Extrusion blow molding technology has become the mainstream production method for hollow plastic products such as detergent bottles due to its advantages of low equipment investment, high production efficiency, and wide adaptability to raw materials. However, in actual production, affected by multiple factors such as raw material properties, equipment status, process parameters, and mold precision, various quality defects and production failures are likely to occur, directly affecting product qualification rate and production efficiency. Combining the production characteristics of detergent bottles, this article systematically sorts out common problems, in-depth analyzes the causes and proposes targeted solutions, providing technical reference for production practice.
I. Process Molding Defects and Solutions
(I) Uneven Wall Thickness
Phenomenon: The wall thickness difference of detergent bottles at the same cross-section or different parts exceeds 10%, with common situations such as thicker top and thinner bottom, and uneven circumferential thickness. This leads to insufficient bottle strength, which is prone to damage during filling and transportation.
Causes: 1. Unreasonable mold design, uneven runner distribution or deviation of air blowing hole position, resulting in unbalanced filling of plastic melt; 2. Large fluctuation of blow molding air pressure, premature or delayed pre-blowing time, affecting the uniform expansion of the parison; 3. Uneven melting temperature of raw materials, unreasonable temperature gradient in each section of the barrel, leading to differences in melt fluidity; 4. Parison sagging, due to excessive melt temperature and slow extrusion speed, the parison stretches unevenly in the longitudinal direction under its own weight.
Solutions: 1. Optimize the mold structure, improve the runner design, calibrate the position of air blowing holes to ensure uniform melt distribution; 2. Adopt a wall thickness control system to stabilize the blowing air pressure, control the air pressure fluctuation within ±0.02MPa, and adjust the pre-blowing timing according to the wall thickness deviation (e.g., delay the pre-blowing time when the top is thick and the bottom is thin); 3. Adjust the barrel temperature gradient, adopt a stepped heating strategy to ensure uniform melt temperature. For HDPE raw materials, it is recommended to control the temperature of each section of the barrel at 170-180℃; 4. Reduce the die head temperature by 10-15℃, increase the extrusion speed by 10%-15%, calibrate the die gap to within ±0.05mm with feeler gauges, and reduce parison sagging.
(II) Surface Defects
Surface defects mainly include bubbles, spots, scratches, orange peel texture, black spots and black streaks, which affect the appearance quality of detergent bottles and reduce consumer trust.
1. Bubbles and Spots
Causes: Raw materials contain moisture or impurities and are insufficiently dried; excessive processing temperature leads to raw material degradation; poor exhaust in the mold cavity, resulting in scorching due to air retention; unsmooth feeding in the hopper, resulting in gaps due to raw material bridging.
Solutions: Select high-quality blow molding grade raw materials, use a dehumidifying dryer to reduce the moisture content of raw materials to below 0.02%; shorten the residence time of raw materials in the barrel, reduce the processing temperature by 5-10℃; add exhaust grooves in the mold cavity with a depth of 0.02-0.05mm to discharge air in time; increase the hopper cone angle to more than 60°, overhaul the vibration motor to avoid raw material bridging.
2. Scratches and Orange Peel Texture
Causes: The mold surface is not smooth, damaged or has residual material accumulation; the parison surface is uneven, resulting in flow marks during extrusion; uneven air volume of the cooling air ring leads to inconsistent surface cooling speed.
Solutions: Regularly polish the mold cavity, clean the die deposits to keep the mold surface smooth; adjust the extrusion speed and temperature to reduce melt flow marks; calibrate the position of the cooling air ring to ensure uniform air volume and control the consistent cooling rate of the bottle outer wall.
3. Black Spots and Black Streaks
Causes: Excessive barrel temperature leads to raw material degradation; wear of the screw and barrel inner wall, resulting in metal debris mixing into the melt; filter blockage, leading to scorching due to impurity retention.
Solutions: Check and adjust the barrel temperature, replace damaged heating coils; replace worn screws or barrels, and perform hard chrome plating on the surface; regularly clean or replace the filter screen to ensure melt cleanliness.
(III) Product Deformation and Dimensional Deviation
Phenomenon: The bottle body has shrinkage deformation, warpage, ellipse and other conditions, and the volume, outer diameter and other dimensions exceed the tolerance range, which cannot meet the filling and packaging requirements.
Causes: Insufficient cooling time or uneven cooling, resulting in insufficient setting of the bottle body; excessive mold temperature, affecting crystallization molding; excessive blow-up ratio, exceeding the material bearing limit; unstable traction speed and poor equipment synchronization.
Solutions: Extend the cooling time by 20%-30%, use chilled water to enhance the cooling effect to ensure the bottle body is fully cooled before demolding; control the mold temperature at 15-25℃, and achieve precise temperature control through a mold temperature machine; adjust the parison control device to control the blow-up ratio within the range of 2:1-2.5:1 to avoid excessive stretching; stabilize the traction speed, regularly calibrate the equipment synchronization valve, measure the bottle size every 2 hours, and adjust parameters in time.
(IV) Melt Fracture and Parison Defects
Phenomenon: The parison surface has corrugations and fractures, and the bottle body has cracks after molding, with seriously insufficient strength and easy damage and leakage.
Causes: Excessively fast extrusion speed leads to excessive melt shear stress; too low raw material melting temperature results in insufficient plasticization; unreasonable mold die design leads to excessive melt flow resistance; excessive moisture content of raw materials leads to water vapor impact during processing.
Solutions: Reduce the extrusion speed to ensure uniform melt extrusion; increase the barrel and die head temperature to ensure sufficient plasticization of raw materials; optimize the die structure to reduce melt flow resistance, widen the mold edge to 0.5mm; strictly control the moisture content of raw materials, and extend the drying time if necessary.
(V) Bottle Mouth Defects
Phenomenon: The bottle mouth has material shortage, deformation, excessive burrs, poor sealing performance, and leakage during filling.
Causes: Failure of the air blowing system, insufficient pre-blowing pressure; loose mold clamping, insufficient clamping force leading to flash; excessive temperature of the bottle mouth mold, insufficient cooling; deviation of parison positioning, leading to incomplete filling of the bottle mouth.
Solutions: Overhaul the air blowing system, clean the solenoid valve impurities to ensure stable pre-blowing pressure; increase the clamping force to 80%-90% of the equipment rated value, replace worn guide pillars and seals; reduce the temperature of the bottle mouth area mold, add local cooling devices; calibrate the parison positioning device to ensure precise docking between the parison and the mold.
II. Equipment Operation Failures and Solutions
(I) Equipment Failure to Start or Missing Actions
Phenomenon: The equipment has no response after pressing the start button, or a single action such as mold clamping, air blowing, or demolding is interrupted.
Causes: Electrical circuit failures, such as loose solenoid valve plugs and damaged contactors; abnormal pneumatic/hydraulic systems, insufficient air source pressure, oil circuit blockage; safety device triggering, such as unclosed safety door and unreset emergency stop button.
Solutions: Detect the electrical circuit with a multimeter, fasten loose plugs, replace damaged components; adjust the air source pressure to 0.6-0.8MPa, clean the oil circuit filter, and check the hydraulic oil level; confirm that the safety door is closed in place, reset the emergency stop button, and troubleshoot the safety interlock device.
(II) Abnormal Die Head Temperature
Phenomenon: The temperature display value deviates greatly from the set value, with local overheating or insufficient cooling, affecting the melt plasticization quality.
Causes: Damaged heating coils or loose wiring; incorrect PID parameter settings of the temperature controller; insufficient cooling water flow of the die head, poor cooling effect.
Solutions: Detect the actual temperature with an infrared thermometer, replace damaged heating coils, and fasten the wiring terminals; recalibrate the PID parameters of the temperature controller to ensure temperature control accuracy; check the cooling water pipeline, clean the blocked impurities, and ensure the cooling water flow rate ≥5L/min.
(III) Material Leakage and Die Blockage
Phenomenon: Material leakage at the connection between the die head and the machine head, or blockage inside the die head, leading to interruption of extrusion.
Causes: Aging and damage of die head seals; wear of screws and barrels, unstable melt transportation; long-term uncleaned filter screens, leading to impurity accumulation; too low die head temperature, poor melt fluidity.
Solutions: Replace aging seals to ensure tight sealing at the connection; check the wear of screws and barrels, and perform surface spray wear-resistant treatment if necessary; regularly clean the filter screen and die head residual material; appropriately increase the die head temperature to improve melt fluidity.
III. Raw Material and Auxiliary System Problems and Solutions
(I) Raw Material Adaptability Problems
Phenomenon: Difficult plasticization of raw materials, unqualified strength and chemical resistance of the formed bottle body, which cannot meet the corrosion resistance requirements of detergent packaging.
Causes: Failure to select blow molding grade special raw materials, such as HDPE raw materials with a melt flow rate (MFR) not in the range of 1-5g/10min; uneven mixing of raw materials, unbalanced proportion of additives; excessive blending of recycled materials, affecting raw material performance.
Solutions: Select blow molding grade raw materials that meet the standards, and prioritize HDPE resins with stable melt flow rate; use a high-speed mixer to ensure sufficient mixing of raw materials and additives, with a mixing time of not less than 15 minutes; strictly control the blending ratio of recycled materials (it is recommended not to exceed 30%) and ensure that the recycled materials are clean and free of impurities.
(II) Feeding System Jamming
Phenomenon: Unsmooth feeding in the hopper, frequent bridging phenomenon, leading to fluctuations in extrusion volume and affecting production continuity.
Causes: Excessive moisture content of raw materials, caking of hygroscopic resins; insufficient hopper cone angle, hindering material flow; failure of the vibrator, unable to assist feeding; wear of the screw feeding section, reduced conveying capacity.
Solutions: Strengthen the drying treatment of raw materials, and hygroscopic raw materials need to be dehumidified and dried before use; increase the hopper cone angle or install an anti-bridging device; overhaul the vibration motor to ensure normal operation; replace the double-thread feeding section screw, and spray wear-resistant coating on the surface to improve conveying capacity.
(III) Abnormal Pneumatic System
Phenomenon: Abnormal noise generated by the pneumatic system during equipment operation, slow cylinder action, affecting blow molding accuracy and production efficiency.
Causes: Improper air source treatment, excessive oil content or moisture content in compressed air; blocked muffler, excessive exhaust back pressure; aging air pipes, resonance caused by inner wall shedding.
Solutions: Install a triple unit (filter + pressure reducing valve + oil mist lubricator), regularly discharge condensed water to ensure clean and dry compressed air; clean or replace the muffler to ensure the exhaust back pressure ≤0.05MPa; replace PU air pipes with wall thickness ≥3mm, control the bending radius ≥5 times the pipe diameter to avoid resonance.
IV. Prevention and Daily Maintenance Suggestions
1. Raw Material Control: Establish an incoming inspection mechanism for raw materials, strictly verify indicators such as melt flow rate, moisture content, and impurity content to avoid unqualified raw materials from being put into production; conduct small-batch verification for different batches of raw materials to ensure process adaptability.
2. Equipment Maintenance: Formulate a standardized maintenance ledger, check the lubricating oil level, cooling water flow rate, and air source pressure before starting the machine every day; clean the die head residual material every week and check the connection status of the heating coil; replace the hydraulic oil filter every month, detect the gap between the screw and the barrel, and reserve vulnerable parts such as heating coils and seals.
3. Process Control: Establish a key process parameter database, and formulate standard operating procedures (SOP) for different specifications of detergent bottles; record key parameters such as temperature, pressure, and wall thickness every 2 hours during production to realize quality traceability.
4. Personnel Training: Strengthen the skill training of operators to make them proficient in equipment operation, parameter adjustment, fault diagnosis and other skills; regularly hold quality analysis meetings, summarize experience in solving common problems, and improve the quality awareness of all employees.
Conclusion
Quality control in the production of detergent bottles by extrusion blow molding machines is a systematic project that needs to take into account multiple factors such as raw materials, equipment, processes, and personnel. In actual production, we should adhere to the principle of “prevention first and rapid response”, and effectively solve various common problems by optimizing process parameters, strengthening equipment maintenance, and strictly controlling raw materials. At the same time, it is necessary to continuously accumulate experience in combination with production practice and constantly optimize production schemes, so as to stably improve the product qualification rate, reduce production costs, and maximize production efficiency.
