Injection Molding Design Guide

Every year, 55 million tons of plastic are processed to manufacture almost any kind of part. Injection molding is the leading process to manufacture these plastic parts. The process is a quick, economical way to mass produce a number of parts, ranging from high-precision engineering components to cosmetic product enclosures.

The Injection Molding Process

Having a basic understanding of the injection molding process will help you understand what features to consider when designing parts for injection molding and also how to forecast production schedules.

01 DESIGN THE CAD MODEL

Choose the color, texture, and resin.

02 DESIGN VERIFICATION

Use China Manufacturing parts, Inc. to verify your design with 3D printed, CNC machined, and urethane cast prototypes to test if the plastic part meets the requirements for function, strength, aesthetics, and durability.

03 DFM ANALYSIS

While most manufacturers won’t provide manufacturability feedback until after the order is purchased, China Manufacturing parts, Inc. will provide full DFM analysis for free with your quote. The aim at this stage is to learn all the potential issues or risks for tool manufacturing and injection molding. Then, you’ll need to improve or resolve any issues by updating the CAD design to avoid future tooling modification. This stage is critical because when the part is in the design stage, most small modifications only take half an hour or a few minutes to complete, but when the tooling is done, it takes at least a few days and hundreds of dollars to make even a small tooling modification.

At this stage, China Manufacturing parts, Inc. will also inform you as to how the tooling and injection molding will affect your plastic parts.

CONSIDERATIONS INCLUDE:

• the location of the gate (the gate shouldn’t scar the cosmetic surface)
• the location of the parting line and split line (are they acceptable from an aesthetic viewpoint?)
• the location of the ejection mark (they should be on the inside and shouldn’t affect the function by being on the sliding track)
• the location of the knit line (the line where two flow fronts meet) and what it will look like
• additional risks, like tolerance and deformation

04 TOOLING DESIGN

Next you will review the tooling drawing for approval. It’s important that you communicate any concerns until you’re satisfied with the design.

05 STEEL CUTTING

Normally, CNC machining, EDM (electrical discharge machining) and wire cutting will be needed to make the core, cavity, inserts, and lifter; while the sliders, the tooling base, and the ejection pin will be purchased as o-the-shelf parts. Bench workers assemble the tooling when every component is ready and fits the tooling to make sure every component is assembled correctly; the side action works well; the cooling system works without leaking; and the cavity and core sit on and shut off well.

06 T1 SAMPLES FIRST TRIAL

When the fitting is done, the tooling factory will make one or two trial molds to resolve as many issues as possible before sending first samples to the customer. After that, they will prepare the T1 sample and send to the customer for function testing and dimensional measurements. The T1 sample typically isn’t perfect, so the tooling factory needs one or two weeks to adjust the gate location and size and to adjust the parameters of the injection machine. Normally, the tooling factory won’t polish or texture the tooling in T1; they polish the tooling or add texture once the customer has approved the T1 sample, and there are no major modifications to the tooling.

07 T2 AND T3 SAMPLES SECOND & THIRD TRIAL

If the tooling isn’t complicated and design requirements are well documented, you should receive samples without significant quality issues by T3. For quality issues caused by the plastic part design (for example, a hole on the plastic part), the knit line will be unavoidable.

08 KICKOFF PRODUCTION

After samples have been approved, it’s time to kickoff production. For new product launches, we recommend teams start by cutting a single cavity tool to accelerate time to market and then ramp up production velocity by cutting additional multi cavity or family tools to

Uniform Wall Thickness

Wall thickness is the most important factor to consider when designing parts to be injection molded. Wall thickness influences many key characteristics of the part, including mechanical performance, cosmetics, and cost. Careful consideration of wall thickness will spare you from expensive delays in your schedule, due to molding issues and mold modifications. While manufacturability is important, the nominal wall thickness should be determined by functional performance requirements. Consideration of acceptable stress and the expected lifetime of the part should help establish the nominal or minimum wall thickness. Walls that are too thin may need excessive plastic pressure or cause air traps where the plastic doesn’t fill completely. Walls that are too thick will be more expensive, due to greater material usage and longer machine cycle times.

RECOMMENDED WALL THICKNESS FOR COMMON RESINS AVAILABLE AT China Manufacturing parts:

Once the nominal wall thickness has been set, it needs to be as uniform as possible across the part, for two reasons:

• The molten polymer takes the path of least resistance as it flows through the mold. Thicker walls cause preferential flow through them, causing air traps and weld lines.
• Plastics are poor conductors of heat. Uniform thickness ensures that the material cools uniformly, causing the plastic to shrink evenly as it solidifies. Differential wall thickness leads to different rates of shrinkage, causing the parts to warp.

If it’s not possible to keep a uniform thickness in your design, you have a few options:

OPTION 1: ROUNDED CORNERS & SMOOTH TRANSITIONS

Rounded corners and smooth transitions limit the stresses on the walls and minimize the differences in shrinkage rates as the material cools. Sharp corners also cause stress risers in the part, becoming a potential failure point. Sharp corners are a big cost driver for injection molded parts, since they require expensive EDM machining to create the mold. Rounded corners allow the material to flow easily through the mold. When designing the corners, maintain uniform thickness through the feature. The interior radius should be at least half of the wall thickness. The exterior radius should be the inside radius, plus the wall thickness.

OPTION 2: DRAFT ANGLES

Shrinkage as the molten plastic cools can cause the part to lock onto the mold. Draft allows for easier part removal. If you don’t add enough draft to the parts, you can get cosmetic flaws on the parts called drag marks that are a result of the parts sticking to the mold. You can add draft pretty easily using most CAD systems. This should be done at the final stages of the design to keep complexity to a minimum. You can find tables with the recommended draft angles for VDI texture and Moldtech textures (the two common mold texturing standards). Typically, 1-2 degrees of draft angle per side is acceptable. For more texture, 3-5 degrees per side, and 5 degrees or more per side for heavy texture.

Bosses

A boss is a localized, raised area used to fasten and connect parts. Bosses can be a helpful way to strengthen your part, without compromising manufacturability, by means of injection molding. In injection molding design, everything must be thin-walled, to increase mold life, part quality, speed, and more. This means that the parts we produce will lack strength and structural support. This is where bosses come into play. We place bosses where we want to introduce more structural integrity in areas that require it, like screw holes and slots.

Another reason to add bosses to your parts is for alignment purposes. We can allow for rapid assembly by creating bosses that can insert into one another for alignment, similar to dowel pins. Boss design is highly critical. One main factor that we consider many times over in injection molding is shrinkage. If we design a boss for a screw hole, we must make sure to design a smaller diameter to compensate for shrinkage. The thickness of the boss is also critical to this. To avoid sink marks from shrinkage, the thickness of the boss should typically not be more than 60% of your overall wall thickness.

Ribs

Ribs are used to add structural integrity to your part and increases its load bearing capacity. They’re a simple concept to introduce to your part that can get complicated with plastics. Plastic injection molded parts should have a uniform thickness, but for ribs, we break this rule. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. The bottom of your rib must also feature a fillet. Typically, this fillet radius should be somewhere near 0.25T-0.5T, where T is your nominal wall thickness. This radius should not be smaller than 0.010 inches. Ribs should have a limit to their height as well. Generally, we want to keep ribs as short as possible and drafted the way other walls are drafted. Ideally, the height of the rib should not exceed 2.5T. Commonly used draft angles are somewhere between 0.5 degrees to 1 degree per side.

Ribs can easily produce sink marks, where heavy shrinkage occurs and cause the surface of your part to have these so-called sinks, which are essentially cosmetic defects. As a result, we must significantly reduce the wall thickness of the ribs, relative to the nominal wall thickness of the part.

Parting Lines

The parting line, where the mold opens and closes, determines the direction of your draft.

Sometimes, the parting line is right down the middle of the part. This is what we all think of when picturing two molds that meet together in the middle, but this is not always the best place. Take, for example, a LEGO brick, one of the most mass-produced products out there. The parting line is not in the center of the LEGO brick; it is at the very bottom. The same goes for a plastic cup. Generally, we never want to place the parting line on a filleted surface. This would require a tight tolerance mold, which increases costs. Any mismatch will lead to flash and a potential cosmetic defect. Placing your parting line on sharp edges is the most ideal placement.

Gates

The four most common types of gate designs are edge, sub, hot tip, and sprue.

EDGE GATES

Edge gates are best suited for flat parts, and, as the name implies, located at the edge. This type of gate will create a scar on the parting line.

SUBGATES

Sub-gates are common but require ejector pins to automatically trim. These types of gates have different variations, such as banana gates, smiley gates, and tunnel gates. These allow you to move the gate away from the parting line, which is useful when your gate location needs to be moved away from the parting line to enhance filling.

HOT TIP GATES

Hot tip gates are located at the top of the mold, typically only in locations where the geometry is round or conical. These type of gates are also only used with hot runner injection molds.

SPRUE GATES

Direct or sprue gates are used for single-cavity molds that are typically large and cylindrical. These are the easiest of the mentioned gates to manufacture and maintain and are also low-cost. Although these are the most simple of gates, they leave a large scar at their point of contact.

Ejector Pins

Ejector pins push the part out of the mold when your part has finished cooling, and most of the time, they leave a mark. You may have noticed these ejector pin marks on some household products. Typically, these are ejected on the surface of the part that may not be visible, such as the inside of a housing. The ejector pin pads need to be placed on a surface that will be perpendicular to the direction the ejection pins will push.

Getting Started

How to minimize risk in launching products

If you’re going into injection molding for the first time, there’s a lot to consider and the process can be daunting. Before you can even start, you need to find the right manufacturing partner for your design requirements, budget, and timeline. If you’re looking overseas, it can be very especially difficult to navigate cultural, language, and time barriers. With China Manufacturing parts, you’ll get instant access to a highly vetted network of local and overseas injection molding manufacturers as well as a dedicated service team in the U.S. and overseas to manage communications with suppliers so your team can focus on designing great products. Additionally, China Manufacturing parts, Inc. has no minimum order quantities so you can minimize risk by ramping up production volume gradually to match customer demand.

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