Robert Slawska has more than 45 years’ experience in blow molding. His firm, Proven Technology Inc., Hillsborough, N.J., provides engineering/consulting services and equipment to industrial blow molders. Reach him at (908) 359-7888 or email firstname.lastname@example.org. Website: industrialblowmolding.com
Know Your Head Tooling Requirements
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Figure 1: Diverge-style tooling heads are used for larger parison diameters. Users of diverge tooling usually experience less “curtaining” than those running converge tools. Figure 2: Converge tools are used for smaller parisons within the same head. Note how the tip extends beyond ring to prevent parison hang-up.
In my last column (Making a Part for the First Time? Here’s How to Get Going, March 2011), I wrote about forming the parison and parison layflat. Let’s now delve more deeply into those issues with a discussion on the available options in head tooling for accumulator-head blow molding machines.
The head tooling consists of the mandrel and bushing. Its purpose is to form the parison size needed to make a given part. There are two categories of head tooling: diverge and converge. Diverge is used for larger parison diameters (see Fig. 1), while converge (Fig. 2) makes a range of smaller parisons within the same head. These “rules” aren’t iron-clad, however, as there is a crossover point between the two tooling types. For example, a 20-lb head will have a total range from 2-in. to 14-in. tooling diam. Converge tooling must be used up to about 5.5 to 6 in. diam. and diverge tooling from 6 to 14 in.
Users of diverge tooling usually experience less parison “curtaining”—vertical fold-overs—than those running converge tools. Diverge-type tooling also generates parisons with less swell ratio than converge tools because the plunger is on the outside and the diverter on the inside of the head. However, diverge tools require more force to program, since there is more area for the plastic to push against. This can require more force or larger shoot cylinders, which in turn could create the need for a larger program cylinder to keep the system in balance. Diverge tools also require finer programming of the differential angle between bushing and mandrel (should be in the 2° to 7° range). In this design, the tip is usually inboard of the bushing ring.
Blow molders that use converge tooling often notice more swell (good and bad) on the parison. Less force is required to program the mandrel. In converge tools, the tip extends beyond the ring to prevent parison hang-up.
DETERMINING ANNULUS GAP
Regardless of type, head tools have an annulus gap between the bushing and the mandrel in the land-length area that must be set up for thick or thin wall distributions as the parison is purged from the tooling. Use this as a guide:
Annulus Gap Land Length Wall Thickness
Above 0.100" 1 to 2" Very thick
0.030 to 0.100" ¾ to 1" Moderate
Below 0.030" ¼ to 3/8" Very Thin
At a gap of 0.100 in., the flow pressure of the material will be balanced around the tooling due to the long land length. If the land length is 2 in. with an annulus gap below 0.030 in., the flow pressure would be very excessive during parison push-out. Most likely, you will not be able to control the parison and the desired profile.
The most common angles for the bushing and the mandrel are 30% and 34%, respectively, and it’s easy to predict what might happen if they are manipulated. For example, if you programmed a 20% bushing angle and a 27% mandrel angle, a large movement of the annulus gap would be necessary with very little difference in the gap opening through this range. Many automotive parts require very thin walls—down to 0.025 in. This is difficult to accomplish, and many parts are thus in the 0.040 in. thickness range.
During setup of the gap between the bushing and the mandrel, you will want to set a “zero-position” mechanical die gap of about 0.010 in. This is done by measuring around the entire gap with feeler gauges. There is also a mechanical adjustment on the program rod extension. This prevents the bushing and mandrel from shutting off the gap completely and hitting metal to metal, which will create an offset pressure between the two. Another adjustment once the zero position is accomplished will be to set the electronic setpoint for the annulus gap. This will add another 0.005 in. to the gap opening. These numbers are usually satisfactory as long as there is no drool from the gap.
Beware of potential mechanical restrictions on the top internal area of bushing and mandrel. Poor design could create an interference for parison programming. A protrusion on the upper mandrel area could cause an interference with the bushing while programming. By moving the mandrel down, the protrusion on the head will minimize flow in this area, causing a very thin area on the parison. This area of the mandrel would have to be redesigned.
When using maximum head tooling on an accumulator head you might have to slow down the shot speed. The high pressure and speed of the parison drop causes the mandrel tooling to open with very little holding control. At this point you have lost parison programming control and the part will be out of spec.
As the parison begins to get pushed out, it will experience its largest percentage of diameter swell. That’s because the material has been sitting in the tooling area under pressure waiting to be purged out of the closed tooling. As the parison gets pushed out, the weight of the parison will actually pull it down. This is called drawdown. Hence the bottom of the parison will be larger in diameter than the top section. This could be good or bad depending upon the type of mold that is in the press. You should study the design of the part and determine that the actual orientation is proper.
Parison curl is caused by uneven distribution of material in the upper accumulator-head area. The pressure drops around the circumference are not equal all the way around the diverter. This often occurs when the majority of the flow comes straight down the extruder inlet side of the material flow area in the head. This can be corrected by adjusting the bushing to be offset by pushing in on the bolts on the heavy-wall side of the parison.
Heater bands are mounted on the accumulator-head body. Many machine builders normally make the heaters line up straight in a vertical line. This looks great, and the actual wiring looks splendid. The trouble is that the design can cause some setup issues. This arrangement leaves two vertical bands on the body of the head that are not being heated, which creates a straight cold area on both sides of the heaters. The best solution is to stagger each horizontal row to overlap the heater above and below it to reduce the cold spots to a minimum. The net result is a more uniform parison wall distribution around the entire body of the head.
The Scoop on Blow-Pin Stands, Pre-Pinch Units & Parison Spreaders
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A bottom blow stand is used for mounting the blow-pin assembly, pre-pinch unit, and parison spreader.
Two spreader pins on a parison spreader unit will ensure that the parison width will be at the same desired position cycle after cycle. They are also useful to prepare the parison for a wide, flat panel.
Most processors spend a lot of time defining what should be included in their new blow molding machine. Yet a precious few spend any time identifying the need for a blow stand and accessories. Few of these machines are supplied with blow stands by the builder. As a result most of them are home-built, poorly designed, inflexible, and unstable. Most often, the basic blow stand is made to serve limited functions such as blow-pin up/down and air on/off.
Other functions such as pre-pinch/pre-blow and parison spread are usually put on the bottom blow frame in order to become part of the basic blow stand. The parison spreader is normally used in conjunction with the pre-pinch. Again, these are hit or miss mountings and have limited flexibility for proper use during the parison forming.
Here are some explanations for using each of these functions:
•A bottom blow stand unit is used for mounting the blow-pin assembly, pre-pinch unit, and parison spreader unit. The stand is used for manual setups during mold changes and setup requirements. Motorized control allows for rapid and accurate up/down position changes. This manual up/down movement should be in the maximum range of at least 17 in. total adjustment.
In addition, the blow-pin cylinder will have a rod stroke of 12 in. to add to this total adjustment. The blow stand should be very flexible and allow rapid changeover of the other functional components. This bottom stand unit has many threaded holes in the top mounting plates for proper positioning of the components. The system utilizes four vertical jacks with couplings and shafting between each jack unit. Miter boxes are also used to allow proper up/down movement of all four jacks. This becomes a very stable frame to handle the forces created during the neck compacting functions.
•The blow-pin assembly allows the parison forming air to enter the parison from the bottom of the mold. Normally, the stroke of this vertical hydraulic cylinder is at least 12 in. The blow pin also forms the neck opening in the molded part. It requires water cooling. Depending on the part requirements, the neck opening could be compacted for a sealing or finished surface in the part. This can be accomplished by having a secondary position or midpoint, and the blow pin will move to this position at full mold closing. Once the mold is closed, the vertical cylinder is fully extended. This stroke will move a small distance (0.020 to 0.050 in.) to the final, fully extended (upward) compacting position.
These positions can be identified with either proximity switches or more precise linear potentiometers. Some parts may require even finer finishes to be provided as a secondary operation.
Once the mold starts to open, the bottom blow pin is used to help demold the parts from the cavities on the bottom, along with the takeout device gripping the upper portion of the parts. After a small movement of the platens, the blow pin is retracted completely from the mold cavity to allow the takeout device to remove the part from the press. Blow pins can also be used as spreaders. For example, with 55-gal drums, blow pins in both openings can spread the parison and then would have an unspin device if internal threads were molded into the ID of the neck.
•A parison pre pinch unit uses two water-cooled aluminum plates in either a bear-trap or horizontal-closure arrangement. Either type of unit is normally used in conjunction with a needle-blow setup. The parison is pushed out, and at the end of the cylinder stroke (accumulator empty) the two pinch bars will close and seal the parison on the bottom. Preblow air then inflates the sealed parison. The preblow air will shut off when the clamp is closed. At this time, main blowing air will be introduced by a needle that is injected into the parison.
Pre-pinch units can also expand or restrict the layflat range of a given size of head tooling. This saves setup time for changing head tooling.
Some pre-pinch units are mounted directly under the mold halves. But if a large blow-up is required, the distance between mold halves most likely will not be big enough to inflate the parison without it hitting the mold.
•Parison spreader units are ideal for making blow molded panels. Two spreader pins will ensure that the parison width will be at the same desired position cycle after cycle. It will also prepare the parison for the wide, flat part shape, which is usually 2 in. to 3-in. thick. It will also tend to move the thin wall of the parison into the flash pocket of the mold and not in the actual part.
Spreader pins also work well when used with the pre-pinch unit. The spreader pins can be used under or over the pinch plates. If used over the plates, notches must be cut into the face of both pre-pinch plates to make room for the spreader pins. You will also need to pay attention to minimum and maximum postions of the stroke of the spreader plates.
It is important to have a copy of the mold cavity drawing to ensure the proper relationship between all the blow-stand positions and the various cavities. Most likely, each mold will have a different width, length, and height. Hence, the setup adjustment range for all molds must be examined. This is extremely important when using a compacting blow-pin arrangement.
It is worth the time to establish the range of parts and molds to be used on a given machine. This examination will help determine the parison size required to make a given part and it will also identify any obstructions preventing a straight parison drop or that could hit into the mold cavity.
Some parts will require a diagonal movement of the blow pin of up to 4 in. or so for offset neck openings. This requires several adjustment positions and moving plates. In some cases, blow pins are set at a 45° angle to the parison. This also needs several different set positions to allow the parison to drop over the angled blow pin and not hang up prior to reaching its final length.
Pre-planning is necessary make this difficult process achievable in production. In most respects, this will not be accomplished in the first parison drop. It could take several hours to make all the adjustments needed for an acceptable part.
The design of the lower blow stand must allow fast part changeovers. Changeover can be well within 30 min using simple lock-down methods and quick disconnects for electrics, air, water, and hydraulic hoses. Fine mounting adjustments are also achievable with the four lock-down bolts on the lower blow stand.
Review Mold Requirements Before Buying a Press
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This is a typical press design with three-guide-bar construction. The press provides a rollout feature to allow more access to the head tooling for mold changeovers.
If you are in the market for a new blow molding press, best to first review all the different types of molds you might be running in that machine. Many of them can be as different as night and day.
Some molds might require a completely different sequence of operation to produce the part. Some of the mold mounting sizes will be different due to outrigger cylinders beyond the mold. Best to do your homework up front and be prepared for changes sooner rather than after the fact.
When you conduct this mold review, make sure to categorize the size of the molds plus any outriggers attached; the tonnage required to pinch the parison and hold the molds closed during the blow cycle; whether you’ll need to mount additional spacer stand-offs on the platen faces to equalize clamp tonnage over the platen surface (if the molds are much smaller than the platen faces); and what type of air-blow system will be required—needle blow with pre-pinch, top or bottom blow pin.
When looking at new equipment, you’re better off buying a press larger than you think you might need. This will allow long-term flexibility and permit production of larger parts later on. If you are looking for a 60 x 60 in. clear-platen press, consider upsizing to 74 x 60. This will give you more room for larger molds and outriggers on the molds.
There are several ways to mount and hold the molds on the platens. There are grouped holes that are drilled and tapped into the platen functioning faces. There are also large holding location holes machined into the plates. These holes are about 4 to 6 in. diam. and ½ in. deep. A matching ring is machined on the back of the molds to center the molds under the head.
Other types of mounting devices are plates that hang off the top face of the platens, with matching hanging plates on the molds. My preference is ¾-in. horizontal “T” slots that run across the face of the platens. Depending upon the platen size, there are normally four to six horizontal rows of slots. The “T” slots are most convenient when using dual heads and side-by-side molds. It is best to mount the molds on a backup plate and then mount the molds to match the dual-head centerline. The holes through the backup plate should match the tapped horizontal “T” slots in the platens.
The “T” bolt arrangements are very safe when removing and mounting the molds on the platens.
Three-guide-bar press construction is preferred, since it is designed to minimize platen or end-frame deflection to ensure proper mold pinchoffs (see illustration). The operator side of the press on the bottom does not have the lower guide rod. This makes it very convenient for the part takeout device to accommodate long molds and also makes mold changeovers easier.
It is also beneficial to have high-speed, repeatable platen movements at up to 1200 in./min combined with a no-shock hydraulic system. The press must provide minimum platen and end-frame deflection.
A motorized adjustment to move the press up and down is useful for mold setups. For the sake of speed, it is possible to leave the safety gates open for setups and mold mountings. But it is wise to not cheat the system. Check the latest ANSI specs on blow molding machine safety guidelines.
Many proportional valves are used on the molded part functions mounted inside the press area. These valves should be mounted outside the press frames and front and rear safety gates.
Many applications utilize moving split molds, which allow removal of molded parts by relieving deeply recessed areas (for deep blow ratios and part entrapment) by repositioning the mold slide and leaving a good radius on the deep area. Also common are core slides, which may have air or hydraulic actuation. These valves must also be positioned outside the press area. And don’t forget about special items such as blow-pin stands, which may be used for in-mold finishing of necks, or even unspin pins for internal threads.
Motorized press rollout allows room for mold changeovers with overhead cranes. Once the press is rolled out, the head tooling (mandrel and bushing) can be replaced with much more clearance to work on the mounted tooling, which is extremely hot. This allows special equipment to be used for tooling changeovers. This is a very safe way to do the changeover with minimum risk. Typically, this will save 30 to 60 min for the complete change.
A servo-actuated press eliminates the need for rack-and-pinion mechanisms. These are replaced with an independent system for controlling each platen speed at all open/close positions. This system allows for fast setups and changeovers. For example, you can leave one mold open and the other mold half at the closed position to set up blow-pin functions. That can save up to 2 or 3 hr of setup time, results in extremely accurate setup with no mechanical backlash, and the preset mold-closed centerline can be offset in either direction.
The lock-up of both platens ensures no pushover on the desired closed centerline position. There is an alarm for positive center-position protection on full close. This will eliminate damage to blow pins or molds and will maintain uniform opening on blow pins.
This system is designed to hold the press platen centerline within less than 0.020 in. Instant high-tonnage lock-up is accomplished by use of the same press cylinders for fast close and lock-up tonnage, which ensures instant pinch-off at lock-up. Excellent pinch-off on parts means faster cycles, as operators can remove flash manually.
Ensure consistent cycle-to-cycle platen position. The PC controller can regulate functions to control ramping of platen movements and positions. This can provide production yield of up to 95% good parts.
The press-position linear transducers are located inside of kicker cylinders (on each side) for efficiency, repeatability, and mold protection. This eliminates the wear resulting from external transducers and saves on replacement costs. This also ensures positive location positions for all press movements. The kicker cylinders are used for fast opening of platens.
Your Pre-Installation Checklist-Part 1
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Once you’ve ordered a new accumulator-head blow molding machine, don’t think you’re done, because you must then prepare for the installation and operation of the equipment.
Sit down with your people and key personnel from the machine builder, preferably at their plant. That way, you’ll see similar equipment and have access to drawings and specifications as required. Project timetables can also be discussed during this initial meeting.
Determine as early as possible the actual overall size of the machine so you can make installation as quick as possible. Allow for proper ceiling heights to give adequate ventilation, access to service the accumulator heads, and future removal of the heads if necessary. Your equipment might require retractable air-blow mechanisms mounted on top of the head. This could add up to 3 ft to the machine height. Review the plant’s internal climate and temperature. Are there any air drafts from doors, windows, air conditioners, or fans? These elements will have a negative effect on the head performance. A cold draft could cause the parison to curl or change drop lengths.
All the necessary utilities are readily available from your machine builder. The important items required are:
•Air: How much CFM is needed? What pressure is available? How much volume is required? Is there a pressure drop in the line (is this machine the last in a row)? Is a surge tank necessary? What line sizes are required? Is filtered/dry air needed?
•Water: What GPM is needed? Where are the hookups located on the machine? What size lines are required? What water temperature is needed? Is a cooling tower required? What size chiller is needed and where is it located?
•Electricity: Consider the voltage, breaker size (full-load amps), and location of the main breaker. Where is power hookup on the machine? Is there enough power in the plant for new equipment?
In addition, find out the weight of each of the machine’s sub-components. You must have the proper equipment to handle each part. It may require a crane, large forklift truck, or a combination of the two.
Once you have determined where the machine will be located on the floor, you can specify the operator position. This is the position on the machine where the blow molded parts will be removed.
Account for space to finish the parts by removing flash (trim) and doing any drilling or cutting required. You’ll also need room for auxiliary equipment.
Stairs are provided for access to the upper level of the equipment. It is important to locate these stairs for safe access, but away from production functions. These stairs must be built to OSHA and ANSI specifications for safety.
When considering machine layout, identify how you’ll be mounting your molds—by forklift truck, overhead crane with a roll-out clamp, etc. Whatever your choice, adequate space must be provided. This may require moving work tables and conveyors.
You should also determine when you order the equipment how you’ll be attaching molds to the platens. The two most common means are bolt holes or T-slots. Locating slots, pins, or bars may also be used for mounting molds in relation to the head centerline.
Mold mounting can usually be accomplished in an hour. Use of quick-connect fittings and manifolds for air, water, and hydraulic connections will drastically shorten changeover times.
When you change molds, you will likely also need to replace the head tooling. Like mold-change procedures, you must determine the best way to change head tooling. If the press is rolled out of the way, head tooling can be changed at the same time as the mold is replaced. Remember that the head tooling is heavy and very hot. You must be set up properly to achieve this switchover.
What material do you plan to run in this equipment? This should be reviewed before the machine is built. Maybe you can use a general-purpose screw if you’re running only polyolefins (though a g-p screw is not a perfect solution, by the way). However, if you plan on also running engineering resins, you might need another screw. If a screw changeover is necessary, remember to provide the space to accomplish this.
LOCATION OF AIR & HYDRAULIC VALVES
It is best to locate the main air valves as close to the inlet of the parison as possible to ensure that the parison inflates quickly. The air lines and the valve and manifold should be as large as feasible—at least 1 in. diam. or 1.25 in. for very large parts. Large lines will facilitate quick exhaust of air within the part during the vent cycle.
The exhaust valve must be located right at the discharge of the blowing needle or pin. Some parts will require “low blow” or low-pressure blowing air. This should be connected to the main air-blow lines to allow for rapid transfer between the two.
Pre-blow air must be set up to be part of main blow air or separate when pre-blow is in a different part location from main air blow. In both cases, if the main air blow is directly connected to pre-blow or low blow, check valves must be used in the connecting lines. This will prevent the main air from leaking out through these auxiliary valves. In most cases, the other air valves used in blow molding are 3/8 in. These are for the gates, needles, knockouts, pre-pinch, strippers, etc. Most often, these valves are manifold mounted on the press or on the main frame.
Like the air valves, locate the hydraulic valves near the actual function they are used to control. Valves for split molds, knockout, unscrewing threaded blow pins, parison spreaders, up/down movement of blow pins, etc. are usually mounted on the press frame. For most of these of these functions, the valves should be mounted in the rear center of the platens (head centerline).
The shooting and programming valves should be mounted near the accumulator heads on top of the main base. The open/close and high-tonnage lockup valve should be mounted on the press. Again, either the valves or the piping to the cylinders should be centered.
Take care with the actual valve mountings. Remember that hydraulic valve and line connections tend to leak due to high pressures and shock. It’s advisable to mount drip pans under the valve to capture small amounts of leaking oil. The pans will also catch oil leakage when changing O-rings.
In our next installment, I’ll discuss what steps you need to take after your machine has been built.