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Plastic Injection Modular Molding Cost Benefits

Manufacturers who need small precision parts in relatively short production runs often find that molding in plastic is too expensive given the cost of tooling, machine time and other factors. Modular molding helps hold down such costs, and so is a way they can gain higher production efficiencies for runs of up to four million pieces.

Modular molding is an advanced version of family molding that molds dissimilar parts in the same frame. This method typically gangs six to 10 inserts in a proprietary frame, although up to 32 inserts can be used for very small parts. Each insert can accommodate several identical parts on the same tree.

Applications for modular molding run the gamut from golf tees to space shuttle parts. It is commonly used to fabricate fasteners, electronic connectors, architectural components and parts for electrical and auto systems.

The potential for savings depends largely on the number of parts per run, part size, turnaround requirements, resin used and tool complexity. In one case, a manufacturer adopted modular molding after learning he could use two cavities costing $10,000 in a modular system versus a $40,000, six-cavity tool in a conventional system and still meet his parts requirements.

In another case, a manufacturer was tooling up to make 160,000 1/8" thick flat washers per month. The washer had a 3/4" OD and a 1/2" ID. In a modular system, this would require a four-cavity tool costing approximately $1,500. Tool fabrication would take four to six weeks. The same part in a conventional system would call for a 16-cavity tool costing at least $5,500 and take 10 to 12 weeks to fabricate.

Production turnaround is also a factor in choosing between the two. Modular systems generally have two-week lead times, although turnaround can be done in as little as 24 hours. Lead times in conventional molding are on the order of one month. Production runs on conventional machines often last a week or more, so new jobs may have to wait several weeks.

Resin usage and fight scheduling also help shorten modular's lead time. Modular molders tend to use the same resin in consecutive runs and move cavities in and out of the frame as the schedule demands. A conventional molder, by contrast, may switch the resin and mold frame at the end of a run. Changing the resin usually means altering run conditions, which requires a period of adjustment. Changing the frame takes more time than just changing inserts.

On-going molding cost savings are also a factor. In a conventional system, the machine's entire capacity is devoted to one customer. Large production numbers are required for economy. Modular molding users pay by the square inch for the insert space they occupy in the frame.

The cost benefit of using a modular system is often a tradeoff between part size and estimated annual volume. For example, a part with a surface area of 0.5 square inches works best in modular frames when production levels are less than four million pieces. A 2.5 square inch part is generally cost effective in a modular system at up to 250,000 pieces.

Modular molding allows for a quick response to seasonal surges and peak demands because inserts can be added rapidly. It also works well for just-in-time procurement because the molder can schedule short weekly or monthly runs.

Modular systems also allow for quick turnaround when there are minor product changes, since individual inserts rather than entire tools-are modified.

When an insert is damaged or needs maintenance, it can be removed and replaced with another without affecting the surrounding inserts. The loss of one insert does not stop production, as n-thought be the case if a cavity is lost in a multicavity tool. Modular molding also cuts prototype lead time to half that of a conventional mold because only inserts are built.

A part is a likely candidate for modular molding if the following questions can be answered with a yes:

• Can a general purpose resin (vs. specialty grade) be used?
• Is parting line surface area less than 9 square inches?
• Is parting line length less than 7"?
• Is part depth less than 2"?
• Is inside diameter less than 1.25" long?
• Is wall thickness between 1/32" and 1/8"?
• Are tolerances greater than .001"?
• Is the finish requirement at or below SPE #3?
• Do part size and annual volume fall within cost-justifications?

The modular frame is also a time saver. It is constructed so that a series of simple keys lock the mold into the frame. Mold cavities often can be switched in as little as five or ten minutes. Switching conventional frames demands hours of downtime and often the use of a crane.

Modular molders operate at high volume to make short-runs pay off. They use sophisticated scheduling and project management techniques to meet fight production deadlines that mate the tight operating conditions with the tight resin.

Modular molders generally use multipurpose engineering plastics. The key to producing the same parts time and again is product consistency and the availability of technical support to keep the entire process on track.

The modular process has some limitations. The demanding schedule and variety of parts molded simultaneously allow little room to customize the process for the needs of a specific part. Modular molding is best used with parts having wall thicknesses between 1/32" and 1/8" and diameters to 3". Optimum tolerance is generally +/- 0.001" or larger. The inserts can accommodate up to two gates and up to two actions plus the direction of the press. As an initial test to see if a part is suitable for modular molding evaluate it against the accompanying checklist.

An industrial distributor supplied office furniture manufacturers with about a million acetal file rollers per year. Before switching to modular molding, he made 500,000 parts at one time in a conventional molding system to control costs and warehoused them for use over a six-month period. His part cost was approximately $30 per thousand, or $15,000 per run.

By going to modular molding, the distributor could run 100,000 parts per month, which decreased per part cost to $25 per thousand per month and his cash outlay to $2,500. He also reduced his tooling expense because he needed only three mold cavities (at roughly $2,700) instead of 16 cavities (at about $10,000).

In another case, a manufacturer of small electric motors for timers and controllers wanted to fabricate a complex brake pawl in nylon 6/6. The 10- to 20 cavity tool required in conventional molding made the process prohibitive. Modular molding reduced this to just two cavities in one insert. The manufacturer absorbed the cost of the tool and kept per-part costs within budget. Modular production also met seasonal upswings in production without sacrificing cost or quality.

As manufacturers seek to occupy niche markets, modular molding provides a highly cost-efficient way to adapt the molding process for small precision parts, especially in low to medium volumes. The method offers flexibility and rapid turnaround for those involved in just-in-time manufacturing or those who must respond rapidly to swings in demand. When all the benefits are considered, modular molding clearly has an important and growing role in today's competitive manufacturing environment.