What Is Blow Molding?

Blow molding also called hollow blow molding, a rapidly developing plastic processing method. 

Blow molding is a manufacturing process for forming and joining together hollow plastic parts. It is also used for forming glass bottles or other hollow shapes.

In general, there are three main types of blow molding, extrusion blow molding, injection blow molding, and injection stretch blow molding.

The blow molding process was used to produce low-density polyethylene vials during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used.

The volume of the hollow container can reach thousands of liters, and some production has adopted computer control. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc.

The resulting hollow containers are widely used as industrial packaging containers. According to the manufacturing method of the parison, blow molding can be divided into extrusion blow molding and injection blow molding. The newly developed multi-layer blow molding and stretch blow molding.

Blow mold

The tubular plastic parison obtained by extrusion or injection molding of the thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state). After the mold is closed, compressed air is injected into the parison to blow the plastic parison.

It expands and clings to the inner wall of the mold, and after cooling and demolding, various hollow products are obtained.

The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use molds. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion.

The blow molding process was used to produce low-density polyethylene vials during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used.

The volume of the hollow container can reach thousands of liters, and some production has adopted computer control. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc. The resulting hollow containers are widely used as industrial packaging containers.

Plastic mold common sense

Extrusion blow molding is a method of manufacturing hollow thermoplastic parts. Well-known blow molding objects are bottles, barrels, cans, boxes, and all containers for packaging food, beverages, cosmetics, medicines, and daily necessities.

Large blow-molded containers are usually used for packaging chemical products, lubricants, and bulk materials. Other blow molded products include balls, bellows, and toys. For the automotive industry, fuel tanks, car shock absorbers, seatbacks, center brackets, armrests, and headrest coverings are all blow molded?

For machinery and furniture manufacturing, blow-molded parts include shells, door frames, racks, clay pots, or boxes with an open face.

Polymer

The most common raw material for blow molding plastic is high-density polyethylene, and most milk bottles are made of this polymer. Other polyolefins are also often processed by blow molding. 

Depending on the application, styrene polymers, polyvinyl chloride, polyester, polyurethane, polycarbonate, and other thermoplastics can also be used for blow molding.

Recently, engineering plastics have been widely accepted in the automotive industry. Material selection is based on mechanical strength, weather resistance, electrical properties, optical properties, and other properties.

Craft

3/4 of the blow-molded products are manufactured by extrusion blow molding. The extrusion process is to force the material through a hole or die to make a product.

The extrusion blow molding process consists of 5 steps:

1. Plastic preform (extrusion of the hollow plastic tube);
2. Close the flap mold on the preform, clamp the mold, and cut off the preform;
3. To the cavity Cold-wall inflation type training, adjust the opening and maintain a certain pressure during cooling;
4. Open the mold, remove the blown parts;
5. Trim the flash to obtain the finished product.

Extrusion

Polymer mixing is defined as a process in which polymers or polymer systems are upgraded through melt mixing. The mixing process ranges from the addition of a single additive to the treatment of multiple additives, polymer alloys, and reactive mixing. It is estimated that one-third of polymer production in the United States is compounded.

Mixing ingredients can be customized according to the performance requirements of the final application. Mixed products have hybrid properties, such as high gloss and excellent impact strength, or precision moldability, and good stiffness.

The compounded polymer is usually pelletized for further processing. However, the industry is increasingly interested in combining compounding with the next steps, such as profile extrusion, to avoid reheating the polymer.

Mixing

People use various types of melt mixing equipment, from roll mills and batch mixers to single-screw and twin-screw extruders. Continuous compounding (extruder) is the most commonly used equipment because it can provide products of consistent quality and can reduce operating costs.

There are two types of mixing: distributed mixed materials can be evenly distributed in the mixed ingredients without using high shear stress. This type of mixed liquid is called extensible mixing or laminar mixing.

Dispersive mixing, also known as intensive mixing, in which high shear stress is applied to break up the solids that agglomerate. For example, when the additive mass is broken up, the actual particle size becomes smaller.

Compounding operations often require two types of mixing in one process.

Application scope of blow molding

Commonly, people’s daily necessities such as bottles, jars, baby products, sporting goods, etc.

Blow molding technology

Blow molding technology is a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of the thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state).

After the mold is closed, compressed air is injected into the parison to blow the plastic parison It expands and clings to the inner wall of the mold, and after cooling and demolding, various hollow products are obtained.

The blow molding process was used to produce low-density polyethylene vials during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used.

The volume of the hollow container can reach thousands of liters, and some production has adopted computer control. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc. The resulting hollow containers are widely used as industrial packaging containers.

According to the manufacturing method of the parison, blow molding can be divided into extrusion blow molding and injection blow molding. The newly developed multi-layer blow molding and stretch blow molding.

Prospects

In recent years, with the development of various functional beverage and wine packaging markets, various multi-layer co-injection-blow molding technologies have become a hot spot in the market.

Flexible one-step production technology

Injection stretch blown hollow molded products have the advantages of uniform wall thickness, good dimensional stability, lightweight, and high strength, so the application range is very wide. Because most of the containers produced by the injection stretch blowing process use PET, PP, PE, PVC, and other materials as raw materials, and the bottles produced using these raw materials have the advantages of a lightweight, portable, non-toxic, transparent, hard to break, and good barrier properties.

It has increasingly become a substitute for medicinal glass bottles and has been used in the packaging of carbonated beverages, fruit juice beverages, drinking water, edible oil, alcohol, daily chemical products, pharmaceuticals, and other industries.

The injection stretch blowing process is divided into two steps and one step. The one-step method is to complete the entire molding process from raw materials to containers on one device.

The two-step process requires two pieces of equipment to complete the molding process from raw materials to products.

The two-step method is mainly suitable for mass production and has great flexibility. You can use one injection machine with multiple blow molding machines, or multiple injection machines with one blow molding machine. Of course, this mainly depends on the output matching of the injection machine and the bottle blowing.

Control points

The process flow of the blown film is rough as follows:

Material plasticizing and extruding on the hopper→blowing traction→air ring cooling→herringbone splint→traction roller traction→corona treatment→film winding

However, it is worth pointing out that the performance of the blown film has a great relationship with the production process parameters. Therefore, in the film blowing process, it is necessary to strengthen the control of process parameters, standardize process operations, and ensure the smooth progress of production. Obtain high-quality film products. 

In the production process of polyethylene blown film, the following process parameters are mainly controlled:

1. Extruder temperature

When blowing low-density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160°C and 170°C, and the temperature of the die must be uniform, the extrusion temperature is too high, the resin is easy to decompose, and the film is brittle, especially Make the longitudinal tensile strength drop significantly;

If the temperature is too low, the resin will not be plasticized and cannot be expanded and stretched smoothly. The tensile strength of the film is low, and the surface gloss and transparency are poor, and even appear like wood growth rings. The pattern and the unmelted crystal core (fisheye).

2. Blow up ratio

The blow-up ratio is one of the control points of the blown film production process, which refers to the ratio between the diameter of the film bubble after inflation and the diameter of the uninflated tube ring. The blow-up ratio is the transverse expansion multiple of the film.

In fact, the film is stretched in the transverse direction. Stretching will produce a certain degree of orientation effect on the plastic molecules, and the expansion ratio will increase, thereby increasing the transverse strength of the film.

However, the blow-up ratio should not be too large, otherwise, the bubble will be unstable and the film will be prone to wrinkles.

Therefore, the inflation ratio should be properly matched with the traction ratio. Generally speaking, the inflation ratio of low-density polyethylene (LDPE) film should be controlled at 2.5-3.0.

3. Traction ratio

The pull ratio refers to the ratio between the pulling speed of the film and the extrusion speed of the tube ring. The traction ratio is the longitudinal stretching multiple, which makes the film have a directional effect in the drawing direction.

If the traction ratio increases, the longitudinal strength will also increase, and the thickness of the film will become thinner. However, if the traction ratio is too large, the thickness of the film will be difficult to control, and the film may even be broken, causing film breakage. The traction ratio of low-density polyethylene (LDPE) film is generally controlled between 4-6.

4. Dew point

The dew point is also called the frost line, which refers to the dividing line of the plastic from the viscous flow state to the high elastic state. During the film blowing process, low-density polyethylene (LDPE) is in a molten state when extruded from the die, and has good transparency. After leaving the die, the blowing zone of the film bubble should be cooled by the cooling air ring.

When the cooling air is blown to the plastic film bubble just extruded from the die at a certain angle and speed, the high-temperature film bubble and cooling air In contact, the heat of the film bubble will be taken away by the cold air, and its temperature will obviously drop below the viscous flow temperature of low-density polyethylene (LDPE), causing it to cool and solidify and become blurred.

On the blown film bubble we can see a dividing line between transparency and blur, which is the dew point (or frost line).

During the film blowing process, the level of the dew point has a certain impact on film performance. If the dew point is high and is located above the inflated bubble, the film is inflated in a liquid state. Inflation only makes the film thinner, and the molecules are not stretched orientated.

At this time, the performance of the inflated film is close to casting film. On the contrary, if the dew point is relatively low, the inflation is carried out in a solid-state.

At this time, the plastic is in a high elastic state, and the inflation is like a transverse stretch, which makes the molecules orientate so that the performance of the inflation film is close to the Orientation film.

Pollutants produced

In the blow molding process, the plastic particles produce a small amount of organic waste gas due to high-temperature volatilization, and the main component is non-methane total hydrocarbons.