The Basics of a Blow Mold


Blow molds are used in the blow molding process to mold an article to the desired shape. Generally
the blow mold is a cavity representing the outside of a blow molded part. The basic structure of a blow mold consists of a cast or machined block with a cavity, cooling system, venting system, pinchoffs, flash pockets and mounting plate.

The choice of material for the construction of a blow mold must take into account such factors as thermal conductivity, durability, cost of the material, the resin being processed and the desired quality of the finished parts. Commonly used mold materials are beryllium, copper, aluminum, ampcoloy, A-2 steel and 17-4 and 420 stainless steel.

Beryllium-copper alloys are the most common mold materials.

Grades of aluminum such as 7075-T6 and QC-7 exhibit good thermal conductivity but are relatively soft. BeCu alloy 165 and 25 are normally used for blow molds. These materials display medium to good thermal conductivity with good durability when used in the high-hardness range. Stainless steels such as 17-4 and 420 are also frequently employed in blow molds where durability and resistance to hydrochloric acid are concerns. Heat-treated A-2 steel is often used as an insert in pinchoffs where thermal conductivity is not a concern and high-quality parts are required.

For blow molding HDPE parts, aluminum is typically used for the base material, with BeCu or stainless steel inserts in the pinchoff areas. For PVC parts, BeCu, ampcoloy or 17-4 stainless steel as employed as the base material, with A-2 or stainless-steel inserts in the pinchoff areas. And for PETG parts, the base mold is usually made of aluminum or BeCu, with A-2 or stainless pinch-offs.

Cooling rates can be a limit on production

Mold cooling is one of the primary concerns to the blow molder. The production speed of blow molded parts is normally limited by one of two factors: extruder capacity on cooling time in the mold. Cooling of the mold is accomplished by a water circuit built into the mold. Flood cooling and case-in tubes are the norm in cast molds; drilled holes and milled slots are most common in machined blow molds.

Multizone cooling systems are employed so the molder can set and control different cooling conditions to different areas of the mold simultaneously. In multiple-cavity molds, series and parallel circuits are used. Series cooling enters and cools one cavity, then moves to the next until all the cavities are cooled. The temperature of the water increases as it moves through the mold, which results in a non-uniform cooling between the first and last cavity.

Parallel cooling enters and exits all cavities simultaneously, thereby cooling all cavities at a uniform rate. Parallel cooling is the preferred method but is not always possible due to limitations. One limitation may be the mold thickness required to carry the required cooling lines versus the thickness available. Another limitation may be insufficient water-line hookups on the blow molding machine.

Regardless of whether a cooling system incorporates a series or parallel design, drilled lines or milled slots, its objective is to maintain a high water-flow rate and consistent cooling to carry away as much heat as possible, as fast as possible, in order to cool the part.

All cooling systems - flood, cast tube, drilled or milled - should be designed not only to cool the finished part, but also to cool the flash. Cooling of the scrap flash is important in order to eliminate part warpage after molding and so that hot flash does not come in contact with other finished parts, possibly sticking to them and destroying them.

The function of the pinchoff is to sever the parison and separate the finished part from the scrap; it also serves to form a seam and to weld the parison together. Some of the factors affecting pinchoff performance are part material, parison thickness and location with respect to the parison drop. Pinchoffs are also used to provide a break line in the scrap for trimming operations. Dams and steps can be incorporated in a pinchoff design to facilitate the proper weld line.

Pinchoff land width is the width of the area on each mold half that nets out against the opposite half. As land width increases, a more substantial seam will be formed, but trimming and finish on the part will be sacrificed.

Venting design is crucial to process quality

Venting must be incorporated into all blow molds. Venting systems allow for the escape of air between the parison and the cavity. Venting is located on the face of the mold, at the split line, between mold sections and beneath inserted pinchoffs. Vent depth is dependent upon location and part material. Poor venting can cause visible air entrapment on the surface of a part and prevent the part from fully contacting the mold.

Vents consist of milled paths or slots that are approximately 0.002 in. to 0.007 in. deep. Core vents (also known as slot vents) are normally installed in areas deep within a cavity where split lines are not located, and also at the last positions parisons contact when they expand to meet the cavity wall.

In applications where evidence of the vent in the finished part is undesirable, small-bore vents are utilized. These vents consist of drilled holes 0.007 in. to 0.013 in. in diameter; they are usually drilled 0.120 in. to 0.180 in. deep from the inside cavity face, with a larger drill from the rear and ported to a central location.

Several changes in the types of products being blow molded are dictating shifts in blow mold design. For example, there is an increasing demand for blow molds that allow application of in mold plastic labels, which can be reprocessed with plastic bottles. The emergence of blow molded interior automotive parts is requiring molds which can produce special grains and textures and which allow in-mold insertion of substrates and structures.

Another trend in the blow mold industry is the increase in customer-supplied computer-aided design data to mold makers. This practice helps reduce lead time on new molds and lessens the risk of misinterpreting customer specifications.