Text/Mate Polymer Reagan Finish
A tubular plastic blank obtained by extrusion or injection molding of thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state), and compressed air is injected into it immediately after mold is closed. make plastic. The blank is inflated and clings to inner wall of mold, and after cooling and demolding, various hollow products can be obtained. The blown film process is very similar in principle to hollow body blow molding, but no molds are used. In terms of classifying plastics processing technologies, blown film process is usually included in extrusion. The blow molding process began to be used to produce low density polyethylene bottles during World War II. In late 1950s, with advent of high-density polyethylene and development of blow molding machines, blow molding technology became widespread. The volume of a hollow container can reach thousands of liters, and in some industries computer control is used. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and resulting hollow containers are widely used as industrial packaging containers.
1. Classification and characteristics of hollow profiles
1.1, extrusion blow molding
Technological process of extrusion blow molding includes: ①The composition of tube billet: it is extruded by an extruder and hung in a pre-separated cavity installed directly under machine head; ②When hanging workpiece reaches qualified Close mold immediately after length and cut workpiece according to cut of mold; ③Insert compressed air tube through small hole on mold parting surface and direct compressed air to make workpiece inflate and form close to mold wall; ④Infusion The air pressure cools product and solidifies it in cavity, after which it can be taken out of mold.
1.2 injection blow molding
Formation of an injection molding blank consists in forming a blank on a metal pipe by injection molding. One end of tube is filled with compressed air, and other end of tube wall has micropores. The injection molding blank is usually taken out after cooling, reheated to a temperature above Tg before blow molding, quickly moved into mold and blown with compressed air, blank expands and separates from metal pipe, and sticks to mold wall for molding and molding. cooling.
1.3 Blow molding
Before blow molding preform, between Tg-Tf, preform is first stretched in axial direction by mechanical means, and then during blow molding, radial size of preform is increased, it is stretched in transverse direction, and then quickly cooled to a shape below Tg.
1.4, multilayer blow molding
Connect with multilayer tubular billets, inflate and cool under same conditions.
The first is blank manufacturing, second is blank positioning, third is blow molding, fourth is cooling.
2. Influence factors of blow molding
2.1. Factors influencing process of manufacturing tubular blanks
Material selection, temperature control, screw speed, die
①Melting index: A low melt index can prevent tube billet from sagging, but it should not be too low, otherwise erratic movements will easily occur.
②Molecular weight distribution: The wide molecular weight distribution contributes to production of high quality tubular blanks.
③Tensile toughness: A material whose tensile toughness increases with increasing tensile stress is advantageous for blow molding processing.
The workpiece sags a lot at high temperature, surface becomes rough at low temperature, plasticization is poor, and cooling is too fast; choose right temperatureRU.
High extrusion speed can increase productivity and reduce billet sag, but too high extrusion speed results in erratic movement, and screw speed must be determined according to specific materials.
The surface of die is smooth and free of foreign matter, and diameter of die must be chosen according to size of product.
2.2. Factors affecting blow molding process
(1) Discharge pressure
Compressed air in blow molding has two effects: inflation and cooling. Parts with low viscosity, small volume, or thick-walled parts should use low pressure; parts with high viscosity, large volume, or thin-walled parts should use higher pressure.
In order to shorten blowing time, so that product can obtain a more uniform thickness and better appearance, blowing speed should be as high as possible, but if speed is too high, following phenomena may occur: ① Vacuum. Formed at air inlet to make this part. The blank has a notch, and when blank is fully inflated, serrated part forms diaphragm; ② The blank in die part is also easily broken by extremely fast airflow.
The inflation factor is usually 2-4 times. If inflation ratio is too large, shape stability of product will be poor. If inflation ratio is too small, it will easily lead to uneven wall thickness.
⑷Mold temperature and cooling time
If mold temperature is too low, extensibility of plastic at contact point will decrease, making it difficult to bulge, and part will be thickened, making molding difficult; If mold temperature is too high, cooling time will be extended and production cycle will be long. If cooling time is too short, insufficient cooling will easily lead to product deformation, increased shrinkage and tarnished appearance.
Including billet extrusion, billet cutting, mold closing, air blowing, cooling, deflation, mold opening and finished product ejection. The principle of choosing a molding cycle is to shorten it as much as possible, based on premise of ensuring final processing of product.
3. Common problems and solutions for hollow blow molds
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