1. Selecting Right Plastic Material
Choice of plastic material plays a crucial role in product's appearance, and different plastic materials exhibit varying appearance qualities. For example, compared to non-glass fiber reinforced materials, glass fiber reinforced materials generally exhibit lower appearance quality after injection molding and are more prone to warping.
2. Avoiding Surface Shrinkage
Surface shrinkage is one of the most common cosmetic defects in plastic parts. Shrinkage occurs on exterior surface of parts where wall thickness is greater, such as where ribs, struts, and walls meet. Where permitted, surface shrinkage can be concealed through use of U-shaped grooves, surface step-downs, and surface texturing. Localized material removal at thickened struts or ribs can significantly reduce likelihood of surface shrinkage. Surface shrinkage is more likely to occur farther from gate.
3. Avoiding Part Deformation
Part deformation not only results in poor dimensional accuracy, easily leading to assembly problems and compromising part functionality, but also affects part's appearance. There are many reasons for deformation of parts, mainly including four aspects: different shrinkage ratios of parts in flow direction of plastic melt and cross-sectional direction, uneven cooling of parts, uneven wall thickness of parts, asymmetric geometric features of parts, etc.
4. Designing Art Groove Between Appearance Parts
When two appearance plastic parts are mated, gaps and step heights (where one part's surface is higher than the other's) will inevitably exist due to manufacturing and assembly variations, affecting product's appearance. Art groove design can conceal these gaps, thereby improving product's appearance quality.
When two appearance plastic parts are front-to-back or top-to-bottom, such as upper and lower covers of a mobile phone, step heights become a factor affecting part's appearance. If rear or lower part is higher than front or upper part, product will look very unsightly. Therefore, art groove should be designed so that rear or lower part is lower than front or upper part.
5. Avoiding Weld Marks on the Surface of Appearance Parts
Weld marks are a common surface defect in plastic parts and should be avoided. Specific methods include: Textured surfaces can partially, but not completely, conceal weld marks. Spray painting can also conceal weld marks. Properly position and number gates should be set to avoid weld marks on important appearance surfaces of part, and ensure smooth mold ventilation.
6. Avoid surface breaks or burrs on exterior of part
Surface breaks or burrs are most likely to occur at intersection of mold core and die, intersection of core and die, intersection of core and die. Therefore, mechanical engineers should carefully examine location of parting surface within mold structure to avoid breakage or burrs on key exterior surfaces of part, which can affect part's appearance quality. Furthermore, avoid placing ejector mechanism on key exterior surfaces, as this can also cause burrs. This is especially important for transparent plastic parts.

Columns protrude beyond plastic wall thickness to assemble products, separate objects, and support other parts. Hollow columns can be used for inserting components, tightening screws, and other applications. These applications require sufficient strength to withstand pressure without breaking.
Columns should rarely be used alone. Instead, they should be connected to exterior wall or used in conjunction with ribs to enhance their strength and facilitate flow of plastic. Furthermore, because columns that are too tall can trap air during plastic part molding, column height is generally limited to two and a half times column diameter. To strengthen columns, especially those located away from exterior wall, in addition to using ribs, triangular reinforcement plates are also commonly used.
A good screw/column design combination depends on mechanical properties of screw and design of column hole. Thickness of typical plastic products is insufficient to withstand stresses generated by most fasteners. Therefore, for assembly purposes, locally increasing plastic thickness is necessary. However, this can lead to undesirable effects such as shrinkage marks, voids, and increased internal stress. Therefore, entry holes and perforations for pillars should be located some distance from outer wall of product. Pillars can be independent and separated from outer wall, or connected to outer wall using reinforcing ribs. The latter not only increases pillar's strength to support greater torsional and bending forces, but also facilitates rubber filling and reduces burning caused by trapped air. For same reason, pillars away from outer wall should also be supplemented with triangular reinforcement blocks, which are particularly suitable for improving rubber flow in thin-walled pillars.

 

Generally, a pillar's outer diameter twice its inner diameter is sufficient. Sometimes, this approach results in a pillar wall thickness equal to or exceeding compound thickness, increasing material weight and causing shrinkage marks and high molding stresses on the surface. Strictly speaking, pillar thickness should be 50-70% of compound thickness. While this design may not provide sufficient strength, it will improve surface shrinkage. Bevel ribs can be added to strengthen pillar and can extend from the smallest dimension up to 90% of pillar's height. If pillar is located close to a sidewall, a rib can be used to connect sidewall to pillar to provide support.

 

Pillars are commonly used for mechanical assembly, such as screw tightening, press fits, and guide assembly. A pillar with an outer diameter twice inner diameter is sufficient for strength. Pillar design is similar to rib design. Excessively thick sections can cause shrinkage and internal vacuum. When pillars are positioned adjacent to sidewall, they can be connected to ribs, maximizing inner diameter.

 

Pillars are mostly used for product assembly, sometimes supporting other objects or separating them. Even very small pillars are eventually heat-melted to secure internal components. Some pillars placed on edge require ribs for mutual attachment to increase pillar strength.

 

PS pillar design close to side wall
Pillars are typically used for driving parts, tightening screws, guiding pins, tapping, or making tight fits. Whenever possible, avoid leaving a single pillar unsupported. Ribs should be added to strengthen it. If pillar is close to side wall, ribs should be used to connect pillar to side wall.

 

Key points in designing PSU pillars
Pillar is used to connect two parts. Its outer diameter should be twice inner diameter, and its height should not exceed twice outer diameter.

Drilling holes in plastic parts to facilitate connection with other components or to increase product functionality is a common technique. Size and location of holes should minimize impact on product strength or increase production complexity. Following are several factors to consider when designing holes.
Distance between adjacent holes or between a hole and adjacent straight edge of product must be at least equal to hole diameter, as shown in key diagrams for hole spacing or inner wall spacing. Furthermore, hole wall thickness should be as thick as possible, otherwise fractures are more likely to occur at perforation site. If hole contains threads, design requirements become more complex, as threaded area can easily create stress concentration areas. Experience has shown that to reduce stress concentration factor at screw hole edge to a safe level, distance between screw hole edge and product edge must be at least three times screw hole diameter.
Perforation:

 

Key points regarding hole position from edge or inner wall
From an assembly perspective, perforations are far more widely used than blind holes and are easier to produce. From a mold design perspective, perforations are also structurally superior because both ends of pins used for perforation are supported. Perforations can be performed with a single pin secured to mold at both ends, or with two connected pins, each secured at one end. Generally speaking, the first method is considered preferable. When using second method, diameters of two pins should be slightly different to avoid undercuts caused by slight misalignment of pin axes. Furthermore, connecting ends must be ground flat.
Blind Vias:

 

Key design points for blind vias
Blind vias are formed by pins in mold. However, these pins are designed to support mold on only one side, making them easily bent and deformed by molten plastic, resulting in an oval shape. Therefore, pins should be kept short. Generally, depth of a blind via is limited to twice its diameter. If diameter of blind via is only 1.5mm or less, depth should not exceed diameter.

 

Methods for eliminating side hole undercuts

 

Hole edge design
Edge of hole should be left with a minimum of 0.4mm of vertical space. Designing a complete chamfer or fillet on hole edge is economically and practically impractical. Please refer to hole edge design drawing.

Snap-on snaps offer a convenient, fast, and economical method for product assembly. Because snap-on components are molded during final production process, assembly requires no additional fastening components such as screws or clips. Simply snap two snaps together.
While snap-on snaps can be designed in a variety of geometric shapes, their operating principle is generally same: when two parts are snapped together, hook-shaped extension of one part is pushed aside by flange of mating part until flange is closed. Elasticity of plastic then causes hook-shaped extension to return to its original position, and groove behind it is immediately engaged by flange of mating part, creating an inverted snap-on position. Please refer to diagram for snap-on operating principle.

 

Principles of permanent and removable buckles
Based on their function, buckle designs can be categorized as permanent or removable. Permanent buckles are easy to attach but difficult to remove, while removable buckles are very easy to attach and remove. Principle behind removable buckles is that hook-shaped extension of a removable buckle has appropriate lead-in and lead-out angles to facilitate fastening and detachment. Size of lead-in and lead-out angles directly influences force required for fastening and detaching. Permanent buckles, however, have only lead-in angles but no lead-out angles. Once fastened, connecting parts form a self-locking lock, making them difficult to remove. Please refer to diagrams illustrating principles of permanent and removable buckles.

 

Wall thickness depends on external forces product must withstand, whether it serves as support for other parts, number of supporting posts, extent of protrusion, and plastic material used. Generally, wall thickness design for thermoplastics should be limited to 4mm. From an economic perspective, excessively thick products not only increase material costs, but also extend production cycle and cooling time, increasing production costs. From a product design perspective, excessively thick products increase likelihood of voids and air holes, significantly weakening product's rigidity and strength.
Ideal wall thickness distribution is undoubtedly uniform across cross-section, but variations in wall thickness are unavoidable to meet functional requirements. In this case, transition from thick to thin areas of plastic should be as smooth as possible. Abrupt wall thickness transitions can lead to dimensional instability and surface defects due to differential cooling rates and turbulent flow. For general thermoplastics, thickness variations are acceptable when shrinkage factor is less than 0.01mm/mm. However, if shrinkage factor exceeds 0.01mm/mm, changes in wall thickness should not exceed this limit. For general thermosets, excessively thin parts often lead to overheating during operation, resulting in scrap. Furthermore, fiber-filled thermosets often lack fill in areas that are too thin. However, some easy-flowing thermosets, such as epoxies, can achieve a minimum thickness of 0.25mm if thickness is uniform.
Furthermore, when using curing molding, runners, gates, and component design should ensure that plastic flows from thicker areas to thinner ones. This maintains appropriate pressure within mold cavity, minimizing shrinkage in thicker areas and preventing incomplete filling. If plastic flows from thinner to thicker areas, structural foaming should be used to reduce cavity pressure.

 

ABS: Generally, a draft angle of 0.5° to 1° is sufficient for application edges. Sometimes, because polished lines align with draft direction, draft angle can be close to zero. For textured sides, increase draft angle by 1° for every 0.025mm (0.001in) of depth. Correct draft angle can be obtained from texture supplier.
LCP: Because liquid crystal copolymers have high modulus and low ductility, undercut designs should be avoided. A minimum draft angle of 0.2-0.5° is required for all ribs, walls, struts, and other areas protruding above glue point. Deep walls, unpolished surfaces, or textured areas may require an additional 0.5-1.5°.
PBT: For parts with a smooth surface finish, a minimum draft angle of 1/2" is required. For textured surfaces, increase draft angle by 1° for every 0.03mm (0.001in) of depth.
PC: Draft angle is required on any edge or raised area of part, including upper and lower molds. Generally, 1.5° to 2° is sufficient for smooth surfaces, but textured surfaces require an additional draft angle of 1° for every 0.25mm (0.001in) of depth.
PET: For ribs, pillar walls, runner walls, and other parts of finished plastic parts, a draft angle of 0.5° is sufficient.
PS: A 0.5° draft angle is considered extremely fine, while a 1° draft angle is standard. A draft angle too small can make part difficult to remove from mold cavity. However, any draft angle is better than no draft angle. If part has texture, such as leather grain, add an additional 1° for every 0.025mm of depth.

Reinforcement ribs are an essential functional component of plastic parts. They effectively increase rigidity and strength of a part, similar to an I-beam, without significantly increasing cross-sectional area. However, they avoid the undercuts and difficult-to-form shapes that often occur with I-beams. They are particularly suitable for plastic parts frequently subjected to pressure, torque, and bending. Furthermore, reinforcement ribs act as internal flow channels, facilitating mold cavity filling and significantly helping plastic flow into component's branching sections.
Reinforcement ribs are generally placed on non-contact surfaces of plastic part. Their extension direction should align with direction of maximum stress and maximum deflection. Rib placement is also influenced by production considerations, such as mold cavity filling, shrinkage, and demolding. Ribs can run the entire length of part, with both ends connected to outer wall, or they can occupy only a portion of part's length to provide localized rigidity. If ribs are not connected to outer wall of product, end part should not end abruptly, but should gradually decrease in height until it ends, thereby reducing problems such as air entrapment, incomplete filling and burn marks, which often occur in locations with insufficient ventilation or closed areas.

 

Rectangular ribs must be reshaped to facilitate production

 

Reinforcement ribs placed along edge of plastic part
ABS: To minimize shrinkage on critical part surfaces, rib thickness should not exceed 50% of intersecting compound thickness. Rib thickness may be up to 70% on noncritical surfaces. In thin-compound structural foam parts, ribs may be up to 80% of intersecting compound thickness. Ribs in thicker compounds may be up to 100%. Rib height should not exceed three times compound thickness. When more than two ribs are used, distance between ribs should be at least twice compound thickness. Rib draft angles should be between one-sided and wide to facilitate demolding.

 

Key Design Considerations for ABS Ribs
PA: Height of an individual rib should not be three times or more thickness of rib base. Behind any rib, there should be some small ribs or grooves. Because ribs will form indentations on the back when they cool, those ribs and grooves can be used for decorative purposes to eliminate defects of shrinkage.

 

PBT: Thick ribs should be avoided as much as possible to prevent bubbles, shrinkage marks, and stress concentration. Rib size is limited by structural considerations. For wall thicknesses under 3.2 mm (1/8 in), rib thickness should not exceed 60% of wall thickness. For wall thicknesses exceeding 3.2 mm, rib thickness should not exceed 40%. Rib height should not exceed three times rib thickness. A 0.5 mm (0.02 in) R-interval is used to connect ribs to plastic wall on both sides to facilitate plastic flow and reduce internal stress.

 

PS: Thickness of rib should not exceed 50% of thickness of wall it touches. Experience has shown that violating above guidelines will result in uneven gloss on the surface.

 

Key points in design of ribs with PS placed in the middle

 

PS rib design points for edge placement