Increase Your Blow Molding Machine

In the early days of industrial blow molding, the processor had quite a few obstacles to overcome, many of which formed bad habits.

Here are a few that come to mind – let’s call these industrial blow molding then:
1. Material and color changeover times ranged from 8 hr to several days depending on the color and material.

2. Poor head designs led to machine downtime and expensive repairs. Many of these occurred every six months or so, amounting to weeks of downtime.

3. Long periods of downtime were spent just waiting for the service tech to arrive to begin to solve the problem.

4. Very basic manuals, schematics, and sequence of operation did not necessarily pertain to your machine as built. Since there are tons of old machines with many “fix up” repairs over the years, these changes have not been recorded in most cases.

5. All of the older machines were semi-automatic and relied on an operator to close the gates and restart the cycle. This gave very unpredictable control of cycle times. And there were no part takeout systems.

6. Mold changeover took a minimum of a full day or even two. Often this also included changing head tooling.

7. Setups from product changes always seem to cause issues, even though the same changeover was done just last week.

8. Cycle times depended on a combination of:

  • parison drop time,
  • press closing time,
  • blow time,
  • exhaust (vent) time,
  • decompression time,
  • clamp opening,
  • gate opening,
  • manual part removal,
  • gate closing,
  • clamp moving to pre-close position.

Due to the many factors involved, these obstacles limited the number of changeovers and different types of parts that could be run on a machine.

Fortunately, developments in machine and processing technologies have removed many of these obstacles, putting blow molders in a better position to keep the machines and profits up.

Let’s look at industrial blow molding now:

1. Current head technology allows color changes in about 20 min to a maximum of 1.25 hr for the most difficult changeover. Since some blow molders make up to three color changes a day, the savings are huge and drop right to the bottom line.

2. Heads now on the market require little repair or maintenance. The industry has switched from sensitive mica-band head heaters to extruded aluminum and Calrod heaters.

After five years of use, these heaters have not experienced one blow-out as compared with hundreds on the mica band heaters, cutting replacement cost and downtime. Further, a properly designed head will eliminate potential galling from metal-to-metal rubbing on the moving internal sections of the flow paths of plastic materials.

This was a major cause of overly long color/material changeover times. There are heads on the market that allow disassembly/cleaning/assembly to change color or material within 1 hr.

3. It is still not uncommon to have to wait for a service tech to arrive at your plant to fix your machine. But I have found out that about 80% of machine problems today are due to improper setup. These types of problems can be solved by technology that links the supplier directly to your machine for troubleshooting.

Most issues can now be identified and resolved in 1 to 2 hr.

4. Machine manuals can now be made viewed on the machine operator’s computer screen. No need to try to find the original paper manuals that are stored in a locked supervisor’s office at 3 am.

5.   Automated Part Takeout has now become a standard component of the blow molding machine. Automation gives a consistent cycle time. In addition, it now makes sense to include a part-holding fixture with the PTO system. In dual-head operations, the holder prevents the part from dropping until the operator is finished with the first part. The gain here is the possible elimination of a second operator for each shift.

6. Molds are now designed with quick-change connections for water, air, and hydraulics. Some processors now make complete mold changeovers in 15 min. This means more uptime and more profit in your pocket.

The costs to accomplish faster mold changes are nominal. Look at mold positioning gadgets like locater hangers.

In many cases, air, water, and hydraulic manifolds mounted directly on the mold will save many hours thanks to fewer connections to be made. Keep your tools on a wagon for the next changeover. This cuts down on walks across the building to get the necessary tools.

The other area of changeover is the head tooling. It’s usually hot, heavy, and large. Simple tables or adjustable stands to support and guide the tooling into place will save time and make changeovers safer.

How much is all this worth?

Every second saved adds directly to your bottom-line profit. For example, if we can save 3 sec on a 60-sec cycle time with a dual-head operation, and the mold runs for 7000 hr/yr, the benefit is 30,000 more parts/yr.

How do we accomplish this?

Make sure there is no more than 1 sec between starting drop of parison and the extruder reaching shot size. It might be possible to bring the pre-close position closer to the mold-close position. This alone could save 1 sec.

Air blow and vent times are usually the longest parts of the cycle. Shorten the time for air blow and see whether that affects the ability to make an acceptable part.

Check the vent/exhaust time of the air inside the part. Make sure the quick-exhaust valve is large enough to vent out a large amount of air in 5 to 6 sec or less. (I have seen this take 10 to 20 sec or longer with an improperly sized exhaust valve.)

Have the part take out start to remove the part from the mold prior to the gate opening. Only open the mold platens just enough to get the part out with the part take out.

On a single-head machine colors changed from white to dark blue and back to white. Both of these color changes were achieved in a total of about 30 shots (30 min)—for not one, but two color changes.

Industrial blow molding Machine & Tooling

Industrial blow molding evolved from art to science with the marriage of PC controls and proportional hydraulic valves.

Today’s machines are highly efficient and predictable, and can generally be relied on to produce sophisticated parts from the first shot.

Newer parts are now being produced with 50-, 75-, and even 100-lb shot sizes.

Regardless of how technology advances, it’s still wise to brush up on some basic guidelines to help you get started, especially if you’re making a particular part of the first time.

First, determine the specifications of the actual parts you’ll be running. Will it require flash only on the top and bottom of the part? Or maybe the only way to make good parts will be to flash all the way around?

Project the actual finished weight of the part, and then estimate the shot size, taking the flash into account.

Flash only on the top and bottom of the part will mean a complete shot weight about 25% to 40% more than the final trimmed part weight. If the flash is all the way around, this could increase the shot weight by upwards of 60%. I have seen that reach 100%–double the final part weight—for certain parts, so be careful with your estimate.

In most cases, flash can be recycled back into the parts.

Shot size and cycle time will also influence the output requirement for the extruder. To make sure the machine is sized right for the job, a rule of thumb is to project the needed output capacity of the extruder at 80% of its maximum screw speed.

The extruder must be working well to ensure a low melt temperature. The hotter the material, the longer the cycle time.

The finished wall thickness of the part also plays an important role in cycle time.

If the part wall thickness is 0.060 in. or less, the cycle time will be in the area of 40 to 50 sec. A wall thickness of 0.080 to 0.100 in. will result in a cycle of around 60 to 70 sec. Very thick parts with walls of 0.120 to 0.180 in. could result in cycles from 90 to 180 sec.

Be careful, as these are only estimated guidelines. Depending upon tack-offs, for example, handles and very thick areas of the part will influence the overall cycle.

Another matter to be carefully considered is the size of the accumulator head that your part and process will require. Here are some guidelines you might find handy:

Determine necessary head tooling size. You might require larger head capacity than output needs alone would indicate, in order to get proper head tooling size for the part.
Find out the parison layflat needed based upon top/bottom or all around part flash. (Layflat = tooling diam. x 3.14 ÷ 2) This equation does not consider die swell or parison preblow inflation.
Factor in parison die swell. Normally, larger tooling diameters have smaller parison swell. Small tooling produces a larger percentage of swell.
Remember that competitive heads may produce different parison sizes for a given tooling size.
Determine if there is a limit on how much regrind can be put back into the finished part. Some flash requirements might be more that the weight of the finished part.
Based upon maximum shot size, determine head capacity needed. Consider what other parts might be run in this machine and to what specifications.
Select tooling size based on head size. Determine if converging or diverging head tooling is needed. If using dual heads, determine the required head center distance.
Next, you’ll need to identify the actual size of the mold, including any outriggers, cylinders for split molds, water connections, and blow pins. This is extremely important in the event you’ll be running dual heads with side-by-side molds on a fixed-head center line. The platens must be sized to fit the molds on the head centerline.
Now you are ready to determine the clamp tonnage required to mold your part. I use this as a guide:

HDPE: 500 to 600 lb of force per linear in.
HMW-HDPE: 600 to 700 lbf/in.
PP homopolymer: 500 to 600 lbf/in.
PP copolymer: 600 to 700 lbf/in.
Next, calculate the pinch clamp force needed to seal the parison:

Length of pinch x lbf/linear in. ÷ 2000

Find the blowing clamp force (to keep molds closed during air blow):

Projected area of blown section x blowing pressure
(around 100 psi) ÷ 2000

Remember that once you clamp up, the pinch clamp force ends and the blowing clamp force takes over. Don’t add these two values together.