As an elastic component, the spring plays a very important role in a plastic injection mold, because a properly located spring is able to provide the necessary driving force that the mold needs. For example, when the ejection mechanism completes the ejection action of a plastic part, a spring can be applied to force the ejection mechanism back to its original position, so as to ensure the work process goes smoothly, i.e. the so-called return spring. To meet this purpose, different spring designs are required for different structural locations.
Usually, rectangular springs are used in plastic injection molds. When mounted in the same space, the loading capacity of a rectangular spring is about 45% higher than that of a cylindrical spring, while the deflection is 13% – 14% larger. A proper spring design is able to simplify mold structure, lower mold maintenance costs and thus guarantee a smooth production process. Through actual examples, this article will introduce the design and selection of springs for the ejection mechanism.
Design Requirements of Return Springs
To meet the work requirements of return springs, the following points should be taken into consideration during spring design:
1. Select the appropriate compression deflection. Spring compression deflection refers to the ratio between its compressed length (L1) to its free length (L), of which the value has a direct influence on spring service life as well as the loading capacity.As an elastic component, one of the biggest disadvantages in spring application is spring failure. During operation, the higher the deflection or compression, the shorter the service life; if the deflection is too low, the spring will not be able to provide sufficient return force for completion of the return task.
Based on the loading capacity, mold springs mainly fall in the categories of minimal load (TL, yellow), light load (TF, blue), medium load (TM, red), heavy load (TH, green) and extra heavy load (TB, brown). Different categories are designed in different colors to indicate springs with different service life as well as different deflections.
2. Ensure spring travel stability. Appropriately designed mounting holes and mandrel sizes are mainly applied to guide the spring and limit its deflection.However, a spring will be over restricted if the mounting holes are too small. At the same time, the mandrel size influences the inner diameter of a spring. Since rectangular springs feature smaller inner diameter, the outer diameter should be as large as possible.
3.Set appropriate spring preload. Preload is also known as preload length. In the preload status, a return spring can effectively maintain stability during the work process by preventing spring fatigue and increasing spring service life. A preload of 10 – 5mm may be applied when a return spring is concerned.
Design Process of Return Springs
1. Select the appropriate compression deflection. Considering the service life and the minimum ejection travel distance, the compression deflection of a return spring usually ranges from 30% to 40%. This value leads to the result that only the light load blue (TF series) springs are applicable when choosing return springs.
2. Outer diameter of a spring. The outer diameter is one the main technical parameters of a spring, of which the value is usually influenced by mold size and mandrel size. The bigger the mold is, the larger the outer diameter of a spring is.
3. Free length of a spring. Free length of a spring is one of the basic parameters for spring selection. Springs of the same outer diameter may have different free lengths. The free length of a spring is influenced by product ejection travel distance – it has to be longer than the travel distance of the ejection mechanism or the spring will not be able to work properly. As a result, the compression of a return spring should at least be the sum of the ejection travel distance and the preload length.