Electroplating Plastic Parts

Electroplating Plastic Part Design and Processing Basics

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Electroplating is the application of a metallic layer to the surface of an object using an electric current. Electroplating can improve the corrosion resistance, hardness, wear resistance, electrical conductivity, and heat resistance of a product while also improving its appearance.

There are many different types of plastics, but not all of them are suitable for electroplating. Because some plastic materials have poor adhesion to the metal layer, converting them into plated parts is difficult. Some plastic materials have physical properties (such as expansion coefficient) that differ significantly from the metal electroplating layer. When these materials are used to make electroplating parts in high-temperature difference environments, it isn’t easy to ensure product performance. ABS and PP are the most commonly used materials for plastic electroplating parts.

On the other hand, some companies use PSF, PC, and PTFE for plastic electroplating parts, but their electroplating processes are more complex.

Choose the Right Electroplating Plastic Material

#1 Choosing a Higher Melting Point Material

The melting point of the plastic material has a significant impact on metal adhesion to the surface of the plastic parts.

Because hotter melt is less viscous, it results in less orientation. It lessens the stretching and shearing forces that can lead to orientation.

Hotter melts also cool more slowly, resulting in more relaxation after total fill. While a colder mold temperature allows polymer orientation to relax more quickly, rapid cooling of the melt introduces cooling stresses into the part. As a result, the melting point of the plastic material must be high enough so that the mold can be filled more efficiently with a lower injection speed.

#2 Choosing a Good Adhesion Material for Electroplating

ABS plastic is the best choice for electroplating. When electroplating ABS plastic, it is simple to achieve a mirror-like gloss and a good bonding layer. With proper electroplating, ABS parts used outdoors can last for several years without deterioration. Although such materials are relatively inexpensive, poor electroplating adhesion is frequently a production issue. In addition to improving adhesion through structural improvements, selecting a material with better adhesion to the electroplating process is required. If the ABS-727/757 does not meet the adhesion requirements, it can be replaced with the ABS-777D to improve the coating’s adhesion. Because this material is more resistant to heat, it can make plated parts more adaptable to external temperature changes.

#3 Choosing a Material with Higher Hardness and Strength

If the plated parts have high stiffness and strength requirements, materials such as ABS CV88B can significantly improve the stability.

#4 Choosing a Material with Heat Resistance

There are numerous electrical components, and the shells are plastic-plated parts. Heat resistance is usually required for these products. If the commonly used ABS is unable to meet the requirements, we can select high heat level ABS, such as ABS-D690, ABS-777B, etc.

2. Design Guide for Plastic Electroplating Parts

#1 Reduce the number of sharp edges and corners

Sharp edges are highly undesirable due to the accumulation of Electroplating thicknesses on these edges, which causes cosmetic, fit, and function issues in many cases.

The current value at the sharp edges and corners is higher in the electroplating process than at the rounded structure. It is simple to form electroplating accumulation, which results in thick local electroplating thickness. Furthermore, reducing sharp edges and corners can help avoid crash deformation and improve structural stability.

In general, all corners should be rounded whenever possible, as should all inside and outside angles.

#2 Address any blind holes

Blind holesshould be avoided whenever possible.

They are not only areas with extremely low current density, but they can also trap Electroplating solution. Furthermore, the depth of a blind hole should not be greater than three times the diameter of the hole. This ratio should be reduced to 2:1 for diameters less than 5 mm. Blind holes should have a bottom thickness greater than 20% of the hole diameter to eliminate surface defects on the opposite surface in injection molding. A better design would ensure that the wall thickness remains consistent and that there are no sharp corners where stress concentrations occur.

#3 Reduce deep concave and protruding parts and transition from thick to thin as smooth as possible.

By reducing deep concave and protruding parts, internal stresses caused by sudden changes in the structure’s shape can be avoided during electroplating. The structural stability of the electroplating parts may be lower if there is residual stress.

Reduced deep concave and protruding parts can prevent uneven current during electroplating due to the structure’s abrupt change in shape. The presence of deep concave and protruding parts will increase the thickness of the electroplating layer and interfere with current stability.

#4 Take care the wall thickness of plastic electroplating parts

Plastic electroplating parts should not be too thick once they meet the structural and functional requirements of the product. The wall thickness should generally be between 0.5 and 4.0 mm, with 2.0 to 3.0 mm being ideal. Thinner wall thicknesses result in shorter molding cycles and lower part weight, which saves money.

Furthermore, the thickness of the plastic electroplating parts should be as uniform as possible to avoid shrinkage due to structural issues during injection molding.

#5 Avoid large areas on flat surfaces

If the surface of plastic electroplating parts has a large flat surface, it will result in uneven layer thickness due to uneven current density distribution. To meet the electroplating process requirements, the area of the flat surface in the design should be less than 10 cm2.

If a large flat and straight surface area cannot be avoided when designing plastic plated parts, the middle part of this surface should be raised 0.1 to 2 mm to ensure the flat and straight surface design.

#6 Add Draft Angle

A draft angle must be added to all vertical walls to facilitate the ejection of the part from the mold. Because of the high friction with the mold during ejection, walls without a draft angle will have drag marks on their surface. A draft angle of at least 2° is recommended. On taller features, larger draft angles (up to 5°) should be used.

In general, the draft angle is determined by several factors, including:

Shrinkage of the material

Certain materials shrink faster than others. In general, the larger the draft angle required to prevent ejection issues, the greater the shrinkage.

Wall or feature height and shape

A shallow single straight rib lacks draft and causes fewer issues than a high cylindrical wall.

The texture of the surface

A textured surface requires a larger draft angle than a polished surface.

Aesthetic considerations

Scuff marks may not be as noticeable on a technical part “hidden” in an assembly, but they may be unacceptable on the assembly’s cover.

#7 Take Care of the Ribs

Ribs are structural elements that contribute to the overall stability of the part. They are perpendicularly extending thin wall protrusions from a wall or plane. Adding ribs instead of thicker walls will provide more structural support.

The thickness of the ribs

Ribs should be half to two-thirds the nominal wall thickness. Thinner has less structure and may be difficult to fill. Thicker ribs are appealing, but they can cause sinks on the cosmetic surface.

Height of the ribs

Rib height should not be greater than three times the thickness of the primary wall. This height cap is advised because too tall ribs may break during use or ejection from the mold. If tall ribs are damaged during ejection, it is most likely due to draft angles tapering them to slivers at their tips. It would help if you tried to keep your ribs as short as possible while remaining functional.

Another issue caused by this combination of tall ribs and draft is that the plastic may not reach the tip of the rib. If the plastic cannot flow into the very end of the rib, this can result in voids. Deep ribs can also be costly for the tool. To avoid these issues, substitute several short ribs for one tall rib. This can help distribute stress more evenly and keep your rib height proportionate while still providing support.

Width of the ribs

Thick ribs can cause significant issues on your part. When a rib becomes too thick, the point where it connects to the primary wall or boss becomes thicker as well. Sink marks, warp, and voids can easily result from this excess material. Sinks, in particular, are visible indicators of poor design, affecting the appearance of your finished part. Sinks and voids, if severe enough, can reduce a part’s strength by acting as concentrated stress centers. To avoid these issues, keep your rib width between 0.5 and 0.75 times the thickness of your primary wall.

When deciding on a rib width, keep in mind that the shrink rate of different materials varies. Plastics with a high shrink rate should be made thinner, while plastics with a low shrink rate can be made thicker. Because low shrink materials do not exert as much pull on the surface or adjacent thin walls as high shrink materials do, they are less likely to sink or warp. Thinner ribs may be considered depending on the intended use of your part or desired cosmetics, but thin ribs are more likely to break.

Radii of the ribs

To avoid the concentrated stress in specific area of the part, the base of a rib should always be rounded with a radius. Sharp corners also reduce the flow rate of the plastic down into the rib, which means it may not be able to fill before the material begins to cool, resulting in voids. For these reasons, the radius of a rib’s base should be 0.25-0.5 times the width of the primary wall, with a minimum radius of 0.010 inches.

Space between Ribs

Even though multiple ribs provide more support, they should not be so numerous that they crowd together. Too close rib spacing results in very low current density areas vulnerable to low electroplating thicknesses and thus corrosion failure in the field.

Ribs should be spaced no closer than their height, which should be no less than three times the primary wall thickness. If the ribs are too close together, they will not be able to cool quickly. This means that the part will have to stay in the mold for longer before being ejected, increasing cycle time and part cost. Another reason spacing is essential is that ribs that are too close together will cause your mold to have thin sections. These are prime areas for mold damage to occur.

Ribs Draft

Like the majority of injection-molded parts’ surfaces, Ribs must be drafted. The draft is vital for removing ribs without damaging or breaking them. Each side of the rib should have a draft angle of 0.25-2 degrees, with 0.5-1 degrees the most commonly used.

3. Take Care of the Electroplating Process

#1 Electroplating Pretreatment

Pretreatment is a required step in electroless nickel electroplating. The pretreatment process aims to remove unwanted contaminants that may impede the bonding process and result in low-quality or unusable results. This can be a time-consuming process with several steps depending on the substrate.

The pretreatment process necessitates cleaning the substrate with a series of base or acid chemicals. The substrate must be rinsed with water to remove residual chemical adhesion between chemical treatments. Degreasing can remove contaminant oils, and further acid cleaning can remove scaling.

For difficult substrates, additional pretreatment considerations may be required. Mechanical finishing techniques are frequently used to improve the surface condition of the plated product. Deburring, bead blasting, and tumble finishing are standard mechanical treatment processes before chemical cleaning.

#2 Experience is important

Choosing the suitable ABS or PC/ABS electroplating grade is as important as choosing the right molding conditions for a specific part. Each plastic part has a unique set of molding parameters that will result in the best overall performance due to the unique design of each part. There are numerous ways to process a specific part, but compromises are always made. In close collaboration with the molding process, it is up to the electroplater to validate the scalability and performance of each new product. Professional experience will provide a starting point for processing, but only trial and error will reveal the best overall injection molding processing for a specific product.

Conclusion

By learning about the electroplating process and how it is affected by the injection molding process and specific design features, We can begin to understand the causes of failures in electroplated plastic parts, 

We can work with design engineers to optimize the design for manufacturing feasibility by better understanding the injection molding process and how specific design features affect it.

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