Design a BETTER INJECTION MOLDED PART
December 25, 2014 by Canadian Plastics
Think it’s tough putting together a decent jacket-shirt-tie combo in the morning? Try mastering the complexities of designing an injection molded appearance part for production runs in the hundreds of thousands. It’s as difficult as it sounds. “Small details in part design can prove to have a huge influence over the lifetime of the production phase, and can make the difference between a part that runs trouble-free and a ‘problem child’ that needs constant adjustment and repair,” said Kip Doyle, president of moldingHELP.com.
The good news is that adhering to some basic part design rules will result in an appearance part that looks good, is easier to manufacture and assemble, and is typically much stronger in service. Here’s a brief look at some of the key factors to keep in mind.
In injection molding as in life, stress kills. Stress is, in fact, the main enemy of any injection molded plastic part. Here’s why. When melted resin is forced into the mold, the molecules are pushed through each feature, bending, turning, and distorting to form the shape of the part. Turning hard or sharp corners exerts more stress on the molecule than taking gentle turns with generous radii. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to. And as the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are locked into the part, and can cause warpage, sink marks, cracking, premature failure, and other problems.
“While some stresses in an injection molded part are to be expected, you should design your parts with as much consideration for stress reduction as possible,” said John Bozzelli, president of Injection Molding Solutions. “Ways to do this include adding smooth transitions between features, and using rounds and fillets in possible high-stress areas.”
Each injection mold design requires a gate to allow the molten plastic to be injected into the cavity of the mold. Gate type, design, and location can impact the part packing, gate removal or vestige, cosmetic appearance of the part, and part dimensions and warping. There are two types of gates available for injection molding — manually trimmed and automatically trimmed — and perhaps the main factor to consider when choosing the proper gate type for your application is the gate design.
According to injection molding technology provider Quickparts Solutions Inc., the four most popular types are the edge gate, which is located on the edge of the part, is best suited for flat parts, and leaves a scar at the parting line; the sub gate, which is automatically trimmed, requires ejector pins, and leaves a pin-sized scar on the part; the hot tip gate, which is often located at the top of the part, is ideal for round or conical shapes where uniform flow is necessary, and leaves a small raised nub on the part surface; and the direct or sprue gate, which is said to be the easiest to design, has low cost and maintenance requirements, and leaves a large scar on the part at the point of contact.
Gate design is definitely not to be taken lightly. “Many problems are a result of part designers and tool designers underestimating and improperly predicting the impact of the decisions relating to the use of hot runner gates,” said Kip Doyle. “It’s an absolutely critical design element.”
It’s hard to overestimate the importance of wall thickness to plastic parts. “Choosing the proper wall thickness for your part can have drastic effects on the cost and production speed of manufacturing,” said John MacDonald, director of sales in Canada for Solid Concepts Inc. “While there are no wall thickness restrictions, the goal is usually to choose the thinnest wall possible, since this uses less material and reduces cost, cooling time, and cycle time.”
On average, MacDonald continued, the wall thickness of an injection molded part ranges from 2 mm to 4 mm (0.080 inches to 0.160 inches). Thin-wall injection molding can produce walls as thin as 0.05 mm (0.020 inches).
Not surprisingly, cooling time varies according to wall thickness, with a ripple effect on quality as the part cools. “During the cooling process, if walls are an inconsistent thickness, the thinner walls will cool first while the thick walls are still solidifying,” said John Bozzelli. “As the thick section cools, it shrinks around the already solid thinner section, causing warping, twisting or cracking to occur where the two sections meet.” How do you avoid these problems? “Try to design with completely uniform walls throughout the part,” Bozzelli said. “When uniform walls aren’t possible, then the change in thickness should be as gradual as possible to avoid stress concentrations and abrupt cooling differences. Also, wall thickness variations shouldn’t exceed 10 per cent in high mold shrinkage plastics.”
And since the amount of shrinkage is polymer-dependent, resin should be chosen with care. But of course that doesn’t always happen. “All too often, the material selection process is rushed, and based mostly on choosing a material that has already been used for a similar part,” said Kip Doyle. “With thousands of variations of materials available, the designer must take time and do the proper research to make the best choice not only for the function of the part, but for the lowest overall cost in the long-run — which isn’t the same thing as picking the least expensive material.”
The material chosen should meet all functional requirements, Doyle continued, and also be easy to handle and process. “Processability, in particular, is rarely given proper consideration,” he said. “Part designers don’t always consider the fact that a lower viscosity material that meets the functional requirements helps to reduce process variation.”
And whatever resin is ultimately selected for the part, the manufacturer’s process guidelines should always be followed for best results.
Other options for complex parts that need variations on wall thickness include using design features such as coring or using ribs. “Rib thickness should be less than wall thickness to minimize sinking effects, and the recommended rib thickness should not exceed 60 per cent of the nominal thickness,” said John MacDonald. “Plus, the rib should be attached with corner radii as generous as possible, and the height of a rib should be limited to less than three times its thickness.” It’s also better to use multiple ribs to increase bending stiffness than to use one very tall rib, he added.
DRAFT AND TEXTURE
Most injection molded plastic parts include features that we’ve already encountered — such as outside walls and internal ribs — that are formed by opposing surfaces of tool metal inside a closed mold. To allow the part to break free as soon as the mold opens, the side walls of the mold are tapered in the direction of the opening, a process usually referred to as “draft in the line of draw”. The amount of draft required can depend on the surface finish of the mold. A smooth, polished tool surface usually allows the part to eject with less draft than a standard tool surface. The amount of draft required (in degrees) varies with the geometry and surface texture requirements of the part. “Allowing for as much draft as possible will permit parts to release from the mold easily,” said John MacDonald. “Typically, one to two degrees of draft with an additional 1.5 degrees per 0.25 mm depth of texture is enough to do the trick.”
On a related note, texturing — a process used to apply patterns to a mold surface — allows flexibility in creating the final appearance of your parts, and can also be used to camouflage gate marks or other part imperfections. Draft for texturing is somewhat dependent on the part design and on the specific texture desired. According to Quickparts Solutions, when applying a texture to a part the CAD drawing must be adjusted to accommodate for this surface variance. “If the texture is on a surface that is perpendicular or angled away from the mold opening, then no draft changes are necessary,” the company said. “If the texture is on a parallel surface with the mold opening, however, increased draft is necessary to prevent scraping and drag marks during part ejection.” And remember, different textures have different impacts on the molded part. “The rule of thumb when designing for texture is to have 1.5 degrees of draft for each 0.001 inch of texture finish depth,” the company said.
There’s no getting away from part lines, which are the lines of separation on the part where the two halves of the mold meet. “While on simple parts this plane can be a simple, flat surface, it’s often a complex form that traces the perimeter of the part around the various features that make up the part’s outer silhouette,” said John MacDonald. Part lines can also occur where any two pieces of a mold meet, he continued, including side action pins, tool inserts, and shutoffs. “Parting lines cannot be avoided; every part has them,” MacDonald said. “When designing your part, remember that the melt will always flow towards the parting line because it’s the easiest place for the displaced air to escape.”
Sharp corners greatly increase the stress concentrations that can lead to part failure. “Sharp corners often occur in non-obvious places, such as a boss attached to a surface or a strengthening rib,” said John MacDonald. “The radius of sharp corners needs to be watched closely because the stress concentration factor varies with radius for a given thickness.” Rule of thumb? “At corners, the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness,” MacDonald said. “A bigger radius should be used if the part design allows it.”
In the end, the old adage that the devil is in the details has rarely been more true than when designing a complex part. “Today’s molders are pushing the limits in so many ways — thinner, smaller, bigger, two-shot molding, insert molding — that there isn’t much room for error,” said Kip Doyle. “I always recommend that the part design phase involve a team of experts from all departments, and also that additional precautionary measures be taken — flow simulation, third party review, and prototyping — before cutting production steel.” Because it’s a whole lot easier to avoid problems in the beginning than to change your design down the line.