MatrixLogo horizontal white

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Injection Molding and Moldmaking
with Surgical Precision

Injection Molding and Moldmaking
with Surgical Precision

Matrix has the ability to perform melt flow testing in our inspection lab. We use this to confirm that an incoming resin's melt properties are within the range specified for that grade.

Melt flow index (MFI) is a measure of how easily the melt of a thermoplastic polymer flows. More specifically, it is the mass of polymer (in grams) that flows in 60 seconds through a tube of specified diameter and length by application of prescribed pressures at prescribed temperatures. The specific method for this testing is described in ASTM standard D1238 and ISO standard 1133. Melt flow rate is inversely proportional to viscosity of the melt under the conditions of the test. Ratios between two melt flow rate values for one material at different pressures are often used as a measure of the broadness of the polymer's molecular weight distribution. Melt flow rate is very commonly used for polyolefins, with polyethylene being measured at 190°C and polypropylene at 230°C.

Something to consider in selecting the proper material grade is to choose one with a melt flow index high enough to easily form the part, yet low enough to provide sufficient mechanical strength for the part's intended use.

Plastics have long been associated with environmental unfriendliness and wastefulness of crude oil and petroleum byproducts. The advent of bioplastics (biodegradable and biocompostable plastics) which are derived from renewable sources such as corn starch or vegetable oil is helping to improve the image of plastics among those concerned with the environment, carbon footprints, sustainability, and being “green.” Bioplastics are slowly but steadily being improved, and in some cases their abilities to process and end-use properties can mimic or even surpass those of traditional petroleum based materials.

Bioplastics, aside from being derived from renewable resources, have the advantage of not releasing harmful toxins during their production, processing or degradation. Many conventional plastics can release known or suspected carcinogens such as formaldehyde or benzene during production, processing or destruction.

Growing the sources for bioplastics also reduces carbon dioxide in our atmosphere. Since the production of conventional plastics produces so much CO2 the use of bioplastics in place of a conventional plastic has a cumulative effect, with the substitution of just one ton of bio for conventional plastic having the net effect of reducing multiple tons of CO2 in the atmosphere. This not only takes into account the production methods for each type of plastic, but also the photosynthesis process in growing biomass or raw material for bioplastics. Bioplastics show great promise in reducing both our industry’s carbon footprint and impact on rising global warming.

What can plastics processors do until bioplastics are perfected in properties and reduced enough in costs to truly compete on a large scale with conventional thermoplastics? This is where the 3 R’s apply in injection molding. The 3 R’s in molding don’t stand for “reading, riting and ‘rithmetic,” but rather: reduce, reuse, and recycle. At Matrix Tooling / Matrix Plastic Products, we have been molding with bioresins, including bioabsorbables for a number of years, but as responsible members of the environmental community, we also have been practicing the 3 R’s.

Reduce: Scrap (and resin usage) is reduced through cold runner and sprue size reduction where possible without affecting moldability. In many cases we have reduced sprues and runners to the prescribed percentage of regrind allowed in the product specification. Hot runners and hot sprue bushings also have been used wherever possible. We have also thinned out wall stocks on parts where the product integrity wouldn’t suffer.

Reuse: We reuse what regrind we can and have come up with applications to use up to 100% in-house regrind. We utilize returnable/reusable packaging where possible and where allowed by the customer. We also have a closed circuit water system to reduce consumption and also filter, monitor and analyze hydraulic oil to avoid indiscriminate unneeded oil changes.

Recycle: Where we can’t reuse in-house regrind, we try to find it a good home. We sell the regrind where possible or even give it away for free if it can be used but there isn’t a paying market. Packaging is recycled also. We even collect our soda pop cans!

Matrix is serious about being environmentally responsible, using bioresins, and abiding by the 3 R’s. It not only makes environmental sense, but favorably affects the bottom line.

Written By:

Brent Borgerson

Senior Process Engineer (Older Molder)

My favorite part about working on R&D projects is that they tend to challenge you to think outside the box, try new things, and learn about the latest technologies.  One of our recent development projects involved injection molding a long, thin-walled tube (picture a miniature drinking straw) with a wall thickness that shrinks down to .0035” over its nearly half-inch length.  By comparison, that’s roughly the same thickness as a human hair.  Even after running dozens of Moldflow® studies for gating locations and flow analysis, the only thing we were confident of was that it was going to be a challenge to fill the parts out completely.

After struggling on our first sampling, the instinct was to look for higher flowing materials to help make the distance more manageable.  We started with a PE material with a Melt Flow Rating (MFR) around 50 g/10min and then moved on to a similar material with a MFR of 110.  We were expecting to see a noticeable improvement in the 110, but what we found was no appreciable difference on the fill.  It was determined that this was primarily due to leakage at the check ring / non-return valve, common to all traditional, reciprocating screw injection machines.

This brought us to one of the more interesting suggestions on the project.  We decided to sample the tool in one of Sodick-Plustech’s (SPT) micro injection machines.  This machine piqued our interests initially because of its two-stage (plunger-style) injection approach, but as we found is well-suited for this type of application for several reasons.

Like a traditional, reciprocating screw machine, Sodick’s two-stage injection technology (shown here) utilizes a small screw to melt and convey material.  But unlike traditional machines the screw is not responsible for injecting plastic into the mold (or any high speed lateral movements).  It feeds a second chamber, which is metered precisely, and then injected into the mold via a high-speed piston.  The feed screw shuts off after material is loaded into the chamber, which eliminates back-flow without the use of a check ring / non-return valve.

Two stage Injection MoldingPhoto courtesy of SPT

On this particular machine, the piston is capable of reaching injection speeds of 450 mm/s, which isn’t particularly impressive by today’s standards.  Many other press manufacturers tout injection speeds well past 1,000 mm/s.  Sodick, too, offers a high-speed/high-pressure line that boasts an impressive 1,500 mm/s injection speed, but their selling point is based more on acceleration than on speed alone.  The Sodick machine utilizes an accumulator that works with the main piston to reach maximum speed almost immediately upon injection.

The next selling point is the consistent shot sizes due to the tightly metered second chamber.  For our application, this is critical because an inconsistent fill could cause a short shot, which would be nearly impossible to detect with the human eye or a vision system during production.  On a project that could expand to a 16 or 32 cavity tool, this becomes increasingly critical to maintain good production parts.

Another positive about the machine is a more consistent melt and material residence time.  Again, the lack of a check ring helps by allowing for a more reliable first-in/first-out material path.  And since the feed screw isn’t creating excess heat via shear, the material is subject to more uniform heat profiles as it moves through the processing stages.

One last positive about the machine is the capability to swap out injection units (smaller or larger) and match them with differently-sized platen and tie bar configurations.  Matrix is running quite a few bioresorbable/bioabsorbable polymers lately which require minimal shot sizes due to the extremely high material costs.  However the molds associated with these projects are often complicated and require multiple side actions, slides, and/or lifters, so running them in a traditional micro-molding machine with a 4-inch max opening and similar small distances between the tie bars doesn’t always lend itself to the mold design.

Written By:
Andy Ziegenhorn



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