In fact, the pin is an immovable cooling device. Therefore, like the core itself, the core pin usually requires a larger draft or taper than the cavity. A material with a higher shrinkage rate exerts a greater holding force than a material with a lower shrinkage rate, so a greater taper is required. The type of material can also determine the required taper and the surface finish of the core pin. If in doubt, you should contact the material supplier for guidance. Sometimes, you may want to add a negative taper so that the core pin acts as a puller to pull the feature out of the cavity.

When installing a core pin in a core insert or B-plate, there is usually a little gap between the pin and the hole. Obviously, the gap needs to be smaller than the venting depth of the plastic material to be molded. The advantage of this small gap is to allow the core pin to float slightly due to the inevitable misalignment. It can also be used as a fixed vent for trapped gas, and can increase the strength of the plastic welding or fusion line when the plastic flows around the pin. Note: If the strength of the weld line is the main issue, especially for load-bearing items such as threaded inserts, it is best to use a very short and sharp core pin. This will form a positioning location for the secondary drilling operation.

Contrary to the tiny gap around the core pin, if you need to maximize the transfer of heat from the exposed part of the pin to the insert that holds it, then the pin should have a slight press fit—not a slip or slight gap fit.

The mold may be perfectly installed on the workbench, but after a few minutes of contact with hot plastic or a sheet heated with hot water or oil, it will begin to expand. There is also a common and more worrying situation where the processor intentionally uses a higher temperature on the A board to prevent parts from sticking in the cavity. The larger the mold, the more likely this is to become a problem. In this case, consider interlocking the cavities and cores instead of just interlocking the A and B plates. When the cavities or cores are individually interlocked, they should be allowed to "float" a bit, otherwise the interlocks may "fight" with each other.

If the cavity or core is cut into a solid body instead of a separate insert, there are few options to counteract the thermal expansion problem between the mold halves. One option is to calculate the amount of thermal expansion and install the casing. The bore of the bushing can be offset from the centerline by the amount of thermal expansion.

The core pin is usually removed by striking the surface of the pin with a piece of brass, aluminum, wood, or a blunt hammer. But the core pin is usually too thin, too small or too fragile to be removed by this method. Therefore, a suitable method is needed to pull the core pin from the back of the board. The threaded hole on the pin head is very suitable for pulling out the core pin. Especially if the mold is leaking and the pin rusts in its hole.

The mold designer should consider the aspect ratio or L/D. This ratio is the unsupported length of the bare core pin divided by its diameter, as shown in Figure 2. The higher the L/D value, the easier it is for the core pin to deflect.

While researching this topic, I found some rules of thumb from various sources. As the diameter of the core pin increases, its maximum L/D ratio also increases. Compared with core pins forming through holes, core pins forming blind holes have a much lower L/D ratio. The design or configuration of how the core pins are retained also has a significant impact on the maximum L/D ratio.

I combined all this information and performed some interpolations to come up with a more user-friendly chart, as shown in the picture. Remember, these values ​​are strict rules of thumb. There is only one way to more accurately predict whether the core sales (or even the core) will be deflected, and that is traffic analysis. Flow analysis can predict the speed and viscosity of the molten material at the precise time when it hits the mandrel.

If you encounter the core needle deflection problem, it can usually be solved by analyzing the injection speed. Reduce the speed before the material hits the pin, then accelerate it to the desired speed at a short distance from the pin. This can increase the effective L/D ratio by two to four points.

The L/D ratio guidelines in the chart are based on materials flowing in one direction. When a part design has a very large L/D ratio, such as a pen holder or syringe, it usually contains two or more gates. Multiple opposing doors can usually achieve L/D ratios as high as 15:1 to 20:1.

If you pay attention to details, design an injection mold that works well and lasts a long time, with minimal maintenance costs and downtime, it's not difficult.

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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