PT Blog

When selecting which die spring you need, start with determining the operating length, which is the amount the spring needs to compress. For the sake of example, let’s say a mold has an ejector stroke of 2 in. Perhaps you want to use the blue, medium-duty springs, which have a recommended operating range of 25% to 35% compression. Assume the annual run quantities of the mold are fairly low—about 100,000 cycles per year and you decide to use the 35% recommended long-life compression value. Therefore, the free length of your spring should be about 2 ÷ 0.35 = 5.7 in. Now round that value up to a standard length of 6 in.

If your mold has ¾-in. diam. return pins and that’s where you would like to mount the springs, you would need springs for a 1½-in. hole diam. Looking at the manufacturer’s chart (see table), a 6-in.-long spring with a 1½-in. diam. hole has a load of 17 lb at 1/10-in. deflection. Now let’s assume you estimated that 100 lb of force is needed to keep the ejector plates firmly against the stop buttons. Let’s also assume the mold is of average size and that you would like to use a total of four springs. Therefore, each spring needs to have 100 ÷ 4 = 25 lb of preload force. The preload distance would therefore need to be 25 ÷ 17 = 1.47 tenths of an inch, or 0.147 in.

The learning never stops. But now, thanks to the novel coronavirus, everything is different. Not a week passes when we don’t learn of yet another trade show, conference, or open house that’s been canceled as a result of COVID-19 social-distancing protocols. This year alone, our parent company Gardner Business Media has had numerous events canceled, including our own Molding and Extrusion Conferences.

But again, the learning never stops. I had one processor tell me that while his packaging business was soaring as a result of the pandemic, he’s concerned about the long-term implications of having his top engineers scattered about, some working at home and only at the plant from time to time. He’s worried about the exchange of ideas and solutions that can happen when technical people huddle during lunch or breaks. He’s worried about an innovation drain, about his technical team losing an edge by not being around each other and around innovators from outside their walls who can provide unique perspectives.

The Gardner Business Index (GBI) for plastics processors—based on monthly surveys of Plastics Technology subscribers—reported its first expansionary reading in July, coming in at 52.3, while the nation and economy continue to battle the spread of the novel coronavirus. Survey results provided by custom processors in particular also revealed better business conditions (Fig. 1).

Among the components that constitute the overall index, new orders and production both showed expanding activity in July vs. June (Fig. 2). All index components had higher readings in absolute terms for the month, with exports and backlogs reporting quickly slowing decline in their respective areas of activity.

Prices increased for each of the major five volume resins, though there was a possibility that they would plateau this month for PP, PS and PET. A key factor behind all these increases was feedstock costs. In addition, prices of  PE and PVC were also driven upward by strong domestic and export demand. Other factors affecting nearly all five volume resins included planned and unplanned feedstock and/or resin plant outages, tighter inventories, and some prebuying activity amidst the hurricane season. Weather-related uncertainties and issues related to COVID-19, including labor shortages, continue to be the “wild cards.”

These are the views of purchasing consultants from Resin Technology, Inc. (RTi), senior editors from PetroChemWire (PCW), and CEO Michael Greenberg of The Plastics Exchange.

Today, with all the focus on recycling, some thought must be applied to avoid processing issues when recycling coextruded or co-injection molded parts. Many plastic products are composed of several different polymer layers to provide special properties such as barrier, gloss, color, wear, etc. that are very thin but have to be considered when designing screws for recycling. This is particularly true for semi-crystalline polymers.

Semi-crystalline polymers do not soften at all until very near their melting point. For example, let’s consider a part made from a semi-crystalline polymer such as HDPE with an additional semi-crystalline layer such as PET co-extruded or overmolded. In that case, the HDPE melts at 266 F and the PET at 500 F. Since the bulk of the recycled article, say more than 90%, is HDPE, its normal processing temperature is typically in the range of 370-400 F. PET would not even begin to soften—let alone melt—at that temperature. To somehow get the overall processing temperature up to where the PET could also melt would degrade or burn the HDPE. The reverse issue also applies: If the main polymer were PET, the required processing temperature would heavily degrade the HDPE.