Lessons from Captain Meticulous’ Adventures in Plastic
by Tom Boyd (aka Captain Meticulous), Blow Molded Specialties President
A man named Mr. Waterman invented a tube.
He was sad because it sprung a little leak.
He said, “Darn it, when I hold my tube on a piece of paper,
The ink leaks out and makes a little streak.”
He said, “I’ve gotta find a way to stop that leak.
I’ll start working on my leakproof tube again.”
I said, “Waterman, you idiot, don’t stop that leak.
You just invented a fountain pen.”
“Good Advice” – Allan Sherman
No, Joe isn’t Going Crazy on the Production Floor
Every hour, on the hour, Joe pulls a strange-looking 3-liter bottle with a graduated cap off
the production line, fills it with water, caps it, and drops it from four feet onto the hard
cement floor. If it breaks, everything stops on that production line and the engineers huddle
together with the production crew until the problem is identified and solved. Fortunately,
thanks to our process control systems, no bottle has broken yet. But we’re not taking any
You see, those bottles will be used by nursing homes and hospitals to collect patients’ 24-
hour urine samples. In 24 hours, those puppies can get mighty full! The last thing any nurse
wants to see is 3000 millilietrs of pee exploding at her feet minutes before drawing it off for
analysis. So we make sure our 24-hour urine bottles won’t split at a bottle’s traditional weak
points: behind the handle or along the bottom seam.
Mr. Waterman may have made a fortune with his leaky tube, but in the blow molded
business, leaks are one of four big enemies. (The other three are sloppy critical dimensions,
inappropriate wall thicknesses, and poor function.) So here at Blow Molded Specialties,
we’ve built a company focused on creating products that do exactly what our clients want
them to do, and nothing they don’t.
Many projects we take on involve creating something that’s never been created before, for
use in some of the most demanding applications in all of industry. Typically, our clients
come to us after discovering that what they need can’t be done by injection molding, or by a
“commodity” blow molder.
Blow molding is very different from injection molding, which is a process that people are
more familiar with. Many of the blow molding nightmares we’ve seen, fixed and prevented
begin with confusion over the differences in the two processes. So let’s start there in
educating you to avoid a blow molding nightmare.
It’s Not Just About the Mold
Injection molding makes solid parts, like a Frisbee, while blow molding makes hollow parts,
like a soap dispenser. From that basic difference, an entire world of differences arise. Here’s
the main one: in injection molding, once you’ve created a good mold, you’ve won 90% of
the battle. The production process is not a major challenge.
With blow molding, a good mold only gets you at most 50% to where you want to be. The
other 50% is the process – and that’s where our knowledge and experience gives us an edge.
There are many indirectly controlled variables. For every part that we produce, we must create a
process to maintain control.
What sorts of problems can arise, aside from exploding urine bottles? How about these:
400,000 bottles of insect repellant packed in corrugated cartons in inventory, all with
pinhole leaks in the bottom
blood testing syringes in automated machines not being squeezable enough, jamming
the machines and wasting thousands of AIDS tests
bellows for water testing at the bottom of a well that can’t stand 100 PSI of pressure
and can’t draw any water in
engine coolant reservoirs that melt in the heat of an ATV engine
rectangular bellows with such uneven wall thicknesses that they don’t expand and
bottles that won’t sit square on the table because the bottoms are rounded
and many more…
In none of these cases was the mold itself the problem. It was something else: the type of
plastic, the process temperature, the velocity or pressure of the blown air, mold close speed,
part design, or something else.
One of the big misconceptions we deal with from new customers is their expectation that
we’ll be able to start shipping product as soon as the mold is complete. At that point, the fun
has just started!
Blow Molding 101: What Matters
(Wall Thickness, Critical Dimension, Controls and Material)
The Problem of Wall Thickness
If you’re injection molding a Frisbee, for example, you inject the plastic into the mold and
get the same product every time. The thickness of the Frisbee is prescribed by the mold.
When we blow mold a hollow part such as a container or a bellows where the opening is
smaller than the diameter of the body, we create that shape by extruding a tube, which is
called a parison, and then closing the mold on it. We pinch that extruded tube of hot plastic
at the bottom, welding it together, and then insert a blow pin in the top, which seals the top
of the mold. Once it’s entirely sealed, we inject air through the blow pin, which inflates the
parison to conform to the shape of the mold. It’s a lot like glass blowing – only higher
production (and with plastic, of course).
Here’s the rub: A perfect mold can still make an unacceptable, useless part.
The molding process itself has a major influence on the end product. The wall thickness is
going to vary from place to place in the blow-molded part, based on how much the material
has to stretch as it is being blown. If the material is forced to go too deep into a cavity or a
mold section, the material may blow a hole before it stretches to the cavity wall.
The more complex the shape, the more wall thickness variation you are going to have over
the entire surface area of the part.
The Little Bellows That Couldn’t
The best way to explain the importance of wall thickness is to tell a story about a
development project that didn’t work. About 12 or 13 years ago, somebody in the furniture
business wanted us to make a bellows that would adjust the firmness of a cushion. It had to
be rectangular, with two very long sides and two very short sides.
They were very anxious to get going, and they knew a lot about injection molding but not
much at all about blow molding. This is where the headache began for all involved. Since
the natures of the processes are so different, their experience proved a detriment to their
A bellows should collapse uniformly as pressure is applied. This requires a uniform wall
circumferentially around the part. With a very sharp or extreme rectangle, as the material
stretches and is blown into the corners, the material in the corners will be thinner than it is
in the center of the long sides where the parison touches the mold first.
That unevenness made that part not behave as uniformly as the customer wanted it to. So
we could not make a part that would function satisfactorily. We had to tell this customer that
given the current part design, we couldn’t produce functionally good parts – back to the
drawing board, literally. From that point on, we’ve only done one rectangular bellows and
that had quite a different functional expectation than that original one.
How to Deal with the Wall Thickness Problem
First, our engineers work with your engineers to figure out what the wall thicknesses must be
in order for the part to function correctly. We do this based partly on experience, partly on
calculations, and partly by utilizing an iterative testing process.
Then we develop the process that can reliably and consistently turn out the optimum wall
thickness distribution. We can work with dozens of plastic formulations, from commodity
resins to engineered plastics. We experiment with dozens of process variables. Most of the
time, we figure out how to make it happen. If I had to point to one thing as our “unique
selling proposition” (glad I stayed awake during that marketing class in college), I’d say it’s
our ability to solve problems that others can’t.
The third step, once we’ve produced sample parts with the right wall thickness, is to validate
the process, measure its repeatability and qualify the process for production. It’s pointless if
we can’t do it successfully every time. The goal is to get the engineers and designers off the
production floor and back into their very neat offices, with all the mechanical pencils lined
up in order of graphite thickness, ready to tackle the next challenge.
At regular intervals during production, our quality team pulls out their magnetic micrometers
and measure wall thicknesses. Just in case you’re ever on the Jeopardy! Geek Challenge, you
should know a little about the magnetic micrometer. It uses a ball bearing and a probe to
measure thickness nondestructively. The ball bearing is dropped inside the part. The
magnetism in the probe attracts the ball bearing and measures the distance between the ball
bearing and this probe. This number is the wall thickness.
The magnetic micrometer is accurate to 0.0001 inch, which is roughly one-tenth the
diameter of a single strand of flaxen human hair. It’s not something you want to eyeball!
Let’s Get Critical – Dimensions
Wall thickness, critical dimensions and material used determine how a part will function.
Different materials can be sampled, but critical dimensions don’t generally change, making
them, well – critical.
Many of the parts we make are components of other pieces of equipment – especially in the
medical device, laboratory, healthcare, and industrial markets. For example, we manufacture
a small part of an assembly that draws blood from a tube used by a blood donor, and tests it
for AIDS and other blood borne diseases. The part, which contains a hypodermic needle,
must fit exactly or the machine will jam. It’s also critical that this part be flexible, but not
too squeezable or it will not draw the blood properly. This is a function of the material and
the wall thickness as well. No sleeping on the job here!
From the point of view of keeping AIDS out of the blood supply for transfusions, our little
part is awfully important. And the critical dimensions of the part are hard to get right – we’re
one of the few companies on the planet that can do it.
One of the reasons is our extensive experience and our willingness to try jobs that others
wouldn’t. We just know more than most blow molders, because we’ve seen more. But
there’s another reason we get results others can’t –
We’re in Control
Soda and milk bottle manufacturers just run the same stuff through the same machines
24/7. They don’t have to worry about cleaning the machines after every run. Soda bottles are
cylindrical, which is the easiest shape to blow mold. Also, dimensions on commodity
products are not as critical.
We engage in obsessive process control and quality assurance. In our business, it’s just too
expensive not to.
Think We’re Finished? Not Quite Yet…
Because we’re a custom solution provider, we run dozens of different plastics through our
machines and meet completely different criteria with each. To make this work, each job has
numerous temperature settings for different segments of the molding machinery, and a
multitude of speed, weight and die settings to create the perfect combination of conditions
for that part. These settings can vary so drastically from job to job, that changeover must be
When we finish a run, we completely clean our machines. As in, we tear them apart and
spend several hours cleaning every nook and cranny. When the machine is cleaned and
stripped, the new mold, tooling and settings are prepared for the next job.
The end result is a production facility that turns out parts that fit and function exactly as they
should, to the tolerances required in very specialized applications.
“Right” is a Moving Target
Often, our clients have a very accurate sense of the critical dimensions and functional
attributes of the part they need from us. Especially if they’ve worked with another blow
molder and have been through a lot of the development experience, then they may have
that pretty well sorted out. They come to us because they can’t get a problem solved, but
they are painfully aware of what the problem is.
Most of the time, particularly in a new product or new development, you have a sense of
what matters because it’s fitting into something else. So you know what you are trying to
accomplish fit-wise. But there are probably another few attributes or dimensions that you
really don’t know until you see what the process produces.
On the other hand, if we are the first blow molders they’re talking to, new specifications
will pop up mid-process and we deal with them. Recently, a client came to us who
wanted a fairly flexible bag blow molded. They had been trying to make a bag by
welding two flat sheets together, and it wasn’t working the way they wanted.
They asked if we could blow mold the bag. We said we thought we could. (My most
common answer to an initial client request is, “That’s a definite maybe.”) We can’t
always guarantee success when developing capability to make a unique new product, but
we do our best to be realistic throughout the process and enjoy the opportunity to attempt
When they saw a prototype, they asked, “Why are the wall thicknesses varying?” (I
hadn’t written this white paper yet.) We explained the process to them and started a small
production run to see if the consistency was satisfactory.
They noticed the surface finish wasn’t always perfect because of optical imperfections
during the extrusion process. They said, “Well, we need the surface finish to be thus and
so.” (“Thus and so” is a complex engineering term which our more sophisticated clients
like to use.)
We said, “Okay, we’ll try.” But to clear the optical imperfections, we needed more heat,
which caused the resin to burn and degrade – back to the drawing board. Perhaps a
different material would work better. The variables are many, but we are dedicated and
persistent – that’s why we’re still here.
Much of the time, we are creating products that have never been created before. That’s
what keeps our technical team at the ready with their mechanical pencils poised for the
next challenge. We love to be busy, and make it a company goal to learn with everything
new we encounter.
It’s All in the Details
I hope I’ve given you a sense of why it pays to pay attention to details in the blow-molding
world. But I don’t want to leave you with the impression that we just hire anal people and
hope for the best. We have developed an eight step plan, which we call the Value
Development Process (VDP) but we think of it as “Captain Meticulous’ Eight Steps for
Getting It Right Every Time.”
If after reading this White Paper you think that blow molding may be the process you need,
I invite you to download and read our VDP White Paper. It’s short and sweet – just a couple
of pages long, and about a 5-minute read – and consists of a description of the eight steps as
applied to a case study, a part we created for a Cannondale ATV.
Whether you end up hiring us or not, this paper spells out best practices, not only in process
planning, but in client and project management as well. It will help you save time, money
and agony – especially if you’re familiar with injection molding and are in jeopardy of
importing those assumptions into a very different process.
Now That You’re a Blow Molding Expert
Congratulations! You now know more about blow-molding than anyone else on your street,
probably. If you have a need that requires blow molding, you also have an understanding of
the process and the challenges that will make you an informed and effective participant in
Obviously, we don’t sell blow molding services over the internet. We have to do a lot of
talking back and forth to figure out what you need and whether we can provide it. So give
us a call about your needs. If we can help, we’ll send you some samples that will help the
communication process. You can reach us at (401) 723-3000 or explore this website.
We pay for the site whether you read it or not, so you might as well.
If you have additional questions, or if anything I’ve written isn’t clear, please drop me a
line and I’ll do my best to help. The more you understand about blow molding, the more
effective we’ll both be at meeting your needs, whether it be product development,
engineering support or high quality production. And, it makes my alter ego, Captain
Meticulous, feel good to share my passion with the world! (Yes, I have my own stash of
mechanical pencils, and I’m quite fond of them thank you very much.)
Tom Boyd, President
Blow Molded Specialties
535 Prospect Street
Pawtucket, RI 02860