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High Quality Molds Start with High Quality Part Design

Part 2 of 6 in the “How to Build Quality Molds in China” Series

Written by Greg Pugh, Project Engineer for GSM

Molders are in the business to produce and sell parts that meet or exceed their customers’ requirements. The first part of this equation is having a high quality part design. Mold longevity and maintenance expense is often dictated by the part design. Are the seal-off angles adequate? Does the design require steel features that will be fragile? Some parts can’t escape such issues and must be worked around on the mold design side, but in most cases, the right part design will allow you to make good, repeatable parts that you can proudly sell to your customers. If you neglect good part design practices, you are already working at a deficit for future process capability, no matter how good your mold is. How do we avoid these pitfalls?

The answer seems simple enough – just follow the rules! Plastics professionals like Bob Hatch have produced article after article over the years troubleshooting parts with capability issues. It would be interesting to note how many times the same rule has been violated with the same result over a few years of articles. When I was in the tech service business with a major resin distributor many years ago, I participated in “Molder’s Workshops” that went through information on best practices in part and mold design along with processing methods. When I was asked to prepare a presentation on part design rules, it struck me just how few rules there really are in injection molding part design and how little time it would take to review them. This is my list:

  1. Where possible, maintain nominal wall thickness. Plastic flows to least resistance. Uneven wall thicknesses create back-filling and gas traps that can be a nightmare to correct after the fact.
  2. Design rib and boss thickness relative to nominal wall thickness. (50-60% of nominal) Very thin ribs do not fill properly. Thick ribs create sinks, dimensional instability, and longer cycles.
  3. Maintain adequate draft angles for vertical surfaces for the surface finish and material shrink characteristics, allowing mold open and part ejection without part distortion. Generally ½ degree on ribs, 1 degree on exterior surfaces minimum. (also follow texture depth guidelines)
  4. Maintain adequate shutoff angles on slots and part stepped features that require vertical steel projections to avoid mold galling and resulting flash. (5 degrees minimum)
  5. Radius sharp corners to maintain nominal wall and reduce material stress.

With the above list, you could then add a couple of part rules related to the gating system.

  1. When you can’t maintain nominal wall thickness, fill parts from thick to thin (this allows you to more fully pressurize thick areas and control shrinkage better)
  2. Gate in such a way to equalize the flow path distance to the ends of the part. On long parts, use multiple gates to equalize pressure across the part, reducing warpage.
  3. On glass fiber-filled parts, either gate at one end to maintain fiber alignment and dimensional predictability or particularly on round parts, gate in multiple places (3 or more) at equal angles to produce even glass orientation distributions. Flow and transverse shrink rates vary widely for glass filled material. In the flow, the shrink rate is determined primarily by the glass fiber (very low) and transverse to flow, it is determined primarily by the resin (very high in semi-crystalline materials).

 

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Above is an example of what can happen when these rules aren’t followed. We’ve all seen them. At GSM, we provide a DFM (design-for-manufacturing) report on every part design as we begin the preliminary mold design process. This generally happens within 2 days of the release of a new order. This process identifies mold and slide parting surfaces, any draft issues, varying wall thickness that could create molding problems, and gate location recommendations. This allows us to alert customers of problems and provide opportunities to correct them up front. Correcting them later gets more costly and adds significant stress to all involved. We understand that our customers are in the business to make parts day after day from the molds we build. If you can’t make parts competitively and of consistently high quality from our molds, you will not come back for more business.

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