The Press-Blow Forming Process for Glass Containers---You Do Not Want To Miss It

The press-blow forming process differs from the blow-blow method. In the press-blow process, the parison (also known as the preform) is pressed upward by a plunger. As shown in Figure 1, when the plunger moves upward to the pressing position, the gap between the plunger, the neck ring, the blank mold, and the head allows the parison with a specified weight to be formed.

(Figure 1: Press-Blow Method Diagram)

The tasks of the engineer are: firstly, to determine the shape and dimensions of the neck ring, blank mold interior, and the forming head. Secondly, to ensure that the volume of the forming parts of the neck ring, blank mold, and head, minus the volume of the forming part of the plunger, equals the volume of the glass gob.

From Figure 1, we can see that the plunger cannot move upward indefinitely (when the parison weight is too light or the blank mold design is too large, the plunger needs to move further up), nor can it retract too much (when the parison weight is too heavy or the blank mold design is too small).

The plunger cannot move too far upward because as it does, the gap between the plunger and the neck ring and blank mold interior decreases, making the parison thinner. When this gap becomes too small, the parison will rapidly harden due to local thinness, obstructing the plunger's upward movement and preventing the glass from fully filling the neck ring, resulting in defects. Conversely, the plunger cannot retract too much because when it does, the conical part of the plunger originally within the neck ring will retract, creating a large gap and causing the glass to be squeezed out of the neck ring, leading to burrs at the bottle neck. Thus, mold designers must pay close attention to the pressing position of the plunger and the gap between the plunger and blank mold interior at this position.

Determining the Shape of the Blank Mold Interior

As shown in Figure 2, the design parameters for the press-blow mold can be determined. This will determine the height of the blank mold interior.

(Figure 2: Press-Blow Mold Design Parameters Diagram)

A. Determining the Extension Value (H_ext)

The function and selection of the extension value have been introduced in the blow-blow method design. Under the same conditions, the extension value in the press-blow method is generally about twice that of the blow-blow method. This is because in the press-blow method, the initial forming of the parison by pressing saves some actions compared to the blow-blow method. The parison in the pressing process contacts the relatively conductive iron mold both internally and externally, resulting in a harder parison than that formed by blowing. Since the plunger cannot press too far upward, the parison’s bottom is relatively thicker, allowing for more time for reheating and stretching during bottle making to produce uniformly thick and bright products.

The extension value in the press-blow method can be determined as shown in Figure 3:

Hfinal/mm	Hext/mm
bottle and jar's weight/g
30-225	225-680	680-1135	1135-1600	1600-2000
12-25	1
26-38	2
39-50	2.5
51-64	3
65-78	3.5	7
79-90	4	7.5
91-102	5	8
103-116	6	9
117-130	7	10
131-142	8	11	13
143-155	9	12	14
156-168	10	13	15
169-182	11	14	16
183-194	12	15	17
195-208	13	16	18
209-220	14	17	19
221-234	15	18	21	25	29
235-246	16	20	23	27	31

(Figure 3: Press-Blow Method Extension Value Table)

B. Determining the Height of the Blank Mold Interior (H_init)

Based on the forming process and Figure 2:

H_init = Final mold cavity height + Depth of the bottom mold concavity - Height of the bottom mold convexity - Neck ring height - Extension value - Forming head depth

Explanations for the related terms:

Final mold cavity height (H_final): The height of the portion of the final mold shape forming the bottle shape.

Depth of the bottom mold concavity (H_concave) and Height of the bottom mold convexity (H_convex): The height dimensions of the two axial parts in the bottom mold involved in forming.

Neck ring height (H_neck): The height of the straight cylindrical section forming the neck at the bottom of the neck ring, which should be 0.3-0.5mm smaller in diameter than the corresponding dimension of the final mold. The neck ring height is generally 2-3mm.

Extension value (H_ext): As previously mentioned, its value is shown in Figure 3.

Forming head depth (H_head): Generally 3-5mm.

2. Determining the Height Dimensions of Other Sections in the Blank Mold Interior.

As shown in Figure 4, the design parameters for the press-blow mold are illustrated. The explanations for the height parameters are as follows:

H_arc: The height of the shoulder arc section, representing the center of the shoulder arc. The value is H_arc = (0.4-0.45) × H_init.

H_waist: The height of the termination of the straight column section, which is H_waist = 0.5 × H_init.

H_bulge: The height at the maximum diameter between the waist and the bottom (if there is a bulge at the bottom). The value is H_bulge = (5/6) × H_init.

(Figure 4: Press-Blow Method Mold Design Parameters Diagram)

3. Determining the Diameters and Shoulder Arcs of Various Sections in the Blank Mold Interior.

The shape of the blank mold interior should generally be as shown in Figure 4. This shape ensures that the plunger’s pressing resistance is not too great and that the glass distribution is relatively uniform. The diameter parameters are explained as follows:

D_neck: The diameter of the parison neck, which should match the neck ring. The value is D_neck = corresponding dimension in the final mold - (0.2-0.5). Its function, as described in the blow-blow method, is to serve as a positioning element during the transfer.

D_waist: The diameter of the parison waist. The value is D_waist = (0.45-0.55) × the bottle body diameter.

D_bulge: The maximum diameter between the waist and the bottom bulge. The value is D_bulge = D_waist + (0.0-0.1) × the bottle body diameter. It should be noted that not all products have this extra bulge size. It is only necessary when the lower part of the product is relatively large or the bottle body is relatively high to ensure that the bottom wall thickness does not become too thin; otherwise, D_bulge should match D_waist.

Additionally, the shoulder arc (R_shoulder) serves as a transition.

If the R_shoulder is well connected with the plunger design, it reduces the working resistance of the plunger during pressing and ensures uniform shoulder wall thickness. The calculation of R_shoulder is purely mathematical and is illustrated in Figure 5:

(Figure 5: Calculation of Shoulder Arc in Press-Blow Method Mold)

a=(D_neck-D_waist)/2

X= R_arc-a

R_arc= √￣(H_arc²+x²）

R_arc ²= H_arc ²+X ² = H_arc+ ( R_arc-a) ²= H_arc+R_arc²-2aR_arc+a²

2aR_arc= H_arc²+a²

R_arc=( H_arc²+a²） /（2a)

Since both H_arc and a are known, R_arc can be calculated.

Designing the Outer Shape of the Plunger Forming Section

The outer shape of the plunger forming section (dimension parameters) should generally be as shown in the left half of Figure 6:

(Figure 6: Press-Blow Method Plunger Dimension Parameters)

A. Explanation of Axial Dimensions

h_neck: The part of the plunger included in the neck ring. The value equals the height of the bottle neck plus the neck ring height.

h_arc: The height of the large arc section of the plunger. The value equals 0.4-0.5 times the H_init in Figure 2, matching H_arc.

h_sphere: The height of the hemispherical part at the top of the plunger. Since its shape is spherical, h_sphere approximates the radius of the sphere, which is half of d_sphere in the diagram.

h_solid: The thickness of the solid material between the plunger and forming head line, generally around 10-15mm. If the bottle bottom is thicker and the material heavier, a larger value is used; otherwise, a smaller value is chosen.

B. Explanation of Radial Dimensions

d_inner_neck: The maximum radial dimension of the plunger within the neck ring. The value equals the inner diameter of the bottle neck, which determines the inner diameter of the product bottle neck during forming. Therefore, designers cannot arbitrarily change this dimension. This diameter should extend downwards for at least 3mm to ensure that when the gob weight increases, the plunger can retract smoothly.

d_neck: The diameter at the junction between the neck ring and the blank mold. The value should meet two conditions: it must be at least 1mm smaller than d_inner_neck and at least 6mm smaller than the corresponding dimension (D_neck) of the blank mold interior. The first condition ensures that the plunger can retract smoothly after pressing, and the second condition reduces the pressing resistance and ensures the plunger can reach the "pressing position."

d_waist: The diameter of the plunger waist. It should be 12-16mm smaller than the corresponding dimension (D_waist) in the blank mold interior. This ensures that the gap between the plunger and the blank mold interior at the pressing position is around 6-8mm, reducing pressing resistance and ensuring uniform wall thickness distribution in the parison for subsequent blowing.

d_sphere: The diameter of the hemispherical part at the top of the plunger.

Generally, this diameter is approximately half the diameter of the waist section (D_waist) in the blank mold interior. The value of r_sphere (half of d_sphere) can be selected based on bottle weight, as shown in the diagram:

Weight ≤ 130g: r = 7mm

Weight 140-279g: r = 8.5

Weight ≥ 280g: r = 11mm

Since the parison is initially in a plastic state, this radius can be adjusted during forming.

Drawing Renderings and Conducting Verification Calculations and Dimensional Adjustments

Renderings involve depicting the shapes of the neck mold, final mold, bottom mold, blank mold, and plunger on a single diagram. This allows for a very intuitive analysis of the previous designs.

(Figure 7: Comparison of Blank Mold and Final Mold in the Press-Blow Method)

As shown in Figure 7, this is the rendering of a certain product. From the diagram, it can be observed that the expansion between the initial shape and the final mold is relatively reasonable. The gap size between the plunger and the internal shape of the blank mold (i.e., the distribution of the glass material during pressing) is also ideal.

To ensure the smooth progress of the pressing process, it is crucial in mold design to ensure the gap distribution between the plunger and the lower part of the neck mold (as shown in the lip protection part in Figure 7) and the internal shape of the blank mold. Generally, the rendering in Figure 7 should achieve the following principles:

a. The gap should gradually increase from top to bottom in a smooth manner.

b. The gap at the waist should generally be twice the gap at the neck.

c. The shape of the plunger should gradually taper from top to bottom, and must not be the opposite.

Verification Calculations and Dimensional Adjustments

Verification calculations involve calculating the volume of hot glass during pressing based on the rendering in Figure 7. Specifically, it checks whether the shaded area in Figure 7 meets the bottle weight requirements. The calculation method for the volume of hot glass is the same as in the blow-blow method for narrow-neck bottles, which is:

𝑉glass-hot=Bottle Weight (g)/ Glass Density×1.025

The results of the verification calculations will generally fall into one of two scenarios besides exactly meeting the bottle weight requirements, which are detailed as follows:

A. Excessive Gap, Overweight Glass

If the verification calculations reveal that the gap at the pressing position is too large and the glass weight is too heavy, the dimensions should be adjusted as per the following steps (refer to Figures 2 and 4):

Increase the diameter of the plunger's spherical section (d_ball).

Simultaneously increase the diameter of the waist section (d_waist) while enlarging the spherical section.

Reduce the waist diameter (D_waist) and the dimensions below the waist in the internal shape of the blank mold.

All three of these adjustments should be implemented simultaneously, but this approach is only adopted when the gap is excessively large and the glass weight significantly exceeds the required amount.

B. Insufficient Gap, Underweight Glass

If the verification calculations reveal that the gap at the pressing position is too small and the glass weight is insufficient, the method for adjusting the dimensions is exactly the same as described above, so it will not be repeated here.

The method for adjusting dimensions is as described above. However, in actual production, it is often neither possible nor necessary to adjust the dimensions to achieve an exact glass weight. Instead, the goal is to bring the glass weight within the allowable fluctuation range for the product (preferably within a slightly lighter range).

After the initial design, the probability of achieving success after the first verification calculation is almost zero. In most cases, success is only achieved after many repeated calculations and adjustments. Therefore, this is a repetitive and tedious task.

As the application of computers in mold design becomes more widespread, this has provided tremendous assistance to designers in completing the above tasks quickly and accurately.

Additionally, it should be noted that because the cross-section of the plunger in the press-blow method is always circular, the cross-section of the hollow part formed by the plunger is also circular. This is different from the blow-blow method, where the cross-section of the hollow shape changes according to the external shape of the initial form. Due to this characteristic of the press-blow method, the shape variation of bottles produced by this process is relatively smaller. However, it is still possible to produce bottles that are close to square or regular polygon shapes with large openings. In these non-circular bottles, the diagonal distance is usually greater than the straight edge distance (such as in bottles with a nearly square cross-section, as shown in Figure 8).

(Figure 8: Cross-section of a Square Bottle)

(Figure 9: Cross-section of the Initial Shape)

If the cross-section of the initial shape is still circular, the expansion at the corners during blowing in the final mold will be larger than in other areas. This can lead to overly thin wall thickness in these areas. To prevent this, we can shape the initial form's cross-section to match the bottle shape. In this way, the hollow part remains circular, while the outer part mimics the bottle shape (as shown in Figure 9).

Obviously, the material at the diagonal corners is thicker, which aligns with the larger expansion in these areas during the final mold blow. This approach helps to achieve a more uniform wall thickness in the final bottle, and this is something that should be considered during the design of the blank mold's internal shape.

By carefully designing the pressing position, plunger dimensions, and blank mold interior, designers can ensure that the glass forming process is efficient and produces uniformly thick and high-quality bottles.