Voice: 812-752-7000, Fax: 812-752-5155, Email: Floyd@apmc.com

The capital investment in market research, project planning, product research, product design, design for manufacturability, gathering of quotations, along with the purchase of molds, dies, fixtures, special assembly machines, and promotion and advertising campaigns can be a severe blow to the corporate pocket before the launch of a new product.

It seems that the last dollars to be spent are for plastic molds. The last items to be ordered are the plastic molds. The most pressure for shorter lead time and lower cost is on the mold builder.

What happens when you get a cheap mold?

Lets just talk …ALIGNMENT. The worst result of bad alignment is uneven wall thickness. Three factors cause bad alignment: machining tolerances, wear of alignment devices, and stiffness of the steel assembly.

Attached is a computer simulation of the flow pattern of a bucket with an un even wall thickness. You will note that the thick side is completely filled while the thin side is only partially filled.

Assume we are making a bucket. We have two sides of the mold typically called cavity which makes the outside of the part and the core which makes the inside of the part. The customer specifies a wall thickness of .090" thick + or - .010". The mold builder assumes that he can use all of that tolerance in his machining process and makes the mold core off center such that on one side the wall thickness is .080" and the other .100".

The plastic material is injected into the center of the part . The plastic material flows easily down the side of the bucket with the thicker wall following the path of least resistance. With one wall already filled at low pressure, the thinner wall must be filled with much greater pressure. This induces uneven stress into the part. If the material freezes off before the thin wall fills, the pressure on the thick wall will tend to cause the core in the mold to shift toward the thin side.

What holds the core centered? That is where extra mold dollars are spent for stiffness. Cores are usually mounted on plates. The plates are held in alignment by leader pins. The larger the better.

A better more precise, longer term solution is to have the mold core enter into the cavity with a tapered steel to steel surface on the large diameter of the bucket to provide positive lock spread out over a large surface area of steel.

Plastic material is injected into molds at pressures sometimes as high as 30,000 pounds per square inch. If the plastic material is perfectly fluid, that means that a force of 15 tons per square inch may be exerted by each square inch of plastic surface area. If the core in a mold for a bucket of 10 inches deep and 10 inches in diameter has that pressure exerted only on one side, the result would be 3,000,000 pounds of side force. In practice the force might be one thirtieth of that due to material viscosity and lower injection pressures. However, a sideward pressure of 100,000 pounds is still a serious force to contend with.

But what about part cost. If a part can be made of acceptable quality at .080" thickness of the wall, then a specification of .082" + or - .002" would still give the molder and mold builder some tolerance to work with. It would result in a higher mold cost, but what happens to the piece price? It goes down by approximately 6%. While the packaging cost would be the same, the material cost would be about 9% less, molding time would be about 5% less, freight would be less, quality problems would be fewer, molded in stress would be uniform. Warpage and distortion would be minimized. By spending a little more for a mold, better part quality and lower piece price are yours.

What about multi-cavity molds? Cavity to cavity variation of wall thickness results in the mold only being able to cycle as fast as the slowest cavity. Parts having different dimensions will cause variation of assembly and possible jamming of automation equipment.

Twenty-six years ago I bought a pretty shinny new red garden tractor for $2,700. After two engine replacements and a few other parts, it died. Then, twenty-two years ago I bought a faded red used farm tractor for $3,700. All I do to it until this day is add gas, oil, and sharpen the blades. The difference is in alignment, stiffness, precision, and weight. Return on investment… no comparison.

Plastic molds are not much different.

For more information; contact Floyd Coates, author of "Our Toilets are Not for Customers, presenter of Plastic Part Design Seminars, President, American Plastic Molding Corporation, 965 South Elm Street, Scottsburg, Indiana, Phone: 812-752-7000, Fax: 812-752-5155, Email: Floyd@apmc.com, Website: www.apmc.com.