The causes and solutions for uneven stretching and deformation of the bottle body

2025-08-29 10:00:42

In the blow molding production of plastic bottles such as PET bottles, uneven stretching of the bottle body (such as local being too thin or too thick, side wall depression or protrusion) and overall deformation (such as ellipticity or tilting) are common problems, which directly affect the strength, appearance and subsequent filling compatibility of the bottle body. The reasons need to be dissected from the four core links of "heating, stretching and blow molding, molds, and raw materials", and targeted solutions should be provided.

I. Core Cause Analysis

The essence of uneven stretching and deformation of the bottle body is that the plastic melt is subjected to unbalanced force, heat and cooling during the blow molding process, resulting in inconsistent molecular orientation or uneven shrinkage after molding. Specifically, it can be divided into four types of causes:

1. Preform heating stage: Uneven heating is the primary cause

Heating tube failure/unreasonable layout Some heating tubes in the heating channel experience power attenuation or burn out, or the distance between the heating tubes and the preform is inconsistent (for instance, the side wall of the preform closer to the heating tube is overheated, while the one far away is not softened), resulting in significant differences in the degree of softening in different areas of the preform - the overheated area is prone to "excessive stretching" (thinning) when stretched, while the unsoftened area is "insufficient stretching" (thickening).

Abnormal rotation of the preform: During heating, the preform does not rotate at a uniform speed (such as the rotating motor getting stuck or the transmission belt slipping). One side of the preform is constantly facing the heating tube, resulting in excessively high local temperatures. During blow molding, the stretching on this side far exceeds that on the other side, causing a thickness difference of more than 0.2mm on the side wall of the bottle (the standard should be ≤0.1mm).

Heating temperature parameter deviation: The overall heating temperature is too high (for example, PET preform exceeds 130℃), the preform is too soft as a whole, and there is not enough support during stretching, making it prone to "sagging and deformation". When the temperature is too low (< 85℃), the preform has high hardness and poor ductility, and is prone to "local breakage" or "uneven stretching" during stretching.

2. Stretch blow molding process: Deformation is caused by unbalanced force

Abnormal position/speed of the stretching rod

The stretching rod is misaligned with the center of the preform (deviation > 0.1mm), and excessive force is applied on one side during stretching, causing the bottle body to shift to one side (such as tilting or being elliptical).

The speed of the stretching rod is too fast (> 500mm/s), and the bottom of the preform is forcibly stretched before it is fully softened, resulting in a "depression" at the bottom. The speed is too slow (< 200mm/s), and the side walls of the preform are not stretched sufficiently, resulting in "local bulges".

Improper blow molding pressure/timing:

The high-pressure blowing pressure is too low (< 2.5MPa), and the plastic does not fully adhere to the mold cavity, resulting in "wavy depressions" on the side wall after stretching. Excessive pressure (> 5MPa) leads to local over-stretching, causing "thin spots" (prone to rupture).

Blowing timing deviation: Blowing starts before the stretching rod is fully in place, causing the bottom of the preform to expand without being stretched, resulting in an "overly thick" bottom. Or after stretching, if the air blowing is delayed, the preform will cool and harden, resulting in poor stretching effect.

Poor stability of the air source: Fluctuations in the air compressor pressure (such as a sudden drop from 0.9MPa to 0.6MPa), or leakage in the high-pressure air pipe, cause the pressure to fluctuate during the blow molding process, resulting in uneven force distribution in different areas of the bottle body.

3. Mold and equipment section: Positioning/structural issues exacerbate deformation

Mold cavity deviation: The mold cavity itself has dimensional errors (such as ellipticity > 0.2mm), inner wall wear (local depressions), or there are gaps at the mold joints (local protrusions caused by material leakage), and the bottle body directly replicates the mold defects after blow molding.

Uneven cooling of the mold: The cooling water channels of the mold are blocked or unevenly laid out (such as one side having unobstructed water channels and the other side being blocked), resulting in differences in the cooling speed of the bottle body - the areas that cool faster solidify first, while the areas that cool slower continue to contract, causing "warping deformation" (such as the bottle mouth and the bottle bottom not being on the same axis).

Wear of equipment positioning components: Loose preform clamping claws, worn mold positioning pins (gap > 0.05mm), causing the center of the preform to shift when placed in the mold, and the overall bottle body to tilt after stretch blow molding (if the verticality deviation between the bottle body and the bottle mouth is > 1°).

4. Raw material and preform stage: The basic quality does not meet the standards

Quality defects of the preform itself: In the upstream preform injection process, the wall thickness of the preform is uneven (such as the thickness difference of the side wall > 0.1mm), eccentric (AA value > 0.2mm), and during blow molding, "thicker parts become thicker and thinner parts become thinner", which aggravates the uneven stretching. There are impurities (such as black spots or bubbles) inside the preform. When stretched, the areas with impurities are prone to breakage or indentation.

Fluctuations in raw material properties: The melt index (MI) of PET raw materials is unstable (for example, MI suddenly rises from 0.8g/10min to 1.2g/10min between batches). When MI is too high, the fluidity of the raw materials is too good, and they are prone to excessive elongation during stretching. When the MI is too low, the fluidity is poor and the force is uneven during stretching.

Ii. Targeted solutions

The problem should be solved step by step following the logic of "first check the heating → then adjust the blow molding parameters → finally inspect the mold and raw materials". The specific measures are as follows:

1. Optimize the heating process: Ensure uniform softening of the preforms

Maintenance of the heating system

Use a multimeter to measure the resistance of all heating tubes in the heating channel, and replace the heating tubes with power attenuation (resistance deviation > 10%) or burnout. Adjust the position of the heating tube to ensure it is at the same distance from the preform (with an error of no more than 2mm). If necessary, add a "supplementary heating zone" (for the parts of the preform that are prone to cooling).

Check the preform rotation mechanism: Replace the slipping drive belt and repair the jammed rotating motor to ensure that the preform rotates at a stable speed (such as 30-50r/min) and rotates without eccentricity (use a dial indicator to measure that the preform swing is ≤0.05mm).

Calibrate the heating temperature

The heating temperature of PET preforms is controlled at 90-120℃ (adjusted according to the bottle type: upper limit for thin-walled bottles and lower limit for thick-walled bottles). "zonal temperature control" is adopted (such as 85-95℃ for the bottle mouth, 95-110℃ for the side wall, and 100-120℃ for the bottom). The surface temperature of the preform is detected by an infrared thermometer to ensure that the temperature difference in each area is ≤5℃.

2. Adjust the parameters of stretch blow molding: Achieve balanced force application

Calibrate the stretching rod

Manually adjust the position of the stretching rod and use a dial indicator to check the coaxiality between the stretching rod and the center of the mold cavity (deviation ≤0.05mm). Replace the worn guide sleeve of the tensile rod to ensure that there is no jamming when the tensile rod moves up and down.

Adjust the stretching speed: Set it according to the height of the preform (for example, for a 100mm preform, the stretching speed is 300-400mm/s), following the principle of "slow at first, then fast, and then slow again" (slow at the bottom → fast in the middle → slow at the top), to avoid over-stretching at the bottom or under-stretching at the top.

Stabilize blow molding pressure and timing

The high-pressure blowing pressure is set at 3-4MPa (4-5MPa for carbonated beverage bottles and 3-3.5 mpa for mineral water bottles). A pressure stabilizing valve is installed at the outlet of the air compressor to ensure that the pressure fluctuation is ≤±0.1MPa. Inspect the high-pressure air pipe joints and valves, replace worn sealing rings to prevent air leakage.

Optimize the blowing timing: Start blowing when the stretching rod descends to "1-2mm from the bottom of the preform" (adjust through equipment timing parameters), and set the blowing time to 0.8-1.2 seconds (adjust according to the size of the bottle type) to ensure that the plastic fully adheres to the mold cavity.

3. Inspect molds and equipment: Eliminate positioning and cooling defects

Repair/calibrate the mold

Disassemble the mold and measure the cavity size with a micrometer (for example, the roundness of the circular bottle cavity should be ≤0.05mm). For slightly worn areas, polish them with diamond polishing paste of 1000# or above. If the cavity is elliptical or concave, return it to the factory for CNC correction or replacement of the cavity inserts.

Clean the cooling waterways of the mold: Use a high-pressure water gun (or circulate and flush with citric acid solution) to unblock the blocked waterways. Check the temperature difference between the water inlet and outlet of the waterway (it should be ≤3℃). If necessary, add branches of the cooling waterway in areas with slow cooling.

Replace the positioning components of the equipment

Replace the loose spring of the preform gripper and adjust the clamping force of the gripper (ensuring that the preform does not slip or deform). Replace the worn mold locating pins (when the gap is greater than 0.05mm) to ensure that there is no misalignment after the mold is closed.

4. Control the quality of raw materials and preforms: Consolidate the foundation

Screening qualified preforms: Before going online, inspect the appearance of the preforms (free of impurities and bubbles), use a wall thickness gauge to measure the wall thickness difference (≤0.1mm) and AA value (≤0.2mm) of the preforms, and remove the unqualified preforms. Confirm the production parameters of the preform (such as injection pressure and cooling time) with the upstream preform injection manufacturer to avoid quality fluctuations between batches.

Stabilize raw material properties: Use PET raw materials from the same manufacturer and of the same grade in the same production batch. Test the melt index (MI controlled at 0.8-1.0g/10min) for each batch. If the MI fluctuates too much, adjust it to the qualified range through "raw material blending" (such as mixing high-MI and low-MI raw materials in proportion).

Iii. Verification and Preventive Measures

Small-batch trial production verification: After adjusting the parameters, first trial-produce 10 to 20 bottle bodies. Use a wall thickness gauge to measure the wall thickness of each area (difference ≤0.1mm), and use a verticality gauge to measure the tilt of the bottle body (≤0.5°). Only after confirming that it is qualified can mass production proceed.

Regular maintenance records: Clean the dust in the heating channel and inspect the guide sleeve of the tensile rod every week. Calibrate the blow molding pressure and test the mold cooling water circuit every month. The quality of preforms is randomly inspected every quarter, and key parameters (heating temperature, stretching speed, blow molding pressure) are recorded to form a "parameter file", which is convenient for subsequent fault detection.

Environmental control: Maintain a stable temperature in the bottle blowing workshop (20-25℃) and a humidity of no more than 60%. Avoid uneven cooling of the bottle body due to sudden temperature changes (for example, increase ventilation in the workshop during hot summer to prevent the mold temperature from being too high).

Through the above measures, the problems of uneven stretching and deformation of the bottle body can be effectively solved, ensuring uniform wall thickness and regular shape of the bottle body, and meeting the requirements of subsequent filling and use.