A reasonable and optimal molded part structure design can not only simplify plastic injection mold manufacture, reduce the cost of the mold, but also to simplify its molding process to improve the good quality rate of products.
Before the design of plastic parts, designer should learn about the function, environmental conditions and loading conditions （including dynamic and static load), understand relations between different parts and the effect of the assembly. Plastic parts function should be accurate and detailed possible, the more comprehensive the plastic parts design is, the better the design of the plastic parts can meet the requirements.
The preliminary design should mainly start from consideration of the shape, size, wall thickness, ribs and holes position. In addition, the plastic parts design should be considered how to help the injection molding processing, mold design and mold manufacturing. After comprehensive consideration of these issues, designer can make a preliminary design of the products, then according to this design, sketch the product after modification then make the final design.
Parting line and mold open direction
at the beginning of the design, the first thing is to determine the mold opening direction and parting line. This design should ensure to reduce the core pulling slides and eliminate the negative effect of the joint line as far as possible.
1, after mold open direction determination, the reinforcing ribs, snaps, convex structure need the same direction as the mold opening, to avoid joint lines of insert and prolongs the life of the mould.
2, after mold open direction determination, the appropriate parting line can be decided to avoid the undercuts on the mold opening direction, which design will improve the appearance and performance of plastic parts.
Draft—A draft taper in deep drawn parts, in addition to making it easier to remove a part from the mold, creates a wedging action as the mold is closed. When thermosetting materials are compression molded, converging tapers increase the density of the plastic in the upper sections. There are no precise formulas for taper, but the most liberal taper the design can allow should be provided.
Tapers of less than 1 degree can be used for deep parts; e.g., television cabinets 6 to 24 in. deep have been molded with tapers as small as 0.001 in. per in.
- the appropriate draft angle can avoid scratching products. The draft angle should be over 0.5 for smooth surface, draft angle is greater than 1 degrees for fine texture (sandblasting surface) surface, while draft angle on the rough surface is greater than 1.5 degrees.
- the appropriate draft angle can avoid the ejection problem, such as ejection white, deformation and cracking.
- the draft angle design of outer surface of deep cavity structure part is bigger than that of the inner surface, in order to ensure no deviation of core to achieve the uniform wall thickness and ensure products strength.
Wall thickness-Strength required both in service and during manufacture must be considered in designing wall thickness. In thermosetting materials， walls less than 0.062 in. thick are extremely difficult to obtain, and a minimum of 0.125 in. is preferred.
- all plastics parts wall have specific thickness range, general 0.5 ~ 4mm. the more than 4mm wall thickness will cause the cooling time too long, sink marks and other issues.so sometimes it should consider to change the wall thickness of the products to solve those problems.
- the uneven wall thickness will cause the surface shrinkage.
- the uneven wall thickness will cause the air bubble and weld lines.
1, the reasonable application of the ribs can increase product rigidity and reduce deformation.
2, the thickness of the ribs must be less than or equal to (0.5 ~ 0.7) T wall thickness, otherwise it will cause surface shrinkage.
3. The single side draft of the rib should be more than 1 degree to avoid the ejection damage.
1, the too small fillet may cause the product stress concentration, and results in product cracking.
2, the too small fillet may cause mold cavity stress concentration, and results in cavity cracking.
3, the reasonable fillet can improve the mold processing technology, such as the cavity can be directly machined with R knife milling to avoid the low efficiency of EDM.
4. Different fillet may cause the shift of parting line.
1, the shape of the hole should be as simple as possible, generally it is round.
2, the hole axial and mold open direction should be same to avoid core pulling.
3. When the hole diameter to height ratio is greater than 2, the draft angle should be designed. The diameter of the hole should be in accordance with the small end.
4, the blind hole height to diameter ratio is generally not more than 4 to avoid pins bending.
5, the distance between the hole and the product edge is generally larger than the diameter of hole.
Mold core pulling and slide mechanism
1, when the plastic parts in the parting direction can not be demolded, the core pulling slider mechanism need be designed. core slider mechanism is capable of forming complex product with undercuts, but it is easy to cause defects joint line and shrinkage on products surface , increase the cost of mold and shorten the life of the plastic mold.
2. Design injection molding products with no special requirements should avoid core pulling structure as far as possible. The hole axial and ribs directions could be designed to the mold opening direction, and formed by shut-off of core and cavity.
Common plastics such as polypropylene, ethylene propylene copolymer, some kinds of ABS, etc., can be directly made of hinge. Common hinge cross section is shown in following pic. The hinge part thickness should be thinned, generally 0.25 ~ 0.4 mm, and melt flow must be through the hinge part, so that linear molecules can bend along their main chain direction. If the flow direction is not right, hinge parts would be easily broken.
The cross section of the hinge shouldn’t be too long, otherwise the bending line is not in one place, and the close is not so good. The wall thickness of the thinned transition shall be the arc. The thickness of the hinge part should be uniform when the mold is made, and the mold temperature must also be consistent, otherwise it will decrease its bending quality. For the injection molding of integrated hinge, the gate can only be designed on one side of the hinge.
1. In the injection molding, products insert can improve the local strength, hardness, size accuracy and small hole screw (shaft) would meet variety of special needs. But at the same time it will increase the cost of the product.
2, The embedded inserts generally are copper, and they can also be other metal or plastic parts.
3. The embedded inserts in the plastic items shall be designed with anti-rotation and anti-pulling features, such as: knurl, hole, bending, flattening, shaft etc.
4, plastic surrounding the plastic should be appropriately thickened to prevent stress cracking.
5, when design insert, it should take full consideration of its positioning in the mold (hole, pin, magnetic).
Product logo generally set in the product surface is flat with the convex form. Its direction and mold opening direction should be same to avoid scratch.
Improving the rigidity of injection product structure could reduce deformation, like to avoid flat structure, reasonable flanging design, concave and convex structure and reasonable ribs design.
Welding (hot plate welding, ultrasonic welding, vibration welding)
1, proper welding can improve the joint strength.
2, proper welding can simplify product design.
Injection process and product performance
1 the contradiction between product appearance, performance and process must be considered in the design of injection product. Sometimes the sacrifice of the process of part can exchange very good appearance or performance.
2 the structure design is unable to avoid the injection defects, as far as possible, the defects occur in the product concealed parts.
Self-tapping screw and screw boss diameter
Self-tapping screw screw boss diameter
1, the screw boss should not be used separately , and it should connect to the outer wall or with the ribs. The design purpose is to strengthen the screw boss and make plastic material flow more smoothly.
2, the height of the screw boss is generally not more than two times of the its diameter,because of the high screw boss can lead to trapped gas (too long screw boss will cause the air hole, burning, short fill and etc.)
3.If screw boss height is more than two and a half times of its diameter, or it is especially away from the outer wall, the added ribs could strengthen the screw boss. Screw boss shape is round mainly, for other shape is difficult to manufacture and flow ability is not good.
4. The screw boss shouldn’t be too close to the corner or outside wall and it need keep a distance away from them.
5, the plastic on the screw boss bottom should be removed to prevent shrinkage sinks
6. The draft angle of screw boss: usually outside wall:0.5 degrees, 0.5 or 1 degrees inside wall.
Allowance must be made for shrinkage of the molding compound in design of the part. In addition to affecting dimensional accuracy of the part, shrinkage can introduce internal stress which may make the part unserviceable. Shrinkage values for the various plastics are given in the data sheets in Section 5 of this issue.
A high degree of accuracy in molded parts is expensive to achieve. On small tolerances as close as -0.002 in. are possible.size is increased to & few inches, over-all tolerances increase to approximately ±0.005 in. On large pieces, tolerances of about 土0.001 to 0.002 in. per in. are obtainable. Tolerances closer than actually mandatory should not be specified; as specified accuracy increases, cost increases disproportionately.
Though it is difficult to generalize about design factors for injection, compression and transfer moldings, the following design rules. should be considered:
- Use sufficient draft on long thin shapes to permit their withdrawal from the mold.
- Minimize coring. When cores are used they should be easy to withdraw.
- Avoid internal and external undercuts; they make withdrawal of parts difficult and require considerably more expensive molds.
- Provide ample fillets on inside comers, and avoid sharp external edges and comers except at the parting line of the die.
- Avoid large flat areas. Dappling or otherwise breaking up the surface is recommended.
- Keep tolerances as liberal as possible. Excessively close tolerances add to cost because of increased die costs and high rejection rate.
- Avoid abrupt changes in wall thickness.
- Locate parting lines so that flash can be removed easily without marring surrounding areas.
- Locate holes for easy coring.
- Use ribs to achieve desired strength and stiffness. Ribs permit materials savings by reducing section thickness.
- Use inserts for threaded holes where high stresses are anticipated or where considerable wear is to be encountered. Round inserts are preferred.