Transparent materials need clean approach

Editor’s note: Cyro Industries (Rockaway, NJ), an acrylic resin supplier, has compiled a troubleshooting guide for clear-part molding exclusively for IMM. The following guide includes recommendations for molding practices and techniques aimed at contaminant-free parts.

Producing clean transparent parts for medical applications is critical for medical device molders. Medical-grade parts need to be molded clean, without the introduction of dirt and outside particles that can affect clarity, sterilization, and overall quality. There are three main areas where material can potentially come in contact with dirt and dust: shipping, in-house material handling, and molding. Each area should be individually evaluated to ensure the cleanest molding process possible. Sometimes it’s the simplest solutions that escape our grasp. Here are some guidelines for cleaner molding.

Starting Off on the Right Foot
The first step is to start with clean material. Make sure your supplier is providing clean product that is packaged and shipped in satisfactory conditions. Adequate liners and durable corrugated gaylords must be used to package the material. Gaylords should arrive intact without dents, punctures, or tears in the corrugated exterior, and interior liners should be securely sealed.

If the packaging is compromised, the resin could be as well. Crushed portions of the corrugated box could mean the pellets inside have been crushed or chipped, even if the liner hasn’t been damaged. Broken pellets and chips can cause inconsistent melting and could adversely affect finished-product clarity. Fines and chips may melt too quickly and eventually burn, causing black spots on the part. On the other hand, if a chip does not receive sufficient heat, it can pass through into the mold and cause white spotting.

Packaging and material should always be quickly inspected upon arrival. Once material is delivered, but before it is introduced into the hopper, the corrugated gaylord should be wiped down with a moist cloth to remove dust and debris that has accumulated during shipping. After this is done, the gaylord should be opened carefully so as not to disturb dust in the area. Spending an extra 2 minutes with this technique can improve mold clarity and reduce dust intake.

Proper Material Handling
A common trouble spot for molders is the conveying system transporting resin from the gaylord to the hopper. This is a potential entryway for outside dust and debris.

Typically, a wand and vacuum system is used to pull resin into a hopper. The gaylord should be opened slowly, and the wand placed inside the liner. Be careful not to press the end of the wand into the liner. This could cause the liner to tear and mix into the material. The liner should be held tight around the wand, allowing only a small opening for air to enter. This will reduce the amount of airborne dust that the vacuum pulls in. Another option is the use of a gaylord cover.

When using a cover, the entire gaylord should be wiped down with a moist cloth to remove as much dust from the area as possible. The cover is lowered over the corrugated gaylord and sealed under a protective hood. Outside air for the vacuum is allowed access through an HVAC filter in the cover to prevent dust from entering the system. This method is the most costly but provides the cleanest method of material transport.

Hose and pipe material can also contribute to potential clean mold problems. Soft pipe material can sliver and chip as resin is pulled through at high speeds. Flexible PVC or aluminum piping, typically used in conveying systems, can chip easily and contaminate the process when a hard material like PMM is molded. Slivers, along with the molding resin, can be pulled into the hopper and produce cloudiness or particles in finished products. The best materials to use for piping are urethane hose for short runs or stainless steel for long runs; steel offers greater surface hardness, preventing chipping and unwanted material from entering the hopper.

Another common problem with conveying systems is mismatched piping. The inner diameter of vacuum piping should be consistent for the entire route the material travels. Retrofitting piping with inconsistent inner diameters will cause dead spots in the line. These dead spots are areas where pellets can strike against edges and break. Again, these smaller pieces can cause inconsistent melting and could affect end-product clarity.

Piping should take the most direct route to keep the length of travel to a minimum, avoiding sharp corners. Vacuum speed should also be monitored and kept to a low but effective pressure so that resin travels rapidly, but not so fast that it causes pellet breakage. One of the most effective methods of removing fine particulates generated during material handling is to incorporate fines removal equipment in the material handling system just prior to the hopper. This significantly reduces or eliminates contamination created in the material handling process.

Clean Processing Conditions
Once material is transported into the facility in a clean, safe manner, it is important that the resin remain pure throughout the molding process. The piping and hose should deliver resin safely into a sealed hopper to prevent airborne dust and debris from entering the compound. This is important because the force of the vacuum can suck and pull airborne dust into the hopper if exposed.

Molding equipment should be regularly inspected for nicks, burrs, or cracks. Nicks can develop on the screw, check valve, or the nozzle and form dead spots. These dead spots are areas where material can sit and burn, causing black carbon spots in the finished product. Equipment inspection is a time-consuming process. If black burn spots appear in the finished product, other more easily identified areas should be examined before disassembling equipment and incurring excessive downtime.

A fully tapered nozzle provides better resin output than a general purpose nozzle, which can again cause material to burn as it is slowly injected into the molds. Gate size and vent size should also be considered if char or burn occurs. Gates may be too small to adequately fill the mold or allow gases to escape, and this can also lead to burning. Extrusion components should also be well matched in the same manner to avoid dead spots.

Operating processes may require fine-tuning in order to reach optimal molding conditions. Sharp corners in the mold are potential dead spots for resin and should be radiused to improve flow. Excessive backpressure can also adversely affect molded part clarity by causing degradation. Typical operating pressures range from 20 to 80 psi, and excessive conditions fall in the 200 to 300 psi range, which should only be used if color mixing is involved. Molders should refer to the manufacturer’s recommended processing conditions for each material.

Regular Maintenance
Material and residue should be cleaned out of processing equipment before it is shut down. After material is run completely out of the system, equipment can be shut down or left running in standby mode at a low temperature.

A purging compound should be run through an injection molding press once a week as standard practice. Even efficient, clean molders may find they can improve mold clarity by instituting some of the simple techniques explained here.

Cyro Industries, Rockaway, NJ
(800) 631-5384; www.cyro.com