Unstable product dimensions and solutions
Part dimensional instability is a common quality issue in injection molding production. This occurs when the dimensions of plastic parts within or across batches fluctuate beyond the permitted range, impacting product assembly precision and performance, and in severe cases, even causing the entire batch to be scrapped. The causes of part dimensional instability are complex and diverse, involving multiple factors, including mold design, raw material characteristics, molding process parameters, and equipment status. Systematic analysis is required to identify the root cause and implement targeted solutions.
Mold factors are a major cause of dimensional instability in finished products. The mold cavity’s dimensional accuracy directly determines the dimensions of the plastic part. Any dimensional changes in the mold cavity due to machining errors, wear, or deformation will be directly reflected in the part. For example, surface wear in the mold cavity can cause the part to be oversized, while thermal deformation due to excessive temperatures can cause the part to be undersized. Furthermore, insufficient mold guide precision can lead to misalignment during mold closing, shifting the relative position of the cavity and core, resulting in dimensional instability. Imbalance in the ejector mechanism can also cause uneven force on the part during demolding, leading to deformation and compromising dimensional accuracy. To address these issues, regular mold inspection and maintenance are essential. High-precision measuring instruments such as coordinate measuring machines should be used to check cavity dimensions, and worn guide and ejector components should be replaced promptly to ensure accurate and well-coordinated operation of all mold mechanisms. For large molds, temperature compensation should also be considered during mold design, with a suitable cooling system in place to prevent excessive cavity deformation due to temperature fluctuations.
Fluctuations in raw material properties are also a key factor contributing to dimensional instability in finished products. Changes in raw material parameters such as molecular weight distribution, melt flow rate (MFR), and moisture content can affect melt fluidity and molding shrinkage, leading to dimensional fluctuations in the molded parts. For example, raw materials with excessively high MFRs exhibit good fluidity but high molding shrinkage, potentially resulting in undersized parts. Conversely, raw materials with excessively low MFRs exhibit poor fluidity, making filling difficult and potentially oversized parts. Furthermore, impurities in the raw materials or uneven mixing of raw materials from different batches can lead to inconsistent melt flow within the mold cavity, resulting in dimensional instability. Solutions to this problem include strictly controlling raw material quality, selecting suppliers with good stability, and testing each batch of raw materials for parameters such as MFR and moisture content to ensure they meet molding requirements. Hygroscopic plastics such as polyamide and polycarbonate require thorough drying before molding to prevent moisture-induced changes in melt fluidity. Compatibility testing of raw materials from different batches before mixing ensures that performance differences are within acceptable limits.
The setting and control of molding process parameters have a significant impact on the dimensional stability of the product. Slight changes in parameters such as injection pressure, holding pressure, injection speed, melt temperature, mold temperature, and cooling time may cause fluctuations in the size of the plastic part. Insufficient or unstable injection pressure will cause insufficient or uneven melt filling, resulting in small or fluctuating dimensions of the plastic part; improper setting of holding pressure and holding time will affect the shrinkage rate of the plastic part. Insufficient holding pressure or too short a holding time will increase the shrinkage rate of the plastic part and the size will be small, and vice versa. Fluctuations in melt temperature and mold temperature will change the fluidity and cooling rate of the melt, thereby affecting the shrinkage and solidification process of the plastic part, resulting in dimensional instability. To address these issues, it is necessary to determine the optimal process parameters through trial molds, and use precise injection molding machines and control systems to ensure the stability of the parameters. For example, an injection molding machine using closed-loop control can monitor and adjust parameters such as injection pressure and holding pressure in real time, keeping fluctuations within ±1%. The mold temperature uses a constant temperature control system to ensure that the temperature deviation in each area does not exceed ±2°C. The cooling time is precisely set according to the thickness and material properties of the plastic part and remains stable to avoid dimensional changes due to insufficient or excessive cooling.
Equipment stability is also crucial for ensuring dimensional stability in finished products. Unstable clamping force in the injection molding machine can cause deformation in the mold cavity during the injection process, impacting part dimensions. Screw and barrel wear can lead to uneven plasticization and transport of the melt, resulting in fluctuations in melt pressure and temperature. Leaks in the hydraulic system can cause unstable injection pressure and speed, impacting the filling and holding processes. To address these issues, regular maintenance and servicing of the injection molding machine are essential. Check the accuracy of the clamping mechanism and promptly replace worn screws, barrels, and hydraulic components. Regularly calibrate the machine’s pressure, temperature, and speed sensors to ensure accuracy. Maintain the cleanliness and quantity of the hydraulic oil to prevent malfunctions in the hydraulic system caused by oil contamination or insufficient fluid. For example, a company was experiencing significant dimensional fluctuations in plastic gears. Inspection revealed that the clamping force sensor in the injection molding machine was inaccurate, causing the unstable clamping force. After replacing the sensor and calibrating it, the gear’s dimensional stability improved significantly, with the dimensional tolerance reduced from ±0.05mm to ±0.02mm.
Environmental factors such as temperature, humidity, and cleanliness can also affect product dimensional stability. Drastic temperature fluctuations can cause molds and plastic parts to expand and contract, a particularly significant impact for parts requiring high dimensional accuracy. Excessive humidity can cause hygroscopic parts to absorb moisture after demolding, leading to dimensional expansion. Vibration and noise within the workshop can affect the proper operation of injection molding machines, indirectly causing fluctuations in process parameters. Therefore, the injection molding workshop environment must be controlled to maintain a stable temperature of 20-25°C and humidity of 50-60%. Seismic foundations should be installed to minimize the impact of external vibration on the equipment. Regular cleaning should be performed to prevent dust and other impurities from entering the molds or raw materials. Furthermore, after demolding, plastic parts require appropriate post-processing, such as annealing, to eliminate internal stresses and minimize dimensional changes. Hygroscopic parts should be sealed and packaged immediately after demolding to prevent moisture absorption and deformation. By comprehensively controlling these factors, product dimensional stability can be effectively improved and quality guaranteed.
