Similarities and Differences Between Blow Molding and Injection Molding

From roll mills and batch mixers to single-screw and twin-screw extruders, people use various types of molten mixing equipment. Continuous mixing equipment (extruders) is the most commonly used equipment as it can provide consistent quality products and reduce operating costs. There are two types of mixing: distributed mixing enables the material to be uniformly distributed in the mixing material without the need for high shear stress. This type of mixing liquid is called extensibility mixing or laminar mixing.

Extrusion blow molding is a method of manufacturing hollow thermoplastic components. Well-known blow molded objects include containers for packaging food, beverages, cosmetics, medicines and daily necessities, such as bottles, barrels, tanks, boxes, etc. Large blow-molded containers are typically used for packaging chemical products, lubricants, and bulk materials. Other blow-molded products include balls, corrugated pipes, and toys. For the automotive industry, fuel tanks, car shock absorbers, seat backs, center supports, and armrest and headrest covers are all blow molded. For the machinery and furniture manufacturing industries, blow-molded parts include housings, door frames, brackets, ceramic jars or boxes with one open face.

The  Differences bewteen blow molding design and injection molding design

Blow molding is injection and blowing; injection molding is injection and compression; rolling molding is extrusion and compression; the design of blow molding molds must have the head left by the blowing air tube, the design of injection molds must have a gate section, and the rolling mold must have burrs without trimming. Generally, the product which is produced by  injection molding is  solid,  while the product which is produced by blow molding and rolling molding are hollow. The surface of the injection-molded part is glossy, while the surface of the blow-molded and rolling-molded parts is not smooth, and at least the blow-molded part has a blow vent.

The shrinkage rate and its influencing factors of blow molding and injection molding

The characteristic of thermoplastics is that they expand after heating and contract after cooling, and of course, the volume will also shrink after being pressurized. In the molding process of injection molds, the molten plastic is first injected into the mold cavity, and after filling, the molten material is cooled and solidified. When the plastic part is taken out of the mold, there will be shrinkage, and this shrinkage is called molding shrinkage. During the time period from the moment the plastic part is taken out of the mold until it stabilizes, there will still be slight changes in size. One change is further shrinkage, which is called post-shrinkage. Another change is that some hygroscopic plastics will expand due to water absorption. For example, when nylon 610 contains 3% water, the size increases by 2%; the size increase of glass fiber reinforced nylon 66 when the water content reaches 40% is 0.3%. However, molding shrinkage plays a major role.

At present, the regulations in the German national standard are generally recommended when we measure the shrinkage rate (molding shrinkage + post-shrinkage) of various plastics. That is, when the temperature is 23℃ and the relative humidity is 45-50%, the shrinkage rate is calculated based on the difference between the mold cavity size at 22.9-23.1℃ and the corresponding plastic part size measured after 24 hours of molding.