Simple Solution To The Deformation Of Injection Mold Expansion

The effects of poor venting and mold expansion deformation on products are discussed, and an idea of ​​controlling the injection pressure distribution by changing the number and distribution of gates is proposed. A method of eliminating the mold expansion deformation of the injection mold template by presetting compressed gas space is introduced with an example.

 

In the production process of injection molded products, it is often encountered that after a long period of use, the mold template is deformed, causing defects such as flash and burrs on the product, resulting in unqualified products. People usually deal with similar problems by overhauling or scrapping. However, for some products with low dimensional requirements, this solution is not worth the cost. This article proposes a simple and easy solution to a common mold expansion deformation situation.

 

1. The influence of poor exhaust and mold expansion deformation on products

Before the plastic melt is filled with mold, the mold cavity is filled with air. During the injection process, the plastic melt will also produce a large amount of gas. During the filling process, all these gases will be driven out of the mold cavity. The gas discharge channels are roughly as follows:
① The gap between the mold inserts and the push rod gap;
② The mold parting surface;
③ Specially opened exhaust holes and exhaust grooves.

 

When the mold is poorly vented, as the plastic melt is continuously injected into the mold cavity, the gas in the cavity will be compressed during the gradual expulsion. The greater the degree of compression, the stronger the effect of blocking the melt from moving forward.

 

During the flow process, the plastic melt loses energy and the temperature decreases accordingly, resulting in poor fluidity. Due to the head-on obstruction of compressed gas, the consequences are nothing more than the following two aspects: First, the melt is not enough to break through the compressed gas blockage and is forced to stop moving forward, resulting in product shortage (short shot) or product burning; second, the melt breaks through the compressed air blockage, but due to excessive pressure (especially for many multi-point gate molds), the mold expands.

After long-term use, the mold (especially the multi-point gate) is most likely to expand because the center gate is directly affected by the injection pressure of the injection machine screw, which is also one of the main factors that ultimately lead to unqualified products.

 

2. Causes and countermeasures of mold expansion and deformation
2.1 Mold example
This example is an abalone plate mold with an outer diameter of 500mm. It is evenly distributed with hundreds of small holes of equal diameter and are through holes. The shape of the product is shown in Figure 1, and the mold casting system is shown in Figure 2.

Since the mold has been used for a long time (5 years) and the production volume is large (300,000 pieces), the mold expands around the central gate of its 5-point casting system under the action of injection pressure, causing flash on the through hole of the product, resulting in a through-hole rate of only 70%, which seriously affects the use function of the product, and the non-through-hole parts are concentrated in the central gate.

 

2.2 Cause analysis
Different flow distance ratios lead to uneven pressure distribution. Since the mold is a central 5-point gate, according to the die formula
△P=jL (1)
△P--die pressure drop
j--die constant
L--die length

It can be seen from formula (1) that the pressure drop at the pouring point is proportional to the flow distance. It can be deduced that the pressure P of the center gate during molding is greater than the pressure P of other runner gates, that is, P>P. Therefore, it can be concluded that excessive pressure at the center gate is the root cause of mold expansion.

 

The molding process of the 5-point gate mold is shown in Figure 3, that is, the center gate is filled first and then expanded outward. In order to make the product completely full, the center part of the product must withstand excessive pressure compensation.

Figure 3 Molding process of 5-point gate mold

 

2.3 Solutions and problems to avoid uneven pressure

The simplest way to solve the above problem is to block the center gate. As can be seen from Figures 1 and 2, after blocking the center gate, the △P values ​​at the four gates have reached a consistent level, and there is no uneven pressure phenomenon. However, a new problem arises. After the product is molded at the center point, it is very easy to form a burning point, which is unacceptable for the product. Obviously, the problem has not been fundamentally solved, as shown in Figure 4. Therefore, we conducted a proofing analysis on the modified mold and found that a burning point of f3~f8mm would be left after the product was formed.

Figure 4 4-point gate mold forming process

 

2.4 Preset compressed air space

Based on the above test and analysis, we adopted the method of presetting compressed air space to solve the above problem. The specific method is shown in Figure 5.

1. Aluminum core 2. Preset compressed air space

Figure 5 Preset compressed gas space a--Before modification b--After modification

 

At the original center gate cavity, based on its upper half diameter and taper, make a truncated cone-shaped aluminum core with a length of 1/2 of the original cavity length to close the upper half of the center gate, and drill and reamer the lower part to a f6mm straight hole.

In this way, during the injection molding process, the gas that has not been completely discharged from the center is squeezed into the preset compressed gas cavity by the melt under the injection pressure, and even part of the melt at the bottom melt junction is pressed into it, forming a conical protrusion with a height of about 5mm, which is equivalent to the diameter of the break mark at the original center gate, and does not affect the appearance of the product, as shown in Figure 5.

 

2.5, Principle diagram of using preset compressed gas space

1. Compressed gas 2. Melt convergence direction