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What is the injection stretch blow moulding process (ISBM)

Process principle

It uses injection stretch blow moulding equipment to heat and melt the resin slices and inject them into the preform mould to form a preform, then place the preform adjusted to a suitable temperature into the blow mould, and then mechanically stretch it in the axial direction. , And pass compressed air into the preform for radial stretching, thereby relying on the blow mould for cooling and shaping to form a fixed-shaped container.

Product moulding of blow moulder generally goes through the following processes:

1. Resin drying (if involved);

2. The resin is plasticized and melted;

3. Injection preform;

4. Temperature adjustment;

5. Stretch blow moulding;

6. Take out the product.

The injection stretch-blow molding process of blow moulder can be divided into two-step injection stretch-blow molding and one-step injection stretch-blow molding process according to its technological characteristics.


Two-step injection stretch blow molding process

Two-step injection stretch blow molding of blow moulder machine is to complete the injection molding preform and stretch blow molding on two equipment.

1. First, a separate preform mould is used to inject the preform on the injection moulding equipment (as shown in the figure below).


2. Reheat the cooled preform on another blow moulding equipment, and then stretch blow moulding (as shown in the figure below).


3. Features of two-step injection stretch blow moulding process:

1. High production flexibility, preforms and blowing bottles can be produced separately, and even preforms can be sold separately;
2. The process is relatively independent and easy to control;
3. Easy to store;
4. The product structure is single, not suitable for producing special-shaped bottles and thick-walled products;
5. The requirements for equipment, moulds and processes are relatively simple.


One-step injection stretch blow molding process

One-step injection stretch blow molding of blow moulder is that the injection preform and stretch blow molding are completed on one piece of equipment. The bottle is directly produced (as shown in the figure below). Simplifies the production process and saves more energy and time.


Compared with the two-step injection stretch blowing method, the one-step injection stretch blowing has the following characteristics:

1. All forming actions are completed on one device, and product hygiene is more guaranteed;

2. Injection of preforms, temperature adjustment and stretch blow moulding on the same equipment can save more energy and be more environmentally friendly;

3. One set of equipment completes the production, and the product quality and stability are higher;

4. Lower requirements for the use of production site area;

5. Save transportation costs;

6. The moldable structure is more complex, and the wall thickness is thicker or thinner;

7. Higher requirements for mould accuracy and process conditions.

Whether it is one-step injection stretch blowing or two-step injection stretch blowing, it is currently the primary production process for injection stretch blowing products on the market. Due to differences in product production costs, structural features, use conditions, and equipment and mould input costs have their own demands and corresponding competitiveness.


The critical points of the structural design of the injection stretch blow bottle product

With the rapid development of current injection stretch blowing equipment and applied materials, injection stretch blowing bottles are widely used in all walks of life. However, due to the characteristics of the injection stretch blowing process and materials, it is necessary to design a beautiful product and meet the use requirements. In addition to selecting methods and materials, product design must follow specific rules.

The following briefly introduces some attention points in the structure design of the injection stretch blow bottle:

(1) Design of preform

Because the material used in the injection stretch-blow process is biaxially stretched, the bottle can have higher strength, better barrier properties, lighter weight and faster second moulds. Whether the design of the preform is reasonable or not is very important. It is related to whether the bottle is well-formed and its performance and has a lot to do with the product’s production mould and manufacturing cost.

Generally speaking, the preform stretch ratio and wall thickness design follow the following two significant principles: one is within the allowable range, the larger the stretch ratio, the better the adjustment of the product wall thickness distribution; the second is within the permissible content, The thinner the wall thickness of the preform, the shorter the bottle production cycle and the lower the production cost.


Design preform of different stretch ratios

However, the stretch ratio design of the preform has a great relationship with the characteristics of the material. The stretch ratio and wall thickness design of the commonly used materials are as follows:

PET: The longitudinal stretch ratio of flat bottles and eccentric bottles is 1.5-1.7 times. Ordinary narrow-necked round bottles can be designed between 1.5-2.5 times according to the weight and wall thickness. Wide-mouth bottles and thin-walled bottles can reach 2.5-3.0 times. The wall thickness is generally suitable for 2.3-4.5mm, and the weight of individual bottles with special requirements can get 5.5mm. But the thicker the wall, the longer the production cycle. It is easy to cause the transparency of the finished product to decrease, and it is easy to crystallize and turn white. Every time the preform’s wall thickness increases by 0.5mm, the production cycle will be lengthened by more than 3.5 seconds. It has a more significant impact on production capacity and production costs.

PP: PP materials are generally more suitable for forming products with a wide-mouth structure, and thick-walled products can be designed at 1.3-1.5 times. When the stretching ratio is 2 times, the moulding of the blow moulder equipment is better, the transparency is also high, and the thin-walled product can be designed to 2.3 times. It can be up to 3 times for thin-necked and thin-walled products, but it is more difficult to form, and it is necessary to increase pre-blowing and other processes. PP preforms are generally controlled at 3-4mm, which is more suitable for moulding in terms of wall thickness. If it reaches 5mm or more, the production cycle will be very long.

PC: Tritan material: 1.2-1.4 times is appropriate. The excessive stretch ratio will cause stress shrinkage or severe product water lines when heated. It is necessary to reduce the blowing pressure or mould temperature to improve it. The thickness of the preform is generally controlled at 4-5mm, which is more suitable for forming.

In addition to the most critical attention to the stretch ratio and wall thickness in the preform design, the taper of the preform also needs attention. The taper is related to the performance’s material, stretch ratio, weight, wall thickness, and mouth size. Under normal circumstances, the taper of the PET bottle preform is 0.75°-2° (see the figure below), and the taper of the part close to the bottle shoulder is larger. The minimum for PP preform is 2°, and for PP and PET wide-mouth bottles, it can reach 5°-25°. PC and Tritan materials can be controlled at 2°-5°.


(2) Design of bottle structure

Note stretch-blow bottles generally select the materials used according to the characteristics of the product and the use requirements, but also need to design the structure of the bottle according to the corresponding material to achieve a more optimized result, which is more conducive to improving the bottle performance and process adaptability, etc.

The following are some details that need to be paid attention to about the structure design of injection stretch blow moulding:

1. For the bottle mouth structure of the product, the following aspects are generally considered: First, the shape. Generally, the shape of the bottle mouth is designed to be round. The round shape is more conducive to ensuring the dimensional accuracy of the bottle mouth and better sealing and blowing with the lid. The distribution of the wall thickness of the bottle body during molding. The second is the bottle mouth structure, generally divided into threaded and buckle structures. The threaded system is more conducive to the sealing effect of the bottle and the lid. It is often used for packaging medicines, liquid beverages, and cosmetic bottles, together with various screw caps, insurance caps, spray heads and pump heads, etc., and the sealing reliability is high. The size and form of the thread can be flexibly selected according to the needs of the product. The buckle structure is often used for solid or paste packaging and can also be used for liquid packaging. The advantage is that it is convenient to use and suitable for high-speed filling. But when used for liquid packaging, pay attention to the design of the lid material, sealing structure and interference, and at the same time do an excellent job of process control to ensure its tightness. The third is the size of the bottle mouth. For PET, the bottle mouth size is more flexible, but for the PP material that is more suitable for forming wide-mouth bottles, the bottle mouth should not be too small. Otherwise, the product forming and wall thickness distribution will be more significant. Influence. Generally speaking, the diameter of the bottle body to the diameter of the bottle mouth is less than 2 times.

2. In bottle body design, injection stretch blow bottles, especially general-purpose PET bottles, have a flexible bottle shape, which can be cylindrical, elliptical, polygonal and various special-shaped structures. The bottle’s surface can be designed with frosted, pattern and concave-convex characters. It can also be designed with ribbed construction, which increases the rigidity of the bottle and makes it more beautiful. For PP, it should be noted that the structure should be cylindrical, regular polygonal and other systems as much as possible. Try not to change the diameter of the bottle too much and do not have sharp corners because if there are sharp corners, the wall thickness will be too thin, and the rigidity will be small. In addition, the difference between the diameter of the bottle body and the diameter of the bottle mouth should not exceed 2 times as much as possible. This is more conducive to product moulding and even distribution of wall thickness. Some unique products (such as infusion bottles) can require special materials and mould designs.

3. The wall thickness of the product. The wall thickness of the bottle should not be too thick without special requirements. Excessive wall thickness will increase the weight and cost of the bottle, but the moulding cycle will be longer and reduce the output. The most widely applicable material for injection stretch blowing is PET. The product’s wall thickness can range from ultra-thin 0.1mm to ultra-thick 2.0mm (unique materials). However, considering the crystallinity of ordinary PET materials, too thick wall thickness will affect the transparency, barrier properties and impact resistance of the product, so try not to exceed 1mm. The wall thickness of the bottle made of PP should not exceed 2mm. PC, Tritan and AS materials should also be about 1.5mm; not more than 2mm is suitable.

4. For products that do not have special requirements for bottle weight and wall thickness, when the rigidity of the product needs to be improved, it is not necessary to blindly increase the weight and wall thickness to increase its rigidity. Various forms of reinforcement or ribs can be added to the bottle surface design. Pattern, while reducing the R angle of different arcs on the surface to achieve the required effect of increasing the rigidity of the bottle. For special-shaped or flat products with secondary processing such as labelling or silk-screening on the surface, do not blindly pursue the uniformity of the product’s wall thickness but focus on ensuring the rigidity of the labelling or silk-screening part.


5. For the bottom of the product, there is no clear requirement for general products. It can be round or other various shapes. But for products with high-pressure resistance, high-temperature resistance or drop requirements. The bottom of the bottle should be designed with a deeper spherical surface or pattern to improve the strength and impact resistance of the bottom of the bottle. Try not to develop the bottom of the bottle with a sharp corner smaller than R3 (referring to the rounded design with a radius of 3mm). If the bottom of the bottle requires text or graphics, try not to be close to the centre of the bottom of the bottle. The farther away from the centre, the clearer the text and graphics. Smaller and shallower text or symbols can also be designed to be directly formed at the bottom of the preform. For PP material, because the material cools slowly, the bottom of the bottle is generally better to have a more profound arc or spherical surface. If it is too flat, it is easy to cause the bottom of the bottle to bulge after cooling.


The characteristics of injection stretch blow bottles compared to other bottles.

Compared with bottles formed by other processes, injection stretch blow moulding has the following characteristics:

1. The injection stretch-blow process is highly adaptable to materials and can be formed with various materials. It can meet product requirements for multiple functions, such as high transparency, high barrier properties, high impact resistance, high-temperature resistance and material safety requirements. It can be widely used in various fields such as food, cosmetics, medicine and various daily necessities.

2. The bottle has good surface precision and gloss and is beautiful and generous. It can be suitable for a variety of secondary processing techniques.

3. The bottle size has good precision and quality stability, which can meet various sealing, matching and automatic canning requirements.

4. Suitable for forming bottles of various shapes, including special-shaped bottles, eccentric bottles, flat bottles, wide-mouth bottles, narrow-neck bottles, small-volume bottles, large-volume bottles, etc.

5. Ultra-lightweight and thick-walled products can be formed to meet unique product requirements or cost-saving needs.

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John Lau.

John Lau.

John Lau, a project manager holding an engineering bachelor's degree, became fascinated with optimizing beverage production equipment during his university days. As an overseas project manager, he firmly believes that educating clients on achieving efficient workflows through customized equipment design is one of the most impactful aspects of his job.

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