Extrusion Blow Molding Of Polyvinyl Chloride (PVC) Films

 

 

After adding plasticizers and other additives to PVC resin, flexible PVC films can be produced via the blow molding process. Depending on the type of stabilizer used, these films can be either transparent or translucent. They are widely used as agricultural seedling-raising films, ginseng covering films, greenhouse films, packaging films for various industrial raw materials, semi-finished products, and finished products, as well as protective sticker films for paper.

 

1. Formulations

The formulations of flexible PVC blow-molded films are presented in Table 6-13.

Table 6-13 Formulations of Flexible PVC Blow-Molded Films

Raw Material	Dosage / Parts by Mass
	Formulation 1	Formulation 2	Formulation 3
Polyvinyl Chloride Resin (PVC)	100	100	100
Dioctyl Phthalate (DOP)	20	12	12
Dibutyl Phthalate (DBP)	10	9	10
Dioctyl Sebacate (DOS)	7	4	-
Petroleum Ester	-	5	-
Epoxidized Octadecene	4	5	6
Barium Stearate	1.6	1.5	0.4
Cadmium Stearate	0.6	0.6	0.8
Organotin Stabilizer	0.5		1.5
Paraffin	0.3	0.3	-
White Oil	-	-	0.3

Notes on the Formulations of Flexible PVC Blow-Molded Films:

Formulation 1 is for agricultural films; Formulation 2 for packaging films; Formulation 3 for sticker films.

Medium-viscosity SG-4 type PVC resin is selected. To reduce "fish eyes" (gel particles) in sticker films, Formulation 3 should use resin with a loose structure and a narrow molecular weight distribution. If Formulation 2 is used for food packaging, non-toxic resin must be chosen.

The dosage of plasticizers is generally 30–40 parts by mass, with a maximum of 45 parts. Exceeding this limit will make the bubble tube too soft, leading to poor shape stability and difficulty in process control. Considering low-temperature application, both agricultural films and packaging films require the addition of DOS (a cold-resistant plasticizer), and agricultural films (used in winter) need a higher dosage of DOS. Epoxy resins also serve as stabilizers.

The stabilizer system adopts organotin or cadmium stearate, which offer good transparency; lead-based stabilizers are not used, as sticker films, agricultural films, and packaging films all require transparency.

Lubricants such as paraffin and white oil are added. Due to the relatively low dosage of plasticizers, a small amount of lubricants is necessary to improve the processability of materials and prevent film adhesion.

If Formulation 1 is used for anti-dripping agricultural films, a small amount of surfactants (e.g., glyceryl monostearate, xylitol stearate) should be added, with the dosage not exceeding 1 part by mass. Surfactants prevent dripping on the inner surface of the film, which is beneficial for seedling growth.

Since agricultural films are used outdoors for long periods, ultraviolet absorbers and antioxidants can be added appropriately based on the light intensity in the application area.

Additives that inhibit plant growth, are absorbed by plants and then transferred to the human body (posing health risks), or cause environmental pollution are strictly prohibited in agricultural film formulations.

 

2. Main Production Equipment

(1) Mixing Equipment: A three-roll mill with 400 mm diameter rolls (equipped with water cooling); a high-speed mixer with a volume of 200 L.

(2) Extrusion Granulation Equipment: An extrusion granulator with a screw diameter of 65 mm, a length-to-diameter ratio (L/D) of 15–20, and a gradually tapered screw.

(3) Blow Molding Equipment: An extruder with a screw diameter of 65 mm, an L/D ratio of 20–25, and a gradually tapered screw. Most dies adopt a mandrel-type structure.

 

3. Production Process Flow

PVC films are produced using either the upward blow molding method or the horizontal blow molding method. There are two blow molding process flows:

One involves direct extrusion blow molding of powder, using a twin-screw extruder or a single-screw extruder with a large L/D ratio (suitable for powder processing).

The other involves granulation first, followed by blow molding. The process flow of the latter is shown in Figure 6-20.

 

4. Production Process Conditions

(1) Batching and Granulation

To remove impurities from materials:

PVC resin is sieved through a 40-mesh sieve (sieve hole diameter: 0.425 mm).

Plasticizers are filtered through a 100-mesh copper screen (screen hole diameter: 0.15 mm).

Other additives are diluted with plasticizers and ground using a three-roll mill to a fineness of less than 80 μm.

The materials are then weighed according to the formulation and placed in a kneader. The kneading temperature is controlled at 100–120 °C, and the material is discharged when it becomes loose and elastic. The kneaded material is fed into an extrusion granulator for granulation, with the temperature controlled at 150–170 °C (excessively high temperatures should be avoided).

(2) Extrusion Temperature

To ensure blow molding quality, preheated granules are preferred, as they help reduce energy consumption and improve film plasticization quality. The extrusion temperatures for PVC films are listed in Table 6-14.

Table 6-14 Extrusion Temperatures for PVC Films

Film Type	Barrel Temperature / °C			Die Temperature / °C
	Zone 1	Zone 2	Zone 3	Connector	Mouth model
Agricultural Film	155 ～ 165	165 ～ 175	175 ～ 185	160 ～ 170	170~180
Packaging Film	145 ～ 155	155 ～ 165	165 ～ 175	165 ～ 175	175~185
Sticker Film	160 ～ 170	170 ～ 180	180 ～ 190	170 ～ 180	180~190

As the dosage of plasticizers increases, the extrusion temperature decreases. However, the extrusion temperature cannot be too low, otherwise, the film surface will become uneven, and defects such as "fish eyes" and hard lumps will occur.

(3) Blow-Up Ratio

The blow-up ratio for PVC films is generally 1.5–3.0. The blow-up ratio affects the film's strength and transparency. For the horizontal blow molding method with a small folded diameter, a larger blow-up ratio is selected. Under the same longitudinal draw ratio, increasing the blow-up ratio improves the film's transverse strength but reduces its longitudinal strength; therefore, the blow-up ratio should not be excessively large.

(4) Draw Ratio

The draw ratio for PVC films is generally 4–6, meaning the drawing speed is 4–6 times faster than the extrusion linear speed. In actual operation, a draw ratio larger than the blow-up ratio ensures better bubble tube stability. However, for balanced longitudinal and transverse properties of the film, it is preferable that the draw ratio is equal to or close to the blow-up ratio.

The increase in drawing speed is also limited by the film's cooling efficiency. Insufficient cooling will cause the film to stick together, making it impossible to separate and affecting its usability.

(5) Position of Cooling Air Ring

The distance between the air outlet of the cooling air ring and the die orifice is generally 50–200 mm. A smaller distance is used for film tubes with larger diameters, and a larger distance for those with smaller diameters. This distance should not be too large, otherwise, the film cooling line will be too high, which is unfavorable for film tube cooling and bubble shape stability. The height of the cooling line is usually controlled at 200–400 mm.

(6) Cooling Air Volume

The cooling air volume directly affects the cooling effect. A larger air volume improves cooling efficiency, but excessively large air volume will cause the film to oscillate, affecting the stability of the bubble tube. Conversely, insufficient air volume will lead to poor film cooling and adhesion.

(7) Angle Control of Cooling Clamping Plates

The angle of the cooling herringbone clamping plates must be well-controlled: 30° for the horizontal blow molding method and 60° for the upward blow molding method. An excessively large angle will cause wrinkles on the film.

 

 

Extrusion blow molding is one of the processes for forming rigid PVC films. Films produced via this process exhibit high transparency, good strength and toughness, excellent impact and tear resistance, good airtightness, and are non-toxic, odorless, and tasteless-effectively preserving the freshness and aroma of packaged goods. Transparent rigid PVC blow-molded films resemble cellophane in appearance and are used for packaging cigarettes, candies, and various electrical and electronic components.

 

1. Formulation

Rigid PVC transparent packaging films are plasticizer-free rigid PVC products. SG-6 type resin is preferred for its good processability. If used for food packaging, food-grade resin must be selected, with the residual vinyl chloride monomer content in the resin less than 5 mg/kg. A typical formulation of rigid PVC transparent packaging films is shown in Table 6-15.

Table 6-15 Typical Formulation of Rigid PVC Transparent Packaging Films

Raw Material Name	Dosage / Parts by Mass	Raw Material Name	Dosage / Parts by
PVC	100	Slippery	0.5~1.0
MBS	5~10	Lubricant	3~4
Processing Modifier	1~3	Colorants	Amount
Stabilizer	2~4	-	-

 

2. Main Production Equipment

A high-speed mixer with a volume of 300 L.

An extrusion unit equipped with an extruder (screw diameter: 100 mm, L/D ratio: 25, compression ratio: 3) with a barrier-type screw structure.

A die with an orifice diameter of 250 mm and a gap of 1.3 mm.

 

3. Production Process

The process flow of rigid PVC transparent films is shown in Figure 6-21.

 

The above process flow involves direct blow molding of powder. If the extruder is not suitable for direct powder processing, granulation should be performed first (excessively high granulation temperatures should be avoided). The transparent PVC films produced by blow molding can be used directly or after antistatic and heat-sealing coating treatments. When used for cigarette packaging, the film surface needs to be coated with a low-temperature heat-sealing material, whose main component is vinyl chloride-vinyl acetate copolymer resin.

Rigid PVC transparent packaging films are generally formed using the horizontal extrusion-upward blow molding method. The main operating conditions are as follows:

Batching and Kneading: Weigh the materials according to the formulation, add them to a high-speed kneader, and knead for 5–8 minutes at a temperature of approximately 102 °C. After kneading, transfer the material to a low-speed kneader to cool it to 40–50 °C.

Extrusion Blow Molding: Powder blow molding is similar to conventional granule blow molding. The extruder temperature is 160–180 °C, the die adapter temperature is 180–190 °C, and the die temperature is 190–210 °C. The film blow-up ratio is 2–3, and the traction speed is 10–30 m/min.