What Is A Blow Mold?

Blow molding is a manufacturing art used to form hollow plastic parts. It is more commonly used to form glass bottles or other hollow shapes. The blow molding process begins by softening the plastic by heating a preform, or parison. A parison is a tubular plastic with a hole at one end through which compressed air can enter.

The plastic workpiece is then clamped into a mold and air is blown into it. The air pressure expands the plastic so that it conforms to the mold. After the plastic cools and hardens, the mold opens and the part ejects. Water channels within the mold aid in cooling.

There are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding.

Extrusion Blow Molding

In extrusion blow molding, plastic is melted and squeezed into a hollow tube, forming a tubular plastic sheet with a hole at one end for compressed gas - called a parison. The parison is captured by enclosing it in a cooled metal mold. Air is blown into the parison, causing it to expand into the shape of a hollow bottle, container, or part. After the plastic cools, the mold opens and the part ejects.

"Direct extrusion blow molding is a forward-pushing material method similar to injection molding, where the plastic material is fed into a heated tube by the rotation of an Archimedean screw. Once the plastic melts, the screw stops rotating and moves linearly to push the melt out. With the accumulator method, the accumulator collects the melted plastic, and after the previous mold cools and accumulates enough plastic, the rod pushes the melted plastic and forms the parison. In this case, the screw can turn continuously or intermittently. In continuous extrusion, the weight of the parison drags on the parison, making it difficult to calibrate the wall thickness. The accumulator head or reciprocating screw method uses a hydraulic system to quickly push out the parison, thereby reducing the effect of weight and allowing precise control of wall thickness by adjusting the mold gap using a parison programming device.

Rotary trimming

Due to the molding process, containers such as jars often have excess material. The excess material is trimmed by rotating a cutting blade around the container, separating the material. The excess plastic is then recycled to make new molded products. Rotary trimmers can be used for a variety of materials such as PVC, HDPE and PE+LDPE. Different types of materials have physical properties that affect trimming. For example, molded items made from amorphous materials are more difficult to trim than crystalline materials. Titanium nitride coated blades are often used instead of standard steel to extend life by 30 times.

Injection Blow Molding

Injection Blow Molded Plastic Bottles
The injection blow molding (IBM) process is used to mass produce hollow glass and plastic items. In the IBM process, polymer is injection molded onto a core pin; the core pin is then rotated to the blow molding station for inflation and cooling. This is the least used of the three blow molding processes and is often used to make small medical bottles and disposable bottles. The process is divided into three steps: injection, blow molding, and ejection.

Injection blow molding machines are based on an extruder barrel and screw assembly that is used to melt the polymer. The molten polymer is fed into a hot runner manifold and injected through a nozzle into a heated cavity and core pin. The cavity mold forms the outer shape and is clamped around a core rod, which forms the inner shape of the preform. . The preform consists of a fully formed bottle/jar neck and an attached thick tube of polymer, which will form the body. The appearance is similar to a test tube with a threaded neck.

The preform mold opens, and the mandrel rotates and clamps into the hollow, cooled blow mold. The end of the mandrel opens to allow compressed air to enter the preform, inflating it to the finished shape.

After a period of cooling, the blow mold opens and the mandrel rotates to the ejection position. The finished product is peeled off the mandrel and can be optionally leak tested before packaging. Preforms and blow molds can have multiple cavities, usually 3 to 16, depending on the size of the item and the required output. There are three sets of mandrels, which can perform preform injection, blow molding and ejection at the same time.

At this stage, more buyers of process products tend to use blow molding process products, mainly because of its low cost and medium plasticity, which allows people to get the desired effect with the least money. The market is currently warming up. If you are interested, join us!