During Mold Development, Which Points on the Product Drawing Must Be Carefully Reviewed?

In mold development, the product drawing is the starting point of the entire process. Only after the mold maker receives the product drawing will they begin to evaluate whether it can be made, how to make it, how much it will cost, and how long it will take. For the buyer, the product drawing seems like something they sent out themselves and are therefore already "familiar" with. But in reality, many disputes, mold repairs, and delays in mold development stem precisely from the places on the product drawing that were not carefully reviewed.

The product drawing answers "what this part ultimately looks like and what requirements it must meet." Many risks in mold development are hidden in the details of this drawing. The following points must be carefully reviewed before the mold is cut.

First, Dimensions and Tolerances: Which Are Critical Dimensions and Which Are Reference Dimensions

When you first look at a product drawing, you usually look at the overall dimensions—length, width, height, wall thickness, hole diameter. But what truly deserves time is which dimensions carry tolerances and which do not.

Dimensions with tolerances are those the designer believes need to be controlled; dimensions without tolerances are usually handled according to unspecified tolerances, with a much wider range. If every dimension on the drawing carries a tight tolerance, it means the machining and molding difficulty of this mold will increase significantly, and the cost will rise accordingly. Conversely, if critical fit dimensions have no tolerances and are only governed by unspecified tolerances, there may be problems of too-loose or too-tight fits during subsequent assembly.

Particular attention should be paid to fit dimensions and functional dimensions. For example, the fit clearance of snaps, the fit tolerance of shaft holes, and the flatness requirements of assembly surfaces. If these dimensions are not clearly annotated on the drawing, or if the annotated tolerances do not match the shrinkage characteristics of the plastic material, the mold may produce parts that cannot be assembled or do not fit tightly.

Another easily overlooked point: what the dimension datum is on the drawing. Which surface or line is used as the datum when annotating dimensions directly affects positioning and measurement during mold machining. If the datum is unclear, or if the datum itself is a process boss that will later be removed, machining and inspection will disagree.

Second, Wall Thickness: Is It Uniform, and Are Transitions Reasonable

Wall thickness is a very critical parameter in mold development, but on product drawings it often appears as just a few lines and is easily glossed over.

What must be carefully reviewed: whether the wall thickness is uniform throughout the product and whether thick-to-thin transitions are gradual. Products with uneven wall thickness will have slow cooling and large shrinkage in thick-wall areas, and fast cooling and small shrinkage in thin-wall areas. Internal stress will develop between the two, causing warping, sinking, or even cracking. If the drawing shows wall thickness suddenly jumping from 3 mm to 6 mm without a gradual transition in between, that is a problem that must be raised before the mold is cut.

For frame-type, shell-type, and flat-type products, also pay attention to the ratio of rib thickness to main wall thickness. If ribs are too thick, the root will sink; if too thin, they will not provide reinforcement. Typically, rib thickness is 50% to 70% of the main wall thickness, but this ratio needs to be adjusted according to the material. If the drawing shows ribs as thick as the main wall, sinking is almost inevitable.

In addition, the relationship between wall thickness and flow length ratio also deserves attention. The thinner the wall, the greater the melt flow resistance, the higher the required injection pressure, and the greater the flash risk. If the product has a very thin overall wall thickness combined with large overall dimensions, the difficulty of both mold design and injection molding process rises significantly.

Third, Draft Angle: Is It Annotated on the Drawing, and Is It Sufficient

Draft angle is the item most easily missing from product drawings but most indispensable in mold development. After the product cools in the mold, it will tightly wrap around the core or become stuck in the cavity. Without sufficient draft angle, the product will be scratched, stress-whitened, or even torn during demolding.

What must be carefully reviewed: whether draft angles are annotated on the drawing, what the annotated values are, and which surfaces have them and which do not. Many product drawings only annotate the dimensions of vertical surfaces without annotating draft, or only write in the technical requirements "unspecified draft angle per 1°." In such cases, it is necessary to confirm whether this 1° applies to all surfaces, especially deep cavities, slender ribs, and textured surfaces.

For products with surface texture, draft angle needs to be increased additionally. The deeper the texture, the greater the draft angle required. If the drawing specifies texture requirements but does not correspondingly increase the draft angle, the textured surface will be marred or whitened during demolding after the mold is made.

One more detail: the direction of the draft angle. Which direction the draft is applied—whether it allows the product to release from the core or from the cavity during demolding—determines the mold's parting surface design and ejection scheme. If the drawing only says "add draft angle" without specifying the direction, it may be applied in the wrong direction during machining.

Fourth, Fillets: Where Fillets Are Needed and Where They Must Not Be

Fillets usually appear on product drawings in the form of R values. They may seem like a minor matter, but they have a significant impact on mold development.

What must be carefully reviewed: whether internal and external corners of the product have annotated fillets and whether the fillet sizes are reasonable. If internal corners have no fillets, stress will concentrate at the sharp corners, and the product will be prone to cracking under load. If external corners have no fillets, the sharp corners of the mold are prone to chipping during machining, and the melt flow will be poor during injection. Generally, the internal corner fillet radius is recommended to be 0.5 to 1 times the wall thickness, and the external corner is the internal corner plus the wall thickness.

But fillets are not a case of "the more, the better" or "the bigger, the better." Fillets near the parting surface require special attention. If the product has a large fillet at the parting surface location and the mold does not have a corresponding sealing structure, flash will escape along the fillet. In addition, assembly surfaces, sealing surfaces, and sliding fit surfaces usually do not allow fillets. If the drawing annotates fillets at these locations, or does not clearly state that fillets are not allowed, subsequent assembly will run into problems.

One more point: the coordination between fillets and wall thickness. If the fillet radius is greater than the wall thickness, the actual wall thickness at the fillet will change, potentially causing sinking or stress concentration. If the drawing only annotates the fillet without considering its relationship to wall thickness, the product appearance and strength may fail to meet standards after the mold is made.

Fifth, Holes, Slots, and Undercuts: Are They There, Where Are They, and Can They Be Made

Holes, slots, and undercuts on the product are the main sources of mold structural complexity. Every hole may mean a core pin or a slide; every undercut may mean a set of side core-pulling mechanisms.

What must be carefully reviewed: the position, diameter, and depth of holes, as well as the direction of the holes. If the hole direction is not consistent with the mold opening direction, side core-pulling or lifters are required. If the hole is very deep and the diameter is very small, the core pin is prone to breaking, and mold life will be affected. If the hole position happens to be on the parting surface, the edges of the hole are prone to flash.

Undercuts are the features that most require vigilance. The product drawing may show only a small snap or groove, but to realize it in the mold, slides, lifters, or complex insert structures may be needed. Carefully review the depth and direction of the undercut, and whether there is enough surrounding space for the slide to retract. If the undercut is too deep or the space is too narrow, the mold may simply be impossible to make, or may be made with very low yield.

In addition, threads, gear teeth, and special textures also require special attention. These features may appear on the drawing as just symbols or notes, but they require dedicated demolding mechanisms or machining processes in the mold.

Sixth, Appearance Requirements: Which Surfaces Are Visible Surfaces, and What Are the Requirements

Product drawings usually contain some notes about appearance, such as "no sinking, flash, stress whitening, or gate marks allowed on the surface" or "visible surface is Class A surface, roughness Ra0.8." These texts may look like quality requirements, but they actually directly determine the mold design scheme.

What must be carefully reviewed: which surfaces are visible surfaces, and what is allowed and not allowed on them. If the product's visible surface happens to be where the parting surface is located, then flash and parting lines will appear on the visible surface—this needs to be confirmed with the mold maker in advance. If gate marks are not allowed on the visible surface, then the gate must open on a non-visible surface, or a special gate type must be used. If stress whitening is not allowed on the visible surface, then ejector pins cannot push on the visible surface, and another ejection method must be used.

For products with surface treatment requirements such as grain, etching, sandblasting, or electroplating, appearance requirements also need to be considered together with draft angle, mold material, and polishing process. The higher the appearance requirements, the greater the mold cost and molding difficulty—all of which need to be assessed clearly before the mold is cut.

Seventh, Material: What Plastic Is It, What Is the Shrinkage Rate, and Are There Special Requirements

Product drawings usually note the material type, such as ABS, PP, PC, PA66+GF30, etc. Material information looks simple, but it directly affects multiple aspects of mold design.

What must be carefully reviewed: what the material's shrinkage rate is, whether it is isotropic or anisotropic, and whether it contains fillers. Different materials have very different shrinkage rates, and the same material may shrink differently in different directions. If the product drawing only states the material name without providing shrinkage data, the mold maker needs to look it up or confirm with the buyer. If the buyer themselves is also unclear, the mold design may be based on an incorrect shrinkage rate, causing product dimensions to be out of tolerance.

Glass-fiber-containing materials require special attention. The shrinkage rate of glass-fiber-reinforced materials has obvious directionality, and their flowability is relatively poor. Both gate design and cooling design need to be adjusted accordingly. In addition, glass-fiber materials cause significant mold wear, and more wear-resistant solutions need to be selected for mold core material and surface treatment.

If the material is corrosive (such as PVC or materials containing flame retardants), the mold material needs corrosion protection. If the material is temperature-sensitive, the cooling channel design needs to be more uniform. All of this information needs to be confirmed starting from the material annotation on the product drawing, combined with the material supplier's technical data.

Eighth, Technical Requirements and Unspecified Notes: The Most Easily Skipped, but the Least Skippable

The corner of a product drawing usually has a "Technical Requirements" section containing unspecified fillets, unspecified tolerances, draft angles, surface treatment, deburring requirements, packaging requirements, and so on. This part is often the most easily skipped, but it is extremely critical in mold development.

What must be carefully reviewed: what standard the unspecified tolerances follow, what the unspecified fillet is, whether there are default draft angle requirements, and what the specific standards for surface treatment are. These texts may seem trivial, but they are the basis for mold design and subsequent acceptance. If the drawing states "unspecified tolerances per GB/T 1804-m" while the buyer requires a stricter standard during acceptance, the two parties will disagree. If the drawing states nothing at all, there is no basis for acceptance, and it will be difficult to determine responsibility when problems arise.

In addition, the version number and date of the drawing also need to be confirmed. Product drawings may undergo multiple revisions. If the buyer sends an old version and the mold maker builds to that old version, then when the product design changes later, responsibility and costs will be unclear. Confirming the drawing version before cutting the mold is a basic action to avoid subsequent disputes.

Ultimately, the Product Drawing Is the "Input" for Mold Development

The quality of mold development depends to a large extent on the completeness and accuracy of the input information. As the most core input, if the product drawing contains missing information, unclear annotations, or contradictory requirements, the mold maker will either proceed according to their own understanding or repeatedly ask for confirmation. Either way, risks and costs increase.