Low-Volume Injection Molding

8/18/2019 Low-Volume Injection Molding

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TOP STRATEGIES TOHELP YOU TAKE FULL

ADVANTAGE OF THE

LVIM PROCESS

AN ENGINEER’SGUIDE TO

LOW VOLUMEINJECTION MOLDING BY RONALD HOLLIS, Ph.D., P.

This eBook contains exper t analysison Low Volume Injection Molding,

including all of the basics you need

to get yourself up to speed on fully

understanding the process. As well as

insider benefits and tips for getting themost out of LVIM, this books contains

the 6 step process that will get your

products to market fastest and the

issues and limitations of LVIM.


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LOW-VOLUME INJECTION MOLDING TO BRIDGE OR NOT TO BRIDGE

As with most things in life, folks tend to focus on the endgame, the score, the finale, but choose to ignore the many

critical steps and decisions that are made during the journey.

DEFINITION WHY YOU NEED IT IDEAL USES

02 | MFG.com | An Engineer’s Guide to Low Volume Injection Molding


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L O W V O L U M E I N J E C T I O N M O L D I N G B A S I C S

An Engineer’s Guide to Low Volume Injection Molding | MFG.com | 0

QUICK TIP: The labels “tool” “mold” ”mould,” “molding,” and “moulding” are all used

interchangeably throughout the industry, causing great consternation to outsiders. Similarly, a

“tool maker,” a “mold maker,” and a “mould maker” all make the tool. Additionally, a “molder,”

a “processor,” and an “injection molder,” make the parts. It’s all good!


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QUICK TIP: Injection molding is the

most common manufacturing method

for making plastic parts. A tool maker

creates the tool from steel or aluminum.

Under high pressure, molten plastic is

injected into the metal “tool” or mold

cavity, filling the inverse or negative

space to make a positive-shaped part.


C O N T R A S T I N G L O W V O LU M E T O P R O D U C T I O N T O O L I N G


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B E N E F I T S T H E B U Z Z I S R E A L


L V I M A P P L I C A T I O N S E V E R Y T H I N G I S E V E R Y T H I N G


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Bridge to Production

Insurance Market Evaluation

T H E K E Y S TO A B R A N D N E W A M P H I B I O U S



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INDUSTRY OVERVIEW A NERD’S EYE VIEW OF LVIM

With current technologies and the growing acceptance of LVIM, applications of this process continue to expand,

making LVIM a standard element of the product development process. Decades before LVIM, a production tool was

predominantly focused on proving that a part could be molded successfully. In other words, product developers

had to use full-on production tooling to validate a part; there was no intermediary refinement process to see how

the part would “behave” in reality. But the LVIM process has evolved significantly with the use of CAD and CAM

technologies. It is now considered a very useful technology in the iterative development process.

There are many ways of using this process to get your products to market faster. Product developers often

use a “bridge tooling” strategy that includes b oth LVIM and production tooling, either in p arallel or in sequence, to

support their goals.

Applications for LVIM are found in every industr y sector: industr ial, automotive, medical, lawn and garden,

and consumer electronics. Close tolerances and high-end appearance are ideal for today’s short run projects.

THE PROCESS OF MOLD MAKING

Making an LVIM is a fascinating process in which you create something to create something else. One of the

major challenges in th e process is that you must c reate a mold or tool that can be used as a receptacle for molten

thermoplastic that holds the inverse or negative shape of the part you desire. While this sounds simple enough,

some special knowledge is required.

Making the physical tool is just a piece of the battle. The part geometry you design must be conducive to

the molding process, and the end-use material must be conducive to the part, as well as the mold. The many variablesof the process — design, materials, actions, and expectations — make the process of getting from tooling to parts a

challenge.

The more efficient LVIM process is similar to the tool making process in that it has existed for more than a

hundred years. As with sculpting, the tool maker eliminates what is not required and keeps only what is essential.


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STEP BY STEP: MAKING A MOLD

Mold making is a complex science that requires a high level of expertise in design, materials, and physics, along with

artistic and intuitive insight, all part of the mold maker’s trade. A highly valued and specialized craftsman “begins

at the beginning.” He starts with a great plan for a well-designed part and follows through with flawless execution,

resulting in a very smooth, high-quality injection mold.

If those of you who are engineers are scratching your heads, you are not alone. For some reason, this valuable mold-

making module is not taught in engineering school, so here’s an important addition to every designer’s knowledge

base. Step by step, this is how you make a mold.

When designing a mold, make sure it is conducive

to injection molding. The design process for

plastic parts is critical, tak ing into account the

“moldability” of a shape. With today’s easy-to-

use CAD software in the hands of very “green”

designers, it is common for parts to be designed

that can be prototyped successfully with SL,

SLS, and FDM, and accepted by the customer.

However, these parts may yet still be unable to be

injection molded. This can often cost your company

thousands of dollars in errors, issues, and lost

opportunities.

Early in the process, the expert tool maker closely

considers all that could go wrong with a design.

Defects that result from poor design arise due to lack

of draft, parting line problems, poorly fitting ejector

pins, and poor materials selection, among other things.

Consequently, the next steps happen electronically in

CAD during your design process.

A) Assess the part for the injection-molding process or Design for Manufacturability (DFM)

PLAN HOW TO MAKE THE MOLD1

09 | MFG.com | An Engineer’s Guide to Low Volume Injection Moldi


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After the tool is electronically designed and all key decisions have been made, the machinist’s physical work begins. The CAM

software technician processes the data for the mold halves to be machined with CNC, making the process very easy and

versatile. Also, some features are processed with an Electrical Discharge Machine (EDM), which uses an electrical charge to

burn away the excess, unwanted material.

Built on the same interrelated model as the CAD data, the CAM output will change automatically if the CAD data changes.

High-speed CNC machines today can also cut metals faster, but the time advantage is incidental. The real power is in the

CAM software and the CNC process.

MACHINE THE MOLD HALVES WITH CNC AND EDM2

After the mold halves have been completely processed and machined, the tool maker mates them together,

a high-precision process. The end result must be very close to perfect, with no gaps or misalignments.

There are many tricks of the trade, such as an ink stamping process called “bluing, which is used to check for the transfer

of ink to the other half of the mold to ensure full mating of mold halves. (An interesting side note is that the U.S. typically

uses blue ink, while China typically uses red ink.) The critical need is for the surfaces to mate perfectly before continuing theprocess. If not, only expensive future rework can fix this error.

Mating has a major impact on the overall quality of the parts that come from the mold, and it can add extra “features” from

the mismatch called “witness” lines. If a small gap between the two halves goes undetected, extra material will be squeezed

into this gap, leaving obvious traces that may ruin the part. It is common that during the mating of the mold halves that the

molds need to be polished so they are very smooth to produce the best parts. Polishing is a very time consuming process.

MATE THE HA LVES FOR FI T3



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BORING BUT NECESSARY LVIM ISSUES AND LIMITATIONS

Plastic injection molding is challenging. As a discipline, it offers a degree of

unpredictability. No matter how well you design your part, the LVIM process will add

other features, errors, and effects that you do not want. These tool design issues are

the consequence of the innate l imitations found in the LVIM process. Discovering that

your trial plastic p art has “annoying” anomalies is par t of the high pr ice of producing

thousands of the parts fast.

While engineers tend to think some issues are the manufacturer’s call, it’s best to

communicate with all collaborators early in the process and incorporate into your

designs front-end decisions. The key elements to successful design are examined below.

PLANNING THE PARTING LINE DESIGN

The par ting l ine happens wherever the halves of the mold come together and mate. This is where the par t halves

will form a tighter bond. While this is not part of the design, the process will add a feature to your part and you

must be prepared to use that feature to your benefit. One of the issues with parting lines are that they can appear

in places visible to the user, which may sometimes be ugly. They can also affect mating of the part with other pieces

in the product, or over time, they can affect the overall tolerance of the part. While you will have parting lines, the

engineer needs to incorporate the parting line into his design to enhance the part’s functionality.

Poordesign ofyour LVIMcan resultin costlyrework.



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DRAFTING THE PART

Draft is the required slant or slope of the walls of the part touching the sides of the tool. Proper draft allows the part

to disengage quickly from the mold when the process is complete. Most engineers struggle with draft because they

don’t understand how the part will actually be molded; others can’t synchronize their CAD software to operate with the

addition of draft. For such a simple feature, draft can be a real nightmare in the CAD world, as applying draft on surfacesin CAD models without the model becoming highly inflexible can be treacherous considering the mathematics required

for CAD surfaces.

It is common for engineers to avoid draft altogether and push the process onto the manufacturer. This can be acceptable,

except when you allow others to control your destiny, you get your destiny controlled. The manufacturer may apply

a larger angle of draft on walls critical to your design and thus prevent it from functioning correctly. Moreover, the

manufacturer may also inadvertently prevent mating parts from mating with an increase in angle. The effect of draft is a

function of the length of the affected surface and the angle of change.

MATERIA LS FOR PARTS

It’s all about the materials. At the end of the day, you are using injection molding to make

use of great materials that will suit the needs of your parts. However, the designer must be

aware of what he expects from the part and materials, as both are interrelated.

The designer must select material that will flow in all parts of the mold before the material

cools to its natural solid state, or the part must be designed in such a way that material

can easily reach all areas of the part. If the part has thin features, such as cooling fins, and

the material is very viscous, then it is likely the tips of the fins will not form completely.

However, if the material was less viscous, then the fins would have no problem forming.

Material selection must always be feasible for the part design. Be sure to choose a material

that lends itself to successful molding of your part design. A material that has a high-warp tendency is not good for

product applications requiring a strict flatness specification. Tool modifications may be necessary to compensate for

material or part design discrepancies.

The Role of Ejector Pins: Ejector pins make features that are remnants of the process. While

typically designed to be flush with the surface, ejector pins can be under the surface or may need to

be located on a critical feature that can cause tolerance or interference issues. If you understand the

process of injection molding, then you can be sure to indicate ejector pin locations and communicate

those to the mold maker.

The keyissue with

materials isviscosity, or

how easy itwill flow inthe mold.



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An online marketplace whe

engineers rapidly connec

with manufacturers

ALUMINUM FOR TOOLING

Most LVIMs are typically made of aluminum, and aluminum has limitations. Compared to steel, it does not offer longevityor consistent production quality. Aluminum is not good for molds that run under higher temperature requirements. It can

have challenges with cosmetic finishes or smooth tooling surfaces provided by harder material tooling. Since aluminum

tools are soft, they can be machined and polished much faster than hardened steel production tools.

MANUFACTURING TOLERANCES

As with all manufacturing, tolerances exist in LVIM. The standard tolerance

is ± 0.005 inch (five thousandths of an inch). When you are designing witha melted material injected into a void set to solidify, maintaining perfection

is nearly impossible. The designer must be aware of variables in design —

such as geometries, materials, tooling materials, pressures, and many others

affecting output — and account for each in the functionality of the part. It is

exceedingly common for great designs to fail because they cannot be made

close enough to perfection to work. This requires that other parts change to

accommodate the imperfection or the product will encounter severe issues.

Moreover, as the material transitions from a solid pellet to liquid flow and to

the solid shape of the part, shrinkage occurs, which can affect the tolerances

of each part.

Lead Times: LVIM typically takes two to six weeks, depending on complexity. CAD data driving

mold design helps alleviate paper drawings used to build molds and thus contributes to shorter and

shorter lead times.

When you are

designing with amelted materialinjected into a

void set to solidify,maintaining

perfection is nearlyimpossible.

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SAVING MON EY, SAVING TIME

Informed engineers always verify their design in LVIM to save money. For example, a medical equipment company spends

$300,000 per year in SLM and then moves into LVIM to verify the part design, while an automotive company uses LVIM

to verify the part design for its customer. Integrating production suppliers into the LVIM process helps everyone with the

learning curve of manufacturing a part. Purchasing LVIMs after the release of the production order gets the customer tosign off early on the parts before submitting designs to production. And of course, always provide the final file versions at

time of order, knowing that the clock cannot start on your job until all data is received.

N MANUFACTURING, ALMOST EVERYTHING YOU DO CAN SAVE MONEY

Build a mold that is able to support the quantity

of parts required.

Forecast accurately to avoid exceeding market

allowance in per-piece price.

Learn the limitations of the process — radii,

tolerances, feature size, and wall thickness.

Design with cutouts or windows for snap features,

making manual tool access easier.

Always verify your part is capable of being injection

molded using DFM2 rules and regulations.

With large parts, use methods of design that allow

for fully CNC-machineable parts to reduce EDM3.

Troubleshoot the design with RP prior to making

a part to avoid rework in mating or function.

Working with a single cavity versues multiple

cavities aids in preventing molding issues.

Produce marketing samples to receive solid

feedback from your target market.

Clearly define and communicate all part and

project specifications at the beginning.

Request sample LVIM parts and use functional

samples in assembly setup.

Keep parts as simple as possible to eliminate

hand loads and additional tooling costs.

Understand the proper and best use of LVIM over

CU1 for certain designs — LVIM can be more cost

xxxeffective after running about 50 parts.

Consider producing parts in large batches over

longer periods and running total parts for one

xxxyear to offset price per part.

When exceeding 10,000 parts at a time, add

automatic slides to reduce cycle times.

Use LVIM when a low quantity of parts is needed

and a production tool is unnecessary.

1 CU: Cast Urethanes2 DFM: Design for Manufacturing3 EDM: Electrical Discharge Machining



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Low-Volume Injection Molding

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