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Design for molding is a niche discipline that challenges even the best engineers. There are many considerations that affect the final composition of your part. These design decisions can lead to manufacturing inefficiencies, costly defects, and delays. But, when a designer takes the time to evaluate their design and mitigate risks, there is opportunity for success.

If you want to design production ready parts – this is the guide to get you there.

Plastic Part Design Process:

Step 1: Define End-Use Requirements

The first step in the product development phase is completing a thorough assessment of the product specifications and end-use requirements. A complete product design specification helps other engineers down the value stream understand the required critical design features. This information should also be quantitative rather than qualitative when possible because this data allows for more informed decisions in the design process. Furthermore, quantitative information, rather than qualitative, allows for better informed design decisions.

However, this can be tricky for prototypes due to the lack of part history. If there is no previous data to inform your design decisions, then a prototype may help inform some of those requirements. As the design matures, use the new data as inputs for the final production part.

Step 2: Create a Preliminary Concept Model

Creating a preliminary model will help your engineering team identify the critical tolerances and design features of the part. This also helps your manufacturing partner understand the features that are critical to quality. If the manufacturer knows what is/ is not critical to quality, they will produce better parts with competitive prices. Your manufacturing partner can also help you optimize part features to simplify tooling.

Step 3: Material Screening

There is an overwhelming array of thermoplastic resin compounds available to you. How do you decide which material best suits your needs? First you want to scout materials with properties that match your end-use requirements. One way to research material properties is to use a resin database like UL Prospector or MatLabs. Start by eliminating the resin families that do not have the material properties you’re looking for such as chemical resistance, thermal properties, or appearance qualities. Once you’ve made your shortlist, compare these material properties side by side. Give yourself enough options so you have flexibility later on.

Step 4: Design Part Around Materials

Now that you have a few materials selected it is time to design your part. But remember, each material has different properties that can affect the geometry of the part. For example, designing with nylon versus a polypropylene would result in thinner wall sections with nylon due to its flow and rigidity properties. Similarly, an HDPE resin would require you to design the part with thicker wall sections which may cause some issues in an assembly where the fit is critical. These are all important factors that make the design process more challenging and cumbersome.

Step 5: Final Material Selection

Once you have compared the design characteristics for each of the selected materials, it’s time to select the one that best matches your needs. Think about this from a few different perspectives: cost, performance, processability, and availability. You want to choose the one that fits the end-use performance criteria. But you also may want to consider how easily that material processes because that can have an effect on the overall manufacturing costs of the product. Hard to process material also results in higher defect rates which triggers higher inspection levels and potentially more scrap. Ultimately, choosing the best material that fits your needs, budgetary and overall performance in both processing and end-use is a key component in successful design projects.

Injection Molding Design Rules

Wall Thickness

Wall thickness is a critical design function for injection molding. This affects how the mold fills as well as the appearance and strength of the molded part. There are three key points to consider:

Uniform wall thickness
Nominal wall thickness
Recommended wall thickness

Uniform Wall Thickness

Uniform wall thickness allows the part to cool evenly and avoid costly defects. When a part consists of varying wall thicknesses it cools and shrinks at different rates. This causes defects such as shrink, warp, and others that affect the strength and appearance of the part. While some designers are okay with these defects in non cosmetic areas, they can cause headaches during inspection if the criteria are not clearly defined. Overall it is best practice to keep a consistent and uniform wall thickness whenever possible.

Nominal Wall Thickness

When a completely uniform wall thickness cannot be achieved, a nominal wall thickness is acceptable. Nominal wall thickness describes the thickness throughout the entire part. This means there are features that are thicker or thinner than the average wall thickness of the part. Be careful, thick and thin walls can affect the molding process negatively and cause quality problems. Let’s define what thick and thin walls are.

Thin Walls: Parts with less than 0.08” of wall thickness are considered thin walls. There are specialized molding techniques that allow you to mold these parts. However, these parts are typically uniformly thin. In non uniform design, thin walls tend to trap air and gas which prevents the plastic from filling the mold cavity completely. This results in short shots – where the molded part is not fully formed. These are typically rejected parts and are thrown away.

Thick Walls: Walls thicker than 0.16” (or thicker than the material recommends) can cause problems too. First, thick walls increase the overall cycle time for each shot. This can impact the cost of each part even if the cycle is just a few seconds longer. Furthermore, thick walls will cool slower than thin walls which causes warp, sink, and voids.

Recommended Wall Thickness

Each resin has a recommended wall thickness specified in its material profile. Whenever you are designing a new part you should refer to these values and adjust your part’s wall thickness appropriately. Below is a quick reference table:

Recommended Wall Thickness by Resin

Resin	in
Acetal	0.030 – 0.120
Acrylic	0.025 – 0.15
ABS	0.045 – 0.14
Nylon	0.030 – 0.115
PBT	0.080 – 0.250
PC	0.040 – 0.150
PEEK	0.020 – 0.200
PEI	0.080 – 0.120
PE	0.030 – 0.200
PPSU	0.030 – 0.250
PP	0.040 – 0.15
PS	0.025 – 0.125
TPE	0.025 – 0.125
TPU	0.025-0.125

These values are derived from publicly available information. Optimal wall thicknesses can vary depending on the geometry of the part. We recommend reviewing your design with a plastics engineer to optimize the wall thickness throughout the part.

Transitions

If your part requires different wall thicknesses, then the transition from thick to thin needs to be blended smoothly. Sharp steps and corners between different wall thicknesses will hinder the flow of material and compromise the part’s integrity. The best way to avoid this is by using fillets and chamfers to round out the transitions.

Fillets – rounded corners or edges
Chamfers – 45deg angled edges connecting two surfaces.

Corners

Where possible, corners should be rounded with proper radii. Rounded corners reduce stress concentrations in the part, which can cause defects that create aesthetic or functional problems. They also minimize plastic shrinkage so that corners are even. Calculating the radius on a rounded corner is simple:

Internal radius 50% of wall thickness (assuming nominal thickness)
For the external radius, use your wall thickness measurement and add the internal radius value (R2=R1 +Wt)
Both internal and external radii need to originate from the same point.
Parts with equal internal and external radii will end up with a corner that is either too thick or too thin.
Sharp corners often require EDM machining, a process that is precise but costly.

Ultimately, using rounded corners not only promotes better moldability, they also simplify the moldmaking process making your part easier to manufacture and buy at a reasonable cost.

Draft

Applying the right amount of draft is a crucial part of the design process for injection molding. Draft is an angle applied to vertical walls that allow the part to eject from the mold. Essentially, the draft allows for the steel to clear away from the molded part without damaging it. A good rule of thumb is to use 1 degree of draft per side for every 4 inches of length. Be sure to consider the pull direction, how the mold separates, to determine the draft direction.

A quick note: draft angle specifications vary from material to material and can be influenced by the texture of the mold. To find out how much draft you need to apply, we suggest using the Society of Plastics Industry (PLASTICS) standards and Mold-Tech for guidance.

Ribs:

Sometimes, an engineer will design a part with thin walls; reducing material usage, speeding up production, and promoting longer tool life. However, these thin walls lack the load-bearing capacity and do not have the structural strength required for the application on their own. Ribs are a design feature that add strength and support to thin walled parts. Using a vertical rib gives these walls additional strength and can even reduce warp with certain geometries.

Like many injection molding design features, ribs require a bit of homework to design properly. Thick ribs have a tendency to produce sink marks, and thin ribs do not provide enough strength and may potentially end up not fully filled.

Here are some more design rules for ribs:

Wall thickness: should be 50-60% of nominal wall thickness
Fillets: should be used at the base of ribs to avoid sharp transitions. An appropriate radius is 25-50% of the nominal wall thickness, but should be no greater than 0.01”
Height: Keep rib heights to a minimum, no more than 2.5X the wall thickness. If you need more support for taller walls then add ribs no less than 2X the wall thickness from each other.
Draft: Apply at least 0.5 degrees per side.

Bosses:

Bosses are another popular design feature used in injection molding. These vertical features are used in assembly applications to receive fasteners such as screws, pins, or even smaller bosses. Like ribs, bosses also increase the structural strength of a part when designed appropriately.

Here are a few golden rules to follow when designing bosses:

Size: the inner diameter of a boss is likely to shrink as the part cools. It is essential that you reference the material’s shrink properties when adding a boss and design the feature accordingly. The overall size of the boss should be between 2-2.5X the size of the inner diameter of the part.
Thickness: Like many other plastic features, thickness matters. Bosses that are too thick are prone to sink, which can be an eyesore. We recommend that bosses are no more than 60% of the overall wall thickness.
Location: Place boxes where structural integrity is needed such as screw holes.
Walls: Alignment when attaching bosses is key. These bosses need to align with other features of the part or mating part to prevent assembly mismatch. Also, avoid thick wall sections when attaching a boss to a wall. One option is to core out sections of the thick area to make a uniform wall section.

Parting Lines:

A parting line refers to the line created at the point where the two halves of the mold meet when closed. Depending on the aesthetic requirements and geometry of the part, the parting line can be moved around to fit your needs. There are a few places where parting lines can be placed to help simplify the mold construction, such as sharp corners. And there are some places where parting lines should not be placed such as on filleted surfaces. Finally, it is important to note some of the common defects that happen at the parting line which is an indication of the overall health of the mold. Defects such as parting line flash, mismatch, and stress marks are indications that either some tool work needs to be done or that the mold was not installed properly.

Gates:

A gate refers to an opening in the injection mold where molten plastic enters and fills the cavity. There are many types of gates in injection molding. Some gates enable better material flow through the cavity which allows the mold to fill fast and complete. While others offer better appearance results or eliminate extra manufacturing steps. This all depends on the part geometry and features.

Before we dive into the different types of gates we need to discuss two critical decisions: location and size.

Gate Size: The size of the gate is important because it controls the flow volume of the material into the mold. Larger parts, and materials with a low melt flow rate (MVR), need larger gates. If the gate is undersized, the part is at risk of not fully forming. However, oversized gates are harder to trim and waste material. Thankfully, thermoplastic materials come with a gate sizing specification which can be used to calculate the proper size for the gate.

Gate location: The gate location has a huge impact on the final part quality. For instance, the location determines the direction of material flows, where weld lines appear, and what features are the last to fill. This can result in defects such as short shots, voids, warping and others that can occur. To avoid potential defects, we offer mold flow analysis and a comprehensive mold design review.

Here are the different types available:

Edge
Fan
Tab
Sprue
Diaphragm
Ring
Spoke
Hot tips
Sub
Pin

Ejection

Ejection in injection molding is the process of removing the molded part from the mold cavity. This is one of the critical phases in the injection molding cycle, so ensuring your part can eject from the mold without any issues is crucial. In order to do this, we need to consider ejection throughout the design process. There are several key factors that affect the parts ability to eject, and of course, some golden rules too.

Let’s look at a few of these factors:

Draft Angles: Draft angles reduce friction during ejection, making it easier for the part to release smoothly from the mold. Deep parts require steeper drafts
Ejector Pins and Contact Points: Place ejector pins on flat, sturdy areas of the part to avoid visible marks and damage. Avoid positioning pins near delicate features, thin walls, or unsupported edges, as these can break or deform during ejection.
Wall Thickness and Reinforcement: Thin or sharp-edged features are prone to breakage. To improve ejection, maintain consistent wall thickness and add ribs or fillets to reinforce critical areas. Avoid thin walls near ejector pins, as they may deform or tear.
Textured Surfaces: For textured or rough surfaces, increase draft angles to prevent sticking and ensure a clean release.
Undercuts and Core Pulls: Avoid undercuts where possible, as they complicate ejection and may require additional mechanisms like core pulls. Designs with minimal undercuts allow for simpler, more reliable ejection.

The information provided in this guide is public information and should be used as a suggestion for part design. Specifications with more stringent requirements may require further investigation. Contact us for help and we can do a comprehensive DFM review with you and your team.

An injection molded part specification tells your molder exactly what the final product requirements are. Specifications are intended to inform engineers and satisfy the end-use requirements for the application. These requirements should include aesthetic, functional, environmental, and economic requirements. Therefore, the final part should match the complete set of molded part specifications.

These specifications should include:

Material (brand name, grade, generic)
Surface finish
Parting line location
Flash limitations
Gate location and weld lines (away from critical stress points)
Locations where voids are not allowed
Allowable warpage
Color (Pantone PMS)
Decorating requirements and artwork
Performance requirements and testing methods (if applicable)

If you’re submitting a request for quote for a new part – download this Design Form to ensure all of your specifications are documented and communicated.

The validation process is like a charcuterie board of overhead costs… And if your quality requirements are stringent – you’re going to need an appetite.

When looking at the overall cost of an injection molding project, it’s easy to quantify and even justify the costs of upfront tooling. Same with the price per unit of each part. But when we start talking about validation processes, things are not always as clear. Therefore, costs can easily begin to compound when the process is not monitored and carried out properly.

First, you want to have the right people.

Validating new products and components requires oversight from Quality and Engineering SMEs. These experts are no slouches – often leveraging specialized skills in processing, measuring, data analysis, and other high-value skills. Their time and knowledge are worth their weight as long as the processes and systems they have to follow can make the best use of them.

That’s where having the system to test, record, assess, and the document becomes so valuable.

It will help you plan, check, and monitor your expenses throughout the validation process. This allows you to make quicker, more informed decisions to stay on budget.

If not, then validation overhead could end up being a significant (or even the biggest) cost driver. You don’t want this process to eat up all your budget.

💡 Here are 8 ways you can keep your validation overhead controlled:

🔎 Utilizing industry standards and guidelines for validation, such as ISO 13485 and ISO 9001, to ensure that all necessary validation processes are being followed.

🚨 Implementing a risk management process to prioritize and focus validation efforts on the most critical aspects of the device.

🚌 Streamlining validation protocols and procedures to minimize duplication of effort and increase efficiency.

💻 Automating validation testing and record-keeping, using electronic systems to reduce manual errors and increase accuracy.

🔎 Continuously evaluating and improving validation processes to ensure that they are current and effective.

🤩 Outsourcing validation services to third-party providers, which can provide specialized expertise and resources.

🔧 Using a modular approach in design, where different parts of the device are validated separately, which can reduce the overall validation effort.

📄 Utilizing device history records, which can provide a detailed record of the device’s history, which can be used to demonstrate compliance with regulatory requirements.

If you struggle to accurately control your validation processes and you’d like to chat about how to improve your process – reach out to us! Plastics Plus Technology operates a QMS that strictly follows ISO 9001:2015 and ISO 13485:2016 requirements.

I recently stumbled upon “The Iceberg Problem”

It is well-known to software developers, but it isn’t unique to just them.

Manufacturers have an iceberg problem too.

This idea originates from the process of building custom products or services for customers. With any custom project, there is a chance for misaligned discourse between what the customer thinks they want and what the contractor perceives the customer’s needs are. This can be frustrating and even painstakingly miserable for both parties.

The common cause – most times customers don’t understand the complete process.

Like an iceberg – 90% of manufacturing happens beneath the surface. The other 10% is what the customer perceives to be the result of the work being done.

When you present the work to the customer:

They care about the looks – does it match their vision?
They care about the performance – does it works as intended?
They care about how much it costs them – did the budget align with their expectations?
They care when they will receive their order – does it match their timeline?

And that’s it. That’s the 10%.

How can that be? From what I have gathered, there are a couple of hang-ups that drive a wedge between the customer and the contractor.

1. Sending in a sample that is less than ideal visual quality

When manufacturers validate a process, they just want to prove that the process is capable of making the product according to the outlined requirements. Sometimes, this sample doesn’t look visually perfect. And when the customer receives their product they are taken back.

The problem isn’t in the quality. In this case, the expectations were not set properly for your customer. You have opened the window for the customer to question the quality of your work. When you go to explain that it will take some time to get a sellable product, then a bit of dissatisfaction creeps in. Is this partnership going to work out? Can this company deliver on its promises?

That of course is for you to answer. But it helps when you establish the expectations upfront.

2. Showing off a finished product does not mean the work is done

Let’s say that the new project has gone through the initial inspection and the resulting parts look great. The first article samples are shipped to the customer, and they are ecstatic. They are ready to place a purchase order for their first production run. Oh, by the way, they need thousands of pieces by the end of the month. HOLD THE BUS! There is still some work to be done to ensure that the process is repeatable and that the equipment is production-ready. They may say “what do you mean, you already sent us good parts!?”.

This is where the iceberg analogy and the idea of above/ below the surface shows its teeth. Above the surface, we have stellar results. The product looks and works great. In the customer’s mind, the majority of the work is complete. But beneath the surface, the process that goes into making that product has to be refined. This often involves training, experimentation, oversight, data capturing and analysis, capital investment, and a whole lot more. Here’s the point – just presenting the results means leaving out everything it takes to get there.

3. The infamous design change midway through the project

There’s nothing that screams “we don’t know what we want!” like a down the value-chain design change.

Now, prototyping is part of the game, we get that. But the amount of work that goes into diverting from the original plan, especially when there are huge capital investments involved, is daunting. While the customer just wants what they think works best for them. But these changes are not subtle, cheap, or quick. These are the types of changes that can make a project stop in its tracks, or at least slow it down.

4. Costs that are hiding in plain sight

Quoting a new project isn’t a straightforward matter. What makes it complicated is when the project budget rises as more labor, equipment, overhead, and time is introduced. Focusing solely on the end price for the product highlights the 10% and completely ignores the work done underneath the surface – the “body” of the iceberg. Below the surface, there are other costs that appear as the work matures. But manufacturers don’t want to spin customers around by scavenging every nook and cranny for pennies. Instead, they often spread these costs across different areas to present a more digestible bill. Some of which are not even passed down to the customer, even if it’s at the contactor’s loss. By the way – we must make some money too.

5. Waiting at the shipping dock wondering when parts will show up

From the time the purchase order is processed until the product is on the truck – manufacturers are diligently working. But sometimes there is a bottleneck in the process that puts us in a holding pattern. It would be a Shakespearian world if this was something we can control but ultimately it is not. Because of supply chain interruptions, labor availability, vendor-driven pauses, and other things that cause unplanned downtime, there is always a chance we miss the deadline.

Here’s the thing – we can’t control everything but we can at least communicate. More times than not, the customer would rather know that a machine had to be sent out for repair and the lead time will have to be adjusted than to be left out in the dark.

Addressing the problem going forward

So how do we address these key points? The answer isn’t simple, but it can be done through a roadmap for continuous improvement. Start with a detailed schedule to set expectations from the start. This will help keep your team and customers on the same page. Next, bring your customers in on the process and educate them. Show them the work that goes into creating the product and they will be more understanding when you want to solidify your processes. Another thing you can do is be a more effective communicator. Communication is always a challenge, but by improving this aspect of your business your customers will be better informed. Not only that, but they will appreciate your willingness to be transparent. And finally, deliver value anytime you talk about costs. When you focus only on price and don’t show everything they get in return, then customers will fight you on costs.

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Every year, PPT donates a portion of its profits to City of Hope to help fund cancer research and the latest treatment efforts.

Our motto is “Contributing to better lives” and PPT strives to have a positive impact on the lives of our customers, employees and the community in which we live.

Tariffs have increased the cost of buying products from China. Components commonly used in injection molded products have been affected as well. These components include terminals that are used in electrical components, various inserts and other materials produced in China. At this time the tariff adds approximately 10% to the cost of the good sold but this could increase to 25% after 2019. Raw materials such as steel have higher tariffs.

A tariff of 25% on Imported Chinese steel has also been put into place. This added cost is cause for concern to molders who use imported Chinese steel to build their tooling. Naturally the injection mold industry is heavily dependent on steel for the production of their tools, for small precision close tolerance molds the raw material cost is only 10% of the total mold costs. Steel cost is a larger percentage of the cost for Large molds and molds that have less stringent quality requirements. For those molders who import finished mold tooling, the cost could be much higher, adding upwards of $10,000 to the mold cost. The good news is that the tariffs on steel produced elsewhere have been lifted. Steels produced in Mexico, Canada and Europe are not affected by these tariffs.

PPT works with local sources for tooling and materials so thus far we have not seen a big impact on tooling costs. Inserts however are another story, early in 2018 we immediately began to see tariff costs passed down from suppliers. Where possible, we searched for alternative sources for inserts and terminals that could be produced in the USA or Mexico.

While it appears that both China and the USA will eventually agree on a new trade deal, nobody can predict when. Until then, there will be pressure to use alternative sources produced anywhere but China.

SME’s (small manufacturing enterprises) rely on lean production processes to eliminate wasted in time and money. This leads to helping customers more efficiently, effectively and for a lower cost. Our eBook/ informational brochure goes in-depth into what lean manufacturing is, how it can help reduce waste and ultimately lower costs.

Click Here to Download eBook!

Injection Molding Design Guide