June 2004 Design Tip

The Hole Picture

You may have heard the gradeschool riddle, "If you dig a four foot hole and fill in two feet of it, how much of a hole do you have left? Answer: All of it, because you can't have half a hole." Holes are just one of those things that are defined by the absence of something else.

When we want to incorporate holes into injection moulded part designs, it can sometimes be complicated just getting them to happen in the right place with the right shape. One approach often used in mould making is to use "core pins", which essentially refers to the use of a cylindrical piece of metal separately inserted into one side of the mould to provide the inner surface of the desired hole?. especially if the hole is deep. In conventional injection moulding these pins are typically made from a hardened steel.

However, in rapid injection moulding these features are milled directly into the mould geometry from a block of aluminium. And because aluminium has less mechanical strength than steel, it is desireable to take this into account when designing you part. Some general guidelines are provided in the following Table and Figure.

Through-hole Guidelines for Rapid Injection Moulding:
The diameter of the hole should be no smaller than .060" (1.5mm).
The length of the hole should be less than eight times its diameter.
The more draft on the hole - the better (even 1/2 degree helps).
A fillet at the entrance to the hole greatly increases the strength of the core.
A deep hole can sometimes be split and drafted to each side.

The following figure illustrates an instance of a through-hole design implemented using Protomold's rapid injection moulding process.

Example of a part with through-hole features created with the
standard rapid injection moulding process.

You can visit the Protomold Design Guide for other helpful Rapid Injection Moulding design information.