01.
Introduction to Blow Molding Process
Principle of the blow molding process: The blow molding process includes three basic steps: preform moulding, blow moulding and bottle cooling.
First, process the plastic into a preform (injection or extrusion) and place the high-temperature preform in a blow mould. Inject compressed air into the preform to inflate it until it is close to the cavity wall, and maintain the blowing pressure until the bottle cools. After finalizing the shape, the hollow bottle can be obtained when the mould is opened.
The steps of the blow molding process are:
02.
Classification and characteristics of blow moulding process:
Classification and characteristics of blow moulding process:
According to different preform preparation and blowing methods, the blow moulding process includes extrusion-blowing, injection-blowing, extrusion-pulling-blowing, injection-pulling-blowing, etc.
1. Extrusion blow moulding process-extrusion blow
2. Extrusion stretch blow moulding process-stretch blow moulding
The plastic is made into a tubular preform through the extruder and the preform head. The hot tube is intercepted by the pre-blow mould.
And perform pre-blowing, the pre-inflated high-temperature preform is moved to the blow mould for stretch blowing moulding.
3. Injection blow molding process-injection blow
The plastic is made into a test tube preform (the preform is attached to the core mould) by the injection machine and the preform injection moulding. The high-temperature preform is moved to the blowing mould for blowing moulding.
4. Two-step injection stretch blow moulding process of blow moulder
The plastic is made into a tube-shaped test preform by injection molding machine and preform. The preform is demolded after cooling and shaping. Before blow moulding, the preform is reheated above the thermal deformation temperature. The preheated high-temperature preform is moved to the blow moulder for stretch blowing moulding.
5. One-step injection stretch blow molding process-injection stretch blow molding
This method uses a multi-station injection stretch-blow molding machine to mold blow-moulded products at one time. Plastic is injected into the preform mold at station a. After cooling and shaping, the preform is moved with the core to station b for heating and temperature adjustment. The high-temperature preform is transferred to the blow mould at station c for stretch blow moulding. The cooled finished product Move with the core to the d station for demolding.
03.
Blow molding process mold composition and characteristics
It can be seen from the blow molding process that the generalized blow molding process mold is composed as follows.
Blow moulding process | Mold structure composition |
| Extrusion blow moulder | Preform extrusion mold + Blow Mold |
| Injection blow moulder | Preform injection mold + blow mold |
| Injection stretch blow moulder | Preform extrusion mold + pre blow mold + blow mold |
| Two-step injection stretch blow moulder | Preform injection mold + blow mold |
| One-step injection stretch blow moulder | Multi-station injection stretch-blow mold with moving core |
04.
Blow molding point
The preform moulding die (extrusion die, injection die) has been mentioned above. Here we mainly discuss the narrow sense.
Blow molding process mould-blow molding mold is referred to as blow mould.As mentioned above, the hollow product is formed by injecting compressed air into the high-temperature preform and pressing it against the surface of the mould cavity by the air pressure in the preform. Only the blow moulder mould’s external structure size is limited, and the internal structure size depends on the shape. And wall thickness.
Therefore, the blow moulder’s moulds are mostly two halves or multiple moulds (when side parting is required). There is only a cavity (regardless of unevenness) of the outer shape of the moulded product and no core; there is an air inlet between the two mould halves Rod or intake needle. This is a hallmark feature shared by blow moulds.
In addition, similar to other two-half cavity moulds such as injection moulds, compression molds, and transfer moulds, the blow mould also has a mould clamping guide positioning mechanism, a cooling system, and an exhaust system.
The specific structure of the blow mould varies with the design of the plastic part and the blowing method of the preform type. For example, the extrusion blow mould has a blank opening and a remaining material slot; the air inlet rod of the stretch blow mould is controllable and expandable, and so on.
05.
Critical points of mould design for the blow moulding process
As mentioned earlier, many blow moulder moulds have different structural compositions and design requirements. Here we only combine the extrusion blow mould (extrusion blow mould) to briefly introduce the critical points of its design.
The main requirements for extrusion blow moulds:
①The blowing ratio is appropriate, and the distribution of each part of the product is as even as possible;
②The preform can be effectively clamped, and the joint strength of the connected component is sufficient;
③Reliable exhaust to ensure rapid blowing of the preform and conform to the mould wall;
④The cooling is fast and uniform.
Mould cavity design:
A Parting surface: usually, the plane passes through the most significant part of the profile of the product section. For products with complex shapes, the parting surface can be a curved surface or a combination of multiple planes. The container handle is generally set on the parting surface.
B cavity surface: low blowing pressure, low moulding temperature, plastic will not enter the trough of microscopic unevenness (roughness), relatively rough profile, not only will not affect the surface state of the product but also facilitate exhaust. The product cavity should be polished for a high gloss, high transparency, and other special requirements.
C. Side parting: To make the wall thickness as uniform as possible for blow moulded products, a more significant slope is used for a gradual transition even if there are undercuts. In addition, the demolding temperature is high, the elasticity is good, and it can be forced to demold. Generally, side parting is not required. For deep grooves with smaller slopes, side parting mechanisms should be set
Clamping mouth and remaining material trough
A clamping blank mouth:
The blank mouth is also called the cutting edge. Extrusion blow moulding mould needs to cut off the remaining material simultaneously, so the corresponding part of the mould must be set with the blank opening.
B remaining trough
The remaining material of the preform cut by the cutting edge falls on the parting surface, affecting the mould’s closing. For this reason, opening a surplus slot in the corresponding position to accommodate the blank opening and the structural size of the surplus slot are the key to the design of the extrusion blow mould. In addition to cutting off the remaining material, the clamping blank also clamping the closed preform. Suppose the width and angle of the clamping blank are not appropriate. In that case, the joint surface will not fit tightly, and the remaining trough capacity will be insufficient. There may be inadequate clamping, continuous preform cutting, or insufficient joint strength after preform cutting.
The remaining material groove is usually set on the upper and lower sides of the cut. Its size should be determined according to the width and thickness of the remaining material after the preform is clamped, and the mould can be closed tightly. The width of the clamping blank is generally 1～2mm, and the angle is 13°～15°.
The structure size of the commonly used blank holder and remaining material trough is shown in the figure.
Exhaust system
The blow mould has a large air displacement and a slight molding pressure. After the mould is closed, the problem of removing the original air in the cavity must be seriously considered. Poor venting, the preform can not fit the cavity well, which will cause defects such as incomplete, unclear product structure and shape, markings, and pits.
The blow mould exhaust method is based on the parting surface with an exhausted groove. Because there is no overflow problem, the size of the exhaust groove is slightly larger than that of the injection mould.
When there are corners, dead corners, etc., and the preform finally clings to the part that is not on the parting surface, consider setting up an exhaust channel on the cavity wall to lead out air.
(1) Parting surface exhaust groove structure size
(2) Exhaust structure size of cavity wall
06.
Commonly used blow mould materials.
Steel: high strength, long life, but poor thermal conductivity, mainly used as pressure-bearing inserts, notch inserts, guide parts, load-bearing structural parts, etc.
Aluminium alloy: good thermal conductivity, lightweight, widely used as cavity material
Copper alloy: Copper-beryllium alloy has good thermal conductivity, high strength, and corrosion resistance. It can be used for corrosive PVC and other plastics. The water channel is not easy to scale, but it is expensive.
Zinc-based alloy: large and complex cavity can be cast at low temperature, good heat conduction, low cost, but low hardness, it needs to be embedded in steel or copper-beryllium alloy clamping blank insert.
07.
Below are several different types of blow moulds
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