Making bottles better
In optimizing throughput, blow molders usually start by considering the way total cycle times break down. The numbers vary by process and application, but in a typical extrusion-blow operation, pariso...
In optimizing throughput, blow molders usually start by considering the way total cycle times break down. The numbers vary by process and application, but in a typical extrusion-blow operation, parison extrusion, mold close and inflation account for 10 to 35 per cent of overall cycle time, while cooling consumes 60 to 90 percent. Part ejection uses only 5 per cent of the total.
The first target for cycle time improvement is clear: the cooling cycle.
Rich Smith, Uniloy Milacron’s product manager for reciprocating systems declares: “In terms of cost effectiveness, molders can get significant bang for their buck by improving the process on existing machines with better cooling of molds to reduce cycle time.”
A commonly quoted rule of thumb is that mold temperature reductions of 1C generate one percent additional productivity. Unfortunately, the mold cooling process is more than just a setting on a controller or PLC. In fact, the extent and number of possible variables is very machine- and process-dependent. Coolant type, flow and heat rejection rates, overall system capacity and temperature stability are among the important parameters that can place an artificial speed limit on a machine cycle.
A problem often encountered when reducing mold temperatures is condensation. Rust on the mold, “orange peel” and moisture on and under the machine are warning signs that mold or ambient air dehumidification is needed. In addition, a thin film of water between polymer and mold can work against the goal of enhanced cooling. Water is roughly 100 times worse than alloy steels as a conductive heat transfer medium, meaning that moisture in the mold is an effective insulator toward the solidifying polymer.
From a process control perspective, if the installed cooling system operates near capacity limits or is unstable, the control process can become a “cat and mouse” chase.
Mike Ryan, CEO of Ryka Blow Molds explains: “We could technically design the best cooling system into a mold, put it into a customer’s plant, but if he doesn’t have the cooling capacity to generate the effective cooling characteristics that he’s looking for, you’ve put a lot of value-added substance into a mold that he can’t use.”
Ryan notes that processors can benefit from talking to their mold supplier before sourcing new production equipment, especially when layout changes introduce possible coolant flow restrictions, or heat-absorbing longer piping runs. Similarly, a cooling system audit often removes an important variable when troubleshooting a tryout.
COOLING FROM THE INSIDE
Optimizing the heat flow through the mold cavity is a “no-brainer” for efficient blow molders, but the inner surface of the resin balloon can also benefit from forced cooling. Fasti-USA Inc.’s CAC System introduces chilled compressed air at the blow pin to cool the part inner wall at rates similar to the outer surface. Fasti-USA claims that the resulting reduction in wall temperature difference lowers residual stresses and distortion while increasing productivity by 15 to 50 percent. Other benefits include improved neck calibration and the availability of residual blow air for mold dehumidification or resin drying. The firm claims a system productivity payback in one to three months.
Post mold cooling is another method for improving productivity and dimensional stability with very short mold cycle times. Marathon Industrial Products Company (Ajax, Ont.) offers Cryo-Jet air impingement conveyorized tunnels which focus jets of high velocity air on product surfaces for accelerated heat transfer and controlled shrinkage. Cryo-Jet systems cool as-molded product up to five times faster than ambient cooling, and minimize total system footprint.
CONVEYORS COUNT, TOO
Getting the product out the door efficiently requires part-handling equipment that integrates well with the rest of the system. Molders stepping up to conveyorized product handling have choices ranging from slat-chain or tabletop manual systems to plant-wide installations which divert and accumulate product for automated packing or palletization.
Accumulation is vital for maximum productivity, since non-accumulating systems deliver product to the filler or packing station directly, requiring the filler, packer, semi-automatic or fully automatic system to match or exceed the speed of the molding machine.
Neck-guided air conveyors are one solution to the throughput problem for this high-volume segment, despite a considerable cost premium per unit length when compared to tabletop installations. Canadian manufacturer Descon Conveyor Systems & Consultants Inc. (Newmarket, Ont.) has introduced an air conveyor system which offers automatic neck guide changeover and rail adjustment for different bottle sizes, an accumulation system designed to minimize footprint, and a “voidless” layering system which the company claims eliminates the need for an operator on bulk palletizers. The Descon system operates at speeds up to 2500 bottles per minute for single-serving containers.
BE CREATIVE ABOUT DOWNTIME
Peter Phelan, plant manager at Richards Packaging’s Etobicoke, Ont. operation notes that automation isn’t the only way to realize greater throughput, especially for smaller custom houses. “It’s about having your set up sheets right and your tooling well organized so downtime (during changeover) is minimal.” Phelan emphasizes that smaller shops need to be creative in reducing downtime when quick change mold technology is cost prohibitive.
Richards often modifies standard machines to suit existing mold and line configurations, allowing a level of standardization which makes maintenance and changeover faster and more repeatable with differing equipment and production volumes in the same plant. With a customer base which packages products ranging from pharmaceuticals to automotive fuel additives, flexibility is profitability.
The number of variables that a blow molder must consider when maximizing output is great. Upgrading the right equipment with the appropriate level of technology remains a major challenge, especially for small- and medium-sized businesses.