How to control the uniformity of wall thickness of Dalian blow molded products in detail?

As an important plastic product, Dalian blow molding products have broad application prospects and market demand. With the continuous advancement of process technology and the improvement of environmental protection requirements, blow molding products will continue to play an important role in the future and achieve innovation and development in multiple fields. By optimizing processes, improving product quality, and using environmentally friendly materials, blow molded products will bring more convenience and value to society.

The production efficiency of blow molding technology is high, the material utilization rate is high, and it can significantly reduce production costs. Blow molding technology can produce various complex shaped hollow products to meet the needs of different customers. Blow molded products are lightweight, easy to transport and use, and meet environmental requirements. Blow molded products have good impact resistance, corrosion resistance, and sealing properties, which can meet the needs of various usage environments.

How to control the uniformity of wall thickness of Dalian blow molded products in detail?

1、 Material selection: Improving uniformity foundation from the source

1. Matching of melt strength and flowability

Priority given to high melt strength materials:

Choosing polymers with long branched structures (such as cross-linked polyethylene XLPE and metallocene polyethylene mPE) can reduce the tendency of billet sagging and enhance the ability to maintain wall thickness during blowing.

Case: When the blowing ratio of traditional HDPE billets exceeds 2:1, it is easy to have a thin bottom. After switching to mPE, the blowing ratio can be increased to 3:1, and the uniformity of wall thickness can be improved by 15%.

Melt index (MI) control:

Low MI materials (such as MI=0.3~1.0g/10min) have high melt viscosity and less billet elongation, making them suitable for large containers (such as 200L storage tanks);

High MI materials (such as MI=5-10g/10min) have good flowability, but they need to be combined with rapid inflation process (≤ 1 second) to avoid excessive sagging.

2. Additive optimization

Anti sagging additive: Add 0.5%~1% nucleating agent (such as talc powder, silica) to enhance the elasticity of the melt and reduce the sagging of the billet before the mold is closed;

Temperature stabilizer: Add antioxidants (such as hindered phenols) and heat stabilizers (such as calcium zinc composite stabilizers) to prevent melt degradation and thinning at high temperatures, which can affect wall thickness distribution.

2、 Mold design: build the physical basis for uniform inflation

1. Optimization of die structure

Uniformity of annular die gap:

The die flow channel adopts streamlined design (such as fish tail and hanger style) to avoid uneven discharge caused by molten retention;

The clearance tolerance of the mold lip should be controlled within ± 0.01mm, and regular testing with a feeler gauge should be carried out to ensure that the deviation of the circumferential wall thickness of the billet is ≤ 2%.

Application of storage cylinder mold head:

Large scale products (such as pallets and car fuel tanks) use storage cylinder molds, which quickly push the melt through plungers to reduce billet sagging and improve wall thickness uniformity to within ± 5%.

2. Design of cavity shape and inflation ratio

Inflation ratio control:

Blowing ratio=product to large diameter/billet diameter, recommended value of 1.5:1~2.5:1. Excessive blowing ratio may cause thin cracks (such as shoulder wall thickness deviation may exceed 20% when blowing ratio>3:1);

Complex shaped products (such as curved shaped bottles) adopt asymmetric inflation ratio, reducing the inflation ratio to 1.2:1 in difficult to fill mold areas (such as bottlenecks).

Stretch ratio collaborative design:

Stretching ratio=product height/billet height, which together with blowing ratio determines the wall thickness distribution. It is recommended that stretching ratio x blowing ratio be 3-6 (such as stretching ratio 2, blowing ratio 3) to ensure balanced force distribution in all directions.

3. Refinement of exhaust system

Exhaust hole layout:

At the highest point of the mold cavity (such as the top of the container) and at the corner (such as the transition zone between the bottle body and the bottle bottom), open exhaust holes with a diameter of 0.5~1mm and a spacing of 50~100mm;

Deep cavity products (such as barrels) use needle valve exhaust, which instantly releases trapped air through pneumatic needle valves during inflation to avoid local wall thickness deviation.

3、 Process parameter adjustment: dynamic balance inflation process

1. Billet extrusion stage

Extrusion temperature control:

The temperature fluctuation of the melt is ≤± 5 ℃, and the temperature in the high-temperature zone (such as the front section of the mold head) is 10-15 ℃ higher than that in the barrel, which improves the uniformity of the melt;

Example: When blow molding PET bottles, setting the barrel temperature to 260-280 ℃ and the die temperature to 270-290 ℃ can reduce the axial wall thickness fluctuation of the preform.

Matching extrusion speed and traction speed:

Traction speed=extrusion speed x (1+billet sag rate), and the sag rate is calculated in real time through weighing method (if the weight of the billet decreases by 5% every 10 seconds, the traction speed needs to be increased by 5% for compensation).

2. Inflation stage

Inflation pressure and speed:

Low pressure and low speed (0.2~0.5MPa, inflation time 1-3 seconds) are suitable for thin-walled or complex shaped products, reducing uneven wall thickness caused by impact;

High pressure and high speed (1-3MPa, inflation time<1 second) are suitable for thick walled products, and rapid mold filling can avoid local incomplete filling caused by cooling and hardening of the billet.

Inflation delay time:

The time from mold closure to the start of inflation is ≤ 0.5 seconds. Prolonged delay can lead to uneven cooling of the billet, especially in low-temperature environments (such as<15 ℃), which needs to be shortened to 0.3 seconds.

3. Cooling stage

Cooling medium temperature and flow rate:

The temperature of the cooling water should be controlled between 15~25 ℃, with a flow rate of ≥ 2m/s, ensuring that the temperature difference on the mold surface is ≤ 3 ℃;

Local strong cooling (such as adding cooling nozzles) is used in thick walled areas (such as the bottom of containers) to extend the cooling time by 20% to 30% and reduce uneven shrinkage caused by differences in cooling speed.

4、 Billet control technology: precise intervention in key links

1. Parison Programming for preform wall thickness

By driving the lip gap change through a servo motor, the local wall thickness is adjusted in real-time during the extrusion process of the billet

For areas of the product that are prone to thinning (such as bottle shoulders), pre program to increase the thickness of the corresponding part of the billet by 10% to 20%, and after inflation, counteract the stretching thinning;

Equipment: Adopting a blow molding machine with an electric die lip, the wall thickness control points can reach more than 50, with an accuracy of ± 0.05mm.

2. Billet sag compensation

Weighing feedback control: Weigh the weight of the heavy billet online, and automatically adjust the extrusion speed or die lip gap when the weight deviation caused by vertical stretching is greater than ± 1.5%;

Gas assisted support technology: During the extrusion of the billet, compressed air of 0.05~0.1MPa is introduced from the center of the die to form an "air core" to support the billet and reduce sagging (especially suitable for large billets with a height greater than 1m).

3. Multi station synchronous control

When using multi-layer blow molding (such as 3-layer co extrusion), the temperature difference between each layer of the melt should be ≤ 5 ℃, and the synchronization error of the extrusion speed should be ≤ 0.5% to avoid uneven wall thickness caused by interlayer slip;

Rotating die head technology: The die head rotates at a speed of 5~10r/min to counteract the circumferential flow velocity difference of the melt caused by the rotation of the screw, resulting in a deviation of less than 1% in the circumferential wall thickness of the billet.

Blow molding is a widely used molding process in the production of plastic products, mainly used to manufacture hollow products such as bottles, containers, cans, buckets, etc. The blow molding process involves blowing molten plastic material into a mold to shape it into the desired shape. Due to its high efficiency, flexibility, and low cost, blow molding technology has been widely used in various industries such as packaging, chemical, food, and medicine.

The basic principle of blow molding technology is to heat thermoplastic to a molten state, and then blow mold the plastic into hollow products through the action of molds and air pressure. The specific process is as follows: Raw material preparation: Select suitable thermoplastic materials, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc. Heating and melting: Heating plastic particles or powders through an extruder or injection molding machine to a molten state, forming tubular or sheet-like plastic blanks. Blow molding: Place the molten plastic blank into a mold, inflate it with air pressure, and make it tightly adhere to the inner wall of the mold to form the desired shape. Cooling and shaping: After blow molding is completed, the product is cooled and shaped in the mold, and then demolded and taken out.