.Principles of thimble arrangement
(1) The ejector pin should be arranged so that the ejection force is as balanced as possible. Parts with complex structures require greater demoulding force, and the number of ejector pins should be increased accordingly.
(2) The thimble should be set in effective parts, such as bone positions, column positions, steps, metal inserts, local thick glue and other structurally complex parts. The thimbles on both sides of the bone and column should be arranged as symmetrically as possible. The edge distance between the thimbles and the bone and column is generally D=1.5mm, as shown in Figure 5.5.8. In addition, the spacing of the thimbles on both sides of the column should be ensured as much as possible. The center line passes through the center of the column.
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(3) Avoid crossing steps or setting ejector pins on slopes. The top surface of the ejector pin should be as smooth as possible, and the ejector pin should be arranged in a structural part where the rubber part is better stressed. As shown in Figure 5.5.9.
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(4) Flat thimbles should be used in deep bone locations (depth ≥ 20mm) or when it is difficult to arrange dome pins. When it is necessary to use a flat ejector pin, try to use an insert at the flat ejector pin to facilitate processing. As shown in Figure 5.5.10
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(5) Avoid sharp steel and thin steel, especially the top surface of the ejector pin from touching the front mold surface. As shown in Figure 5.5.11
(6) The ejector pin layout should consider the edge distance between the ejector pin and the water conveyance channel to avoid affecting the processing and water leakage of the water conveyance channel. See Chapter 10, Section 10.2 for specific requirements.
(7) Consider the exhaust function of the ejector pin. In order to exhaust the ejector during ejection, the ejector pin should be arranged in the area where vacuum is easily formed. For example, in the larger plane of the mold cavity, although the tightening force of the plastic parts is small, it is easy to form a vacuum, resulting in an increase in the demoulding force.
(8) For plastic parts with appearance requirements, the ejection pin cannot be arranged on the appearance surface, and other ejection methods should be used.
(9) For transparent plastic parts, the ejector pin cannot be placed in the area that needs to be light-transmissive.
B. Principles for selecting thimbles
(1) Choose a thimble with a larger diameter. That is, if there is enough ejection position, an ejection pin with a larger diameter and size priority should be selected.
(2) The specifications of the thimble should be as small as possible. When selecting an ejector pin, the size of the ejector pin should be adjusted to minimize the size specifications, and at the same time, try to select the preferred size series.
(3) The selected ejector pin should meet the ejection strength requirements. When ejecting, the ejector pin must bear greater pressure. In order to avoid bending and deformation of the small ejector pin, when the diameter of the ejector pin is less than 2.5mm, a supported ejector pin should be used.
The problem of difficult demoulding of the mold and easy breaking of the ejector pin can be reduced through various means but cannot always be eliminated. Later maintenance costs are high. Some reasons and suggestions are as follows:
1. The high temperature of the production environment causes fatigue and annealing of the ejector pin material, resulting in failure.
2. Whether the dimensional accuracy meets the usage requirements
3. Stress concentration.
When machining steps, the shaft is prone to stress concentration in parts where the diameter changes suddenly (other workpieces change suddenly in shape), which can lead to cracking or breakage when external forces (especially radial forces) are encountered during use.
4. Heat treatment
Most ejector pins require heat treatment during the manufacturing process. Products that are not tempered after quenching or tempered for insufficient time are prone to quality problems such as excessive residual stress;
5. For surface treatment, in order to improve wear resistance, nitriding treatment is often used. Check whether the nitriding process is standardized. Higher nitriding temperature will cause the material to be tempered or even annealed to cause failure.
6. Try to avoid subjecting the ejector pin to radial force during use.
7. When designing the mold, consider using rounded transitions or adding stress relief grooves at the reducing part of the hole.
8. Check the size and roughness of the hole before installing the ejector pin. If the ejector pin is loose after reaming, the ejector pin hole will be easy to feed and the ejector pin will be easily broken. If it is tight, it will be easily burned at high temperatures. Die, so the fit between the thimble and the hole must be mastered.
9. Routine mold maintenance and process debugging
It is better to apply anti-seize oil every time when loading the mold. The main daily maintenance is to apply lubricating oil or ejector oil regularly. The ejector pins of the mold all have a gap of 3-5μ. After applying general lubricating oil, the mold can be supported for up to 3 to 5 days. As dust and metal powder formed by mold friction accumulate, they enter the gaps between the ejector pins, sliders, and inclined roof tunnels. The gaps in the mating surfaces become smaller and smaller, the smoothness becomes lower and lower, and finally they are completely ablated and stuck until they break. Therefore, it is necessary to wipe the dirt on the ejector pin and the mold surface and re-apply clean lubricant. During maintenance, pay attention to applying high-temperature resistant lubricating paste to the inclined ejector and ejector pin as much as possible to form a thin film. Do not apply forcefully with a brush, otherwise it will easily cause oil stains to contaminate the injection molded parts.
10. In pursuit of a shorter cycle, the ejection and ejection speeds are set too fast to exceed the mold's capacity.
11. If the mold is demoulded too fast, there will be insufficient air intake at the bottom of the product, which will form a negative pressure damping state and increase the resistance to demoulding. The push rod will receive great resistance
12. If the ejector pin is relatively thin, it will be easily damaged.
13. The resistance is too great when ejecting. Check whether there is any strain or deformation on the beer parts. Improve the mold ejection angle appropriately. For the first few molds, shorten the mold opening time to one or two seconds. This is not a problem with the system, but with the product. The reason is that the buckle force is too large.
14. If the cooling water is not turned off after the machine is shut down, the mold temperature will drop. When pressing the first mold, the cooling time will be long and the tightening force will be strong, shortening the mold opening time.
15. For the material problem of the ejector pin, you can choose the new material Toolox44 Toolox steel from Swedish SSAB Steel Group to solve the material and heat treatment problems. Toolox is the hardest pre-hardened steel in the world. It is pre-hardened to 45-48HRC when leaving the factory, which reduces the heat treatment Risk and cycle, excellent nitriding performance, almost no stress, no deformation, 2-3 times higher toughness than traditional materials, can withstand high temperatures below 640°C, can completely solve the problem of fracture or deformation, and is currently the most stable in high temperature resistance Best choice of materials.
