Head tooling – extrusion blow molding

>> Wednesday, 19 August 2009

Mandrel and die dimensions are estimated based on container dimensional data, container symmetry, blow-up ratio, targeted container weight, neck finish requirements and the type of material (degree of parison swell) that will be used to produce the container.

An initial blow-up ratio must be calculated using the container design dimensions and the required parison diameter.
The required parison diameter will depend on the relative size of the container, the container design (handle or no handle) and the container neck finish requirements. Initial blow-up ratios may be calculated using the following equation.

Blow up ratio = Bd / Nd

where:
Bd = Bottle diameter, in
Nd = Minimum neck diameter, in

The blow-up ratio is compared with the maximum recommended blow-up ratio of the selected material.
Blow up ratios of 2 or 3 to 1 are considered normal when molding commodity resins such as polyethylene.
A blow-up ratio as high as 4:1 is a practical upper limit.
The blow up ratio for large containers with a small neck, is generally extended to 7:1 so that the parison fits within the neck and so that there is no mold parting line mark on the neck finish.
Blow up ratios for a containers with a handles are generally in the 3 or 5 to 1 range as the die diameter must be larger to allow the handle to be blown.

Figure shows a typical blow molded container with dimension and design nomenclature for reference


In order to properly estimate and ‘size’ mandrel and die geometry for the blow mold(s), and to effectively control the process, a thorough understanding of parison swell and draw down phenomena is required. Parison swell is a combination of diameter swell and weight swell. It is a difficult blow molding property to estimate and to control. The parison diameter swell is a complex function of the weight swell, the rate of swell, and the melt strength.

Parison swell behavior varies significantly depending on material type, material processing conditions, machine processing parameters, basic die design (diverging vs. converging), container geometry (required parison diameter), container weight (shuttle process) and type of blow molding process. Some of the wheel type blow molding processes clamp (pinch off) and hold the parison at both ends during the blowing sequence in the process. The parison swell effects are normally more readily controlled on the wheel process compared with the shuttle process.

Parison swell data for a given material is often not available for mandrel and die calculations. The alternative is to proceed in a stepwise approximation towards the desired mandrel and die dimensions, and through trial and error, towards the targeted container weight with the aid of an interchangeable set of dies.

Internal die design dimensions including approach angles and land lengths vary significantly with blow molding machine capabilities and machinery manufacturers experience. Calculations for these dimensions are beyond the scope of this document and will not be discussed here.

However, as a rule of thumb, when blow molding commodity materials (PE, PP), a die land length of at least 8 times the annulus gap (die gap) is typical.

A simplified approach for calculating and estimating mandrel and die dimensions is presented here to serve as a general guide. The following equations may be applied in cases where the container geometry is symmetrical and there is no handle on the container.

Case A
When the neck size of a container or the smallest diameter of the container is the controlling feature (as when the parison must be contained within the smallest diameter) , the following approximations may be used to calculate the dimensions of the mandrel and die. Use of these equations assume a free-falling parison and they can be used with most PE blow molding materials.

Dd = 0.5 N d (Equation 1)

Pd = ( D d2 - 2Bdt + 2t2)1/2 (Equation 2)

where:
Dd = Diameter of die bushing, in
Nd = Minimum neck diameter, in
Pd = Mandrel diameter, in
Bd = Bottle diameter, in
t = Bottle thickness at Bd, in

Case B
When the container weight is specified instead of the wall thickness for a process using inside-the-neck blowing, the following equations may be applied:

(The equation may be applied for a free-falling parison, and is applicable to most irregularly shaped containers.)

Pd = (Dd 2 - 2 (W/T2) Ld)1/2 (Equation 3)

where:
Pd = Mandrel diameter, in
Dd = Diameter of die bushing, in
W = Weight of container, g
L = Length of container, in
d = Density of material, g/cc
T = Wall thickness, in

Case C
When a parison is partially controlled by tension, i.e., the rotary wheel blow molding process, the following relationships may be used. The assumption here is that the parison is not free-falling.

Dd = 0.9 N d (Equation 4)

Pd = ( Dd 2 - 3.6 Bd t + 3.6 t 2)1/2 (Equation 5)

Pd = ( Dd 2 - 3.6 ( W / T 2 ) Ld) 1 / 2 (Equation 6)

where:
Pd = Mandrel diameter, in
Nd = Minimum neck diameter, in
Dd = Diameter of die bushing, in
W = Weight of container, g
L = Length of container, in
d = Density of material, g/cc
T = Wall thickness, in
Bd = Bottle diameter, in
t = Bottle thickness at Bd, in

Case D
Estimating head tooling dimensions for containers with handles requires an empirical method based on container size and geometry. When the container has a molded handle use the following equations to determine the estimated dimensions for the head tooling die and mandrel.

The die diameter can be estimated using equation 7 with the container dimension data.

D = ( 0.8 Z ) / 3 (Equation 7)

Where:
Z = Maximum container width or diameter
D = Diameter of die bushing, in.

The die diameter, D, is then substituted in equation 3, equation 5 or equation 6 depending on available container data and the blow molding process that will be used.

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