Common Defects and Solutions for PET Preform Injection Molding

In the PET preform injection molding production process, despite continuous technological advancements, various defects still occur from time to time. A thorough understanding of common defects and their causes, as well as effective solutions, are key to ensuring the quality of PET preform production.

 

Defect on the Bottle Mouth Sealing Surface

The accuracy of the bottle mouth sealing surface is directly related to the sealing performance of the PET bottle. In actual production, even if the bottle mouth size meets tolerance requirements, microscopic irregularities may still occur on the sealing surface, resulting in air leakage after the bottle cap is tightened. This is mainly due to insufficient microroughness of the mold surface, injection pressure fluctuations, and excessively rapid pressure drop during the holding stage. Specific solutions include: using high-precision electrospark machining technology to mirror the mold sealing surface and controlling the Ra value of the surface roughness to below 0.2 μm; introducing a servo-hydraulic system to achieve linear pressure attenuation during the pressure holding stage to prevent shrinkage and deformation of the sealing surface due to a sudden pressure drop; and developing special testing equipment to use a laser interferometer to perform three-dimensional contour scanning on the bottle mouth sealing surface, increasing detection accuracy to 0.1 μm, ensuring that 100% qualified products enter the market.

Residual Stress Concentration Points

Stress concentration points are easily generated at the thread root, shoulder transition, and other parts of the bottle embryo due to design or process issues. These small areas are subjected to local overloads during the blow-molding or filling process, which can lead to cracks. Traditional designs often rely on empirical formulas and cannot accurately predict stress distribution; melt turbulence and uneven cooling during the injection molding process further exacerbate stress concentration.

Innovative solutions begin with the design. Finite element analysis software is used to simulate and optimize the structure of the bottle embryo. By adjusting parameters such as the fillet radius and wall thickness gradient, the stress concentration factor is reduced by more than 30%. Cooling inserts are placed in the mold to perform enhanced local cooling on key parts for more even stress distribution. Variable mold temperature injection molding technology is used in production. The mold temperature rises during the filling stage to reduce resistance to melt flow, and the temperature is rapidly reduced and solidified during the cooling stage to reduce residual stress at the source.

 

Uneven Preform Wall Thickness

Uneven preform wall thickness will affect the subsequent blow molding process, leading to problems such as bottle deformation and inconsistent strength. The main causes of this defect are unreasonable mold design, uneven injection pressure distribution, and an uneven cooling system. The dimensional accuracy of the mold cavity, the position and size of the gate, will affect the flow and filling of the melt, resulting in differences in wall thickness. Furthermore, uneven cooling will cause uneven shrinkage of different parts of the bottle embryo, exacerbating the uneven wall thickness.

To improve this problem, it is necessary to start from the beginning of the mold design, optimize the mold structure through mold flow analysis software, reasonably set the number, position, and size of the gates, and ensure that the melt fills the cavity evenly. During the production process, the injection pressure and speed are precisely controlled, and the segmented injection method is adopted to ensure the melt fills the mold smoothly. At the same time, the mold cooling system is optimized to ensure that the cooling medium is evenly distributed so that the cooling rate of each part of the preform is constant.

Difficulty demolding preforms

Difficult demolding not only reduces production efficiency but also causes the preform to deform and break easily. Common reasons include high mold surface roughness, an insufficient demolding angle, an unreasonable ejection mechanism design, and so on. If the mold surface is not smooth enough, friction between the preform and the mold will increase; and if the demolding angle is small or the ejection position is incorrect, it will be difficult for the preform to be removed smoothly from the mold. To solve this problem, the mold surface can be polished to reduce surface roughness and friction. During the mold design stage, the draft angle should be reasonably increased. Generally, the draft angle of a PET preform mold should be between 1° and 2°. In addition, the ejection mechanism design is optimized to ensure that the ejection force is evenly distributed to prevent damage to the preform due to uneven force. You can also spray a release agent on the mold surface, but be careful to choose a release agent that meets food hygiene standards to avoid preform contamination.

 

 

PET preform quality issues are often the result of multiple factors and require a systematic analysis of aspects such as raw materials, equipment, process, and environment. It is recommended that companies establish a comprehensive quality traceability system to record the complete process parameters of each batch of products and provide data support for quality improvement. At the same time, strengthening operator training and ensuring strict implementation of process discipline can truly achieve stable production.