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.

Blow Molded Parts Manufacturing

How to make Blow Molded Parts

First time parison programming for a new part can be lengthy and tedious.

Here’s some advice on making the process a bit easier:

The accumulator head is the most important component of an industrial blow molding machine. Likewise, forming a parison profiled to the proper wall thickness for a given part is mandatory to get consistent parts that meet specifications.

Blow Molding Machine

On dual-head machines with two molds of the same or different products, it is advisable to run just one head first to develop the necessary processing points. This will make it quicker to accomplish good parts.

First-time parison programming for a new part can be a very lengthy and difficult process. It all depends on what type of part is being made: Is it round or rectangular with a small height? What about the blow ratio of the parison into the mold areas? Those are just two of the issues that need to be addressed.

Then relate those factors to the original wall thickness of the parison at various segments along its length. Blow molding machine and head performance and optimal settings are usually different from machine to machine. Every machine has its own personality, and you need to learn these differences. The key issues are how to set up the parison length and profile points to make an acceptable part.

First, estimate the length of parison required. Measure the mold length and the distance from face of the tooling mandrel to the top of the mold. Determine the additional parison length below the bottom of the mold required for pre-pinch, blow-pin stand, and parison spreader (depending on what is required to make the part). Then determine the layflat of the parison to size head tooling.

Next, determine the actual length of the parison. Experience is especially important when doing this for a new mold and part design. Refer to any similar parts previously produced to assist in factoring the proper shot size. It is better to be longer on the parison than too short. If you have a short shot you’ll wind up with a glob of plastic lying in the bottom of the mold or floor. Making an actual part will enable you to make critical changes on the next shots.

This is not yet the time to configure any parison profile steps. Instead, check the finished weight of the part and measure its wall thickness in critical areas. Review those results and consider areas that require major profile changes.

Make sure the parison drops straight on a consistent basis.

Now consider how much space is needed between the bottom of the head tooling face and the top of the mold. Round parts can allow the tooling face to be closer to the top of the mold. This minimizes the amount of top and bottom flash. But flat panels and molds with small shut height (thickness) must be farther away from head tooling due to parison arching angles back to the head tooling.

This might require using a pre-pinch unit to “balloon up” the parison diameter. (It’s like a blown up bag.)

The parison must be long enough to allow pinch bars to seal the parison bottom and clear the mold. The job might also require parison spreader pins (or blow-pin movement for offset necks). These features could require either longer parisons or cutouts in the molds for the spreader pins. Once you pre-pinch the parison, you might need to slow closing of the press to allow the inflated parison to fit into the deep-draw areas of the mold.

If you are running a dual-head machine with two molds of the same or different products, run just one head first to develop the necessary processing points. This will make it quicker to make good parts. Once you can make a series of good parts, you can transfer the details and setpoints to the second head if it is the same part on both heads.

Most likely, minor changes will be required for the second head, which can be programmed accordingly.

To estimate the weight of the parison (best guess), determine how much flash might be required on the top, bottom, and sides of the parts. Once you have repeated the parison weight and length for three or four shots, you can start profile programming of the thin and thick parts of the parison and part. It is a time-consuming chore to measure wall thickness in various areas of the part and parison. It will be a great help if the parison can be manually purged out at the proper speed and profile steps to determine the set-up for automatic operation.

Some parison programming units allow you to place a spike (a thick parison numbered point) in a given area that might be too thick or thin. This will enable you to quickly find the correct location (profile numbered point) for the change.

Some systems allow you to accurately trace the actual profile path after the parison is dropped, as compared with the setup profile. This will improve the speed at which you can achieve the correct area thicknesses. It will also indicate places that you are asking the machine to accomplish too abrupt a change in thickness and the mechanical parts cannot react fast enough. That will produce a wide variation in wall thickness in those spots.

Use as many “master” setpoints as needed, but don’t overdo it. There are systems available that allow smoothing of the master points by either holding the original master points’ position or allowing them to “travel” with the smoothing feature. Make only one programming change at a time and stick with it until you are sure it can or cannot accomplish the desired results. Remember the drawdown effect from gravity on the top section of the parison. In most cases, this area must be thickened.

How fast should the parison be pushed out? This certainly will affect parison swell and the total amount of flash. Fast parison drop in most cases will increase the layflat. Trying to vary the parison drop speed is very difficult to control in relation to exact positions along the parison.

In many cases, the mold might stay in the machine only a few days. This is not enough time to set up the proper parison profile. You could make many reject parts trying to get the acceptable part dimensions needed in such a short time in the machine. The saving grace, in many cases, is to ship parts that are on the heavy side to ensure they won’t be rejected by the customer. The negative consequence will be additional costs for the heavier parts, which can eat up your profit.

You may even need to change the head tooling size in order to make an acceptable part. If so, you will probably have to do the above functions all over again with new tooling.

Allen has more than 20 years’ experience in blow molding parts manufacturing. His firm, GC MOLD., provides engineering/consulting services and equipment to industrial blow molders, plastic mold, die casting, metal tooling serce to the ords. Reach him by contact us page. Website:

Technical blow molding

Welcome to our blow molding blog post.  We hope to chat about Industrial/Technical blow molding here. We would like to post actively on this blog and promise to answer any of your questions if possible.  If we cannot answer a question, then we will try to find the answer.

We look forward to bringing items of interest to the Industrial Blow Molding Market.

SPE Blow Molding Div Annual Conference ABC-2008

I just returned from the 24th SPE Blow Molding Division’s Technical Conference.  This was located at Chevron Phillips in Bartlesville, OK.  This was an excellent conference with several interesting and well presented talks.  This conference was split evenly between the packaging and the Industrial area. 

The actual attendance was fairly light with about 60 paid attendees.  The technical facility is well equipped with 6 blow molding machines being demonstrated during the Plant tour. This was a must not miss occasion that many of you did miss.  Please contact the SPE BLOW MOLDING WEB SITE for additional information.

The Blow Molding Division did a great job in putting on this Conference.  There was plenty to learn. Based upon the background of the Board of Directors, Speakers and Exhibiters, this was the place to learn and get your questions readily answered.

Hundreds of years of hands-on experience were present to help you.

The Chevron Phillips Technical Center is a very well equipped facility.  They can do just about every major type of processing as well as testing of the products being made on the equipment. 

This of course includes the evaluation of the basic material used with all types of testing means.  The technical staff is well trained and very friendly.  Next time you are there, just ask them and be pleased with the results.

Industrial Blow Molding History

I have been in the Industrial Blow Molding Market for over 45 years. Most of this time has been in the machinery portion of the industry.

Every 10 years or so, there were tough times for one reason or another. Take a look at this cycle below:


Material/Gasoline shortages and claimed storage offshore on large ships drive up material cost.

Uniloy built molds for very lightweight bottles (milk) to survive.


New Machine Sales stopped. You could not even sell used machinery.


Again Machine Sales stopped.

However, more new machines were sold in 1995 than any other years since the early 1960’s.


There was almost a whole decade of the worst period ever and nearly 10 years in the making. The typical total USA market ranged from a low of 50 machines to a maximum of 150 machines a year. There were some used machines sold during this period.

Now the industry is fortunate to sell 10 new machines a year. There are a lot of used machines sold between 20 to 35 machines a year during this long period.


What about the producers of Industrial Blow Molding parts? Just what are they doing?

Please see the following chart as researched and produced by Plastics News every November since 1994. My record keeping starts at year 1996.


You quickly see an increase in sales volume for every year including 2008. The respondents go from a high of 194 in 1999 to a low of 148 in 2008.

There have been a number of consolidations and some bankruptcies. The Industrial market ranges from a low of 18.7% in 1996 to a high of 29.9 in 2000 for the sales numbers shown on the charts.

These numbers do not include several large Captive Blow Molding Corporations. In most years, the same companies did not respond to this report.

We all know about the financial bliss bailout of the Big 3 Automotive USA car makers. If you look at the Plastics News List, you will see that there is a strong reliance upon the automotive market by USA producers. Of course, some of these companies also provide goods to the Japanese Automotive Companies doing business in the USA.

Look at these stats for companies doing work for the American Auto Parts Producers. Some of these companies only do automotive parts work. Other plants do various types of custom blow molding.

2Top 10
5Top 25
14Top 50
25Top 80
30+Top 100

This is a total of 30% + of the top 100 companies that do work for the automotive builders in the USA.

If the Big 3 goes down where does this leave these companies? What is next for these plants?

I have listened to several options. Several blow molding companies have folded in the last 2 years. What about the machinery builders? Can they survive this mess?

Maybe Julie Brown of Plastech fame did get the last laugh at the BIG 3 PLUS with her bankrupted company.

Yes, I do remember when the USA Machinery Companies did not have any real available money. They still developed innovative machines and their customers were willing to take a chance to get a couple of years lead over their competitors.

They not only took the chance but actually were willing to fine tune the process and equipment.

Many of these small companies were very successful and the owners became millionaires and retired.

Those customers also demanded a machine that would be up to date for their current needs and options.

A couple of years ago, I visited a Chinese machine producer. The machine they were producing could not meet the needs of the USA processor and their specifications.

Even though the Chinese machine were inexpensive there were many missing basic items.

I made up a list of items about 4 or 5 pages long. This list was what the processor wanted/expected on the equipment when it arrived in his USA plant.

The Chinese machines did not have these items or components. I am sure that this would require much more costs to supply the Chinese machine as desired.

Perhaps, I have been in this business too long. However, I still have dreams that the USA can be very innovative and there are many opportunities to grow this business. There has not been any real development in techniques in a while.

The current design of PC controls leads the machine developments in the right direction. It is easy to understand and functions very efficiently.

Today’s machines are producing at a higher level of production. The parts and cycles are definitely more consistent. Older machines will not produce on the levels of today’s machines.

There are also energy savings to achieve every hour of production. The result is more money on the bottom line for the blow molder. Just ask and these savings can be justified.

Changeover times such as material and color changes can now be consistently done in the matter of one hour or less as compared to 8 hours or longer. This is money saved with more up time to raise the production yield level.

Modems back to the machinery builder’s plant assures more up time as the machine can be troubleshot in many cases without visiting the processors plant. This does work.

New machine payback is very rapid when yields and efficiencies are high. When did you last check how much it costs you to run your old machine? You will be surprised with the numbers.

Also, the machines are more flexible on the various types of parts, colors and materials that can be run on them.

Any thoughts? Look forward to hearing your comments. Just respond below.