Tooling
Why injection mold design matters
Mold design is the last inexpensive moment in a molding program. Almost every cost, defect, and delay that appears later was decided here.
Design fixes the unit cost for the life of the tool
Cycle time is the number you pay on every part, and it is set almost entirely by the tool. Cooling layout determines how quickly heat leaves the part. Wall thickness in the part design determines how much heat there is to remove. Cavitation determines how many parts come out per cycle. None of these can be changed meaningfully once steel is cut, so a tool designed for a two second shorter cycle keeps paying that back across hundreds of thousands of parts.
It decides where the defects will be
Most molding defects are designed in rather than caused on the floor.
| Defect | Decided by |
|---|---|
| Sink marks | Rib and boss thickness relative to the nominal wall |
| Weld lines | Gate position, which sets where flow fronts meet |
| Warp | Cooling layout and wall thickness uniformity |
| Short shots | Gate size, venting, and wall thickness at the extremity |
| Flash | Parting line placement and plate rigidity under pressure |
| Drag marks | Insufficient draft and tool finish direction |
It decides how expensive a change will be
Steel can be removed cheaply and added back only with difficulty. That asymmetry is why good tool design cuts critical features steel safe, deliberately leaving extra metal so a dimension can be corrected downward after first samples. A tool designed without that allowance may need welding or a replacement insert to fix a dimension that a steel-safe tool would have corrected in an afternoon.
The parting line is the first decision in tool design, because everything else follows from it. Split the tool on the widest silhouette and the two halves draw straight apart. Move it and material that used to clear the opening becomes an undercut, which has to be freed by a side action. That is a permanent addition to tool cost, cycle time and maintenance.
It decides whether the part can be inspected
Parting line placement determines where a witness line falls and whether a critical dimension crosses it. Dimensions that cross the parting line inherit the clamp variation and are always looser than dimensions formed within one half. If a tight tolerance has to be held, the tool has to be designed so that feature sits in a single half.
What a good design review covers
- Where the gate sits, and what the resulting fill pattern does to weld lines and packing
- Where the parting line falls, and whether any critical dimension crosses it
- Where ejector pins land, and whether their marks are acceptable there
- Whether the cooling circuit reaches the sections that dictate cycle time
- Which features need slides or lifters, and whether the part could be redesigned to avoid them
- Which dimensions are cut steel safe, and by how much
The economics, stated plainly
While the part is a model, a change costs minutes. After the tool layout is approved, it costs days. After steel is cut, it costs money and lead time, and sometimes a new insert. The mold design review is the cheapest engineering hour in the entire program, and it is the one most often skipped.
Let’s get started on your part
Send your CAD files and target volumes. We come back with a price, a lead time, and any design notes that would reduce either.