Buyers Guide to Selecting an Injection Molder | Crescent Industries

What is the importance of Injection Mold Design? Will it significantly impact the final product? This article will introduce the concepts, procedures, and applications of Plastic Injection Mold Design to be able to better understand how the overall system works and avoid being misled by the manufacturers when there are issues arise with the final product.

Further reading :Why do we need custom injection molding?

What is Injection Mold Design?

Injection mold design is a technique used for mass production of plastic parts. Done by melting the Polymer Pellets and inject it into a mold under pressure, where the liquid plastic will cool down and solidify in the mold, forming the final desired part.
The advantages of this process are its efficiency and cost effectiveness; since the per unit cost is relatively low, it is suitable for mass production; the molding is fast, offering variety of material options; also, it provides excellent repeatability with high tolerances, enabling it to be able to create complex geometry shapes without the need for additional precision machining. In addition, the scrap rate is also very low.
However, injection mold design also has its limitation; the initial investment for the molds are high and also design modification for it are relatively expensive. In addition, the lead time from design to actual production can be long, it typically took at least 4 weeks.

Further reading : What are the common types of plastic fasteners?

The 10 factors to consider for Injection Mold Design!

Material Selection

Injection Molding has a wide range selection of materials that can be used. Below are some common Injection Molding material and its characteristics:

• Polyoxymethylene (POM): Has an excellent rigidity and thermal stability, low water absorption, and good chemical resistance.
• Acrylic: Strong, lightweight, shatterproof, optical transparency, UV Light Resistance, and good weather resistance.
• Acrylonitrile Butadiene Styrene (ABS): Strong and has impact resistance, even at a lower temperature environment.
• Nylon (PA): Has high heat resistance, high wear resistance and good fatigue resistance with tough structural integrity.
• Polybutylene Terephthalate (PBT): Creep resistance, suitable for thin section parts.
• Polycarbonate (PC): Strong and lightweight, natural transparency, able to stably work across a wide temperature range.
• Polyether Ether Ketone (PEEK): Has an outstanding mechanical properties, chemical resistance and thermal degradation.
• Polyetherimide (PEI): Has both rigidity and stability, with low flammability and low smoke generation.
• Polyethylene (PE): Commonly used in indoor application, resistant to chemical corrosion, available in high density and low density forms.
• Polyphenylsulfone (PPSU): High toughness, with high bending flexibility and tensile strength, as well as chemical resistance and good heat resistance.
• Polypropylene (PP): Excellent chemical resistance, will not degrade in moist environment or water.
• Polystyrene (PS): Lightweight, relatively cheap, moisture resistance and resistant to bacterial growth.
• Thermoplastic Elastomers (TPE): The processing method is similar to plastic but with the elasticity and properties of a rubber.
• Thermoplastic Polyurethane (TPU): Has rubber-like elasticity with good load capacity.

Wall Thickness

Wall thickness will affect the strength, costs and appearance of the parts. It is
necessary to first understand the following three terms related to wall
thickness:

• Uniform Wall Thickness: The ideal design is to maintain a consistent wall thickness throughout the parts, to ensure uniformity during the cooling process and reducing shrinkage discrepancies.
  If the wall thickness is uneven, it may lead to defects such as dents or warping, because different parts with different thickness will have different cooldown and shrinking rates.
• Nominal Wall Thickness: This refers to the average thickness of the parts. Even though Uniform Wall Thickness is the best choice, but it is also necessary to avoid excessive or insufficient wall thickness. Excessive thickness will lead to increase usage of plastic materials and machine cycle time, thus raising costs. While insufficient thickness may result in incomplete filling of the mold, leading to incomplete parts.
• Recommended Wall Thickness: Based on different materials, below are the recommended wall thickness:

If uniform wall thickness can’t be maintained in the design, it is necessary to design a smooth transitions between areas of different thickness to minimize the potential manufacturing issues.

Transitions

The best practice is to have transitions between areas of different wall thickness to reduce stress concentrations that may lead to parts failure. There are two main methods to achieve this:

• Chamfer: Forming a sloped edge at the intersection of two surfaces.
• Filleting: Create a rounded edge at the corners.

Corners

Sharp edges will cause stress concentrations, and at the same time increase parts costs, since these edges typically require Electrical Discharge Machining (EDM) to fabricate the mold.
Even though Sharp Corners can be useful at the parting line, but should be used with cautious. Choosing rounded corners design will be better to help reduce stress concentrations, minimize the shrinkage differences during the cooling process, lowering mold costs and allow smoother flow of molten plastic through the mold. When designing rounded corners, it is essential to ensure that the inner corner radius is at least 50% of the wall thickness, the outer corner radius equals the sum of the inner corner radius and the wall thickness, and both inner and outer corners must start from the same point.

Parting Lines

Injection molds have a parting line where the mold open and closes, which typically located along the centerline of the molded part. This is a common practice, but not the best or most effective approach.
Taking plastic bottles as example, the parting line is not located at the centerline of the bottle, because it will affect the appearance and feel of the bottle. Instead, it is often located at the side or bottom of the bottle, so that when the bottle is standing, the parting line will not be easily seen or touched, which maintain its overall aesthetic appeal.
Injection Molding Parting Lines usually will place it on sharp edges to simplify mold fabrication and reduce costs, but it must avoid placing it on rounded surfaces due to the possibility of following issues occurred:

• Rounded surface requires a high precision molds, which significantly increases costs.
• Rounded surface also increase the risk of flash occurrence.

Gates

Gates are the openings of injection molds that allow molten plastic to enter the mold cavity, and their dimensions and position will directly impact the quality and appearance of the parts. The dimensions of the Gate should be adjusted based on the dimensions of the parts, while its position can affect the issues like warping, sink marks, and weld lines. When designing, Gates should be placed in the less visible area, typically along the Parting Line.
Manually Trimmed Gates

• Edge or Standard Gate: Suitable for flat parts with a rectangular cross section that can taper gradually.
• Fan Gate: Larger openings with thickness that can vary, suitable for quick filling large size parts and delicate mold areas.
• Tab Gate: Used for thin and flat parts that require low shear stress, the stress concentration of this kind of Gate is located in the Gate area.
• Direct or Sprue Gate: Suitable for large cylindrical parts, quickly delivering material directly into the cavity through the sprue.
• Disc or Diaphragm Gate: Suitable for conical or cylindrical parts that require concentricity, but this kind of Gate is usually difficult to remove and have high trimming costs.
• Ring Gate: Allow material to flow freely, then filling the mold through a tubular extension.
• Spoke Gate: Has a cross structure in the center, suitable for producing tubular parts, but very difficult to achieve perfect concentricity.

Automatically Trimmed Gates

• Hot Tip Gate: Suitable for conical or cylindrical parts that require uniform flow, which typically used in Hot Runner System that keeps the plastic in molten state.
• Submarine and Sub Gate: Has a tapered channel that effectively hides the trace left by the Gate, which sometimes referred as Tunnel Gate.
• Pin Gate: Suitable for fast-flowing resins and high quality appearance parts, typically used for components that must not have traces on either side of the parting line.

Ribs and Bosses

Utilizing a thin wall design can achieve a faster production speed and extend mold life, but it may lack sufficient strength. To enhance the strength of the parts, Ribs and Bosses can be included into the design:
Ribs are vertical structured use to improve structural strength and load capacity. However, thick ribs may shrink, which will cause indentation on the opposite side, affecting the parts’ appearance. But this condition can be improved through the following methods:

• Wall Thickness: The thickness of ribs should be 50%-60% of the standard wall thickness (0.5T-0.6T)
• Fillets: Adding fillets at the base of the ribs with a radius close to 0.25Tto 0.5T, where T is the standard wall thickness, and the corner radius should not excel 0.010 inches.
• Height: Ribs should be as short as possible, ideally no more than 2.5T. If higher Ribs are needed, it is preferred to use multiple shorter ribs.
• Draft: Apply draft angle to the ribs, with at least 0.5 degrees on each side.
  Bosses are a vertical structured used to support assembly and enhance structural strength, which can accommodate fasteners such as screws. Also, smaller bosses can be inserted into larger ones.
• Position: Place bosses at where additional structural support is needed, such as near screw slots.
• Diameter: Avoid making the holes too small, since they may shrink during cool down process.
• Walls: Consider how bosses will be connected to the walls, and ensure they aligned properly.

Draft

Draft is an angled applied to a vertical walls to help the parts easily be removed from the mold, which can reduce wear on the mold and shorten the cooldown time, helping to efficiently control the costs. When the draft angle applied is confirmed, the following factors should be considered:

• The Material Used: Different materials has different requirements for the draft angle.
• Standards Followed: The industry standards and company regulation is set by Society of Plastics Industry (SPI) and the Association of German Engineers (VDI), which will affect the required draft angle.
• Surface Finishing of the Parts: The final appearance of the parts will affect the draft required, for example, smooth surfaces finishing will require smaller draft angle, while rougher surface require larger angles. Every 0.001 inch (About 0.025mm) of textured line will be recommended to add another 1.5° draft angle.
• Mold Design: The structure of the mold and how it is separated also will affect the selection of draft angle, or else the parts might get stuck at one half of the mold or stick to the half of the mold that contains the mold release system.

Tolerances

When the injection molded parts is used for larger assemblies, it must maintain a precise and consistent dimensions. However, any manufacturing process will have a certain degree of dimension bias. The designer must specify the acceptable tolerance ranges of the dimensions, which the tolerances are categorized into two types:

• Commercial Tolerances: Lower precision, using less expensive mold, which result in lower production costs.
• Precision Tolerances: Higher Precision, require higher costs’ mold, which leads to higher production costs.

Tolerances types also varies depending on the injection mold materials and the overall dimensions and particular features:

• Dimensions: The overall size of the parts.
• Flatness and Straightness: Involves deformation in large flat areas.
• Hole Diameter: Larger holes requires larger tolerances due to their greater shrinkage.
• Concentricity/Ellipticity: Thin wall cylindrical parts may shrink unevenly, which affect their roundness.

For assemblies involving multiple parts, stack tolerances must also be considered, meaning that all and individual tolerances must match correctly.

Ejector Pins

Ejector pins are used to push the parts out of the mold after cooldown, but sometimes the parts may stick, which leave Ejector Pins marks on the finished product.
Ejected Pins should be placed on areas of the parts that are not visible, and there are also some design guidelines that needs to be lookout, not only related to mold manufacturing, the designer should also understand this to easily evaluate the mold design.

• Distribute the force as evenly as possible when pushing the parts out of the mold, to prevent deformation of the parts.
• Apply the force to push out the parts on the strongest and most rigid areas of the parts.
• Avoid placing Ejector Pins on thin wall or sloped areas.
• Don’t place the Ejector Pins on the mold’s moving slides.
• Use the ejection mechanism that has enough strength and durability.

Further reading : Introduction To Manufacturing for Designers: What is it and How to Choose?

The 3 procedure of Plastic Injection Mold Design Process

After understanding the factors affecting mold design, next will be introducing the complete procedure of plastic injection mold design, which is divided into three stages. If any issues are found after receiving the samples, further modifications can still be made to ensure the final parts will achieve the desired production outcome.

What are the common applications of Plastic Injection Mold Design?

There are currently five industries that commonly require plastic injection mold design. Whether you are working in these fields now or plan to enter them in the future, mastering knowledge of injection mold design will be beneficial for you.

• Household items: cups, nail clippers, razors, etc.
• Beauty products: lipstick tubes, makeup brushes, face masks, etc.
• Office supplies: staplers, pen holders, file folders, etc.
• Toys and entertainment: playing cards, board game pieces, Frisbees, etc.

• Cost efficiency: Consumer goods usually require low cost mass production, so the injection mold design needs to ensure an efficient production process and low manufacturing costs.
• Material selection: Use affordable plastic materials (e.g., polypropylene, polyethylene) to balance the costs and performance.
• Durability: Although these are consumables, durability and reliability during use must be considered in the design.
• Production efficiency: The design should support fast production and high output to meet market demand.

• Consumer use electronics: smartphones, tablets, laptops, headphones, etc.
• Communication equipment: routers, switches, base station box, fiber optic connectors, etc.
• Accessories and casings: device’s casing, buttons, connectors, heat sinks, etc.
• Internal structural components: PCB mounts, connectors, connector’s casing, etc.

• Precision and tolerances: Electronics and communications equipment often have strict requirements for parts’ dimension and shape. Mold design must ensure high precision and strict tolerance control.
• Material selection: Choose the most suitable plastic materials (e.g., polycarbonate, PEEK) to meet requirements for mechanical strength, thermal stability, and electrical performance.
• Heat management: Electronics equipment usually has heat problem, so mold designing should consider heat dissipation, potentially needing heat sinks or ventilation holes.
• Design Complexity: The outer case and internal components of electronics may have complex geometries, which require mold designs that support efficient production of these shapes.

• Food and beverage packaging: bottles, cans, caps, containers, etc.
• Cosmetic packaging: bottles, jars, lids, dispensers, etc.
• Medical packaging: medicine bottles, syringes, medical storage containers, etc.
• Consumer goods packaging: detergent bottles, shampoo bottles, chemical containers, etc.
  Design Consideration
• Product safety: Packaging must provide adequate protection against contamination and damage. Mold design should consider sealing and anti-leak functions.
• Size and shape: The packaging size and shape must be precise to accommodate the product and meet the transport and storage requirements.
• Material selection: Choose the most suitable plastic materials (e.g., polyethylene, polypropylene, PET) to meet food safety, chemical resistance, and transparency requirements.
• Aesthetic and branding: Packaging design often need to incorporate brand logos and images, and mold design must consider printing and texture details.

• Industrial mechanical parts: gears, connectors, brackets, rails, etc.
• Electronic equipment components: casings, connectors, switch buttons, etc.
• Automotive parts: dashboard panels, external decorations, interior fittings, etc.
• Household appliance components: switch panels, fan blades, motor housings, etc.
  
  Design Consideration
• Strength and durability: Mechanical parts often bear heavy loads, so the choice of plastic material and mold design must ensure that the parts have sufficient strength and durability.
• Precision and tolerances: Mechanical parts require high dimensional accuracy and tolerances, and mold design must be precise to ensure the parts meet the design requirements.
• Material selection: Choose the most suitable plastic materials (e.g., polycarbonate, nylon, ABS) with good mechanical properties and chemical resistance for high-stress environments.
• Surface treatment: The surface treatment for mechanical parts should consider the wear resistance, corrosion prevention, and improving appearance, where the mold design should account for.

• Daily use items: toothbrushes, cutlery, bathroom accessories, etc.
• Home decor: vases, lampshades, tabletop decorations, etc.
• Sporting equipment: water bottles, yoga mat stands, sports guards, etc.
• Transportation accessories: bicycle accessories, car interior parts, motorcycle guards, etc.
  
  Design Consideration
• Costs control: General plastic products often require mass production, so mold design should ensure efficient production with the most economical way, to reduce unit costs.
• Functionality and comfort: Even though these products may be daily consumables, but their design must consider its practicality and comfort.
• Material selection: Choose the most suitable plastic materials based on the product’s functional needs. For example, choose the most wear-resistant materials for sporting equipment and easy-to-clean materials for home decor.
• Production efficiency: The design should achieve quick and stable production in the mold to meet market demand. For example, multi-cavity molds can increase production efficiency and shorten the production cycle.

Can Injection Mold Design Fail?

Although the scrap rate for injection mold design is low, there are still potential issues that may arise during the manufacturing process. This section summarizes 16 common problems and their causes. However, you can rest assured that if you choose a reliable manufacturer to assist in production, these problems can be effectively resolved.

What is Injection Mold Design? | FAQ

Creating a successful Injection Mold Design

If you’re concerned about communication difficulties or meeting an unreliable manufacturer, you might consider consulting to Hsin Hung Yih Technology or Hsin Hung Yih Plastic, which use advanced software to analyze potential issues like weld lines, shrinkage, and stress. Additionally, offer suggestions for drawing modifications and provide model creation services before mold production, to avoid unnecessary revisions and troubleshooting later in the process.

Are you interested in learning more about automotive injection molding? Contact us today to secure an expert consultation!