10 Tips for Tooling Plastic Parts

06 Jan.,2025

 

10 Tips for Tooling Plastic Parts

A perfect, precision part begins with the mold. Building the tool takes time and a great deal of accuracy. It can also represent the largest investment in the manufacturing process, so getting it right is critical to the success of a project. If your goal is to manufacture parts with a high degree of precision in large volumes, the tooling becomes even more complex.

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1. Know Your Plastic

The tool and the molding process are customized based on the type of plastic. Plastics that are amorphous are less free-flowing and tend to shrink less than crystalline or semi-crystalline plastics, which offer better flow, but higher shrinkage. For this reason, many projects call for engineering resins that provide a better melt and less shrinkage. Plastic suppliers provide information on the shrinkage rate of their resins along with temperature and melt flow rate recommendations.

2. Add A Draft Angle To Your Parts

Adding draft angles to a part's face, also called drafting, eases its release from the injection mold. Because a draft angle can interfere with mating parts and cause other design issues, they must be precisely calculated. Two main drafting rules of thumb are:

  • At least 1° angles for untextured molds
  • At least 3° angles for textured molds

Maintaining these minimum angles ensures the drafted parts are easily released from molds without damage. For a tight part mating area, restrict the zero-draft area to be as close as possible to the mating section.

3. Ensure Resin Flows From Thick to Thin Sections in the Mold

Plastics tooling often requires a combination of wall thicknesses to create the desired structure and increase its strength. This requires foresight into where thicker sections are positioned relative to the gate because molten resin loses pressure and temperature as it flows throughout the mold.

Resin flowing from thinner to thicker walls can have problems filling completely, so the gate is best positioned closest to the thickest sections. In other words, the part should be designed so that resin flows from thick to thin sections. Because efficient material use is another aspect of good plastics tooling and design, the ideal wall thickness provides a balance between a part's strength, weight, durability, and cost.

4. Reduce Sizes of Strengthening Ribs As Much As Possible

Ribs make parts stronger and stiffer while minimizing warp. Cross-hatched rib patterns create even greater strength while avoiding sink, an undesirable surface depression caused when overly thick areas cool slowly.

A properly formed rib balances three basic design features:

  • Base thickness no greater than 60% of wall thickness
  • Rib height less than three times the part thickness
  • Overall rib thickness less than the base thickness

The draft angle may also play a role in the rib's dimensions. Note that more or bigger ribs don't always strengthen parts-the fewer, the better.

5. Be Attentive to Resin Shrinkage

Resin shrinkage impacts the design and machining of the tool cavities. The cavities must accommodate the amount of shrinkage that can occur. Using modern CAD software, the design engineer will create cavities that are larger than the actual finished part. The size of this allowance is based on the specific properties of the resin.

Some of this shrinkage could be addressed by regulating the packing and holding rate in the mold, but all plastic shrinks as it cools, even after the part is ejected from the mold. Worst case, warpage can occur when a part has molded-in stress. This stress can be a result of issues with pressure, temperature, flow rate, gate location, or venting.

6. A Strong Mold Design Leads to High-Volume Quality

Precision parts are only accomplished by meeting exacting standards not only in the cavities but the design of the mold components. Gates must be correctly placed to allow for proper melt flow and pressure. The appearance of the final part can be improved by positioning the gates in an inconspicuous location on the part. The size of the gate is also an important consideration. The gate must be large enough to provide for proper packing of the material without extending the cycle time. If the gate is too small, the packing may be insufficient to fill the cavity (also called a short shot) or the part may display other defects.

The design of the mold must also include vents. Vents allow the air that is displaced by the melted resin to escape the tool. As with gating, the size and position of the vents are key factors in producing a quality part. Vents that are too large can allow the plastic material to escape and cause flashing. Vents that are too small may not release enough of the trapped air and gas. These gas bubbles can cause an improper fill (short shot) or worse. The gas could combust and cause burn marks on the part.

When possible, simpler designs are preferred and will result in stronger molded parts with minimal risk of damage. Designers should avoid using undercuts, sections that can&#;t be formed by the opening and closing direction of the tool. An undercut creates a back-drafted area that inhibits mold ejection while making the part more complex.

To reduce or eliminate undercuts, experienced parts design engineers can modify designs for maximum plastics tooling efficiency, reduced costs, and improved structural integrity. If undercuts or other complex features can&#;t be avoided, they can be formed in the main pull direction using sliders and lifters. These require two to three times the feature&#;s width for appropriate lifter or slider travel.

7. Proper Cooling Is Key

An efficient and effective cooling system is the hallmark of a quality injection mold. The mold needs to maintain a consistent temperature to avoid shrinkage and warping while minimizing the cycle times to maximize production output. This delicate balance is achieved with a well-designed cooling system.

8. Size and Place Ejector Pins

The final step in the molding process is releasing or ejecting the parts from the mold. The part geometry, type of resin, and mold finish are all considered when designing the ejector system. The placement of the ejector pins, the type of ejection mechanism, and the cycle times need to be calculated with precision to avoid any defects in the part. All of these are accomplished with a series of carefully placed ejector pins, the size and position of which are determined by the shape, size, and wall thickness of the part.

9. Utilize Samples Prior to the Full Run

Avoiding any unnecessary rework of a mold cavity will save time and money in the long term. Experienced molders create a sample mold that is used to produce a test run of the part. This step is vital in determining if any adjustments to the mold, the resin selection, or molding parameters such as temperature and flow rate are needed. If there are any quality issues, the project team will work together to determine the cause and re-sample the parts until they meet the customer&#;s and the molder&#;s standards.

Rapid prototypes enable faster design improvements, greater manufacturability, and more efficient secondary processes. It allows engineers to detect design flaws that aren&#;t always obvious with 3D models alone.

Plastic injection molders can employ numerous 3D printing options for rapid prototyping, including:

  • CNC Machining - A time-tested, affordable option for plastic and metal parts with a quality finish and tight tolerances.
  • Metal 3D Printing - Capable of creating high-strength, low-weight, and highly complex samples.
  • Laminated Object Manufacturing (LOM) - Applies layers of thin laminates to form plastic, paper, or metal materials into simple prototypes at a lower cost.
  • Stereolithography (SLA) - Uses light-reactive resins to produce low-volume runs with a high-quality finish and great strength.
  • Digital Light Processing - Fulfills tight tolerance levels while providing an attractive surface finish.
  • Selective Laser Melting (SLM) - The most suitable choice for parts with a balance of high strength, durability, and complexity.
  • Selective Laser Sintering (SLS) - SLS is ideal for plastic or metal parts with complex internal designs.
  • Fused Deposition Modeling (FDM) - An affordable, easier process capable of combining multiple colors and materials into a single part.
  • Binder Jetting - Affordably creates multiple parts at once.
  • Rapid Injection Molding - A fast, low-cost process suitable for low-volume runs.

10. Be Mindful of Cost

Plastic tooling cost can be influenced by several factors, such as the number of cavities, the mold base, part complexity, the core metal, and more. Working with an experienced plastic injection molding manufacturer can help you optimize your parts for performance and cost.

The Rodon Group Difference

At The Rodon Group, we have been providing turnkey manufacturing solutions for over 67 years. We specialize in building high-volume molds that last for decades. If you would like to learn more about how we can design and build an injection mold for your project, please give us a call at 215-822- or us at .

Our eBook "How to Manufacture a Perfect Plastic Part" reviews the four key factors that go into producing a perfect plastic part. First, the part must be designed for manufacturability. Tool design and building is step two in the process. Third, suitable resins for the application must be selected. The final phase of manufacturing the part depends on getting the right type and size press. Proper press size is the key to minimizing the price per piece cost.

Don't let questions about plastic injection molding weigh you down. Get the answers you need in your free copy of our eBook today and learn "How to Manufacture A Perfect Plastic Part."

Injection Molding 101 – Everything You Need To Know

Injection molding (or injection moulding) is a widely used manufacturing process in many different industries such as aerospace, automotive, medical, and even consumer products. Here you&#;ll find the answers to the most common questions we get asked about injection molding

What is injection molding?

Injection molding is a method of producing parts by injecting material into a mold. Metals (for which the process is known as die-casting), glasses, elastomers, confections, and, most commonly, thermoplastic and thermosetting polymers can all be used in injection molding. The part&#;s material is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the cavity&#;s configuration. After a product is designed, usually by an industrial designer or engineer, molds are made from metal, usually steel or aluminum, and precision-machined to form the desired part&#;s features. 3D printing materials like photopolymers which do not melt during the injection molding of some lower temperature thermoplastics can be used for some simple injection molds. Injection molding is widely used for producing a wide range of parts, from very small to very large. The ability to produce parts with varying geometrical shapes and sizes is determined by the type of machine used in the operation.

The history of injection molding

Injection molding equipment originates from metal die casting processes. For example, John Wesley Hyatt is credited with inventing a patent for the first plastic injection molding machine. This patent was granted in August . However, the origins of injection molding date back to when Brothers Francis and John Downing invented and patented the bottle-making machine that produced embossed bottles from a continuous sheet of glass. This technology made it possible to create seamless bottles that could not be created with traditional blow-molding techniques.

How does plastic injection molding work?

Injection Molding takes plastic resin, heats it up, and forces it into a mold. The mold is usually made of steel or aluminum. The mold is made in two halves, called the A-side, and B-side. The A-side is attached to the injection unit on the injection machine. When the molten plastic comes out of the barrel of the machine, it fills the mold and then hardens.

Once the plastic has hardened in the mold, both sides are opened and the part is ejected. The process starts again when you close the mold back up and pressurize it to push more plastic in.

The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

What are the benefits of plastic injection molding?

#1 Very high production rates

One of the most appealing characteristics of plastic injection molding as opposed to other processes is its very high production rates. Depending on the complexity of the design, the size of the part being molded, and other factors, individual molds can produce hundreds or even thousands of finished parts per hour. This allows manufacturers to keep costs low, while still benefiting from a fast turnaround time for their products.

#2 High tolerance precision

With the use of this process, you can produce high tolerance precision parts. A mold is used to create the shape of your part. The molds are held to very close tolerances by design. For example, if you need to make a thousand identical parts, you can use the same mold and they will all be exactly the same size and shape.

#3 Low labor costs

Compared to other manufacturing processes, plastic injection molding is relatively labor-free once a mold has been manufactured. Many companies that offer injection molding services are able to produce large quantities at a very low price point because they do not need to hire much labor for this type of work. This allows them to remain competitive in today&#;s market and keep prices low for consumers.

#4 Environmentally friendly

When compared to other manufacturing processes like CNC machining, which creates a lot of waste by cutting away at raw material, plastic injection molding is a much more eco-friendly solution.

#5 Very durable

Another great benefit of plastic injection molding is the ability to create very durable products that do not scratch or break easily. You can also choose from different types of plastics based on your product&#;s needs, like an impact-resistant or heat-resistant plastic.

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#6 Variety of plastics available for choices

There are a variety of plastic resin material options to choose from for use in the plastic injection molding process. Each material has its own unique properties; therefore, understanding the differences between them is crucial to ensure selecting the most suitable material for your intended application.

What are the typical materials for plastic injection molding?

Injection-molded parts can be made from a variety of thermoplastic materials including ABS, nylon, polycarbonate (PC), polyoxymethylene (POM), polypropylene (PP), polystyrene (PS), polyurethane (PUR), thermoplastic elastomers (TPE) and TPU.

#1 Acrylonitrile Butadiene Styrene(ABS)

ABS is a commonly used plastic injection molding material with three main ingredients: acrylonitrile, butadiene, and styrene. Each of these monomers imparts specific properties and provides ABS terpolymer with a robust combination of features. ABS offers high strength, toughness, and resistance to impact and temperature. It is easily molded and gives a high-quality glossy surface finish. This plastic polymer does not have a specific melting point.

#2 Polycarbonate (PC)

A polycarbonate is a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonate has a high degree of stiffness and thermal resistance due to its molecular structure and a reasonably high viscosity when processed. Even so, polycarbonates can be molded and thermoformed with great ease, making them a popular choice for a wide range of products.

#3 Polyoxymethylene (POM)

Polyoxymethylene also known as polyacetal/acetal/polyformaldehyde/Delrin, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas. POM is characterized by its high strength, hardness, and rigidity to &#; 40°C.

#4 Polypropylene (PP)

Polypropylene also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and is partially crystalline and non-polar. PP is inexpensive and easy to access, and due to flexible consistency, PP is widely used for manufacturing storage containers, such as bottles and plastic boxes. Polypropylene has a strong resistance to fatigue and chemical corrosion, making it useful for most types of plastic storage containers, kitchenware, water bottles, and even insulation and piping systems.

#5 Nylon Plastic (PA)

Nylon plastic (PA) is a synthetic thermoplastic polymer commonly used in injection molding applications. It&#;s a versatile, durable, flexible material often used as a more affordable alternative to other materials like silk, rubber, and latex.

#6 Acrylic (PMMA)

Acrylics are a group of polymers prepared from acrylate monomers. These plastics are noted for their transparency, resistance to breakage, and elasticity. They are also commonly known as acrylate polymers or polyacrylates. Acrylate polymer is commonly used in cosmetics, such as nail polish, as an adhesive. The most common acrylic plastic is polymethyl methacrylate (PMMA).

#7 Polyethylene (PE)

Polyethylene thermoplastic materials are generally divided into multiple groups, based on density. These include low-density polyethylene (or LDPE), medium density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE or UHMW). In general, the higher the density, the higher the tensile and flexural strength, chemical and abrasion resistance, and surface hardness.

#8 High Impact Polystyrene (HIPS)

This cost-efficient material offers excellent machinability, dimensional stability, impact resistance, and aesthetic properties. It is highly customizable and can be glued, printed, bonded, and decorated with ease.

Design tips for injection molding parts

Designing parts for injection molding can be simple, but there are a few basic rules you should follow to avoid common pitfalls and make sure your part is made in the best way possible.

To design parts for injection molding, try to keep these rules in mind:

#1 Uniform wall thickness

Wall thickness should be no less than 4mm (0.16&#;) and no greater than 8mm (0.31&#;). Part walls thinner than 4mm (0.16&#;) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. Walls thicker than 8mm (0.31&#;) will require more material and longer cooling times, which can lead to warpage or increased cycle times.

A wall thickness between 1.2 mm and 3 mm is a safe value for most materials.

Below are the recommended wall thicknesses for some of the most commonly used injection molding materials.

MaterialRecommended wall thickness [mm]Recommended wall thickness [inches]Polypropylene (PP)0.8 &#; 3.8 mm0.03&#; &#; 0.15&#;ABS1.2 &#; 3.5 mm0.045&#; &#; 0.14&#;Polyethylene (PE)0.8 &#; 3.0 mm0.03&#; &#; 0.12&#;Polystyrene (PS)1.0 &#; 4.0 mm0.04&#; &#; 0.155&#;Polyurethane (PUR)2.0 &#; 20.0 mm0.08&#; &#; 0.785&#;Nylon (PA 6)0.8 &#; 3.0 mm0.03&#; &#; 0.12&#;Polycarbonate (PC)1.0 &#; 4.0 mm0.04&#; &#; 0.16&#;PC/ABS1.2 &#; 3.5 mm0.045&#; &#; 0.14&#;POM (Delrin)0.8 &#; 3.0 mm0.03&#; &#; 0.12&#;PEEK1.0 &#; 3.0 mm0.04&#; &#; 0.12&#;Silicone1.0 &#; 10.0 mm0.04&#; &#; 0.40&#;

#2 Round all edges and corners

The uniform wall thickness limitation also applies to edges and corners: the transition must be as smooth as possible to ensure good material flow. For interior edges, use a radius of at least 0.5 x the wall thickness. For exterior edges, add a radius equal to the interior radius plus the wall thickness. This way you ensure that the thickness of the walls is constant everywhere (even at the corners). Wherever possible, round all corners to increase both the looks and the durability.

#3 Add draft angles
To make the ejection of the part from the mold easier, a draft angle must be added to all vertical walls. Walls without a draft angle will have drag marks on their surface, due to the high friction with the mold during ejection. A minimum draft angle of 2° is recommended. Larger draft angles (up to 5 °) should be used on taller features.

#4 Take care of the ribs

Ribs serve as structural features that help maintain the part&#;s overall stability. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.

#5 Avoid undercuts if possible

Injection molds are made up of two halves, which means they have parting lines and a &#;blindside&#; that cannot be reached by the injection nozzle or runner system.

#6 Transition from thick to thin

Parts will form better if plastic flows through features moving from greater to lesser wall thickness starting from the gate(s) (where the plastic first flows in to fill the part).

Besides, injection molding part design is not just creating an injection molded part that functions in its environment, but one that will delight your customer. It&#;s a given that you need to understand what it is you&#;re trying to design for manufacturability &#; but taking the step further, here are some suggestions on how to make your part stand out:

1. Think in terms of functional design

2. Make it easy to assemble

3. Make it easy to manufacture

4. Minimize the cost of manufacturing

5. Follow the design for manufacturability and assembly (DFMA) principles

Start Injection molding with LEADRP

Nothing compares to injection molding as a mass-production process. It is an efficient way to produce large quantities of parts ranging from a few grams to several kilograms. The injection molding process requires an injection molding machine, raw plastic material, and a machined mold. As a manufacturing process, injection molding is one of the most versatile ways to produce plastic parts. It allows you to create complex shapes at low costs; however, it can be challenging to get your product off the ground with an injection molder without first investing in tooling.