Two- and four-side slide molds are standard for housings with side openings, and every molder has a story about a parting line that looked perfect at trial and became visible two weeks into the run. The reason is thermal, not cosmetic.
The mechanism
A slide sits in the moving half and is usually cooled differently from the cavity block. The two run at different temperatures, so they grow differently. At the start of a batch the fit clearance is the one set in the tool; later, as the temperatures stabilise or drift, the clearance changes. Too little clearance and the slide marks the part; too much and the material flashes into the gap, which is then a visible parting line and often a hand-feel defect as well.
Housings make this harder because they are molded in PC. PC does not flow easily, so the injection pressure is high, and high pressure pushes the slide back against whatever clearance exists.
Calculating instead of guessing
- Set the molding window for the part: mold temperature on the cavity side, on the core side and on the slides.
- Calculate the thermal expansion of the slide and of the cavity block from their actual dimensions and the temperature difference.
- Derive the fit clearance from those numbers, allowing for the pressure load on the slide face.
- Refine the tool in production, using measured parts rather than assumptions.
On a 108 mm tall four-side slide housing this method keeps the parting line below hand-feel across the batch. It is also why we ask for the side-opening geometry early: the clearance calculation depends on the loaded area of the slide, which is a product dimension, not a tool dimension.
The question to ask
When a housing has side openings and the cosmetic requirement says “no hand-feel parting line”, ask how the slide clearance is calculated and what happens to it when the mold temperature moves by five degrees. If the answer is “we polish it”, the line will come back.