From Product Design to Production: How to Prepare a Plastic Part for Injection Molding

Preparing a plastic part for injection molding means reviewing the part design, material, tooling requirements, tolerances, and production needs before the injection mold is built.

A part can look complete on a computer screen and still need changes before it is ready for molding.

That is normal.

Designing the product and designing a product that can be manufactured through plastic injection molding are related, but they are not exactly the same job.

Injection molding places physical requirements on the part. Molten plastic has to enter the mold cavity. It has to flow through the part geometry. The material then cools and contracts. Finally, the finished part has to release from the mold.

Wall thickness, corners, draft angles, undercuts, ribs, bosses, material selection, tolerances, and surface requirements can all affect those steps.

At RMC Plastics, we work with manufacturers, product developers, and businesses that need to move from a product concept or existing design into custom injection molding. Our services include product design support, mold design, tooling, material consultation, injection molding, production, and related manufacturing services.

If you have a part that needs to move into production, here are the areas we recommend reviewing before committing to tooling.

Start With What the Plastic Part Actually Needs to Do

Before discussing the mold, we need to understand the part.

What is its job?

Where will it be used?

What will it touch?

What will it need to withstand?

Those questions help establish the requirements that guide later decisions about geometry, material, tolerances, and tooling.

Depending on the application, we may need to understand:

• Whether the part will be used indoors or outdoors
• Expected temperatures
• Exposure to water, oils, cleaners, fuels, or other chemicals
• Whether the part needs to flex or remain rigid
• Loads placed on the part during normal use
• Potential impact or vibration
• How the part connects with other components
• Appearance requirements
• Expected service conditions
• Required certifications or specifications
• Estimated production volume

These are not minor details.

They define the job.

For example, two parts with nearly identical dimensions may require different materials when one is installed indoors and the other is exposed to outdoor heat, sunlight, moisture, and chemicals.

The application comes first.

Then we can evaluate how to manufacture the part.

Review the Part for Injection Molding

A design-for-manufacturing review, often called a DFM review, evaluates whether the geometry of a part is suitable for the intended manufacturing process.

For injection molding, that means examining how the plastic will fill the cavity, how different sections of the part will cool, how the molded part will be removed, and how the mold itself can be constructed.

This review should take place before final tooling decisions whenever possible.

Changing a CAD model is different from changing finished mold steel.

Once tooling has been manufactured, some design changes may require machining, replacing mold components, adding actions, welding and recutting an area, or building different tooling. The required work depends on the mold design and the change being requested.

That makes early design review valuable.

Not as a slogan.

As a manufacturing step.

RMC includes product design, tool design, fabrication, material selection, and manufacturing within its injection molding services, allowing these related decisions to be considered as part of the same production program.

Evaluate Wall Thickness

Wall thickness is one of the major geometric considerations when preparing a plastic part for injection molding.

Plastic has to flow through the mold cavity before it cools enough to solidify.

Very thin sections may be more difficult to fill under certain combinations of material, flow length, geometry, gate location, and molding conditions.

Very thick sections can cool differently from thinner neighboring areas. That difference can contribute to visible sink or dimensional changes as the material contracts.

For many molded parts, designers therefore look for reasonably consistent wall sections where the function of the product permits them.

That does not mean every wall has to be identical.

The appropriate thickness depends partly on the selected resin and the requirements of the part. Different plastics have different processing ranges and flow characteristics.

This is one reason part geometry and material selection should be reviewed together.

Add Draft Where the Part Needs to Release From the Mold

An injection mold opens after the plastic has cooled sufficiently for the part to be removed.

Draft is a slight angle added to surfaces that run in the direction the mold opens.

As molded plastic cools, it can contract around features within the mold. A properly selected draft angle gives the surface additional clearance as the part moves away from the tooling during ejection.

The required draft is not one fixed number for every project.

It can vary with:

• Part depth
• Resin
• Surface texture
• Part geometry
• Tool design
• Ejection method

Textured surfaces commonly require additional consideration since the molded plastic conforms to the texture of the tool surface.

Draft can look almost insignificant on a CAD drawing.

Inside a mold, it matters.

Review Sharp Internal Corners and Radii

Sharp internal corners deserve attention during an injection molding design review.

Adding an appropriate radius can change how material moves through that section of the cavity and can distribute stress over a larger transition area rather than concentrating it at a sharp corner.

That does not mean every edge of a molded part needs to be rounded.

External appearance, mating surfaces, part function, mold construction, and other design requirements still matter.

The goal is to identify where sharp geometry is required and where a radius may be a better fit for the application and molding process.

Use Ribs and Bosses With the Entire Part Design in Mind

Injection molded parts frequently include ribs and bosses.

A rib is a relatively thin feature used to support another section of the part.

A boss is commonly used around a mounting point, fastener, pin, insert, or other connection.

These features cannot be evaluated only by looking at their height or shape.

Their thickness relative to the surrounding wall matters.

A thick boss or rib connected directly to a thinner cosmetic wall may cool at a different rate than the surrounding area. Under some combinations of resin, dimensions, processing conditions, and surface requirements, that difference may appear as sink or distortion on the opposite surface.

The answer is not simply “make everything thicker.”

Sometimes less material in the right place is more appropriate.

Sometimes a rib provides the needed support.

Sometimes the geometry needs to change.

It depends on the part.

Identify Undercuts Before Tooling Begins

An undercut is a feature that interferes with straightforward removal of the molded part from the core or cavity.

Think about a hook.

A side hole.

A retaining feature.

A recessed area that wraps behind another feature.

If the finished plastic mechanically locks around part of the mold, simply opening the two mold halves may not release it.

Depending on the geometry, tooling may use slides, lifters, side actions, shutoffs, inserts, collapsible components, or another mold-design approach.

In other cases, the part itself can be redesigned to eliminate the undercut.

Neither option is automatically better.

The manufacturer needs to look at the function of the feature, mold construction, expected volume, material, tolerances, and production requirements before selecting an approach.

Finding the undercut after the mold is built is late.

Finding it in the CAD file is early.

Early is where we want to find it.

Define Tolerances Based on What the Part Actually Requires

Every dimension does not need the same tolerance.

Some dimensions may control whether two components fit together.

Some may locate a bearing, insert, seal, fastener, or other functional feature.

Others may have little effect on how the product works.

This distinction matters when preparing a part for injection molding.

A drawing filled with unnecessarily tight tolerances can create manufacturing requirements that the application itself may not need. At the same time, leaving an important mating dimension poorly defined can make it difficult to determine whether a molded part meets the functional requirement.

We want to know which dimensions are critical.

Those are the dimensions that deserve particular attention during design review, tooling, sampling, inspection, and production.

Choose the Plastic Material Before Finalizing the Mold

Material selection should be part of the product development process rather than an afterthought once the tool is complete.

Different plastic resins can have different:

• Shrink characteristics
• Flow characteristics
• Temperature ranges
• Stiffness
• Flexibility
• Impact properties
• Chemical resistance
• Moisture behavior
• Surface characteristics
• Processing requirements

The correct choice depends on the application.

A material selected for an outdoor industrial enclosure may be evaluated differently from a material used for an interior cap, flexible component, structural bracket, or cosmetic housing.

RMC provides material consultation as part of its manufacturing capabilities and works with customers to evaluate material options in relation to the intended application and molding process.

Design and material interact.

They should be reviewed that way.

Decide What the Surface Needs to Look Like

Appearance requirements also belong in the conversation before the mold is finalized.

Does the part need to be smooth?

Textured?

Glossy?

Matte?

Does the customer care about the appearance of every surface, or are some surfaces hidden after assembly?

Does the part have molded lettering, logos, symbols, identification numbers, or other markings?

Surface requirements can influence tooling decisions, draft requirements, gate considerations, material choices, and which side of the mold forms the visible surface.

A hidden industrial component and a customer-facing housing may have very different cosmetic requirements even when both are injection molded.

Specify what actually matters.

Understand How the Parts Fit Together

Many molded components do not operate alone.

They snap into another piece.

Bolt to a housing.

Slide into an assembly.

Accept a gasket.

Hold an insert.

Connect to metal, rubber, electronics, or another plastic component.

If the molded part has to interact with other components, those relationships should be reviewed during product design.

Providing mating-part drawings, assemblies, samples, or CAD files can give the molding and tooling team additional context.

We are not just trying to manufacture a shape.

We are trying to manufacture a part that performs its intended role within the customer’s product.

Determine How Many Parts You Expect to Manufacture

Production volume affects tooling decisions.

A mold intended to produce a limited quantity of parts may have different requirements from tooling expected to support repeated OEM production.

The appropriate approach can depend on:

• Expected annual quantity
• Total anticipated program volume
• Number of cavities
• Resin
• Part geometry
• Cycle requirements
• Mold maintenance expectations
• Tool life expectations
• Production schedule

More cavities can produce multiple parts during a molding cycle, but cavity count is only one part of the decision.

The mold becomes more complex as features and cavities are added. The molding machine also needs adequate capacity for the total shot, mold size, clamping requirements, and other operating needs.

Tooling should match the production program.

Not an arbitrary rule.

Consider Prototypes Before Production Tooling

A prototype can be useful when the manufacturer needs to evaluate aspects of the product before committing to production tooling.

What needs to be tested determines what type of prototype is appropriate.

A prototype may be used to examine:

• Overall shape
• Fit with mating components
• Ergonomics
• Assembly
• General appearance
• Certain functional characteristics

A prototype made through another manufacturing process should not automatically be expected to behave exactly like the final injection molded part.

Material, geometry, manufacturing method, surface, and mechanical characteristics may differ.

The question should therefore be specific.

What are we trying to learn from this prototype?

Once that is clear, we can determine whether prototyping is useful and what type of prototype fits the test.

Get the Injection Molder Involved Before the Design Is Locked

One of the most useful things a manufacturer can do is involve the injection molding and tooling team while design decisions can still be evaluated.

A product designer understands what the part needs to accomplish.

A tooling specialist looks at how the cavity and core can be constructed.

A molding specialist considers how the resin will be processed through the tool.

A material specialist evaluates plastics in relation to application and processing requirements.

Those are different perspectives on the same part.

RMC’s Houston operation brings product design, mold design, tooling, material consultation, molding, and production capabilities together within its manufacturing services.

That makes it possible for us to discuss the product as both a design and a manufacturing project.

What to Send an Injection Molding Company for a New Project

You do not need to have every detail figured out before contacting an injection molder.

Send what you have.

That might include:

• 3D CAD files
• 2D drawings
• Existing parts or samples
• Assembly drawings
• Material requirements
• Expected quantities
• Desired colors
• Surface requirements
• Critical dimensions
• Operating conditions
• Photos
• Drawings of mating components
• Current prototypes
• A description of what the product needs to do

If something has not been decided yet, identify it as an open question.

That is more useful than treating an assumption as a requirement.

From Product Design to Injection Molding With RMC Plastics

Moving from product design to injection molding involves more than sending a CAD file to a toolmaker.

The part needs to be evaluated as a manufactured product.

Geometry matters.

Material matters.

Tooling matters.

Volume matters.

The application matters most.

At RMC Plastics, we work with customers from early product and part development through mold design, tooling, custom injection molding, production, and related manufacturing services at our Houston facility. RMC has provided plastic molding services since 1966.

If you have a new plastic part, an existing design that needs to move into production, or a product that needs to be reviewed for injection molding, talk with our team about the project before committing to tooling.

A few decisions made on a computer screen can become permanent features in steel.

Wouldn’t you rather examine those decisions while they are still easy to discuss?

Frequently Asked Questions About Preparing a Part for Injection Molding

How do I prepare a plastic part for injection molding?

Preparing a plastic part for injection molding generally includes reviewing the part’s function, geometry, wall thickness, draft, radii, ribs, bosses, undercuts, tolerances, material, appearance requirements, production volume, and tooling needs. The exact review depends on the part and its application.

What is design for injection molding?

Design for injection molding is the process of reviewing a part in relation to how it will be molded. The review can include material flow, cooling, mold construction, part ejection, wall thickness, draft, undercuts, tolerances, and other features that affect how the tool and molded part will be produced.

Do I need a finished CAD design before contacting RMC Plastics?

Not necessarily. RMC provides product design and injection molding services, and customers can contact us during product development to discuss design, tooling, materials, and production requirements.

Should I select the plastic material before designing the injection mold?

Material should generally be evaluated before mold design is finalized. Resin selection can influence shrinkage, processing conditions, wall-thickness considerations, flow, and other tooling and production decisions.

Why do injection molded parts need draft?

Draft provides an angle along surfaces that move past the mold during ejection. As molded material cools and contracts around mold features, appropriate draft provides additional clearance as the part is removed. The amount needed depends on factors such as geometry, depth, material, and surface texture.

Can RMC Plastics help take a product from design into production?

RMC provides product design services along with mold design, tooling support, material consultation, custom injection molding, production, and other manufacturing services from its Houston operation.