BLOW MOLDING OF PLASTICS
In its infancy, blow molding was used primarily for the production of plastic bottles for packaging detergents and bleaches. The process was more art than science. Now a highly developed technology, it can produce parts which either cannot be made by another process, or be made as economically. Containers remain a major application.
The most commonly used blow molding materials include high, medium and low density polyethylene; polypropylene; nylon; thermoplastic elastomers (TPR, TPO, TPE); polyester; BDS and others. Polyethylenes have properties suited for low temperature use (-40°F) and excellent environmental stress/crack resistance. The maximum temperature range for HDPE (high density polyethylene) is between 180-225°F.
Polypropylene presents the problem of cracking in cold weather and should not be subjected to stresses at temperatures below 01°F. On the other hand, it can withstand temperatures approximately 25°F higher than HDPE. Also, it is less expensive on a cost/volume basis.
Both materials can be painted but surfaces must first be prepared for painting using flame or chemical treatments.
Many blow molded parts have little or no resemblance to containers, in the general sense, yet they can be produced most cost effectively using blow molding. Usually, if the part has uniform wall thicknesses, and the outside shape is the major consideration, it is a candidate for the savings offered by blow molding's comparatively low tooling costs and high production rates. If there are openings in a part, consider them sealed off for evaluation purposes. Parts can be produced as sealed units and the required openings provided by secondary operations. This is where the tooling economies are evident. Tooling for the inside of the part is compressed air, far less expensive than metal forms, if indeed the geometry of the part would permit their use.
Also, making the part in one piece is less costly than molding and assembling two or more components.
Typical parts include: coolant overflow jars for trucks and automobiles, dehumidifier buckets, drinking water storage tanks, flexible bellows, hoses, boots, sprayer tanks, toys, tool cases, medical products, air ducts, and much more.
In making hollow parts by other processes (vacuum or pressure forming, injection molding, machining, thermoforming, etc.) multiple molds, multiple molding or machining operations and costly assembly procedures are required for completion. These include: welding, heat or ultrasonic sealing, staking, riveting, gluing, stitching, and snap-together, to name just a few. In addition, where individual components are joined, the seams may be unsightly, irregular, cosmetically deficient, or pose a strength or leakage problem.
A blow molded part, on the other hand, is a single, strong, attractive unit, even to the extent that careful moldmaking and processing procedures can minimize or eliminate unsightly mold parting lines. It must be remembered that the inside shape of a blow molded part is the same basic shape as the outside. The differences which can be measured relate only to thinning of the walls in varying amounts, depending on the shape of the part, as the parison balloons out to fill the mold cavity. "Preblowing" the parison by clamping its open lower end and injecting air before the mold closes, can reduce the result to some extent. As a rule, thickness of the parison walls is usually calculated so as to be sure wall thickness is as required for the sections that will be subjected to the thinning effect. It must also be remembered that any portion of the outer surface which is raised above the rest results in a "pocket" of the same height and configuration being present on the inside of the part. This factor should be taken into account when calculating the volume of containers which will be used for storage of liquids or powders.
To more clearly understand the thinning effect, consider what would happen if a bubble of gum were blown inside a box. When the gum hits the center of the inner six sides, it sticks. The thickness at these contact points is heaviest. The remainder of the bubble stretches to fill out the box, its walls thinning progressively as the gum reaches into corners. In the end, you have a six-sided bubble, heavywalled at the center of each side, thinner moving outward to the edges, and thinnest at the corners.
Compared to the cost of tooling for injection molding a part, blow molding tooling weighs considerably less and its cost is 25-40 percent less, on average. On the other hand, coring elements needed for injection molding do result in uniformity of wall thickness. If this is a vital part requirement, the use of blow molding should not be considered.
Molds can be cast, machined, EDMed or any combination of the three. Most cast molds are made from aluminum with the casting having a hollow back or water jacket, required for cooling in the case of large parts or those with extreme contours. Molds with simple geometric shapes which can be easily machined are produced from aircraft aluminum blocks. A series of cross-drilled holes are usually provided for the water cooling system. When high volume production is called for, on the order of 60,000 or more pieces per month, molds are cast in beryllium-copper to take advantage of the hardness of the copper alloys. It also gives an excellent parting line surface that takes maximum wear. Also, since copper offers twice the thermal conductivity of aluminum, faster molding cycles are possible, although molds are approximately one-third more costly than the same molds in aluminum due to the cost of the alloy.
The cavities of the mold are usually sandblasted to produce a matte finish on the part. This helps air to escape as the parison expands and also hides any minor surface imperfections on the parison. All cavities must be vented. To provide for air escape, slotted 1/16" to 1/2" diameter brass pins are pressed flush to the cavity surface at its deepest points. The slots have insignificant, if any, effect on the part surface. If even a minor effect is not acceptable, other venting methods are used. Without vents, air trapped between the parison and the cavity walls would prevent the part from filling out and cause irregular surfaces.
Dependent on part size, shape, production volume and other factors, the number of molds needed will vary. Extruder capacity is another variable, as is the mold size capacity of the blow molding machine. Machines are built to take one, two or four molds. Each mold station can be fed by 1, 2, 3 or more vertically-dropping parisons. (Each mold can have one or more adjacent cavities.) Also, cavities can be repeated in multiples stacked one above the other in single or multiple rows.
The type and combination of mold possibilities used are determined by the factors previously mentioned, with production volume required usually the dominant factor.
Cycle time depends on cooling time. Until the blown-to-shape parison solidifies, the part cannot be removed from the mold. For this reason, all molds have cooling systems fed by a central water chiller. To further expedite cooling, some systems use chilled air to blow the parison. Proper cooling is especially important on heavy-walled parts to prevent distortion.
The minimum tolerance specified should not be less than 0.020". Requirements for 0.125" tolerances are common on large parts, and must be much higher on parts which may be 4 feet long and have 0.180" thick walls.
Secondary machining operations to provide holes or slots can be very accurately controlled.
Parts should be dimensioned according to the outside form. The wall thicknesses can vary widely based on many factors. It is not uncommon for walls to be 1/8" thick at the mold parting line and as thin as 1/64" in a corner away from it. The thinning which occurs as the plastic stretches must be taken into account when designing a part and finished parts should be approved on a functional basis, not on rigid dimensional checks.
Inserts can often be molded in, or added after blow molding. Parts can be drilled, die cut, milled, etc. to meet functional requirements.
Simply explained, parts are produced by trapping a melted tube of plastic (a parison) between two mold halves and then introducing high pressure air to stretch the parison out to fill the mold cavity.
The following sequence produces the parison and the part:
- Plastic pellets (approximately 1/16" cubes) are fed into the hopper of an extruder and feed by gravity down to the extruder screw.
- The screw chamber is equipped with a heating unit which melts the plastic as it is pumped by the screw toward the die head of the extruder.
- In the die head is a "flow pin" around which the molten plastic flows horizontally at first, and then downward. It emerges from the end of the flow pin as a seamless tube. The tube is extruded to the length required for the part. Extrusion is then halted while horizontally moving mold sections clamp it preparatory to air pressure being applied internally.
- Air can be applied by a tube (blow pin) inside the parison flow pin, or by needles built into the mold which pierce the parison as the mold closes.
- As pressure is applied, the parison balloons out to fill the mold cavity. Molds are constructed with internal water lines to provide chilled water for cooling the blown part.
- Depending on the size of the part, the production rate required, and the capacity of the extruder, multiple molds or molds with multiple cavities may be used and more than one parison at a time extruded.
In considering whether a part can be produced economically, or more economically, by blow molding, look at it first from the viewpoint of not having any openings. If what you visualize is basically hollow, request an evaluation from an experienced blow molder.
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