Product shapes for injection moulding

Buckled internal strut, due to shrinkage of a thin part of a moulding (Hasenauer, J. et al.. Top 10 Design Tips, Du Pont, UK, reprinted with permission). 

Once a mould is full, there is effectively one-dimensional heat flow in the direction perpendicular to the mould wall, while the plastic soHdifies. The moulding must solidify all the way through, before it can be ejected, so the cycle time is determined mainly by this through-thickness solidification time s- In Appendix A, Fig. A.4 is a dimensionless graph of mouldings temperature profiles versus time. If the plastic is solid at T — Tq = 0.4, the melt is fully solid when the Fourier number Fo = 0.5. Hence, the solidification time is given by 

2 Part thickness that decreases away from the gate 

Convex regions of the product surface (A in Fig. 13.8), cool more rapidly than do internal corners (B) due to the difference in heat flow geometry. Hence, the solid skin is thinnest at concave locations. This can be partly rectified by radiusing both internal and external corners, and keeping the part thickness uniform. The stress concentration factor at rounded corners is lower than at sharp corners when the product is bent (Section 9.3.1). Rounded corners make flow into the mould easier, and are unlikely to cause injury to the user of the product. 

Hand-held electric tools, such as electric drills or hot-air blowers, usually have clam shell mouldings. When the two casing halves close, tongues or 

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As moulds have a high capital cost, injection moulding is usually economic, only if more than 10000 products are made. It is preferable to reduce the cycle time than to invest in a second mould to achieve the required production rate. The mould only has a high productivity if the cooling time, which dominates the cycle time, is minimised. Consequently, the product wall thickness is minimised (Eq. 5.4 predicts the effect on the cooling time). Sections 3 and 4 of Chapter 13 consider further suitable product shapes for injection moulding. 

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