Plastic Product Design Guidelines – A Complete Beginner's Guide
- mailtoritheesh
- Jun 2
- 3 min read
Updated: Jun 9
Table of Contents
Designing a plastic product isn't just about how it looks or functions—it’s also about how easily and efficiently it can be manufactured. Many product designs, although great in theory, fail during production due to issues like molding defects, impractical geometries, or safety hazards. To avoid such pitfalls, designers follow established plastic product design guidelines. This blog provides a beginner-friendly overview of these essential guidelines.
Before diving into the rules, let's understand why these guidelines are critical:
Ease of Manufacturing
Lower Production Cost by reducing time and simplifying the process
Eliminate Defects in the final product
Improve Safety for end users
Ensure High Product Quality
Plastic Product Design Guidelines
1. Tooling Direction
The tooling direction is the direction in which the mold opens and the part is ejected. Molds have two halves—core and cavity. The core and cavity close to fill molten plastic and then separate in a straight line to eject the part.
All features in the product should be designed with respect to the tooling direction.
It becomes the first reference line in any plastic product design.
2. Parting Line
The parting line is the dividing line between the core and cavity of the mold. It usually:
Marks the boundary where draft angles change direction
Appears as a sharp edge that separates the inside and outside of the part
Is the only sharp edge intentionally present in a plastic part
Correctly placing the parting line is essential for clean part ejection.
3. Draft Angle
A draft is a slight taper given to vertical walls of the part to facilitate easy removal from the mold.
A minimum of 0.5° draft is required
Draft is always aligned with the tooling direction
Without draft, parts may stick inside the mold or get damaged during ejection
4. Part Thickness
Maintaining proper part thickness is vital for quality
Recommended: 2–4 mm
Plastic shrinks during cooling, and excessive thickness causes:
Voids
Sink marks
Internal stresses
Warping or deformation
To avoid these defects:
Maintain uniform thickness throughout the part
If change is necessary, it should be gradual
The transition length should be at least 3× the wall thickness
5. Undercuts
Undercuts are part features that interfere with straightforward ejection from the mold.
They are usually perpendicular to the tooling direction
Require additional mechanisms like sliders or lifters to remove
This increases cost and complexity
To solve this, adjust the parting line to split the undercut feature between core and cavity or use mechanisms.
6. Remove Sharp Edges
Sharp edges in plastic parts should be avoided due to:
Stress concentration, increasing risk of cracks
Restricted plastic flow, affecting part filling
Poor heat dissipation, leading to voids
Use fillets (rounded corners) instead
Minimum radius: 0.3–0.5 mm or 0.3–0.5 × wall thickness*
7. Shut-Off Angle
Shut-off angles are used in mold regions where core and cavity meet to block the flow of plastic and form slots or holes.
Helps prevent core-cavity collision
increases mold life and ensures proper sealing
Recommended shut-off draft angle: 3° to 5°
Types of Shut-Offs:
1. Flat – Perpendicular surface-to-surface shut-off
2. Wipe – Creates side openings with simple rectangular profiles
3. Saddle – Most common; wraps around the side surface with a single shut-off wall
Thick Steel/Core Features
Since molds are the *negative* of the product:
Closely spaced features lead to thin mold walls
Thin walls wear out quickly, reducing mold life
Ensure adequate spacing between features to maintain strong mold walls and extend mold life.
9. Thick Cavity
Very thin part features make it hard for molten plastic to fill the mold completely.
Plastic might not reach narrow corners, causing defects
Ensure enough thickness for complete and defect-free molding
Conclusion
Designing a plastic product goes beyond appearance and functionality. It’s about making the design manufacturable, reliable, and cost-effective. Following these standard guidelines ensures:
Fewer defects
Smooth production
Better end-user safety
Lower costs and waste
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