Blow molding, also known as hollow blow molding, is a molding way that the heated plastic parison gets inflated and molded in the blow mold to achieve blow molded parts, which is adopted in the manufacture of hollow products.
Blow molding is widely used in the industry of toy, household commodity, chemical, cosmetics, food and etc. Our blow molded products include:
- toy car,
- mineral water bottle,
- pots, barrels,
- pipeline ,
- automobile conduit,
- depot equipment,
- stadium seating, and so on.
Advantages: compared with injection molding, extrusion blow molding has the following advantages:
- Blow molding machine (especially blow mold) cost is low (similar product), blow molding machinery cost is about 1 / 3 – 1 / 2 that of injection molding machine, also the production cost is low;
- In the blow molding, the heated parison is inflated in the blow mold under low pressure (mostly 0.2 – 1.0MN) via die head on the blow molding machine. As a result of the small residual stress, the blow molded parts is with good performance: tensile resistance, impact resistance, bending resistance and environment friendly. In the injection molding, the melt plastic should be injected in high pressure (15 – 140yPa) through the mold runner and runner, which will lead to uneven distribution of stress;
- The relative molecular mass of a blow grade plastic (e.g. PE) is much higher than that of the injection grade plastics. Therefore, blow molding product features the characters with high impact toughness, resistance to environment and stress cracking. So it is suitable for production of packaging of transport detergent, chemical reagent container and barrel;
- Because the blow mold is only composed of the 2 cavities, simply adjusting the die head gap or extrusion conditions can change the thickness of the product.it is with advance to make the blow molded part with the required wall thickness which couldn’t be predicated. While for the injection molding, it is very hard to change the wall thickness of the injection molded product;
- Blow molding can produce products with very small wall thickness, which kind of products cannot be produced by injection molding;
- Blow molding can produce an integral, complicated and irregular product. While as to injection molding, to assemble two or more products, snap fit, screw and ultrasonic welding are needed.
The disadvantage of blow molding is that the accuracy of blow molded products is generally not as high as that of the injection molding products.
In-house Blow Mold Making
One of the primary reasons for the rapid development of blow molding, once equipment was commercially available，was the economies afforded a molder in the tooling costs. Regardless of materials used, a blow mold consists of a set of pins and bushings for alignment of the two halves； pinch-off plates at both ends, usually made of mild steel; a basic mold body or shell；and adequate cooling channels for rapid and effective heat transfer.
Due to the limited number of blow mold and blow molded part manufacturers in China at present it is necessary for the molder to know as much about mold design and mold shrinkage as possible. In a majority of cases the initial development of a blow-molded item is done by the molder. The part produced is actually a compromise between the molder’s machine capabilities and the customer’s desires. For this reason a molder must work extremely closely with the mold maker in order to make required changes and do all the testing prior to production.
The type of material selected for a mold will depend on:
- The number of pieces to be produced.
- Type of material to be blow molded.
- Number of molds required.
- Speed of production.
- Quality of the finished part.
Molds are presently produced from the following materials:
- Aluminum (cast and machined)
- “Kirksite” (pressure and gravity cast)
- Machined steel
Besides the four materials mentioned above, two others are being used to some extent in experimental work. They are:
- Epoxy filled with aluminum or steel.
- Hard copper shell molds.
Aluminum molds are one of the most commonly used types. In the cast aluminum type, the model maker produces a full-scale plaster model for customer approval. These samples are either hand-made or machine-made, depending on the circumstances. At this time the model maker determines the shrinkage factor for casting (approximately .040″ per inch) and the shrinkage for the template of about .058-.060″ per inch for plaster and casting. Once a model has been constructed, a plaster mold is produced and from this is produced the first rough lead casting. The lead casting is finished and doweled and used as the master for producing the production cavities. In some cases, cooling channels are cast into place during the casting operation； in others, channels are drilled into the mold.
Since aluminum is relatively a soft metal, it is easily scarred or damaged by rough treatment in the shop. Also if used in constant production, the pinch-off areas degrade. For this reason a pinch plate of mild steel should be inserted to obtain maximum efficiency from the molds. This is especially true of a mold whose shortest inside diameter is smaller than the parison diameter.
This material, probably more commonly known as “Kirksite“，is preferred by many molders because of its castability, stability, excellent heat transfer properties, machinability and reasonable cost. The grain structure of this type of mold gives a tensile strength of approximately 50,000 psi and compression strength of 100,000 psi. The casting process is basically the same as for aluminum molds. Figure 5.2 illustrates a mold half with cast-in-neck threads and bottom pinch-off. These inserts accomplish three objectives.
- Since the pinch-off is the part of greatest wear, it can easily be replaced.
- It gives access to the mold for parting line finishing.
- It enables separate cooling to be installed in the pinch-off areas.
Zinc alloys can be cast, hobbed or machined, and although they are more expensive than aluminum, they exhibit better heat-transfer characteristics and wear slightly better than aluminum.
Beryllium-copper molds are used extensively throughout the doll industry for the blow molding of doll bodies. This type of mold is roughly 2-3 times the cost of the equivalent aluminum mold but has excellent heat-transfer characteristics and, all things being equal, gives faster production.
This type of mold is best used in two specific areas:
- If an article to be produced is within the diameter of the parison and an excess of material will result all around the molded piece, then steel should be used.
- In multiple-cavity blowing, again due to excessive pinch-off areas, steel is recommended.
Both the above types require extraordinary clamping pressure and result in more than normal wear on the pinch-off areas. The disadvantages of a steel mold are that the heat-transfer properties are only fair to good, and that it is the heaviest of all mold materials and requires more set-up time than the lighter molds.