Opening a production mold is one of the most important commitment points in a custom silicone molding project.

Before tooling begins, many decisions are still inexpensive to change in CAD.

After the mold has been machined, even a small design change can require:

• insert modification;
• welding;
• re-machining;
• new EDM work;
• mold re-polishing;
• repeated sampling;
• additional tooling cost.

For this reason, a proper Design for Manufacturability, or DFM, review should be completed before mold construction starts.

A silicone part may look perfectly acceptable on a 3D model but still create manufacturing problems such as:

• excessive flash;
• difficult demolding;
• unstable dimensions;
• tearing;
• sink or deformation;
• impossible parting-line placement;
• visible gate marks;
• high tooling cost.

This checklist covers 12 important items that engineering and purchasing teams should review before approving a custom silicone mold.

1. Is the Wall Thickness Reasonable?

Wall thickness is one of the first features that should be reviewed.

Silicone can mold thin sections, but extremely thin walls may create problems with:

• filling;
• venting;
• tearing;
• demolding;
• dimensional stability.

Very thick sections can create different problems.

They may require:

• longer cure time;
• more material;
• longer molding cycles.

This increases production cost.

A good silicone design should avoid unnecessary transitions between extremely thin and extremely thick regions.

Whenever possible, wall thickness should be relatively consistent.

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Why Sudden Thickness Changes Matter

Consider a silicone cover with:

• 1.0 mm side wall;
• 8.0 mm solid mounting block.

These areas cure differently.

The thick region may require more heat and time to cure completely, while the thin region may already be fully cured.

Large thickness differences can therefore create:

• longer cycle time;
• uneven shrinkage;
• dimensional variation.

If the thick section is not functionally required, consider:

• coring it out;
• adding ribs;
• reducing unnecessary material.

This can lower both:

material cost and cycle time.

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2. Are There Unnecessary Undercuts?

Silicone is flexible, so some undercuts can be demolded by stretching the part.

However, not every undercut is easy or safe.

Examples include:

• deep internal hooks;
• reverse lips;
• side holes;
• locking grooves;
• aggressive internal ribs.

These may require:

• manual demolding;
• removable inserts;
• sliders;
• collapsible cores.

Every additional mechanism increases:

• mold cost;
• mold maintenance;
• cycle time.

Before approving an undercut, ask:

Can this geometry be changed without affecting product function?

A small geometry modification may eliminate a much more expensive mold structure.

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3. Where Will the Parting Line Be?

Every multi-part mold requires surfaces where mold sections separate.

The resulting line on the molded product is called the:

parting line.

This is not simply a tooling detail.

Parting-line location can affect:

• sealing;
• appearance;
• assembly;
• tactile feel;
• flash.

For example, placing a parting line directly across a sealing lip may increase the risk of leakage.

Placing it on the front surface of a consumer-facing product may create an unnecessary cosmetic issue.

Parting-line location should therefore be reviewed before mold machining starts.

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Mark Critical Surfaces

During DFM, classify surfaces such as:

Sealing Surface

Avoid parting lines where practical.

Cosmetic Surface

Try to move the mold split to a hidden edge.

Hidden Surface

More molding witness may be acceptable.

A simple drawing note can prevent major disagreements after the mold is complete.

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4. Where Will the Gate Be Located?

For LSR injection molding, silicone must enter the cavity through a gate.

The gate can leave a small visible mark called:

• gate vestige;
• gate witness;
• gate mark.

The buyer should identify areas where gates are:

Allowed

och

Not Allowed.

Till exempel:

Surface A: no gate permitted

Surface C: gate permitted

This is particularly important for:

• sealing surfaces;
• Class A cosmetic areas;
• thin membranes;
• tactile surfaces.

Do not wait until first samples arrive to decide that the gate is in the wrong place.

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Gate Location Also Affects Filling

Gate location influences more than appearance.

It can affect:

• material flow;
• air trapping;
• weld or flow lines;
• filling balance;
• pressure.

For multi-cavity molds, runner and gate balancing become even more important.

The manufacturer should therefore select the gate position based on both:

part function + molding performance.

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5. What Flash Is Acceptable?

Silicone can flow into very small gaps before curing.

Therefore, flash control must be considered during mold design.

Do not rely on vague requirements such as:

“No flash.”

Instead, identify critical areas.

Till exempel:

Sealing Surface

Very limited residual flash.

No loose flash.

Functional Hole

Flash must not restrict the opening.

Hidden Edge

Standard molded flash may be acceptable.

More realistic flash requirements make tooling and inspection easier to control.

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Why Extremely Tight Flash Requirements Increase Tooling Cost

Very low flash may require:

• precision shut-off surfaces;
• better mold fitting;
• tighter clamping;
• additional polishing;
• more frequent mold maintenance.

This is especially relevant for low-viscosity LSR.

Therefore, apply strict flash limits where function requires them rather than over the entire part.

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6. Are the Tolerances Realistic?

A common DFM problem is applying:

±0.05 mm

or:

±0.10 mm

to every dimension.

Silicone is an elastomer.

It behaves differently from machined metal.

Tolerance capability depends on:

• part size;
• hardness;
• geometry;
• mold structure;
• parting line;
• measurement method.

Large flexible dimensions generally require wider tolerances than small fixed features.

Before tooling, classify dimensions as:

Critical

Directly affects assembly, sealing or function.

Functional

Needs reasonable control.

Non-Critical

General molded tolerance is acceptable.

This can significantly reduce unnecessary mold complexity and inspection cost.

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Fixed Dimension vs Closure Dimension

Another important distinction is whether the dimension is:

formed entirely within one mold feature

or

created across the mold closing direction.

Closure dimensions may be more sensitive to:

• flash;
• mold compression;
• clamping;
• parting-line behavior.

These dimensions often require wider tolerances than fixed internal features.

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7. Can the Part Be Demolded Without Damage?

A silicone part is not complete when it cures.

It must also come out of the mold.

DFM should evaluate:

• demolding direction;
• stretching;
• stripping force;
• thin membranes;
• fragile features.

A design that requires excessive stretching may create:

• tearing;
• permanent deformation;
• longer cycle time.

If every part requires an operator to spend 20 seconds carefully removing it, the labor cost becomes significant in mass production.

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Consider Automation

For high-volume projects, ask:

Can the part be automatically demolded?

LSR molding may support:

• ejector systems;
• air blow;
• robotic removal;
• automatic stripping.

A geometry optimized for automated demolding can reduce unit manufacturing cost substantially.

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8. Are Holes, Slots and Thin Features Moldable?

Small holes and openings can be molded using core pins.

But very small or long features may be difficult.

Review:

• hole diameter;
• hole depth;
• pin strength;
• hole position tolerance.

A very thin mold pin may:

• bend;
• break;
• wear.

In some cases, it may be more practical to mold a simpler shape and create the final hole through:

• punching;
• cutting;
• laser processing.

The best method depends on production volume and tolerance.

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Thin Membranes Need Special Attention

Silicone valves and diaphragms often contain very thin sections.

These can be sensitive to:

• filling;
• venting;
• thickness variation;
• tearing.

The manufacturer should review the membrane geometry before tooling because small thickness changes can have a major effect on performance.

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9. Are Inserts and Overmolding Features Properly Located?

Some silicone components are molded around:

• metal inserts;
• plastic frames;
• cables;
• electronics.

The mold must hold the insert in a precise position during molding.

DFM should evaluate:

• locating points;
• insert tolerance;
• insert movement;
• shut-off areas.

If the insert can move during injection or compression, final part dimensions may become unstable.

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Mechanical Lock or Chemical Bond?

Another important question is whether the silicone only needs to:

mechanically surround the insert

or

chemically bond to it.

Permanent bonding may require:

• primer;
• plasma treatment;
• surface preparation;
• compatible substrate material.

This should be determined before the mold is designed.

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10. Is the Silicone Material Fully Defined?

Do not approve production tooling with only the note:

“Silicone.”

At minimum, clarify:

• LSR or HCR;
• hardness;
• color;
• curing system.

Depending on the application, also specify:

• food-contact requirement;
• medical requirement;
• flame retardancy;
• electrical conductivity;
• high-temperature performance.

Different silicone formulations can have different:

• shrinkage;
• viscosity;
• curing behavior.

This affects mold design.

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Material Shrinkage Affects Tool Dimensions

The mold cavity may need to be larger than the final component to compensate for shrinkage.

If the silicone grade changes after tooling has been completed, shrinkage behavior may also change.

This can affect dimensions.

Therefore, the production material should ideally be selected before final mold machining.

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11. Is the Hardness Appropriate?

Silicone hardness affects much more than feel.

It can influence:

• demolding;
• sealing force;
• compression;
• deformation;
• measurement.

A very soft silicone part may deform under a caliper.

A harder silicone part may require more assembly force.

Typical custom silicone projects may use hardness levels such as:

• 30 Shore A;
• 40 Shore A;
• 50 Shore A;
• 60 Shore A;
• 70 Shore A.

The correct value depends on product function.

Avoid selecting hardness simply because:

“50 Shore A is standard.”

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Hardness and Geometry Work Together

The same geometry made in:

30 Shore A

and:

70 Shore A

may behave very differently during:

• assembly;
• sealing;
• demolding.

For sealing products, hardness must be evaluated together with:

• groove depth;
• compression;
• sealing pressure.

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12. How Will the Part Be Inspected?

Inspection should be considered during DFM, not after production begins.

Ask:

How will this dimension actually be measured?

For flexible silicone, conventional calipers may not always provide repeatable readings.

Depending on the feature, inspection may use:

• calipers;
• optical measurement;
• vision systems;
• profile projector;
• go/no-go gauge;
• custom fixture.

The drawing should define practical measurable requirements.

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Define Inspection Datums

A flexible silicone component may sit differently depending on how it is placed.

For complex parts, inspection should define:

• measurement position;
• reference surface;
• fixture condition.

Otherwise, the buyer and supplier may obtain different results while measuring the same part.

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Bonus Review: Does the Design Need a Cosmetic Surface Classification?

Although not always part of the core 12-item checklist, cosmetic classification is highly useful.

Consider defining:

Surface A

Highly visible.

Strict cosmetic requirement.

Surface B

Secondary visible surface.

Minor molding witness acceptable.

Surface C

Hidden.

Standard molded finish acceptable.

This allows the tooling engineer to optimize:

• gates;
• parting lines;
• surface finish.

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Bonus Review: Does the Product Need Compression Stops?

For silicone seals and gaskets, a mechanical compression stop can prevent:

• over-compression;
• inconsistent assembly;
• premature compression set.

If the product relies on a controlled sealing compression, the housing design should be reviewed together with the silicone part.

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A Practical Silicone DFM Checklist

Before approving mold construction, review:

No.	DFM Item	Key Question
1	Väggtjocklek	Is thickness reasonably uniform?
2	Underskärningar	Can they be removed or simplified?
3	Parting Line	Is it away from critical surfaces?
4	Gate Location	Is the gate hidden or acceptable?
5	Flash	Are realistic limits defined?
6	Tolerances	Are only critical dimensions tight?
7	Demolding	Can the part be removed safely?
8	Holes / Thin Features	Are core pins and membranes practical?
9	Inserts	Are inserts located securely?
10	Material	Is the production grade defined?
11	Hårdhet	Does hardness match the function?
12	Inspection	Can the requirements be measured reliably?

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Example DFM Review

Consider a custom silicone housing seal.

The original design contains:

• 1 mm thin wall;
• 6 mm solid block;
• deep side undercut;
• gate planned on cosmetic surface;
• ±0.05 mm tolerance on all dimensions.

During DFM, the supplier may recommend:

Change 1

Reduce the thick solid block by coring it out.

Benefit:

• less material;
• shorter cure time.

Change 2

Modify the undercut.

Benefit:

• eliminate side action;
• simplify demolding.

Change 3

Move the gate to hidden Surface C.

Benefit:

• better cosmetic appearance.

Change 4

Keep ±0.05 mm only on one critical sealing feature.

Benefit:

• lower tooling and inspection cost.

This is exactly why DFM should happen before machining begins.

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What Should the Supplier Provide After DFM?

A useful silicone DFM report may include:

• mold opening direction;
• parting-line proposal;
• gate proposal;
• flash areas;
• undercut analysis;
• tolerance comments;
• demolding concerns;
• tooling recommendations.

For complicated parts, screenshots from the 3D model can make the review much easier to understand.

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Who Should Approve the DFM?

Ideally, approval should involve:

• product engineer;
• purchasing;
• silicone manufacturer;
• tooling engineer.

Why purchasing?

Because DFM decisions affect:

• mold cost;
• unit price;
• mold lead time.

Why engineering?

Because geometry changes may affect:

• sealing;
• assembly;
• performance.

Both functions should understand the trade-offs.

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Do Not Approve the Mold Based Only on Price

A low tooling price is not helpful if the mold produces:

• unstable dimensions;
• excessive flash;
• difficult demolding.

Before approving tooling, confirm that the quotation and DFM are based on the same:

• product revision;
• material;
• tolerance;
• annual volume.

This avoids changes after the purchase order has already been issued.

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Recommended Pre-Tooling Approval Package

Before the manufacturer begins cutting steel, confirm:

Final 3D CAD

Correct revision.

Final 2D Drawing

Critical dimensions and tolerances.

Material Specification

Exact silicone grade where available.

Hårdhet

Example:

50 ±5 Shore A.

Färg

Approved standard.

DFM Report

Parting line, gate, flash and demolding reviewed.

Quantity

Annual forecast.

Inspection Requirement

FAI and critical features defined.

Only after these items are aligned should production tooling begin.

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Why DFM Saves Money

Suppose a mold costs:

$5,000

Changing the CAD model before machining may cost almost nothing.

Changing the same geometry after mold completion may require:

• welding;
• insert replacement;
• machining;
• new sampling.

This can add substantial cost and delay.

DFM therefore should not be treated as an unnecessary review step.

It is one of the most effective methods for controlling tooling risk.

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DFM Also Helps Reduce Unit Price

Good DFM does not only reduce mold cost.

It can improve:

• cycle time;
• cavity count;
• automation;
• production yield.

Till exempel:

Removing a difficult undercut may reduce manual demolding by:

10 seconds per part.

At an annual production quantity of:

500,000 parts

that change can represent a large amount of avoided labor.

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Questions Buyers Should Ask Before Opening the Mold

Before approving tooling, ask the silicone manufacturer:

• Where is the parting line?
• Where is the gate?
• What flash should we expect?
• Are there difficult undercuts?
• Can the part demold automatically?
• Which dimensions are hardest to control?
• Is the silicone grade confirmed?
• What shrinkage assumption is being used?
• Does the tool support the expected annual volume?
• Are any secondary operations required?
• How will critical dimensions be inspected?
• What design changes would reduce cost?

If these questions have not been answered, the DFM review is probably not complete.

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Slutsats

The best time to solve a silicone molding problem is before the mold is built.

A good pre-tooling DFM review should confirm 12 core areas:

wall thickness, undercuts, parting line, gate, flash, tolerances, demolding, small features, inserts, material, hardness and inspection.

These factors directly influence:

• tooling cost;
• part quality;
• cycle time;
• production yield;
• long-term unit cost.

The goal of DFM is not to make every design simpler at the expense of function.

The goal is to identify features that create manufacturing difficulty without adding real product value.

A few hours of engineering review before opening the mold can prevent weeks of tooling correction later.

Before approving your next custom silicone mold, ask one final question:

“Has the product been designed only to function—or has it also been designed to manufacture reliably?”

Vanliga frågor

What does DFM mean in silicone molding?

DFM means Design for Manufacturability. It is the engineering review performed before mold construction to identify features that may create tooling, molding, demolding or inspection problems.

When should a silicone DFM review be completed?

DFM should be completed after the product geometry is sufficiently mature but before the production mold is machined.

What are the most important silicone DFM issues?

Common issues include wall thickness, undercuts, parting-line location, gate position, flash, tolerances, demolding and material selection.

Can silicone mold undercuts without sliders?

Sometimes. Because silicone is flexible, certain undercuts can be stripped during demolding. Deep or aggressive undercuts may still require removable inserts or mold mechanisms.

Why should gate location be approved before tooling?

The gate can affect appearance, sealing surfaces and material flow. Moving the gate after the tool is completed may require expensive modifications.

Can DFM reduce silicone part cost?

Yes. Simplifying demolding, reducing unnecessary material, relaxing non-critical tolerances and improving cavity design can reduce both tooling investment and mass-production unit cost.