Blow-Blow Method in Glass Bottle Manufacturing: Processes and Key Considerations

The blow-blow method is a forming process used to produce narrow-necked glass bottles. This process is characterized by the initial glass parison (also known as the preform) being blown into shape by compressed air after being pre-blown by the blow-head. The purpose of pre-blowing is to ensure that the glass drop and the neck ring mold fully integrate to form the bottle's neck. Subsequently, the parison is inverted and further blown into the final shape.

A complete set of molds should include the following components:

1. Neck Mold: Includes the neck ring and neck mold sleeve.

2. Core and Sealing Sleeve.

3. Blank Mold and Plunger.

4. Funnel and Blowhead.

5. Final Mold and Bottom Mold.

6. Blow Head.

7. Grippers.

While all these components are essential for production, the primary design challenge lies in determining the shape of the blank mold's inner cavity. Below is a detailed explanation of these components and their roles in the blow-blow process.

1. Concept and Application of Overcapacity Rate

1. Overcapacity Rate: This refers to the ratio of the total volume of the blank mold's inner cavity to the volume of the hot glass required for the product. The blank mold must accommodate not only the volume of hot glass but also the space needed for blowing air into the preform.

To illustrate, if a product has a bottle weight of 350g and a full volume capacity of 530mL, the calculation of the preform's maximum and minimum volumes would be as follows:

• Hot Glass Volume:

V glass=350/2.5*1.25=143.5ml

(Note: 2.5 is the assumed glass density ratio, and 1.025 accounts for the thermal expansion of glass.)

• Maximum Preform Volume = 143.5+530=673.5mL

• Minimum Preform Volume= 143.5mL

In practice, the preform volume should not be at these extremes. Experience shows that the volume of the hollow part of the preform should be about 40% of the hot glass volume:

P=(Vblank total-Vglass)/Vglass×100% 

Where P is the overcapacity rate, V blank total is the total volume of the blank mold’s inner cavity, and V glass​ is the volume of hot glass.

For the example product, with an overcapacity rate of 40%, the appropriate total volume of the preform would be:

V blank total = V glass(1+P)=143.5*(1+0.4)=200.9ml

1. Application of Overcapacity Rate: The overcapacity rate is critical for determining the total volume of the blank mold's inner cavity, which is essential for mold design. Different bottle wall thicknesses require different overcapacity rates, with thin-walled bottles needing a higher rate and thick-walled bottles needing a lower rate.

2. Extension Value: Role, Determination, and Application

1. Reheating of Preform: In the blow-blow process, the preform is reheated to ensure uniform temperature and viscosity before being blown into the final mold. This reheating helps achieve even wall thickness and a smooth surface. The extension value (H_extension) plays a crucial role in this process.
   

• H_mold: The total height of the final mold's inner shape plus the recessed depth of the bottom mold.

• H_guard: The height of the neck mold's guard, which is generally around 2mm.

• H_plunger: The depth to which the plunger is involved in forming.

• H_protrusion: The protrusion amount of the bottom mold. Extension value refers to the vertical distance between the lowest part of the preform and the highest part of the bottom mold at the moment the preform is transferred into the final mold. The extension value allows the preform to stretch under gravity before blowing, enhancing reheating efficiency and enabling high-speed forming.

1. Determination and Application of Extension Value: The extension value is determined based on equipment specifications or reference tables that consider the height and weight of the bottle. It ensures the proper alignment of the preform within the final mold, accounting for factors like mold depth and core involvement.

Hinitial​=Hmold​−Hguard​−Hextension​−Hplunger​−Hprotrusion

3. Determining the Inner Shape of the Blank Mold

The inner shape of the blank mold should mimic the final mold but with allowances for the preform's expansion. Key considerations include:

1. Neck Diameter: The blank mold's neck diameter should be 0.3-0.5mm smaller than the final mold to avoid material pinching and ensure proper alignment.

2. Bottom Dimensions: The bottom of the blank mold, where the plunger operates, should be about 0.55-0.65 times the corresponding dimension of the final mold to facilitate smooth material reception.

3. Shoulder Dimensions: For bottles with shoulders, the corresponding blank mold dimensions should be 0.5-0.6 times the final mold. For bottles without shoulders, this step is unnecessary.

After determining these dimensions, smooth transitions between sections should be designed to facilitate glass flow.

4. Finalizing the Blank Mold’s Inner Shape

Once the initial shape and dimensions are established, the total volume is calculated and adjusted to meet the required overcapacity rate. This iterative process ensures that the blank mold's design is optimized for production.

For complex bottle shapes, the principle remains to follow the final mold's profile, with specific design adjustments made to accommodate the round cross-section of the glass drop during loading.