Bubbles on the surface of PVC soft rubber cup lids are a common problem affecting appearance quality during production. Bubbles not only reduce the product's aesthetics but can also affect its sealing performance and lifespan. The causes of bubbles are complex, involving multiple aspects such as raw material characteristics, processing conditions, equipment status, and operating procedures, requiring systematic optimization and refined control to avoid them. The following discusses how to effectively prevent the formation of bubbles on the surface of PVC soft rubber cup lids from seven dimensions: raw material management, processing parameters, equipment maintenance, operating procedures, mold design, environmental control, and quality inspection.
Raw material management is the foundation for preventing bubbles. PVC resin itself is hygroscopic; if the storage environment has excessive humidity, the raw material easily absorbs moisture, which evaporates during high-temperature processing, forming bubbles. Therefore, raw materials must be stored in a dry, well-ventilated warehouse, avoiding direct contact with damp floors or walls. Before production, raw materials must be pre-dried using hot air circulation or vacuum drying equipment to remove moisture, ensuring the moisture content meets process requirements. Furthermore, the selection and proportioning of additives are crucial. If plasticizers, stabilizers, or other additives have poor compatibility with PVC, or are added in excessive amounts, it may lead to a decrease in melt viscosity, making it easier for gas to become trapped and form bubbles. Therefore, it is necessary to optimize the formulation based on the characteristics of the raw materials, select additives with good compatibility and low volatility, and strictly control the addition ratio.
Precise control of processing parameters is key to avoiding bubbles. PVC soft rubber cup lids are usually produced using injection molding or compression molding processes. Processing temperature, pressure, and speed are the core parameters affecting bubble formation. Excessive temperature will cause PVC to decompose and produce gas, while simultaneously reducing melt viscosity, making it easier for gas to diffuse and form bubbles; excessively low temperature will result in poor melt flowability, preventing gas from escaping in time and trapping it inside the product. Therefore, it is necessary to rationally set the barrel temperature, mold temperature, and nozzle temperature according to the characteristics of the raw materials and the mold structure to ensure that the melt maintains suitable flowability and stability during the mold filling process. Regarding pressure, the injection pressure and holding pressure must be matched. Insufficient pressure will result in incomplete melt filling, while excessive pressure may cause melt backflow or poor mold venting, both of which easily lead to bubble formation. Regarding injection speed, excessively fast injection speeds can lead to excessively high melt shear heat, causing localized overheating and gas buildup; conversely, excessively slow injection speeds can cause the melt to cool and solidify, preventing gas from escaping. Therefore, it is necessary to optimize the injection speed curve through process testing to achieve stable mold filling.
Equipment maintenance is a crucial aspect of ensuring production stability. Wear or aging of components such as the screw, barrel, and mold of injection molding or compression molding machines can cause uneven melt flow, creating stagnation zones and dead zones where gas can easily accumulate and form bubbles. Therefore, regular equipment maintenance is essential, including checking the clearance between the screw and barrel and replacing worn parts promptly; cleaning the mold runners and venting channels to ensure smooth gas escape; and checking the heating system and temperature control devices to ensure temperature uniformity and accuracy. Furthermore, equipment cleanliness also affects bubble formation. Residual impurities or mixing of different batches of raw materials can lead to impure melts and gas generation. Therefore, thorough cleaning of the equipment is necessary when changing materials or colors to avoid cross-contamination.
Strict adherence to operating procedures is key to preventing human error. During production, parameters such as material feeding, holding time, and cooling time must be strictly followed according to the process documents to avoid bubble formation due to improper operation. For example, excessive material feeding will lead to excessively high mold cavity pressure, preventing gas from escaping; insufficient material feeding will result in incomplete filling of the melt, forming shrinkage cavities and bubbles. Insufficient holding time will cause the mold to open before the melt is fully solidified, allowing gas to easily seep into the product; excessive holding time will cause melt backflow, generating dark bubbles. Insufficient cooling time will cause the product to be demolded before it is fully solidified, easily generating bubbles due to stress release; excessive cooling time will reduce production efficiency and increase costs. Therefore, it is necessary to improve the skill level of operators through training to ensure that they can master the process requirements and equipment operating procedures.
The rationality of mold design directly affects the bubble venting effect. The mold's runner, gate, and venting system need to be optimized according to the product structure to ensure that the melt can smoothly fill the mold and that gas can be vented in a timely manner. Runner design should avoid being too long, too narrow, or excessively curved to reduce pressure loss and gas retention. Gate design should select the appropriate type based on the product's wall thickness and shape, such as side gates or submarine gates, to ensure uniform melt filling and smooth gas escape. The venting system should be located at the final filling point of the product or at the end of the melt flow, such as the parting line, the core-cavity mating area, etc. The depth of the venting grooves should be determined based on the raw material characteristics and product requirements; too deep will cause flash, while too shallow will not provide effective venting.
Environmental control is a supplementary measure to ensure production stability. The temperature, humidity, and cleanliness of the production workshop must meet process requirements to avoid raw material moisture absorption or equipment malfunction due to environmental factors. For example, excessive humidity in the workshop will cause raw materials to absorb moisture, excessive temperature will cause equipment overheating, and insufficient cleanliness will allow impurities to enter the melt. Therefore, the workshop environment must be controlled using equipment such as air conditioners and dehumidifiers, and the workshop must be cleaned regularly to ensure a clean and orderly production environment.
Quality inspection is the last line of defense to ensure product quality. Real-time inspection of products is necessary during production to promptly detect and address issues such as air bubbles. Inspection methods include visual inspection, dimensional measurement, and sealing tests, with particular attention paid to critical dimensions and sealing areas. For products that have already developed bubbles, the causes must be analyzed and appropriate measures taken, such as adjusting process parameters, cleaning equipment, or replacing raw materials, to prevent the problem from escalating.
