Topic: Molding Quality

Mold Design For Injection Molding

Injection molding, for the most part, is the method used to fabricate plastic parts for electroplating. If a new plastic item is to be plated, there are some very basic mold design provisions that must be incorporated into the part design to enhance the overall results. Mold and part design should focus on three major areas: Uniform and adequate wall thickness; provisions for racking the part; and providing generous contours.

Mold Performance

While we cannot elaborate here on the details of mold-making, we are stressing its importance in producing plastic parts for plating. Problems are often encountered because of existing or uncomplimentary mold-design factors.

What often happens in manufacturing is that the plastic part to be plated is not originally designed for plating. As a result, molds are built to satisfy design features and injection-mold the part economically. When someone decides after the fact that the part should be plated, problems may already be built in for the plater.

In other manufacturing cases, the design engineer who is responsible for building the mold even when he knows the plastic part will be plated does not always take plating factors into consideration .

Residual Mold Problems

The results of trying to plate a plastic part with molded-in problems are higher plating costs (which can show up in a variety of ways) and decreased plating quality. The following information is provided concerning mold-design features that should be considered when building or re-building a mold for any plastic part, to bring about an improvement in the plating process.

Ribs are used to strengthen and make the part lightweight at the same time. Many small, thinner ribs are preferred over larger or thicker ones, which may leave visible sink marks after plating.

Bosses are used for attachment purposes. Designing thinner walls will leave less evidence from sink marks seen before and after plating, similar to that which occurs with heavy rib thickness, as noted above.

Planes should have a curvature to help camouflage minor surface irregularities.

Recesses should be as generous as possible to reduce the effects of low current-density-plating-appearances.

Edges and comers should be rounded as much as possible to reduce undesirable plating build-up or the effects of plating at extreme high current densities.

Other mold design features that should be considered in regard to plating are drafts, threads, wall thicknesses, gating, parting lines, protuberances, part size and shape, and radiuses.

Molding Process Control

After the best possible mold design has been chosen, molding parameters are established. Process control is responsible for temperatures, pressure and cycle time for the molding operation. Once the proper parameters are established, the molding operation should go smoothly. Even when set-up seems to be perfect, however, there are times when things go awry and significant molding defects appear. The following are common molding defects and other problems:

Resin contamination, the result of erroneously mixing resins, affects overall plating quality. Unacceptable quality includes poor or no adhesion, partial plating coverage, cracked plastic substrates observed after plating, and poor thermal cycling values.

Humidity in the resin occurs when the resin is not thoroughly dried prior to and du ri ng the actual mold i ng operation . Hopper dryers are a popular addition to the molding machine, to keep the resin moisture-free just prior to molding.

Molding moisture results in poor plating adhesion. This problem occurs when the parts contain moisture blisters and splays.

Splays are a by-product of resin moisture. When parts with these mold defects are plated, the result is a variety of poor surface qualityóusually blotches and excessive porous or pitted areas randomly seen on any area of the part.

Careless handling of molded parts prior to plating can result in poor aesthetic quality.

Because parts coming directly out of the molding machine are very warm and easily deformed, some minor defects can occur, such as distortion, scratches, excessive sink marks and scuffs. These flaws can be corrected easily by the molder. Good communications between departments are essential . When the Plating Department advises the Molding Department, communication with diplomacy and respect lead to resolution of the problem.

Other molding defects include improper de-gating, excessive plating lines, objectionable knit lines, and excessive surface porosity. Although it may be upsetting and difficult to do, the order should be given to stop plating immediately, if a molding problem manifests itself at any time. Thereafter, plating tests are required and a complete evaluation is in order, to determine the next course of action.

About the Columnist

Nick Anis is a consultant who specializes in plating on plastics. With more than 30 years experience in all facets of plating, he has published many articles and is considered one of the pioneers of plating on plastics. Anis is a member of AESFs New York Branch and the Society of Plastics Engineers.