Design of injection molds must adhere to guidance of scientific theory. It should integrate qualities and abilities of professional technicians, draw on experience of classic cases, utilize modern scientific, technological conditions and corresponding mechanical engineering design concepts to continuously optimize design scheme of injection molded part. Let's take a look at core considerations for injection mold design!
I. Mold Opening Direction and Parting Line
When designing each injection molded product, mold opening direction and parting line must first be determined to minimize impact of core-pulling mechanisms and parting line on appearance. After determining mold opening direction, product's reinforcing ribs, snaps, protrusions, and other structures should be designed to align with mold opening direction as much as possible to avoid core pulling, reduce seam lines, and extend mold life; After determining mold opening direction, an appropriate parting line can be selected to avoid undercuts in mold opening direction, thereby improving appearance and performance.

 

II. Draft Angle
A proper draft angle can prevent product scratches (roughening). Draft angle for smooth surfaces should be ≥0.5 degrees, for fine-textured (sand-textured) surfaces >1 degree, and for coarse-textured surfaces >1.5 degrees; A proper draft angle can prevent product ejection damage, such as whitening, deformation, and breakage; When designing deep-cavity products, draft angle of outer surface should ideally be greater than that of inner surface to ensure that mold core does not shift during injection molding, resulting in uniform product wall thickness and ensuring material strength of opening area.
III. Product Wall Thickness
Various plastics have a certain wall thickness range, generally 0.5–4 mm. When wall thickness exceeds 4 mm, it will cause excessively long cooling time, resulting in shrinkage marks and other problems. Product structure should be modified accordingly; Uneven wall thickness will cause surface shrinkage; Uneven wall thickness will cause porosity and weld lines.
IV. Reinforcing Ribs
Proper application of reinforcing ribs can increase product rigidity and reduce deformation; Thickness of reinforcing rib must be ≤ (0.5～0.7)T * product wall thickness; otherwise, surface shrinkage will occur; Single-sided slope of reinforcing rib should be greater than 1.5° to avoid top damage.
V. Fillets
Insufficient fillet radius may cause stress concentration in product, leading to cracking; Insufficient fillet radius may cause stress concentration in mold cavity, leading to cavity cracking; Setting reasonable fillet radius can also improve mold processing technology; for example, cavity can be directly milled with an R-cutter, avoiding inefficient EDM; Different fillet radii may cause parting line movement; different fillet radii or corner clearings should be selected based on actual situation.
VI. Holes
Shape of hole should be as simple as possible, generally circular; Axial direction of hole should be consistent with mold opening direction to avoid core pulling; When length-to-diameter ratio of a hole is greater than 2, a draft angle should be set. In this case, hole diameter should be calculated based on minor diameter (maximum material size); Length-to-diameter ratio of blind holes generally should not exceed 4 to prevent hole pin from bending; Dstance between hole and product edge is generally greater than hole diameter.
VII. Core-pulling and sliding mechanisms in injection molds and their avoidance
When plastic part cannot be easily demolded in mold opening direction, a core-pulling sliding mechanism should be designed. Core-pulling sliding mechanisms can mold complex product structures, but they easily cause defects such as seam lines and shrinkage, increase mold costs and shorten mold life; When designing injection molded products, avoid core-pulling structures as much as possible unless there are special requirements. For example, change hole axis and rib direction to mold opening direction, or use methods such as core-cavity penetration.
VIII. Integrated hinges
Utilizing toughness of PP material, hinges can be designed as an integral part of product; Film size of hinge should be less than 0.5mm and kept uniform; When injection molding an integrated hinge, gate can only be designed on one side of hinge.
IX. Inserts
Inserting inserts into injection molded products can increase local strength, hardness, dimensional accuracy, and allow for creation of small threaded holes (shafts), meeting various special requirements. However, this will increase product cost; Inserts are generally made of copper, but can also be other metals or plastic parts; Portion of insert embedded in plastic should be designed with anti-rotation and anti-pull-out structures. Examples include: knurling, holes, bends, flattening, and shoulders; Plastic around insert should be appropriately thickened to prevent stress cracking of plastic part; When designing inserts, their positioning method in mold (holes, pins, magnetism) should be fully considered.
X. Markings
Product markings are generally placed on relatively flat areas of inner surface of product and are raised. Markings should be placed on surfaces where normal direction may coincide with mold opening direction to avoid damage.
XI. Precision of Injection Molded Parts
Due to unevenness and uncertainty of shrinkage during injection molding, precision of injection molded parts is significantly lower than that of metal parts. Dimensional tolerances of mechanical parts cannot be simply applied; appropriate tolerance requirements should be selected according to standards. China also issued GB/T14486-93 "Dimensional Tolerances for Engineering Plastics Molded Parts" in 1993. Designers can determine dimensional tolerances of parts according to plastic raw materials used and requirements of parts, based on provisions in standard. At the same time, appropriate design tolerance precision should be determined based on factory's overall capabilities and design precision of competitors' products.
XII. Deformation of Injection Molded Parts
Improve the rigidity of injection molded product structure and reduce deformation. Avoid flat structures as much as possible, and reasonably set up flanges and concave-convex structures. Set up reasonable reinforcing ribs.
XIII. Snap-fit Devices
Design snap-fit device with multiple snap-fit points that can be used simultaneously, so that the entire device will not fail to operate due to damage to individual snap-fit points, thereby increasing its service life. Also, consider adding more fillets to increase strength; Tolerance requirements for dimensions related to snap-fit are very strict. Too many snap-fit positions can easily lead to snap-fit damage; conversely, too few snap-fit positions make assembly difficult to control or result in loose parts. Solution is to allow for easy re-molding during mold changes.
XIV. Welding (Hot Plate Welding, Ultrasonic Welding, Vibration Welding)
Welding can improve connection strength; Welding can simplify product design.
XV. Reasonably Consider Conflict Between Process and Product Performance
When designing injection molded products, conflict between product appearance, performance, and process must be comprehensively considered. Sometimes sacrificing some processability can achieve excellent appearance or performance; When injection molding defects cannot be avoided in structural design, try to place the defects in concealed parts of product.
XVI. Relationship between Screw Prong Hole Diameter and Self-Tapping Screw Diameter
Self-Tapping Screw Prong Hole Diameter: M2 1.7mm, M2.3 2.0mm, M2.6 2.2mm, M3 2.5mm.
XVII. BOSS Design Principles:
Prongs should not be used alone as much as possible. They should be connected to outer wall or used with reinforcing ribs to increase strength of prongs and facilitate smoother material flow; Height of prong should generally not exceed two and a half times diameter of prong. Excessively tall prongs can cause air entrapment during molding of plastic parts (excessive length can cause porosity, burning, incomplete filling, etc.); If support column height exceeds two and a half times column diameter, especially for columns far from outer wall, strength of column should be increased by using reinforcing ribs; BOSS shape is mainly circular; other shapes are difficult to process; BOSS position should not be too close to corners or outer walls; it should maintain a certain distance from outer wall of product; Removing part of wall thickness around BOSS (i.e., creating a crater) can prevent shrinkage and sagging; BOSS draft angle: usually 0.5° outwards and 0.5° or 1° inwards.