Economical Way to Clean Plastic & Rubber Molds

Micro-Abrasive Blasting
An Effective and Economical Way to Clean Plastic and Rubber Molds

     Injection molds used to produce molded parts develop a buildup of residue from
lubricating compounds, burnt polymers, and cooling liquids. To maintain cosmetic and dimensional quality, the build up of residue needs to be cleaned from the cavity periodically. The job can be accomplished by using chemicals to soak the molds clean or by hand or mechanical scrubbing. Soaking can take hours and scrubbing by hand can be labor intensive for cleaning complex molds and can cause damage to the mold if unskilled employees are used.

 

     Micro-Abrasive Blasters are ideal for cleaning molds because they are effective at removing surface build up without damaging or changing the mold base structure. In addition, these blasters allow the operator to clean areas of very intricate molds better and more quickly than by hand or chemicals. The blasting operation can be performed in an enclosed lighted work chamber where the blasting pen and media tanks are fully integrated or with a mobile blasting system that can be taken to the cleaning site. Micro-Abrasive Blasters allow a multitude of different media to be used as well as different nozzle configurations and air pressure settings to achieve the desired results. Industrial quality Mobile blasters can cost around $500.00 compared to an entry level, fully integrated blasting system costing $1000.00. The choice of which media to use depends on the make up of the residue on the mold, removal time, and at the same time, protect the mold from dimensional changes as a result of the cleaning.

 

Most commonly used abrasive media:
Plastic media is typically used to clean a mold when a polymer layer needs to be removed. It is aggressive enough to remove these stubborn deposits without damaging the mold surface. Sodium Bicarbonate is very soft but has sharp features that allow it to cut through the deposits without compromising the mold structure.


 

Removal of Conformal Coating with Small Sandblasters

Removal of Conformal Coating with Small Sandblasters

The development of conformal coating technology was driven to a large degree by the military and aerospace industries. While conformal coatings are mostly used on populated printed wiring boards (PWBs), they are also used to protect components such as transistors, diodes, rectifiers, resistors, integrated circuits (ICs) and hybrid circuits including multi-chip modules (MCMs) and chip on board (COB).

Recent environmental regulations and concerns have had a significant impact on both coating materials and application methods, particularly with regard to control of volatile organic compounds and chlorofluorocarbon compounds. Both VOCs and CFCs have been extensively used as solvent carriers. Manufacturers and suppliers of conformal coating materials have responded by developing non-solvent based coatings and environmentally acceptable methods of application, curing and removal.

It is important to consider how the choice of a conformal coating material affects the rework and repair issues. The need for rework or repair of a conformal coating can occur any time after completion of an assembly due to a variety of process or product requirements and component replacement issues.

A number of methods are available for rework of conformal coatings. These include thermal, chemical, mechanical, plasma and laser-based systems, and small sandblasters or "micro abrasive blasters", which will be the focus of this article.

Micro-abrasive blasters used for conformal coating removal are small sandblasting systems that are commonly used for metal deburring and etching and surface preparation. The cutting media is introduced into a compressed air stream and is ejected through a hand piece utilizing tips as small as .026". This is directed at a component or surface area on PCB where the conformal coating has to be removed. This system can remove conformal coating from a single test node, an axial leaded component, a through-hole IC, a SMT component or an entire PCB without any modification to the system for a variety of coating materials. This method provides the most practical and environmentally friendly means for removing conformal coating from PWBs.

Although these small Micro Abrasive Blasters provide the most practical and environmentally friendly means for removal, they also pose a problem. Micro Abrasive Blasters can generate static electricity as the high velocity air and particles impinge on the PWB surface. The ESD voltage generated at the point of contact can cause damage to components and electrical circuits on a PWB.

Type of Device

Voltage Range

VMOS

30-1.800

MOSFET

100-300

GaAsFET

100-300

EPROM

100

JFET

140-7000

SAW

150-500

OP-AMP

190-2500

CMOS

250-3000

SCHOTTKY DIODES

300-2500

RILM RESISTORS

300-2500

BIPOLAR TRANSISTORS

380-7000

ECL

500-1500

SCR

580-1000

SCHOTTKY TTL

1000-2500

Equipment manufacturers have used several different approaches to solving the ESD problem. These are:

 

  • The installation of AC or DC pulsed ionizer bars in the chamber results in a rapid decay of ESD voltages in the work cell and tubing.
  • The installation of a point ionizer at the end of the nozzle to dissipate any static charge built-up in the media stream at the point of contact.
  • The use of an in-line, auto balanced ionizer where the air source is split, one side flowing to the media and the other side flowing to the in-line ionizer. This ionized air is then injected into the media stream just before it leaves the nozzle, eliminating the static charge build up in the media chamber. The ionized air is also pumped into the work chamber. With this type of system, ESD levels are reportedly in + 10 volts range.

Types of Cutting Media

 

  • Sodium bicarbonate is a popular abrasive but it generates high ESD levels and it must be thoroughly cleaned off the PWB before reapplication of a coating.
  • Aluminum oxide is a very aggressive abrasive which can damage PWB substrates. Typical ESD level range from 500-700 volts.
  • Biological media such as wheat starch and walnut shells are not as aggressive as aluminum oxide and they usually leave a residue which must be cleaned prior to recoating and generate ESD.

There are several types of plastic cutting media available which have the lowest ESD levels and are recyclable.

Small sandblasters with the ESD feature can range in price from $2000 up to $15,000.