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.
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 215-822-5544 or email us at email@example.com.
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."