Surface Finish of Injection Molding

Surface Finish of Injection Molding

Molded components are prevalent in nearly every industry, from highly aesthetic housings to internal components which do not require fine polishing. Individuals do not pay much attention to how the surface of these components is finished. But engineers and product designers must consider it when designing components.

Surface finishes are defined based on a few key factors such as development and production stages, materials used, and the end-use application of the parts.

The injection molding process offers the ability to apply polished and textured parts directly to the mold itself. Since secondary finishing isn’t possible, material selection is crucial in some cases. Surface finishes and materials have a very small correlation; however, some materials, like glass or mineral, can produce parts that are not always indicative of the finish you are looking for.

When it comes to materials containing glass and minerals, they provide properties that other materials do not, but they should be used with caution if they are intended for highly cosmetic uses. Often, minerals and glass cause streaking on the surface that makes grading virtually impossible. You may wish to speak with applications engineers to verify this or request a material sample plate if you intend to use a particular filled resin.

By interacting with our quote, you will learn which areas require more attention – textured surfaces may need more draft requirements than polished surfaces, for example.

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.

injection molding

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. ” You need a draft for two reasons: to prevent scratching and drag marks on your parts, and to avoid damaging them when applying a bead blast finish. The minimum draft required for PM-T1 is 3 degrees, and the minimum for PM-T2 is 5 degrees. The issue of accessibility may also prevent deep ribs and thin ribs from being finished within a mold.

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.

FinishSPI* standardFinishing MethodTypical surface roughness Ra (μm)
Super High Glossy finishA-1Grade #3, 6000 Grit Diamond Buff0.012 to 0.025
High Glossy finishA-2Grade #6, 3000 Grit Diamond Buff0.025 to 0.05
Normal Glossy finishA-3Grade #15, 1200 Grit Diamond Buff0.05 to 0.10
Fine Semi-glossy finishB-1600 Grit Paper0.05 to 0.10
Medium Semi-glossy finishB-2400 Grit Paper0.10 to 0.15
Normal Semi-glossy finishB-3320 Grit Paper0.28 to 0.32
Fine Matte finishC-1600 Grit Stone0.35 to 0.40
Medium Matte finishC-2400 Grit Stone0.45 to 0.55
Normal Matte finishC-3320 Grit Stone0.63 to 0.70
Satin Textured finishD-1Dry Blast Glass Bead #110.80 to 1.00
Dull Textured finishD-2Dry Blast #240 Oxide1.00 to 2.80
Rough Textured finishD-3Dry Blast #24 Oxide3.20 to 18.0
As machined-Finished to the machinist's discretion3.20 (with visible machining marks)

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