Alexei Volkov
Designing products in plastic requires a good understanding of the capabilities of each production method and the properties of each plastic type. Injection moulding is the most common and versatile method of manufacturing plastic products, but other methods include cutting, extrusion, and 3D printing. The design process must consider factors such as circularity, efficiency, cost, and safety regulations. For example, the wall thickness of a plastic product will depend on its size and the external forces it will endure. The complexity, reliability, and accuracy of the design will also impact the workload for product structural design, requiring superb technical and engineering abilities from designers.
| Characteristics | Values |
|---|---|
| Design stage | Tooling concepts, injection molding, safety, functionality, performance, manufacturability, cost efficiency, and aesthetics |
| Injection molding | Melting plastic granules, injecting a predefined amount of melt into the mold, and cooling and solidifying the plastic |
| Tooling | Two mold halves that form a cavity to define the final part's shape |
| Prototyping | 2-D prints, 3-D models, and physical prototypes for evaluation |
| Wall thickness | 2.4-3.2mm for large components, 1.0mm for smaller components, with reinforcing ribs to increase strength |
| Hole depth | About two times the diameter of the core pin for small pins, four times for larger pins |
| Hole spacing | At least two times the thickness of the part or the diameter of the core pin |
| Draft angle | Minimum of 1 degree per side, preferably between 2 and 5 degrees per side, to assist in part ejection and improve productivity |
| Ribs | Used for strengthening and decoration, with a minimum base radius to reduce stress and improve flow and cooling |
| Bosses | Basic design element used for assembly, mounting, location, or reinforcement |
| Inserts | Used as fasteners or support elements, typically made of metal with specific shapes, strength, and bonding to the plastic matrix |
| Raw materials | Plastic resin pellets |
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What You'll Learn
Injection moulding
The basic process involves injecting plastic into a mould through the sprue orifice. The mould is clamped to the platen of the injection moulding machine, also known as a press, which consists of a material hopper, an injection ram or screw-type plunger, and a heating unit. The press is rated by tonnage, which refers to the clamping force that keeps the mould closed during the process.
There are several advantages to injection moulding. It offers high tolerance precision, repeatability, a large selection of materials, low labour costs, minimal scrap losses, and little need for finishing parts after moulding. Additionally, injection moulding can produce parts with very good physical properties, which can be tailored by using additives or mixing different pellets to achieve the desired strength, stiffness, or impact resistance.
However, there are also some disadvantages to consider. Injection moulding has high upfront tooling investment costs, and there are process limitations. The mould design itself can be expensive, ranging from USD 3,000 to USD 100,000+, depending on complexity, material, and accuracy.
Designing plastic parts for injection moulding is a complex task. It requires considering various factors, such as application requirements, functional and structural issues, and processing constraints. Adhering to basic design rules will result in parts that are easier to manufacture and assemble, and are typically stronger. For example, allowing plastic to flow and cool uniformly within the mould results in a stable and accurate part. Additionally, keeping wall thicknesses constant and avoiding thick sections improve part quality and mouldability.
Overall, injection moulding is a versatile and widely used technique for creating plastic parts, offering high precision and repeatability but requiring careful design considerations and upfront investments.
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Wall thickness
The wall thickness of plastic parts is determined by considering the specific application requirements and the conditions of plastic production. The choice of raw material significantly influences the overall thickness of the product. For instance, thinner walls are suitable for plastics with high fluidity, such as nylon, polyethylene, and polypropylene. On the other hand, the wall thickness can be increased for polymers with low fluidity, like PC and PSF, to enhance structural integrity.
To achieve uniform wall thickness, walls in plastic-molded parts should be no less than 40 to 60 percent of adjacent walls and should adhere to the recommended thickness ranges for the selected material. A Design for Manufacturability (DFM) analysis is crucial before the design phase to identify potential issues and improve manufacturing outcomes.
Reinforcing ribs can be incorporated into plastic product designs to increase strength without adding to the overall wall thickness. The thickness of these ribs is typically 0.5 to 0.75 times that of the overall wall thickness. Additionally, the distance from the edge of a hole to a vertical surface or another hole should be at least twice the thickness of the part or the core pin's diameter.
The wall thickness also impacts the cooling time of the plastic injection molding process. Thicker walls result in longer cooling times, while thinner walls may lead to high flow resistance during injection and difficulty in filling the cavity. Thus, it is essential to consider the balance between strength, rigidity, and thickness when designing plastic products.
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Draft angles
Plastic tends to shrink onto the mould core, creating higher contact pressure and friction between the core and the part, thus making ejection difficult. Draft angles are designed to assist in part ejection, reducing cycle time and improving productivity. The draft angle ensures that once the plastic part separates from the mould, it no longer contacts the mould, eliminating friction.
The mould itself must also be designed with features that accommodate a proper draft angle. Both the core and the cavity should incorporate draft angles to ensure proper ejection. The choice of plastic material can impact the required draft angle, as different materials have varying coefficients of friction. Some materials may require a larger draft angle to ensure smooth ejection. The surface finish of the mould and the part can also affect ejection, with a polished or textured mould surface impacting the friction between the part and the mould.
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Ribs and spacing
Ribs are an essential part of plastic design, providing reinforcement, improving rigidity, and distributing loads effectively within the part. They are thin, elongated projections that extend perpendicularly from the walls or surfaces of a plastic component. These ribs are strategically placed to enhance the strength and performance of plastic parts.
The spacing between two parallel ribs is important as it affects the mold wall thickness. If ribs are placed too close to each other or the walls of the parts, thin areas are created which can be hard to cool and can negatively impact quality. It is recommended that the spacing between ribs should be at least twice the nominal wall thickness.
The height of the ribs should not exceed three times the thickness of the adhesive or the wall. The rib thickness should be smaller than the wall thickness of the plastic material to prevent issues like sink marks or excessive material usage. The recommended rib thickness is between 0.4 to 0.5 times the thickness of the plastic material, providing a balance between reinforcement and material efficiency.
Draft angles are also important in rib design. The ideal draft angle for the outer rib is 0.5 degrees, while for the inner rib it is 0.25 degrees. Draft angles assist in part ejection, reducing cycle time and improving productivity.
Ribs can be arranged in different patterns depending on the shape of the plastic part. Long grid ribs form a lattice-like structure and are often used in large, flat components such as panels or enclosures to prevent warping. Circular ribs are used in components with a circular or cylindrical shape, like pipe fittings or tanks, providing radial support and distributing stress uniformly.
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Inserts
There are several types of standard inserts available on the market, including press-fit inserts, tapping inserts, heat-set inserts, helical inserts, mold-in inserts, and dowel pin inserts. Press-fit inserts, also known as press-in inserts, are designed to be pressed into a straight post-mold hole without additional heat, making them ideal for softer plastics. They feature knurls, which can be helical or diamond-shaped, to provide torque and pull-out resistance. The standard press tooltip is lowered to push the insert into the pilot hole, with the plastic deforming and flowing around the knurls, fins, and undercut features of the insert.
Self-tapping inserts, also known as self-threaded inserts, are like self-tapping screws for plastic. They are screwed into a pilot hole using threaded drivers, which can be manual, electric, or pneumatic. These inserts are designed for hard or brittle plastics and have a thin thread profile to avoid exerting high stress on the plastic. They also have a course thread pitch to maximise plastic shear surface and resist pull-out.
Heat inserts, also known as heat-set inserts, are designed for post-mould installation on thermoplastics using an electric heat insertion press. They are placed on a pre-drilled or pre-moulded pilot hole, and the electrical insertion press tooltip is lowered to sit on top of the insert. The heat from the press softens the plastic, allowing it to flow around the insert. However, heat inserts are not suitable for thermoset plastics.
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Frequently asked questions
Injection moulding is one of the most popular ways to design plastic parts. This method can be used to create automotive interior parts, electronic housings, housewares, medical equipment, compact discs, and doghouses.
There are several factors to consider when designing plastic parts, including the use of inserts, the working environment, the type of gate and placement, the finish, the cost, and the assembly. It is also important to consider the wall thickness, with large components ranging from 2.4-3.2mm and smaller components at around 1.0mm.
To avoid defects, it is important to consider the thickness of the plastic and ensure that it is consistent. Inconsistent thickness can lead to defects such as sink marks, voids, stresses, and warping. It is also important to consider the type of plastic used, as some materials are more prone to thermal shrinkage than others.
