Surface Finish of Injection Molding
Several factors affect cost, including the finish of mold parts, the surface of the mold, and the joints between mold parts. A higher mold price will result from a finer machining finish and the necessity for hand finishing. In spite of its apparent obviousness, too often it is overlooked.
Tolerance and surface finish are intimately related, as are surface finish and cost.
Finish of Molding During the molding process (the area in contact with the product), the molding surfaces are finished in the following manner:
- Assuring that the product fulfills its intended function or appearance
- It should not be difficult to eject the product from the mold, but:
If necessary, a rough surface can be applied to certain areas on the mold to keep the product attached.
A high polish, however, can also make it difficult for the product to be ejected easily, depending on its design.
To specify a proper finish in such cases, it is the responsibility of the mold designer.
During a plastic molding process, the surface finish of a very thin walled product interferes with the flow of plastic. Filling and cycle times are shortened when a better finish is achieved. It is known that flash chrome plating (particularly with PS) can improve the productivity of a mold by approximately 15% in some situations.
In order to finish (polish, etc.) mold parts, a substantial amount of handwork is required, and polishing should be restricted to areas that need it. Several automatic methods and hand-operated machines are now available for finishing mold surfaces. In many industries, however, hand finishing still plays an important role due to the lack of access to mechanical equipment.
Surface finishing is the process of eliminating tool marks found on a piece of work. It is often acceptable to leave a rough, “as machined” finish following chip removal operations (turning, milling, etc. ), for example on the inside surface of a technical product (enclosures, boxes, television cabinets, etc.), but this may not always be satisfactory for the ejection of the product, because the plastic will not easily (or not at all) slide over too rough a surface.
Additionally, it should be considered to consider which direction the rough machining grooves lie: if they run parallel to the ejection, they are acceptable, but if they are across it, they are usually not.
The draft angle is also important for wall construction (or for rib construction). The surface finish must be much better with little draft (a small draft angle), while a large draft angle (approx. 5° or more) permits rougher finishes, such as “as machined.”
Draft angles, especially for ribs, are constantly required a good finish, although they are not polishable. Interior structures should be designed for less mass. In most cases, an ejection motion finishes with a good finish (“draw stoning”).
A finish for such ribs would become less problematic if ejectors were placed under them. When making the mold, it is always important to consider what can happen if a plastic snaps inside a rib: speeding up the mold-making process, but causing severe production delays when service personnel frequently have to remove plastic fragments and parts from it.
The work surface left after grinding and electric discharge machining (EDM) may not require further finishing except for polishing if necessary. You may not need to polish an EDM finish if it is rough to very fine. High current and fast cutting speeds result in rough finishes, which therefore require more time.
Moreover, new methods of finishing hardened surfaces are often as good as a ground finish and do not require additional polishing.