Blog | The Best Plastic Injection Molded Design Tips - RL Hudson

Design and molding. When it comes to custom plastic injection molded parts, these two undertakings must be considered together. As a top manufacturer of custom plastic injection molded parts, we know a thing or two about design. In this article, we’ll break down the absolute best plastic injection molded design tips. Here’s what you need to know.

If you are looking for more details, kindly visit Yihua Mould.

Plastic Injection Molding Tip #1: Begin with the End in Mind

Have you read “The 7 Habits of Highly Effective People” by Stephen Covey? You might remember habit no. 2: “Begin with the end in mind.” This principle also applies when it comes to designing and molding plastics. For plastics, that means starting where the plastic injection molded part will end up — the environment.

Perhaps you already know the material properties of plastic are sensitive to temperature, chemicals, stress and time. And when it comes to optimal design and molding of plastics, you may also know the importance of considering these stressors as they relate to one another, and as they relate to the environment. Remember: when dealing with these stressors, they cannot be considered individually.

Just take Nylon, for example. Nylon handles high heat well, and, apart from being hygroscopic, Nylon does not mind water. But put Nylon in hot water, at say -95°C, and it resembles a thick mucus. The deformity of a solid material, or creep, occurs due to time and stress, and extreme temperatures increase the rate at which a material creeps. This only begins to scratch the surface of things to consider when it comes to design and molding.

With your designs, be sure to keep in mind the implications for finite element analysis (FEA). The properties listed in material data sheets are prepared from samples with particular sizes and shapes, samples with certain mold filling directions, and samples that have been subjected to a specific rate of load, at a specific temperature and humidity. For this reason, the properties listed in material data sheets may be inapplicable.

Usually, design guides have information that can help you “knock down” properties. Molding itself can have a large impact on the performance of a part. Processing parameters, gate orientation, use of regrind and even storage, can affect strength. Again, look to Nylon: it becomes stronger when soaked in water. When it comes to pressing together parts in an assembly, water soaking is an effective way to condition parts prior to applying the stresses of assembly.

Plastic Injection Molding Tip #2: Design for Uniform Wall Thickness

When it comes to designing custom molded plastic parts, it is important to design for uniform wall thicknesses.Here are some examples of common design flaws and their solutions.

Plastic Injection Molding Tip #3: Design for Ribbed Walls

As a general rule and except for thin-walled parts as noted below, support ribs should be approximately 1/2 as thick as the primary wall to prevent sink at the intersection. Radii at the intersection should be 1/4 of the primary wall thickness. The diagram below outlines common issues and solutions for ribbed walls.

Plastic Injection Molding Tip #4: Design for Areas Prone to Stress Concentration

Stress risers should always be avoided in plastic. Below are common design flaws and associated solutions here.

Plastic Injection Molding Tip #5: Design for Assembly

Because of the flexibility that plastic parts allow, many people will try to get a little too creative. Here are some common errors and the preferred designs.

Beyond Tips for Plastic Injection Molded Parts

Apart from the tips we’ve covered, there are also design considerations involving compression limiters, adhesive joints, markings, cavities and thin-walled plastic parts. Let’s take a closer look.

Compression Limiters and Custom Molded Plastic Parts

What do you do when you have to bolt a plastic piece to another part? That’s where compression limiters come into play. At RL Hudson, we have executed a wide variety of designs with compression limiters. Whether it’s molded compression limiters with grooves, knurling or incorporating other retention features directly into parts, our experience with custom molded plastic parts and compression limiters runs the gamut. In fact, in order to retain the part, we have executed assemblies using retention features and a groove in the limiter.

Adhesive Joints

Adhesive joints can be very strong when designed correctly. We prefer to use the adhesive just for structural purposes. If a pressure tight seal is required, it may be advisable to incorporate a seal in the joint. The image below shows the preferred joint design for perimeter joints.

Markings, Identification, etc.

Material Codes

Material Marking Codes, e.g. , should be to ASTM D etc.,etc., etc. for plastic.

Date Codes

We prefer to use indexable date code of the form below. If there is not sufficient space, other forms such as changeable pins with single digits may be used.

A complete line of standard date code wheels is available through RL Hudson and in Asia.

Cavity ID

If there is more than one cavity in the tool, we prefer to have a cavity ID to assist in possible corrective action situations where one bad cavity may exist. The preferred format would be a number. However, if space is tight or pulls make it difficult, a series of dots, i.e. one dot for cavity one, two for cavity two, etc, is more than adequate.

Thin-Walled Plastic Parts

Parts are considered “thin-walled” when the ratio of low length to wall thickness exceeds 100:1

Thin-walled parts require very fast filling at 10-20″ per second. This requires increased ejection forces. As a result, the tooling may need to be much stouter than ordinary tools and special molding machinery may be required. A smaller barrel will alleviate problems caused by “residence time” or how long material stays heated in the barrel. Draw polishing the mold in the direction of injection will help ejection.

Ribs in thin-walled parts may be thicker than in ordinary parts. Rib thicknesses up to 100% of wall thickness may be used. 3:1 thickness transitions (length of transition 3 times the depth of the transition) should be used at a minimum.

How Does Injection Molding Work?

Tooling fabrication: Once an injection molding design is finalized the first step in the manufacturing process is to mill the tooling, which is typically fabricated from steel or aluminum. In most cases, the metal block of material is placed in a CNC mill, which then carves out a negative of the final plastic part. Additional treatments like polishing or laser etching can then be applied to the tooling to achieve specific surface finishes.

Contact us to discuss your requirements of custom mold plastic. Our experienced sales team can help you identify the options that best suit your needs.

Part production: The actual production of plastic parts begins by loading resin pellets into a barrel. The temperature of the barrel is raised until the resin pellets reach a molten state and are then compressed. Next, the molten plastic is injected into the metal tool through a runner system, which then feed into the mold cavity through gates. The part then cools down, solidifies, and is ejected from the tool with ejector pins.

Types of Injection Molding

The term injection molding encompasses a handful of processes that inject liquid resin into a tool to form plastic parts. Here are four common types:

Thermoplastic injection molding: Thermoplastic injection molding is the most common type of molding. It injects thermoplastic resin into the mold where the material cools to form the final part.

Liquid silicone rubber molding: Liquid silicone rubber uses thermoset materials and a chemical reaction creates the plastic part.

Overmolding: Overmolding is a process used to manufacturing plastic parts with two or more materials. You’ll often find this on parts to improve grip by adding rubber to the handle.

Insert Molding: Insert molding is process that begins with an insert component placed into the mold before resin enters. The material is then injected and flows around the insert, typically metal, to form the final part. This is frequently used for parts that require metal threads.

Basic Design Principles for Injection Molding

Tolerances
With our injection molding process, we can hold about ±0.003 in. machining accuracy. Shrink tolerance depends mainly on part design and resin choice. It varies from 0.002 in./in. for stable resins like ABS and polycarbonate to 0.025 in./in. for unstable resins like TPE.

Wall Thickness
Wall thickness is important because it can lead to defects such as sink and warp. It is best practice to maintain a uniform thickness throughout an injection-molded part. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin.

Core Geometry
Core out parts to eliminate thick walls. You get the same functionality in a good molded part. Unnecessary thickness can alter part dimensions, reduce strength, and necessitate post-process machining.

Draft
Applying draft to molded parts is critical to ensure parts do not warp during the cool down process and it helps the part easily eject from the mold. Applying 1 to 2 degrees works well in most scenarios. If there are vertical faces, we advise incorporating .5 degrees of draft.

Side Actions
A portion of the mold that is pushed into place as the mold closes, using a cam-actuated slide. Typically, side-actions are used to resolve an undercut, or sometimes to allow an undrafted outside wall. As the mold opens, the side action pulls away from the part, allowing the part to be ejected. Also called a “cam.”

Undercuts
A portion of the part that shadows another portion of the part, creating an interlock between the part and one or both of the mold halves. An example is a hole perpendicular to the mold opening direction bored into the side of a part. An undercut prevents the part from being ejected, or the mold from opening, or both.

Bosses
A raised stud feature that is used to engage fasteners or support features of other parts passing through them. There can be a tendency to design thick bosses which will increase the likelihood of sink and voids in a part. Consider reinforcing bosses with ribs or gussets for extra strength

Gates
A gate is an opening in the injection mold tool that allows resin to enter and fill the cavity. There are three common types of injection molding gates.

• Tab gates are the most common type of gate since it works well with additives and is the most cost effective option.
• Hot Tip gate is best for parts that cosmetic appearance is a priority. These gates can also reduce wear on tooling and flash.
• Pin, Post, or Tunnel gates  are ideal for cosmetic parts that don’t require a vestige. Sometimes not an option depending on material and geometry.

Ribs
A rib is a thin, wall-like feature parallel to the mold opening direction, its used to add strength and support to features like bosses and walls. To prevent sink, ribs should be no more than 60% of the wall’s thickness.

Ejector Pins
Ejector pins are installed in the B-side of the mold and help to release the plastic part from the tool after the part has cooled sufficiently. Designing in sufficient draft can help reduce the need for ejector pins on a part.

Logos and Text
Sans serif fonts will be the easiest to mill into a mold with text. We recommend font larger than 20 pt. and no deeper than 0.010 in to 0.015 in.

Commodity Resins

Polypropylene (PP): PP is a cheap material and good when cosmetics and rigidity aren’t a priority. Chemical resistant and good for living hinge designs.

Polyethylene: High and low density options. Is durable and chemical resistant.

Polystyrene: A hard thermoplastic that is cheap and clear.

Check out our guide to injection molding materials if you want to dig deeper into plastic selection.

Colorants and Resin Additives for Injection Molding

Stock colors from the resin vendor are typically black and natural. Natural might be white, beige, amber or another color. Semi-custom colors are created when colorant pellets are added to natural resins. For available colors, visit our materials page. There is no added charge for our inventory colors. They may not be an exact match and may create streaks or swirls in parts.

Resin Additives

Short glass fibers are used to strengthen a composite and reduce creep, especially at higher temperatures. They make the resin stronger, stiffer, and more brittle. They can cause warp due to the difference in cooling shrink between the resin and the fibers.

Carbon fiber is used to strengthen and/or stiffen a composite and also to aid in static dissipation. It has the same limitations as glass fibers. Carbon fiber can make plastic very stiff.

Minerals such as talc and clay are often used as fillers to reduce the cost or increase the hardness of finished parts. Since they do not shrink as much as resins do when cooled, they can reduce warping. 

PTFE (Teflon) and molybdenum disulfide are used to make parts self-lubricating in bearing applications.

Long glass fibers are used like short glass fibers to strengthen and reduce creep, but make the resin much stronger and stiffer. The downside is that they can be particularly challenging to mold parts with thin walls and/or long resin flows.

Aramid (Kevlar) fibers are like less-abrasive glass fibers only not as strong.

Glass beads and mica flakes are used to stiffen a composite and reduce warping and shrinkage. With high loading, they can be challenging to inject.

Stainless steel fibers are used to control EMI (electromagnetic interference) and RFI (radio frequency interference) typically in housings for electronic components. They are more conductive than carbon fiber.

UV inhibitor for outdoor applications.

Static treatments make resins dissipate static.

The company is the world’s best mould company supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.