Injection molding forced demoulding
Forced demolding in injection molding is a demolding method used under specific conditions. It is mainly suitable for plastic parts with relatively simple structures and a certain degree of elasticity and plasticity. In particular, when the undercut or raised portion of the plastic part is small, demolding is achieved by utilizing the elastic deformation of the material itself, eliminating the need for a complex core-pulling mechanism, effectively simplifying the mold structure and reducing production costs. For example, when producing polyethylene bottle caps, because the undercut on the edge of the bottle cap is shallow and polyethylene has good elasticity, the use of forced demolding not only ensures production efficiency but also reduces mold manufacturing costs. However, forced demolding is not suitable for all plastic parts. If used improperly, it can easily cause problems such as deformation and cracking of the plastic part. Therefore, when applying it, it is necessary to comprehensively consider factors such as the material properties, structural shape, and production batch of the plastic part.
From a material perspective, plastic materials suitable for forced demolding typically require high elastic modulus and elongation to withstand significant deformation during the demolding process without permanent damage. Materials such as polyethylene (PE), polypropylene (PP), and polyamide (PA) possess excellent elasticity and toughness, making them common candidates for forced demolding. However, for more brittle materials like polystyrene (PS) and polycarbonate (PC), forced demolding can easily lead to cracking in the parts. For example, a factory attempting forced demolding in the production of PS toy parts experienced cracking in nearly 60% of the parts, ultimately forcing them to switch to a core-pulling mechanism. Therefore, before selecting a forced demolding method, it is crucial to thoroughly evaluate the material’s mechanical properties to ensure it can withstand the stresses of demolding.
The structural design of a plastic part significantly impacts the feasibility of forced demolding. Generally speaking, the larger the undercut angle and the smaller the height, the easier forced demolding is. Conversely, parts with a shallow undercut angle and a large height are difficult to remove using forced demolding and may even be damaged during the demolding process. Furthermore, the wall thickness of the plastic part should be moderate. Parts that are too thin lack strength and are prone to fracture during demolding. Parts that are too thick have poor elasticity and are difficult to deform, also hindering forced demolding. For example, in the production of a plastic clip, if the undercut height is 1mm and the angle is 15°, forced demolding is relatively smooth. However, when the undercut height increases to 3mm and the angle decreases to 5°, forced demolding can cause fracture at the base of the clip, necessitating the use of alternative demolding methods. Therefore, during the plastic part design phase, the possibility of forced demolding should be fully considered, and the undercut size and angle should be appropriately designed.
The structural design of the mold is also a key factor affecting the effect of forced demolding. The surface roughness of the mold cavity and core should be low, which can reduce the friction during demolding and reduce the risk of scratching or damage to the plastic parts. At the same time, the mold demolding angle should be reasonable. Properly increasing the demolding angle can effectively reduce the demolding resistance. In addition, the design of the ejection mechanism is also particularly important. The ejection points should be evenly distributed to ensure that the plastic parts are evenly stressed during the ejection process and avoid deformation due to excessive local stress. When a company was producing PP handle plastic parts, due to the uneven distribution of the mold ejection points, the plastic parts showed obvious bending deformation during forced demolding. By readjusting the ejection point position, the plastic parts were evenly stressed and the deformation problem was effectively solved.
In actual production, the setting of process parameters also has a certain impact on the success of forced demolding. Excessively high injection and holding pressures can cause the plastic part to fit too tightly within the mold cavity, increasing demolding resistance and hindering forced demolding. Excessively low pressures can lead to underfilling of the part, compromising product quality. Injection and mold temperatures also need to be properly controlled. Excessively high temperatures prolong part cooling time, reduce production efficiency, and increase part shrinkage, potentially making demolding difficult. Excessively low temperatures can reduce part elasticity and make it prone to cracking during forced demolding. A factory producing PE pipe fittings reduced demolding resistance and part deformation by adjusting the injection temperature from 190°C to 170°C and the mold temperature from 50°C to 40°C. This resulted in a 25% improvement in the pass rate for forced demolding.
