How to control molding in extrusion blow molding process

The extrusion blow molding process includes plastic melting and plasticization, extruding the tube blank, inflating the tube blank, cooling, opening the mold, taking out the product, trimming, and checking. The control of product molding can be carried out from the following four aspects.

1. Blowing pressure.

   In the blow molding process, the compressed air has two functions: one is to use the pressure of compressed air to inflate the semi-molten tube blank and close to the cavity wall to form the desired shape; The second is to cool blow molding products. The air pressure depends on the type of plastic and the temperature of the parison, and is generally controlled at 0.2 to 1.0 MPa. For plastics with low melt viscosity and easy to deform, take a lower value; for plastics with higher melt viscosity, take a higher value, and take a higher value for the wall thickness of the parison. The blowing pressure is also related to the volume of the product. Large-volume products should use a higher inflation pressure, and small-volume products should use a lower inflation pressure. The most suitable inflation pressure should be able to make the appearance and pattern of the product clear after molding.

2. Blowing speed, in order to shorten the blowing time and make the product obtain a more uniform thickness and better appearance, it is required to inject a large flow of air at a low air velocity to ensure that the parison can be uniform and fast in the mold cavity Ground expansion, shorten the cooling time in the mold cavity, and help improve the performance of the product. The low air velocity can also avoid the occurrence of the Venturi effect in the parison and the formation of a partial vacuum, causing the parison to collapse. This can be ensured by using a larger blowing tube.

3. Blow-up ratio, blow-up ratio refers to the ratio of the maximum diameter of the plastic part to the diameter of the parison. When the size and quality of the parison are constant, the larger the size of the product, the greater the inflation ratio of the parison, but the thinner the thickness of the product. The blow-up ratio is usually determined according to the type and nature of the plastic, the shape and size of the product, and the size of the parison. The expansion ratio increases, the thickness of the product becomes thinner, the strength and rigidity decrease, and molding becomes difficult at the same time.
   

4. Blow mold temperature,

   The temperature of the blow mold has a greater impact on the quality of the product (especially the appearance quality). Generally, the temperature distribution of the mold should be uniform, so that the product can be cooled as uniformly as possible. The temperature of the mold is related to the type of plastic, the thickness and size of the product. For different types of plastics, the temperature control of the hollow blow mold is shown in Table 2-1. There are a few plastic molds whose temperature should be controlled in sections. Production practice has proved that if the mold temperature is too low, the elongation of the plastic at the jaws will be reduced, and it will not be easy to inflate, making the product thicker in this part, while molding is difficult, and the contour and pattern of the product surface are not clear; the mold temperature is too high , The cooling time is prolonged, the production cycle is increased, and the productivity is reduced. At this time, if the cooling is not enough, it will cause the product to be demolded and deformed, the shrinkage rate increases, and the surface gloss becomes worse. Generally, the mold temperature should be higher for plastics with greater molecular chain rigidity; and for plastics with greater molecular chain flexibility, the mold temperature should be lower.

   During the blow molding process, control the molding from four points of inflation pressure, blowing speed, inflation ratio, and inflation mold temperature.